socat(1) socat(1) NAME socat - Multipurpose relay (SOcket CAT) SYNOPSIS socat [options]
socat -V socat -h[h[h]] | -?[?[?]] filan procan DESCRIPTION Socat is a command line based utility that establishes two bidirec‐ tional byte streams and transfers data between them. Because the streams can be constructed from a large set of different types of data sinks and sources (see address types), and because lots of address op‐ tions may be applied to the streams, socat can be used for many differ‐ ent purposes. Filan is a utility that prints information about its active file de‐ scriptors to stdout. It has been written for debugging socat, but might be useful for other purposes too. Use the -h option to find more infos. Procan is a utility that prints information about process parameters to stdout. It has been written to better understand some UNIX process properties and for debugging socat, but might be useful for other pur‐ poses too. The life cycle of a socat instance typically consists of four phases. In the init phase, the command line options are parsed and logging is initialized. During the open phase, socat opens the first address and afterwards the second address. These steps are usually blocking; thus, especially for complex address types like socks, connection requests or authentication dialogs must be completed before the next step is started. In the transfer phase, socat watches both streams’ read and write file descriptors via select() , and, when data is available on one side and can be written to the other side, socat reads it, performs newline character conversions if required, and writes the data to the write file descriptor of the other stream, then continues waiting for more data in both directions. When one of the streams effectively reaches EOF, the closing phase be‐ gins. Socat transfers the EOF condition to the other stream, i.e. tries to shutdown only its write stream, giving it a chance to terminate gracefully. For a defined time socat continues to transfer data in the other direction, but then closes all remaining channels and terminates. OPTIONS Socat provides some command line options that modify the behaviour of the program. They have nothing to do with so called address options that are used as parts of address specifications. -V Print version and available feature information to stdout, and exit. -h | -? Print a help text to stdout describing command line options and available address types, and exit. -hh | -?? Like -h, plus a list of the short names of all available address options. Some options are platform dependend, so this output is helpful for checking the particular implementation. -hhh | -??? Like -hh, plus a list of all available address option names. -d Without this option, only fatal and error messages are gener‐ ated; applying this option also prints warning messages. See DI‐ AGNOSTICS for more information. -d -d Prints fatal, error, warning, and notice messages. -d -d -d Prints fatal, error, warning, notice, and info messages. -d -d -d -d Prints fatal, error, warning, notice, info, and debug messages. -D Logs information about file descriptors before starting the transfer phase. -ly[] Writes messages to syslog instead of stderr; severity as defined with -d option. With optional , the syslog type can be selected, default is "daemon". Third party libraries might not obey this option. -lf Writes messages to [filename] instead of stderr. Some third party libraries, in particular libwrap, might not obey this option. -ls Writes messages to stderr (this is the default). Some third party libraries might not obey this option, in particular lib‐ wrap appears to only log to syslog. -lp Overrides the program name printed in error messages and used for constructing environment variable names. -lu Extends the timestamp of error messages to microsecond resolu‐ tion. Does not work when logging to syslog. -lm[] Mixed log mode. During startup messages are printed to stderr; when socat starts the transfer phase loop or daemon mode (i.e. after opening all streams and before starting data transfer, or, with listening sockets with fork option, before the first accept call), it switches logging to syslog. With optional , the syslog type can be selected, default is "daemon". -lh Adds hostname to log messages. Uses the value from environment variable HOSTNAME or the value retrieved with uname() if HOST‐ NAME is not set. -v Writes the transferred data not only to their target streams, but also to stderr. The output format is text with some conver‐ sions for readability, and prefixed with "> " or "< " indicating flow directions. -x Writes the transferred data not only to their target streams, but also to stderr. The output format is hexadecimal, prefixed with "> " or "< " indicating flow directions. Can be combined with -v . -r Dumps the raw (binary) data flowing from left to right address to the given file. -R Dumps the raw (binary) data flowing from right to left address to the given file. -b Sets the data transfer block [size_t]. At most bytes are transferred per step. Default is 8192 bytes. -s By default, socat terminates when an error occurred to prevent the process from running when some option could not be applied. With this option, socat is sloppy with errors and tries to con‐ tinue. Even with this option, socat will exit on fatals, and will abort connection attempts when security checks failed. -t When one channel has reached EOF, the write part of the other channel is shut down. Then, socat waits [timeval] sec‐ onds before terminating. Default is 0.5 seconds. This timeout only applies to addresses where write and read part can be closed independently. When during the timeout interval the read part gives EOF, socat terminates without awaiting the timeout. -T Total inactivity timeout: when socat is already in the transfer loop and nothing has happened for [timeval] seconds (no data arrived, no interrupt occurred...) then it terminates. Useful with protocols like UDP that cannot transfer EOF. -u Uses unidirectional mode. The first address is only used for reading, and the second address is only used for writing (exam‐ ple). -U Uses unidirectional mode in reverse direction. The first address is only used for writing, and the second address is only used for reading. -g During address option parsing, don’t check if the option is con‐ sidered useful in the given address environment. Use it if you want to force, e.g., appliance of a socket option to a serial device. -L If lockfile exists, exits with error. If lockfile does not ex‐ ist, creates it and continues, unlinks lockfile on exit. -W If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues, unlinks lockfile on exit. -4 Use IP version 4 in case that the addresses do not implicitly or explicitly specify a version; this is the default. -6 Use IP version 6 in case that the addresses do not implicitly or explicitly specify a version. ADDRESS SPECIFICATIONS With the address command line arguments, the user gives socat instruc‐ tions and the necessary information for establishing the byte streams. An address specification usually consists of an address type keyword, zero or more required address parameters separated by ’:’ from the key‐ word and from each other, and zero or more address options separated by ’,’. The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some keywords there exist synonyms (’-’ for STDIO, TCP for TCP4). Key‐ words are case insensitive. For a few special address types, the key‐ word may be omitted: Address specifications starting with a number are assumed to be FD (raw file descriptor) addresses; if a ’/’ is found be‐ fore the first ’:’ or ’,’, GOPEN (generic file open) is assumed. The required number and type of address parameters depend on the ad‐ dress type. E.g., TCP4 requires a server specification (name or ad‐ dress), and a port specification (number or service name). Zero or more address options may be given with each address. They in‐ fluence the address in some ways. Options consist of an option keyword or an option keyword and a value, separated by ’=’. Option keywords are case insensitive. For filtering the options that are useful with an address type, each option is member of one option group. For each ad‐ dress type there is a set of option groups allowed. Only options be‐ longing to one of these address groups may be used (except with option -g). Address specifications following the above schema are also called sin‐ gle address specifications. Two single addresses can be combined with "!!" to form a dual type address for one channel. Here, the first ad‐ dress is used by socat for reading data, and the second address for writing data. There is no way to specify an option only once for being applied to both single addresses. Usually, addresses are opened in read/write mode. When an address is part of a dual address specification, or when option -u or -U is used, an address might be used only for reading or for writing. Considering this is important with some address types. With socat version 1.5.0 and higher, the lexical analysis tries to han‐ dle quotes and parenthesis meaningfully and allows escaping of special characters. If one of the characters ( { [ ’ is found, the correspond‐ ing closing character - ) } ] ’ - is looked for; they may also be nested. Within these constructs, socats special characters and strings : , !! are not handled specially. All those characters and strings can be escaped with \ or within "" ADDRESS TYPES This section describes the available address types with their keywords, parameters, and semantics. CREATE: Opens with creat() and uses the file descriptor for writing. This address type requires write-only context, because a file opened with creat cannot be read from. Flags like O_LARGEFILE cannot be applied. If you need them use OPEN with options create,create. must be a valid existing or not existing path. If is a named pipe, creat() might block; if refers to a socket, this is an error. Option groups: FD,REG,NAMED Useful options: mode, user, group, unlink-early, unlink-late, append See also: OPEN, GOPEN EXEC: Forks a sub process that establishes communication with its par‐ ent process and invokes the specified program with execvp() . is a simple command with arguments separated by single spaces. If the program name contains a ’/’, the part af‐ ter the last ’/’ is taken as ARGV[0]. If the program name is a relative path, the execvp() semantics for finding the program via $PATH apply. After successful program start, socat writes data to stdin of the process and reads from its stdout using a UNIX domain socket generated by socketpair() per default. (exam‐ ple) Option groups: FD,SOCKET,EXEC,FORK,TERMIOS Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, login, sigint, sigquit See also: SYSTEM FD: Uses the file descriptor . It must already exist as valid UN*X file descriptor. Option groups: FD (TERMIOS,REG,SOCKET) See also: STDIO, STDIN, STDOUT, STDERR GOPEN: (Generic open) This address type tries to handle any file system entry except directories usefully. may be a relative or absolute path. If it already exists, its type is checked. In case of a UNIX domain socket, socat connects; if connecting fails, socat assumes a datagram socket and uses sendto() calls. If the entry is not a socket, socat opens it applying the O_AP‐ PEND flag. If it does not exist, it is opened with flag O_CREAT as a regular file (example). Option groups: FD,REG,SOCKET,NAMED,OPEN See also: OPEN, CREATE, UNIX-CONNECT IP-SENDTO:: Opens a raw IP socket. Depending on host specification or option pf, IP protocol version 4 or 6 is used. It uses to send packets to [IP address] and receives packets from host, ignores packets from other hosts. Protocol 255 uses the raw socket with the IP header being part of the data. Option groups: FD,SOCKET,IP4,IP6 Useful options: pf, ttl See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV, UDP-SENDTO, UNIX-SENDTO INTERFACE: Communicates with a network connected on an interface using raw packets including link level data. is the name of the network interface. Currently only available on Linux. Op‐ tion groups: FD,SOCKET Useful options: pf, type See also: ip-recv IP4-SENDTO:: Like IP-SENDTO, but always uses IPv4. Option groups: FD,SOCKET,IP4 IP6-SENDTO:: Like IP-SENDTO, but always uses IPv6. Option groups: FD,SOCKET,IP6 IP-DATAGRAM:
: Sends outgoing data to the specified address which may in par‐ ticular be a broadcast or multicast address. Packets arriving on the local socket are checked if their source addresses match RANGE or TCPWRAP options. This address type can for example be used for implementing symmetric or asymmetric broadcast or mul‐ ticast communications. Option groups: FD, SOCKET, IP4, IP6, RANGE Useful options: bind, range, tcpwrap, broadcast, ip-multi‐ cast-loop, ip-multicast-ttl, ip-multicast-if, ip-add-membership, ip-add-source-membership, ttl, tos, pf See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO, IP-RECVFROM, IP-RECV, UDP-DATAGRAM IP4-DATAGRAM:: Like IP-DATAGRAM, but always uses IPv4. (example) Option groups: FD,SOCKET,IP4,RANGE IP6-DATAGRAM:: Like IP-DATAGRAM, but always uses IPv6. Please note that IPv6 does not know broadcasts. Option groups: FD,SOCKET,IP6,RANGE IP-RECVFROM: Opens a raw IP socket of . Depending on option pf, IP protocol version 4 or 6 is used. It receives one packet from an unspecified peer and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process. This allows a behaviour similar to typical UDP based servers like ntpd or named. Please note that the reply packets might be fetched as incoming traffic when sender and receiver IP address are identical be‐ cause there is no port number to distinguish the sockets. This address works well with IP-SENDTO address peers (see above). Protocol 255 uses the raw socket with the IP header be‐ ing part of the data. See the note about RECVFROM addresses. Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE Useful options: pf, fork, range, ttl, broadcast See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV, UDP-RECVFROM, UNIX-RECVFROM IP4-RECVFROM: Like IP-RECVFROM, but always uses IPv4. Option groups: FD,SOCKET,IP4,CHILD,RANGE IP6-RECVFROM: Like IP-RECVFROM, but always uses IPv6. Option groups: FD,SOCKET,IP6,CHILD,RANGE IP-RECV: Opens a raw IP socket of . Depending on option pf, IP protocol version 4 or 6 is used. It receives packets from multi‐ ple unspecified peers and merges the data. No replies are pos‐ sible. It can be, e.g., addressed by socat IP-SENDTO address peers. Protocol 255 uses the raw socket with the IP header be‐ ing part of the data. Option groups: FD,SOCKET,IP4,IP6,RANGE Useful options: pf, range See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM, UDP-RECV, UNIX-RECV IP4-RECV: Like IP-RECV, but always uses IPv4. Option groups: FD,SOCKET,IP4,RANGE IP6-RECV: Like IP-RECV, but always uses IPv6. Option groups: FD,SOCKET,IP6,RANGE OPEN: Opens using the open() system call (example). This operation fails on UNIX domain sockets. Note: This address type is rarely useful in bidirectional mode. Option groups: FD,REG,NAMED,OPEN Useful options: creat, excl, noatime, nofollow, append, rdonly, wronly, lock, readbytes, ignoreeof See also: CREATE, GOPEN, UNIX-CONNECT OPENSSL:: Tries to establish a SSL connection to [TCP service] on [IP address] using TCP/IP version 4 or 6 depending on ad‐ dress specification, name resolution, or option pf. NOTE: Up to version 1.7.2.4 the server certificate was only checked for validity against the system certificate store or cafile or capath, but not for match with the server’s name or its IP address. Since version 1.7.3.0 socat checks the peer certificate for match with the parameter or the value of the openssl-commonname option. Socat tries to match it against the certificates subject commonName, and the certificates exten‐ sion subjectAltName DNS names. Wildcards in the certificate are supported. Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY Useful options: cipher, verify, commonname, cafile, capath, cer‐ tificate, key, compress, bind, pf, connect-timeout, sourceport, retry See also: OPENSSL-LISTEN, TCP OPENSSL-LISTEN: Listens on tcp [TCP service]. The IP version is 4 or the one specified with pf. When a connection is accepted, this ad‐ dress behaves as SSL server. Note: You probably want to use the certificate option with this address. NOTE: The client certificate is only checked for validity against cafile or capath, but not for match with the client’s name or its IP address! Option groups: FD,SOCKET,IP4,IP6,TCP,LIS‐ TEN,OPENSSL,CHILD,RANGE,RETRY Useful options: pf, cipher, verify, commonname, cafile, capath, certificate, key, compress, fork, bind, range, tcpwrap, su, reuseaddr, retry See also: OPENSSL, TCP-LISTEN OPENSSL-DTLS-CLIENT:: Tries to establish a DTLS connection to [UDP service] on [IP address] using UDP/IP version 4 or 6 depending on ad‐ dress specification, name resolution, or option pf. Socat checks the peer certificates subjectAltName or commonName against the addresses option openssl-commonname or the host name. Wildcards in the certificate are supported. Use socat option -b to make datagrams small enough to fit with overhead on the network. Use option -T to prevent indefinite hanging when peer went down quietly. Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY Useful options: cipher, verify, commonname, cafile, capath, cer‐ tificate, key, compress, bind, pf, sourceport, retry See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT OPENSSL-DTLS-SERVER: Listens on UDP [UDP service]. The IP version is 4 or the one specified with pf. When a connection is accepted, this ad‐ dress behaves as DTLS server. Note: You probably want to use the certificate option with this address. NOTE: The client certificate is only checked for validity against cafile or capath, but not for match with the client’s name or its IP address! Use socat option -b to make datagrams small enough to fit with overhead on the network. Use option -T to prevent indefinite hanging when peer went down quietly. Option groups: FD,SOCKET,IP4,IP6,LIS‐ TEN,OPENSSL,CHILD,RANGE,RETRY Useful options: pf, cipher, verify, commonname, cafile, capath, certificate, key, compress, fork, bind, range, tcpwrap, su, reuseaddr, retry See also: OPENSSL-DTLS-CLIENT, OPENSSL-LISTEN, UDP-LISTEN PIPE: If already exists, it is opened. If it does not ex‐ ist, a named pipe is created and opened. Beginning with socat version 1.4.3, the named pipe is removed when the address is closed (but see option unlink-close Note: When a pipe is used for both reading and writing, it works as echo service. Note: When a pipe is used for both reading and writing, and so‐ cat tries to write more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat might block. Consider using socat op‐ tion, e.g., -b 2048 Option groups: FD,NAMED,OPEN Useful options: rdonly, nonblock, group, user, mode, un‐ link-early See also: unnamed pipe PIPE Creates an unnamed pipe and uses it for reading and writing. It works as an echo, because everything written to it appeares im‐ mediately as read data. Note: When socat tries to write more bytes than the pipe can queue (Linux 2.4: 2048 bytes), socat might block. Consider, e.g., using option -b 2048 Option groups: FD See also: named pipe PROXY::: Connects to an HTTP proxy server on port 8080 using TCP/IP ver‐ sion 4 or 6 depending on address specification, name resolution, or option pf, and sends a CONNECT request for hostname:port. If the proxy grants access and succeeds to connect to the target, data transfer between socat and the target can start. Note that the traffic need not be HTTP but can be an arbitrary protocol. Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY Useful options: proxyport, ignorecr, proxyauth, resolve, crnl, bind, connect-timeout, mss, sourceport, retry See also: SOCKS, TCP PTY Generates a pseudo terminal (pty) and uses its master side. An‐ other process may open the pty’s slave side using it like a se‐ rial line or terminal. (example). If both the ptmx and the openpty mechanisms are available, ptmx is used (POSIX). Option groups: FD,NAMED,PTY,TERMIOS Useful options: link, openpty, wait-slave, mode, user, group See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM READLINE Uses GNU readline and history on stdio to allow editing and reusing input lines (example). Due to licensing restrictions the readline feature is disabled in Debian. See BUGS. You can use STDIO instead. SCTP-CONNECT:: Establishes an SCTP stream connection to the specified [IP address] and [TCP service] using IP version 4 or 6 depending on address specification, name resolution, or option pf. Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY Useful options: bind, pf, connect-timeout, tos, mtudiscover, sctp-maxseg, sctp-nodelay, nonblock, sourceport, retry, read‐ bytes See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN, TCP-CONNECT SCTP4-CONNECT:: Like SCTP-CONNECT, but only supports IPv4 protocol. Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY SCTP6-CONNECT:: Like SCTP-CONNECT, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY SCTP-LISTEN: Listens on [TCP service] and accepts an SCTP connection. The IP version is 4 or the one specified with address option pf, socat option (-4, -6), or environment variable SOCAT_DE‐ FAULT_LISTEN_IP. Note that opening this address usually blocks until a client connects. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY Useful options: crnl, fork, bind, range, tcpwrap, pf, max-chil‐ dren, backlog, accept-timeout, sctp-maxseg, sctp-nodelay, su, reuseaddr, retry, cool-write See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN, SCTP-CONNECT SCTP4-LISTEN: Like SCTP-LISTEN, but only supports IPv4 protocol. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY SCTP6-LISTEN: Like SCTP-LISTEN, but only supports IPv6 protocol. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY SOCKET-CONNECT::: Creates a stream socket using the first and second given socket parameters and SOCK_STREAM (see man socket(2)) and connects to the remote-address. The two socket parameters have to be speci‐ fied by int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_fam‐ ily and (BSD) sa_len components. Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you ap‐ ply socat option -g. Option groups: FD,SOCKET,CHILD,RETRY Useful options: bind, setsockopt, See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO SOCKET-DATAGRAM:::: Creates a datagram socket using the first three given socket pa‐ rameters (see man socket(2)) and sends outgoing data to the re‐ mote-address. The three socket parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components. Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you ap‐ ply socat option -g. Option groups: FD,SOCKET,RANGE Useful options: bind, range, setsockopt, See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV, SOCKET-RECVFROM SOCKET-LISTEN::: Creates a stream socket using the first and second given socket parameters and SOCK_STREAM (see man socket(2)) and waits for in‐ coming connections on local-address. The two socket parameters have to be specified by int numbers. Consult your OS documenta‐ tion and include files to find the appropriate values. The lo‐ cal-address must be the data representation of a sockaddr struc‐ ture without sa_family and (BSD) sa_len components. Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you ap‐ ply socat option -g. Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY Useful options: setsockopt, setsockopt-listen, See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO, SOCKET-SENDTO SOCKET-RECV:::: Creates a socket using the three given socket parameters (see man socket(2)) and binds it to . Receives arriv‐ ing data. The three parameters have to be specified by int num‐ bers. Consult your OS documentation and include files to find the appropriate values. The local-address must be the data rep‐ resentation of a sockaddr structure without sa_family and (BSD) sa_len components. Option groups: FD,SOCKET,RANGE Useful options: range, setsockopt, setsockopt-listen See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECVFROM SOCKET-RECVFROM:::: Creates a socket using the three given socket parameters (see man socket(2)) and binds it to . Receives arriv‐ ing data and sends replies back to the sender. The first three parameters have to be specified as int numbers. Consult your OS documentation and include files to find the appropriate values. The local-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components. See the note about RECVFROM addresses. Option groups: FD,SOCKET,CHILD,RANGE Useful options: fork, range, setsockopt, setsockopt-listen See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM, SOCKET-DATA‐ GRAM, SOCKET-SENDTO, SOCKET-RECV SOCKET-SENDTO:::: Creates a socket using the three given socket parameters (see man socket(2)). Sends outgoing data to the given address and re‐ ceives replies. The three parameters have to be specified as int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components. Option groups: FD,SOCKET Useful options: bind, setsockopt, setsockopt-listen See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO, SOCKET-DATAGRAM, SOCKET-RECV SOCKET-RECVFROM SOCKS4::: Connects via [IP address] to [IPv4 ad‐ dress] on [TCP service], using socks version 4 protocol over IP version 4 or 6 depending on address specification, name resolution, or option pf (example). Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY Useful options: socksuser, socksport, sourceport, pf, retry See also: SOCKS4A, PROXY, TCP SOCKS4A::: like SOCKS4, but uses socks protocol version 4a, thus leaving host name resolution to the socks server. Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY STDERR Uses file descriptor 2. Option groups: FD (TERMIOS,REG,SOCKET) See also: FD STDIN Uses file descriptor 0. Option groups: FD (TERMIOS,REG,SOCKET) Useful options: readbytes See also: FD STDIO Uses file descriptor 0 for reading, and 1 for writing. Option groups: FD (TERMIOS,REG,SOCKET) Useful options: readbytes See also: FD STDOUT Uses file descriptor 1. Option groups: FD (TERMIOS,REG,SOCKET) See also: FD SYSTEM: Forks a sub process that establishes communication with its par‐ ent process and invokes the specified program with system() . Please note that [string] must not contain ’,’ or "!!", and that shell meta characters may have to be pro‐ tected. After successful program start, socat writes data to stdin of the process and reads from its stdout. Option groups: FD,SOCKET,EXEC,FORK,TERMIOS Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, sigint, sigquit See also: EXEC TCP:: Connects to [TCP service] on [IP address] using TCP/IP version 4 or 6 depending on address specification, name resolution, or option pf. Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY Useful options: crnl, bind, pf, connect-timeout, tos, mtudis‐ cover, mss, nodelay, nonblock, sourceport, retry, readbytes See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT, UNIX-CON‐ NECT TCP4:: Like TCP, but only supports IPv4 protocol (example). Option groups: FD,SOCKET,IP4,TCP,RETRY TCP6:: Like TCP, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6,TCP,RETRY TCP-LISTEN: Listens on [TCP service] and accepts a TCP/IP connection. The IP version is 4 or the one specified with address option pf, socat option (-4, -6), or environment variable SOCAT_DE‐ FAULT_LISTEN_IP. Note that opening this address usually blocks until a client connects. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY Useful options: crnl, fork, bind, range, tcpwrap, pf, max-chil‐ dren, backlog, accept-timeout, mss, su, reuseaddr, retry, cool-write See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, SCTP-LISTEN, UNIX-LISTEN, OPENSSL-LISTEN, TCP-CONNECT TCP4-LISTEN: Like TCP-LISTEN, but only supports IPv4 protocol (example). Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY TCP6-LISTEN: Like TCP-LISTEN, but only supports IPv6 protocol. Additional useful option: ipv6only Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY TUN[:/] Creates a Linux TUN/TAP device and optionally assignes it the address and netmask given by the parameters. The resulting net‐ work interface is almost ready for use by other processes; socat serves its "wire side". This address requires read and write ac‐ cess to the tunnel cloning device, usually /dev/net/tun , as well as permission to set some ioctl()s. Option iff-up is re‐ quired to immediately activate the interface! Note: If you intend to transfer packets between two Socat "wire sides" you need a protocol that keeps packet boundaries, e.g.UDP; TCP might work with option nodelay. Option groups: FD,NAMED,OPEN,TUN Useful options: iff-up, tun-device, tun-name, tun-type, iff-no-pi See also: ip-recv UDP:: Connects to [UDP service] on [IP address] using UDP/IP version 4 or 6 depending on address specification, name resolution, or option pf. Please note that, due to UDP protocol properties, no real con‐ nection is established; data has to be sent for `connecting’ to the server, and no end-of-file condition can be transported. Option groups: FD,SOCKET,IP4,IP6 Useful options: ttl, tos, bind, sourceport, pf See also: UDP4, UDP6, UDP-LISTEN, TCP, IP UDP4:: Like UDP, but only supports IPv4 protocol. Option groups: FD,SOCKET,IP4 UDP6:: Like UDP, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6 UDP-DATAGRAM:
: Sends outgoing data to the specified address which may in par‐ ticular be a broadcast or multicast address. Packets arriving on the local socket are checked for the correct remote port only when option sourceport is used (this is a change with Socat ver‐ sion 1.7.4.0) and if their source addresses match RANGE or TCP‐ WRAP options. This address type can for example be used for im‐ plementing symmetric or asymmetric broadcast or multicast commu‐ nications. Option groups: FD,SOCKET,IP4,IP6,RANGE Useful options: bind, range, tcpwrap, broadcast, ip-multi‐ cast-loop, ip-multicast-ttl, ip-multicast-if, ip-add-membership, ip-add-source-membership, ttl, tos, sourceport, pf See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-DATAGRAM UDP4-DATAGRAM:
: Like UDP-DATAGRAM, but only supports IPv4 protocol (example1, example2). Option groups: FD,SOCKET,IP4, RANGE UDP6-DATAGRAM:
: Like UDP-DATAGRAM, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6,RANGE UDP-LISTEN: Waits for a UDP/IP packet arriving on [UDP service] and `connects’ back to sender. The accepted IP version is 4 or the one specified with option pf. Please note that, due to UDP pro‐ tocol properties, no real connection is established; data has to arrive from the peer first, and no end-of-file condition can be transported. Note that opening this address usually blocks until a client connects. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6 Useful options: fork, bind, range, pf See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN UDP4-LISTEN: Like UDP-LISTEN, but only support IPv4 protocol. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4 UDP6-LISTEN: Like UDP-LISTEN, but only support IPv6 protocol. Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6 UDP-SENDTO:: Communicates with the specified peer socket, defined by [UDP service] on [IP address], using UDP/IP version 4 or 6 depending on address specification, name resolution, or option pf. It sends packets to and receives packets from that peer socket only. This address effectively implements a datagram client. It works well with socat UDP-RECVFROM and UDP-RECV ad‐ dress peers. Option groups: FD,SOCKET,IP4,IP6 Useful options: ttl, tos, bind, sourceport, pf See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-SENDTO UDP4-SENDTO:: Like UDP-SENDTO, but only supports IPv4 protocol. Option groups: FD,SOCKET,IP4 UDP6-SENDTO:: Like UDP-SENDTO, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6 UDP-RECVFROM: Creates a UDP socket on [UDP service] using UDP/IP ver‐ sion 4 or 6 depending on option pf. It receives one packet from an unspecified peer and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process. This allows a behaviour similar to typi‐ cal UDP based servers like ntpd or named. This address works well with socat UDP-SENDTO address peers. Note: When the second address fails before entering the transfer loop the packet is dropped. Use option retry or forever on the second address to avoid data loss. Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE Useful options: fork, ttl, tos, bind, sourceport, pf See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-RECVFROM, UNIX-RECVFROM UDP4-RECVFROM: Like UDP-RECVFROM, but only supports IPv4 protocol. Option groups: FD,SOCKET,IP4,CHILD,RANGE UDP6-RECVFROM: Like UDP-RECVFROM, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6,CHILD,RANGE UDP-RECV: Creates a UDP socket on [UDP service] using UDP/IP ver‐ sion 4 or 6 depending on option pf. It receives packets from multiple unspecified peers and merges the data. No replies are possible. It works well with, e.g., socat UDP-SENDTO address peers; it behaves similar to a syslog server. Note: if you need the fork option, use UDP-RECVFROM in unidirec‐ tional mode (with option -u) instead. Option groups: FD,SOCKET,IP4,IP6,RANGE Useful options: pf, bind, sourceport, ttl, tos See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM, UDP-CONNECT, UDP-LISTEN, IP-RECV, UNIX-RECV UDP4-RECV: Like UDP-RECV, but only supports IPv4 protocol. Option groups: FD,SOCKET,IP4,RANGE UDP6-RECV: Like UDP-RECV, but only supports IPv6 protocol. Option groups: FD,SOCKET,IP6,RANGE UNIX-CONNECT: Connects to assuming it is a UNIX domain socket. If does not exist, this is an error; if is not a UNIX domain socket, this is an error; if is a UNIX domain socket, but no process is listening, this is an er‐ ror. Option groups: FD,SOCKET,NAMED,RETRY,UNIX ) Useful options: bind See also: UNIX-LISTEN, UNIX-SENDTO, TCP UNIX-LISTEN: Listens on using a UNIX domain stream socket and ac‐ cepts a connection. If exists and is not a socket, this is an error. If exists and is a UNIX domain socket, binding to the address fails (use option unlink-early!). Note that opening this address usually blocks until a client connects. Beginning with socat version 1.4.3, the file system entry is removed when this address is closed (but see option un‐ link-close) (example). Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX Useful options: fork, umask, mode, user, group, unlink-early See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV, TCP-LISTEN UNIX-SENDTO: Communicates with the specified peer socket, defined by [] assuming it is a UNIX domain datagram socket. It sends packets to and receives packets from that peer socket only. Please note that it might be necessary to bind the local socket to an address (e.g. /tmp/sock1, which must not exist before). This address type works well with socat UNIX-RECVFROM and UNIX-RECV address peers. Option groups: FD,SOCKET,NAMED,UNIX Useful options: bind See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT, UDP-SENDTO, IP-SENDTO UNIX-RECVFROM: Creates a UNIX domain datagram socket []. Receives one packet and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each ar‐ riving packet - from arbitrary peers - is handled by its own sub process. This address works well with socat UNIX-SENDTO address peers. Option groups: FD,SOCKET,NAMED,CHILD,UNIX See the note about RECVFROM addresses. Useful options: fork See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM, IP-RECVFROM UNIX-RECV: Creates a UNIX domain datagram socket []. Receives packets from multiple unspecified peers and merges the data. No replies are possible. It can be, e.g., addressed by socat UNIX-SENDTO address peers. It behaves similar to a syslog server. Option groups: FD,SOCKET,NAMED,UNIX See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV, IP-RECV UNIX-CLIENT: Communicates with the specified peer socket, defined by [] assuming it is a UNIX domain socket. It first tries to connect and, if that fails, assumes it is a datagram socket, thus supporting both types. Option groups: FD,SOCKET,NAMED,UNIX Useful options: bind See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN VSOCK-CONNECT:: Establishes a VSOCK stream connection to the specified [VSOCK cid] and [VSOCK port]. Option groups: FD,SOCKET,CHILD,RETRY Useful options: bind, pf, connect-timeout, retry, readbytes See also: VSOCK-LISTEN, VSOCK-LISTEN: Listens on [VSOCK port] and accepts a VSOCK connection. Note that opening this address usually blocks until a client connects. Option groups: FD,SOCKET,LISTEN,CHILD,RETRY Useful options: fork, bind, pf, max-children, backlog, su, reuseaddr, retry, cool-write See also: VSOCK-CONNECT ABSTRACT-CONNECT: ABSTRACT-LISTEN: ABSTRACT-SENDTO: ABSTRACT-RECVFROM: ABSTRACT-RECV: ABSTRACT-CLIENT: The ABSTRACT addresses are almost identical to the related UNIX addresses except that they do not address file system based sockets but an alternate UNIX domain address space. To achieve this the socket address strings are prefixed with "\0" inter‐ nally. This feature is available (only?) on Linux. Option groups are the same as with the related UNIX addresses, except that the ABSTRACT addresses are not member of the NAMED group. ADDRESS OPTIONS Address options can be applied to address specifications to influence the process of opening the addresses and the properties of the result‐ ing data channels. For technical reasons not every option can be applied to every address type; e.g., applying a socket option to a regular file will fail. To catch most useless combinations as early as in the open phase, the con‐ cept of option groups was introduced. Each option belongs to one or more option groups. Options can be used only with address types that support at least one of their option groups (but see option -g). Address options have data types that their values must conform to. Ev‐ ery address option consists of just a keyword or a keyword followed by "=value", where value must conform to the options type. Some address options manipulate parameters of system calls; e.g., option sync sets the O_SYNC flag with the open() call. Other options cause a system or library call; e.g., with option `ttl=value’ the setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int)) call is applied. Other options set inter‐ nal socat variables that are used during data transfer; e.g., `crnl’ causes explicit character conversions. A few options have more complex implementations; e.g., su-d (substuser-delayed) inquires some user and group infos, stores them, and applies them later after a possible ch‐ root() call. If multiple options are given to an address, their sequence in the ad‐ dress specification has (almost) no effect on the sequence of their ex‐ ecution/application. Instead, socat has built in an option phase model that tries to bring the options in a useful order. Some options exist in different forms (e.g., unlink, unlink-early, unlink-late) to control the time of their execution. If the same option is specified more than once within one address spec‐ ification, with equal or different values, the effect depends on the kind of option. Options resulting in function calls like setsockopt() cause multiple invocations. With options that set parameters for a re‐ quired call like open() or set internal flags, the value of the last option occurrence is effective. The existence or semantics of many options are system dependent. Socat usually does NOT try to emulate missing libc or kernel features, it just provides an interface to the underlying system. So, if an operat‐ ing system lacks a feature, the related option is simply not available on this platform. The following paragraphs introduce just the more common address op‐ tions. For a more comprehensive reference and to find information about canonical option names, alias names, option phases, and platforms see file xio.help. FD option group This option group contains options that are applied to a UN*X style file descriptor, no matter how it was generated. Because all current socat address types are file descriptor based, these options may be ap‐ plied to any address. Note: Some of these options are also member of another option group, that provides another, non-fd based mechanism. For these options, it depends on the actual address type and its option groups which mecha‐ nism is used. The second, non-fd based mechanism is prioritized. cloexec= Sets the FD_CLOEXEC flag with the fcntl() system call to value . If set, the file descriptor is closed on exec() family function calls. Socat internally handles this flag for the fds it controls, so in most cases there will be no need to apply this option. setlk Tries to set a discretionary write lock to the whole file using the fcntl(fd, F_SETLK, ...) system call. If the file is already locked, this call results in an error. On Linux, when the file permissions for group are "S" (g-x,g+s), and the file system is locally mounted with the "mand" option, the lock is mandatory, i.e. prevents other processes from opening the file. setlkw Tries to set a discretionary waiting write lock to the whole file using the fcntl(fd, F_SETLKW, ...) system call. If the file is already locked, this call blocks. See option setlk for information about making this lock mandatory. setlk-rd Tries to set a discretionary read lock to the whole file using the fcntl(fd, F_SETLK, ...) system call. If the file is already write locked, this call results in an error. See option setlk for information about making this lock mandatory. setlkw-rd Tries to set a discretionary waiting read lock to the whole file using the fcntl(fd, F_SETLKW, ...) system call. If the file is already write locked, this call blocks. See option setlk for information about making this lock mandatory. flock-ex Tries to set a blocking exclusive advisory lock to the file us‐ ing the flock(fd, LOCK_EX) system call. Socat hangs in this call if the file is locked by another process. flock-ex-nb Tries to set a nonblocking exclusive advisory lock to the file using the flock(fd, LOCK_EX|LOCK_NB) system call. If the file is already locked, this option results in an error. flock-sh Tries to set a blocking shared advisory lock to the file using the flock(fd, LOCK_SH) system call. Socat hangs in this call if the file is locked by another process. flock-sh-nb Tries to set a nonblocking shared advisory lock to the file us‐ ing the flock(fd, LOCK_SH|LOCK_NB) system call. If the file is already locked, this option results in an error. lock Sets a blocking lock on the file. Uses the setlk or flock mecha‐ nism depending on availability on the particular platform. If both are available, the POSIX variant (setlkw) is used. user= Sets the (owner) of the stream. If the address is member of the NAMED option group, socat uses the chown() system call after opening the file or binding to the UNIX domain socket (race condition!). Without filesystem entry, socat sets the user of the stream using the fchown() system call. These calls might require root privilege. user-late= Sets the owner of the fd to with the fchown() system call after opening or connecting the channel. This is useful only on file system entries. group= Sets the of the stream. If the address is member of the NAMED option group, socat uses the chown() system call after opening the file or binding to the UNIX domain socket (race con‐ dition!). Without filesystem entry, socat sets the group of the stream with the fchown() system call. These calls might require group membership or root privilege. group-late= Sets the group of the fd to with the fchown() system call after opening or connecting the channel. This is useful only on file system entries. mode= Sets the [mode_t] (permissions) of the stream. If the address is member of the NAMED option group and uses the open() or creat() call, the mode is applied with these. If the address is member of the NAMED option group without using these system calls, socat uses the chmod() system call after opening the filesystem entry or binding to the UNIX domain socket (race con‐ dition!). Otherwise, socat sets the mode of the stream using fchmod() . These calls might require ownership or root privi‐ lege. perm-late= Sets the permissions of the fd to value [mode_t] using the fchmod() system call after opening or connecting the chan‐ nel. This is useful only on file system entries. append= Always writes data to the actual end of file. If the address is member of the OPEN option group, socat uses the O_APPEND flag with the open() system call (example). Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND) call. nonblock= Tries to open or use file in nonblocking mode. Its only effects are that the connect() call of TCP addresses does not block, and that opening a named pipe for reading does not block. If the address is member of the OPEN option group, socat uses the O_NONBLOCK flag with the open() system call. Otherwise, socat applies the fcntl(fd, F_SETFL, O_NONBLOCK) call. binary Opens the file in binary mode to avoid implicit line terminator conversions (Cygwin). text Opens the file in text mode to force implicit line terminator conversions (Cygwin). noinherit Does not keep this file open in a spawned process (Cygwin). cool-write Takes it easy when write fails with EPIPE or ECONNRESET and logs the message with notice level instead of error. This prevents the log file from being filled with useless error messages when socat is used as a high volume server or proxy where clients of‐ ten abort the connection. This option is experimental. end-close Changes the (address dependent) method of ending a connection to just close the file descriptors. This is useful when the connec‐ tion is to be reused by or shared with other processes (exam‐ ple). Normally, socket connections will be ended with shutdown(2) which terminates the socket even if it is shared by multiple processes. close(2) "unlinks" the socket from the process but keeps it active as long as there are still links from other pro‐ cesses. Similarly, when an address of type EXEC or SYSTEM is ended, so‐ cat usually will explicitly kill the sub process. With this op‐ tion, it will just close the file descriptors. shut-none Changes the (address dependent) method of shutting down the write part of a connection to not do anything. shut-down Changes the (address dependent) method of shutting down the write part of a connection to shutdown(fd, SHUT_WR). Is only useful with sockets. shut-close Changes the (address dependent) method of shutting down the write part of a connection to close(fd). shut-null When one address indicates EOF, socat will send a zero sized packet to the write channel of the other address to transfer the EOF condition. This is useful with UDP and other datagram proto‐ cols. Has been tested against netcat and socat with option null-eof. null-eof Normally socat will ignore empty (zero size payload) packets ar‐ riving on datagram sockets, so it survives port scans. With this option socat interprets empty datagram packets as EOF indicator (see shut-null). ioctl-void= Calls ioctl() with the request value as second argument and NULL as third argument. This option allows utilizing ioctls that are not explicitly implemented in socat. ioctl-int=: Calls ioctl() with the request value as second argument and the integer value as third argument. ioctl-intp=: Calls ioctl() with the request value as second argument and a pointer to the integer value as third argument. ioctl-bin=: Calls ioctl() with the request value as second argument and a pointer to the given data value as third argument. This data must be specified in form. ioctl-string=: Calls ioctl() with the request value as second argument and a pointer to the given string as third argument. form. NAMED option group These options work on file system entries. Please note that, with UNIX domain client addresses, this means the bind entry, not the target/peer entry. See also options user, group, and mode. user-early= Changes the (owner) of the file system entry before ac‐ cessing it, using the chown() system call. This call might re‐ quire root privilege. group-early= Changes the of the file system entry before accessing it, using the chown() system call. This call might require group membership or root privilege. perm-early= Changes the [mode_t] of the file system entry before ac‐ cessing it, using the chmod() system call. This call might re‐ quire ownership or root privilege. umask= Sets the umask of the process to [mode_t] before access‐ ing the file system entry (useful with UNIX domain sockets!). This call might affect all further operations of the socat process! unlink-early Unlinks (removes) the file before opening it and even before ap‐ plying user-early etc. unlink Unlinks (removes) the file before accessing it, but after user-early etc. unlink-late Unlinks (removes) the file after opening it to make it inacces‐ sible for other processes after a short race condition. unlink-close Removes the addresses file system entry when closing the ad‐ dress. For named pipes, UNIX domain sockets, and the symbolic links of pty addresses, the default is 1; for created files, opened files, and generic opened files the default is 0. OPEN option group The OPEN group options allow setting flags with the open() system call. E.g., option `creat’ sets the O_CREAT flag. See also options append and nonblock. creat= Creates the file if it does not exist (example). dsync= Blocks write() calls until metainfo is physically written to me‐ dia. excl= With option creat, if file exists this is an error. largefile= On 32 bit systems, allows a file larger than 2^31 bytes. noatime Sets the O_NOATIME options, so reads do not change the access timestamp. noctty= Does not make this file the controlling terminal. nofollow= Does not follow symbolic links. nshare= Does not allow sharing this file with other processes. rshare= Does not allow other processes to open this file for writing. rsync= Blocks write() until metainfo is physically written to media. sync= Blocks write() until data is physically written to media. rdonly= Opens the file for reading only. wronly= Opens the file for writing only. trunc Truncates the file to size 0 during opening it. REG and BLK option group These options are usually applied to a UN*X file descriptor, but their semantics make sense only on a file supporting random access. seek= Applies the lseek(fd, , SEEK_SET) (or lseek64 ) system call, thus positioning the file pointer absolutely to [off_t or off64_t]. Please note that a missing value defaults to 1, not 0. seek-cur= Applies the lseek(fd, , SEEK_CUR) (or lseek64 ) system call, thus positioning the file pointer [off_t or off64_t] bytes relatively to its current position (which is usu‐ ally 0). Please note that a missing value defaults to 1, not 0. seek-end= Applies the lseek(fd, , SEEK_END) (or lseek64 ) system call, thus positioning the file pointer [off_t or off64_t] bytes relatively to the files current end. Please note that a missing value defaults to 1, not 0. ftruncate= Applies the ftruncate(fd, ) (or ftruncate64 if avail‐ able) system call, thus truncating the file at the position [off_t or off64_t]. Please note that a missing value defaults to 1, not 0. secrm= unrm= compr= fs-sync= immutable= fs-append= nodump= fs-noatime= journal-data= notail= dirsync= These options change non standard file attributes on operating systems and file systems that support these features, like Linux with ext2fs and successors, xfs, or reiserfs. See man 1 chattr for information on these options. Please note that there might be a race condition between creating the file and applying these options. PROCESS option group Options of this group change the process properties instead of just af‐ fecting one data channel. For EXEC and SYSTEM addresses and for LISTEN and CONNECT type addresses with option FORK, these options apply to the child processes instead of the main socat process. chroot= Performs a chroot() operation to after processing the address (example). This call might require root privilege. chroot-early= Performs a chroot() operation to before opening the address. This call might require root privilege. setgid= Changes the primary of the process after processing the address. This call might require root privilege. Please note that this option does not drop other group related privileges. setgid-early= Like setgit but is performed before opening the address. setuid= Changes the (owner) of the process after processing the address. This call might require root privilege. Please note that this option does not drop group related privileges. Check if option su better fits your needs. setuid-early= Like setuid but is performed before opening the address. su= Changes the (owner) and groups of the process after pro‐ cessing the address (example). This call might require root privilege. su-d= Short name for substuser-delayed. Changes the (owner) and groups of the process after processing the address (exam‐ ple). The user and his groups are retrieved before a possible chroot() . This call might require root privilege. setpgid= Makes the process a member of the specified process group . If no value is given, or if the value is 0 or 1, the process becomes leader of a new process group. setsid Makes the process the leader of a new session (example). READLINE option group Due to licensing restrictions the readline feature is disabled in De‐ bian (see BUGS). These options apply to the readline address type. history= Reads and writes history from/to (example). noprompt Since version 1.4.0, socat per default tries to determine a prompt - that is then passed to the readline call - by remember‐ ing the last incomplete line of the output. With this option, socat does not pass a prompt to readline, so it begins line editing in the first column of the terminal. noecho= Specifies a regular pattern for a prompt that prevents the fol‐ lowing input line from being displayed on the screen and from being added to the history. The prompt is defined as the text that was output to the readline address after the lastest new‐ line character and before an input character was typed. The pat‐ tern is a regular expression, e.g. "^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex(7) for details. (example) prompt= Passes the string as prompt to the readline function. readline prints this prompt when stepping through the history. If this string matches a constant prompt issued by an interactive pro‐ gram on the other socat address, consistent look and feel can be achieved. APPLICATION option group This group contains options that work at data level. Note that these options only apply to the "raw" data transferred by socat, but not to protocol data used by addresses like PROXY. cr Converts the default line termination character NL (’\n’, 0x0a) to/from CR (’\r’, 0x0d) when writing/reading on this channel. crnl Converts the default line termination character NL (’\n’, 0x0a) to/from CRNL ("\r\n", 0x0d0a) when writing/reading on this chan‐ nel (example). Note: socat simply strips all CR characters. ignoreeof When EOF occurs on this channel, socat ignores it and tries to read more data (like "tail -f") (example). readbytes= socat reads only so many bytes from this address (the address provides only so many bytes for transfer and pretends to be at EOF afterwards). Must be greater than 0. lockfile= If lockfile exists, exits with error. If lockfile does not ex‐ ist, creates it and continues, unlinks lockfile on exit. waitlock= If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues, unlinks lockfile on exit. escape= Specifies the numeric code of a character that triggers EOF on the input stream. It is useful with a terminal in raw mode (ex‐ ample). SOCKET option group These options are intended for all kinds of sockets, e.g. IP or UNIX domain. Most are applied with a setsockopt() call. bind= Binds the socket to the given socket address using the bind() system call. The form of is socket domain dependent: IP4 and IP6 allow the form [hostname|hostaddress][:(ser‐ vice|port)] (example), UNIX domain sockets require , VSOCK allow the form [cid][:(port)]. connect-timeout= Abort the connection attempt after [timeval] with er‐ ror status. so-bindtodevice= Binds the socket to the given . This option might require root privilege. broadcast For datagram sockets, allows sending to broadcast addresses and receiving packets addressed to broadcast addresses. debug Enables socket debugging. dontroute Only communicates with directly connected peers, does not use routers. keepalive Enables sending keepalives on the socket. linger= Blocks shutdown() or close() until data transfers have finished or the given timeout [int] expired. oobinline Places out-of-band data in the input data stream. priority= Sets the protocol defined [] for outgoing pack‐ ets. rcvbuf= Sets the size of the receive buffer after the socket() call to [int]. With TCP sockets, this value corresponds to the socket’s maximal window size. rcvbuf-late= Sets the size of the receive buffer when the socket is already connected to [int]. With TCP sockets, this value corre‐ sponds to the socket’s maximal window size. rcvlowat= Specifies the minimum number of received bytes [int] until the socket layer will pass the buffered data to socat. reuseaddr Allows other sockets to bind to an address even if parts of it (e.g. the local port) are already in use by socat (example). sndbuf= Sets the size of the send buffer after the socket() call to [int]. sndbuf-late= Sets the size of the send buffer when the socket is connected to [int]. sndlowat= Specifies the minimum number of bytes in the send buffer until the socket layer will send the data to [int]. pf= Forces the use of the specified IP version or protocol. can be something like "ip4" or "ip6". The resulting value is used as first argument to the socket() or socketpair() calls. This option affects address resolution and the required syntax of bind and range options. type= Sets the type of the socket, specified as second argument to the socket() or socketpair() calls, to [int]. Address resolu‐ tion is not affected by this option. Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3 means raw socket. protocol Sets the protocol of the socket, specified as third argument to the socket() or socketpair() calls, to [int]. Address resolution is not affected by this option. 6 means TCP, 17 means UDP. reuseport Set the SO_REUSEPORT socket option. so-timestamp Sets the SO_TIMESTAMP socket option. This enables receiving and logging of timestamp ancillary messages. setsockopt=:: Invokes setsockopt() for the socket with the given parameters. level [int] is used as second argument to setsockopt() and spec‐ ifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or SOL_SOCKET for the socket layer (1 on Linux). optname [int] is the third argument to setsockopt() and tells which socket option is to be set. For the actual numbers you might have to look up the appropriate include files of your system. For the 4th and 5th setsockopt() parameters, value [dalan] specifies an arbi‐ trary sequence of bytes that are passed to the function per pointer, with the automatically derived length parameter. setsockopt-int=:: Like setsockopt, but is a pointer to int [int] setsockopt-listen=:: Like setsockopt, but for listen type addresses it is applied to the listening socket instead of the connected socket. setsockopt-string=:: Like setsockopt, but is a string. This string is passed to the function with trailing null character, and the length parameter is automatically derived from the data. UNIX option group These options apply to UNIX domain based addresses. unix-tightsocklen=[0|1] On socket operations, pass a socket address length that does not include the whole struct sockaddr_un record but (besides other components) only the relevant part of the filename or abstract string. Default is 1. IP4 and IP6 option groups These options can be used with IPv4 and IPv6 based sockets. tos= Sets the TOS (type of service) field of outgoing packets to [byte] (see RFC 791). ttl= Sets the TTL (time to live) field of outgoing packets to [byte]. ip-options= Sets IP options like source routing. Must be given in binary form, recommended format is a leading "x" followed by an even number of hex digits. This option may be used multiple times, data are appended. E.g., to connect to host 10.0.0.1 via some gateway using a loose source route, use the gateway as address parameter and set a loose source route using the option ip-op‐ tions=x8307040a000001 . IP options are defined in RFC 791. mtudiscover=<0|1|2> Takes 0, 1, 2 to never, want, or always use path MTU discover on this socket. ip-pktinfo Sets the IP_PKTINFO socket option. This enables receiving and logging of ancillary messages containing destination address and interface (Linux) (example). ip-recverr Sets the IP_RECVERR socket option. This enables receiving and logging of ancillary messages containing detailed error informa‐ tion. ip-recvopts Sets the IP_RECVOPTS socket option. This enables receiving and logging of IP options ancillary messages (Linux, *BSD). ip-recvtos Sets the IP_RECVTOS socket option. This enables receiving and logging of TOS (type of service) ancillary messages (Linux). ip-recvttl Sets the IP_RECVTTL socket option. This enables receiving and logging of TTL (time to live) ancillary messages (Linux, *BSD). ip-recvdstaddr Sets the IP_RECVDSTADDR socket option. This enables receiving and logging of ancillary messages containing destination address (*BSD) (example). ip-recvif Sets the IP_RECVIF socket option. This enables receiving and logging of interface ancillary messages (*BSD) (example). ip-add-membership= ip-add-membership= ip-add-membership= ip-add-membership= ip-add-membership= Makes the socket member of the specified multicast group. This is currently only implemented for IPv4. The option takes the IP address of the multicast group and info about the desired net‐ work interface. The most common syntax is the first one, while the others are only available on systems that provide struct mreqn (Linux). The indices of active network interfaces can be shown using the utility procan. ip-add-source-membership= Makes the socket member of the specified multicast group for the specified source, i.e. only multicast traffic from this address is to be delivered. This is currently only implemented for IPv4. ip-multicast-if= Specifies hostname or address of the network interface to be used for multicast traffic. ip-multicast-loop= Specifies if outgoing multicast traffic should loop back to the interface. ip-multicast-ttl= Sets the TTL used for outgoing multicast traffic. Default is 1. ip-transparent Sets the IP_TRANSPARENT socket option. This option might re‐ quire root privilege. res-debug res-aaonly res-usevc res-primary res-igntc res-recurse res-defnames res-stayopen res-dnsrch These options set the corresponding resolver (name resolution) option flags. Append "=0" to clear a default option. See man resolver(5) for more information on these options. Note: these options are valid only for the address they are applied to. IP6 option group These options can only be used on IPv6 based sockets. See IP options for options that can be applied to both IPv4 and IPv6 sockets. ipv6only= Sets the IPV6_V6ONLY socket option. If 0, the TCP stack will also accept connections using IPv4 protocol on the same port. The default is system dependent. ipv6-recvdstopts Sets the IPV6_RECVDSTOPTS socket option. This enables receiving and logging of ancillary messages containing the destination op‐ tions. ipv6-recvhoplimit Sets the IPV6_RECVHOPLIMIT socket option. This enables receiving and logging of ancillary messages containing the hoplimit. ipv6-recvhopopts Sets the IPV6_RECVHOPOPTS socket option. This enables receiving and logging of ancillary messages containing the hop options. ipv6-recvpktinfo Sets the IPV6_RECVPKTINFO socket option. This enables receiving and logging of ancillary messages containing destination address and interface. ipv6-unicast-hops=link(TYPE_INT)() Sets the IPV6_UNICAST_HOPS socket option. This sets the hop count limit (TTL) for outgoing unicast packets. ipv6-recvrthdr Sets the IPV6_RECVRTHDR socket option. This enables receiving and logging of ancillary messages containing routing informa‐ tion. ipv6-tclass Sets the IPV6_TCLASS socket option. This sets the transfer class of outgoing packets. ipv6-recvtclass Sets the IPV6_RECVTCLASS socket option. This enables receiving and logging of ancillary messages containing the transfer class. TCP option group These options may be applied to TCP sockets. They work by invoking set‐ sockopt() with the appropriate parameters. cork Doesn’t send packets smaller than MSS (maximal segment size). defer-accept While listening, accepts connections only when data from the peer arrived. keepcnt= Sets the number of keepalives before shutting down the socket to [int]. keepidle= Sets the idle time before sending the first keepalive to [int]. keepintvl= Sets the interval between two keepalives to [int]. linger2= Sets the time to keep the socket in FIN-WAIT-2 state to [int]. mss= Sets the MSS (maximum segment size) after the socket() call to [int]. This value is then proposed to the peer with the SYN or SYN/ACK packet (example). mss-late= Sets the MSS of the socket after connection has been established to [int]. nodelay Turns off the Nagle algorithm for measuring the RTT (round trip time). rfc1323 Enables RFC1323 TCP options: TCP window scale, round-trip time measurement (RTTM), and protect against wrapped sequence numbers (PAWS) (AIX). stdurg Enables RFC1122 compliant urgent pointer handling (AIX). syncnt= Sets the maximal number of SYN retransmits during connect to [int]. md5sig Enables generation of MD5 digests on the packets (FreeBSD). noopt Disables use of TCP options (FreeBSD, MacOSX). nopush sets the TCP_NOPUSH socket option (FreeBSD, MacOSX). sack-disable Disables use the selective acknowledge feature (OpenBSD). signature-enable Enables generation of MD5 digests on the packets (OpenBSD). abort-threshold= Sets the time to wait for an answer of the peer on an estab‐ lished connection (HP-UX). conn-abort-threshold= Sets the time to wait for an answer of the server during the initial connect (HP-UX). keepinit Sets the time to wait for an answer of the server during con‐ nect() before giving up. Value in half seconds, default is 150 (75s) (Tru64). paws Enables the "protect against wrapped sequence numbers" feature (Tru64). sackena Enables selective acknowledge (Tru64). tsoptena Enables the time stamp option that allows RTT recalculation on existing connections (Tru64). UDP option group This option may be applied to UDP datagram sockets. udp-ignore-peerport> Address UDP-DATAGRAM expects incoming responses to come from the port specified in its second parameter. With this option, it ac‐ cepts packets coming from any port. SCTP option group These options may be applied to SCTP stream sockets. sctp-nodelay Sets the SCTP_NODELAY socket option that disables the Nagle al‐ gorithm. sctp-maxseg= Sets the SCTP_MAXSEG socket option to [int]. This value is then proposed to the peer with the SYN or SYN/ACK packet. UDP, TCP, and SCTP option group Here we find options that are related to the network port mechanism and thus can be used with UDP, TCP, and SCTP client and server addresses. sourceport= For outgoing (client) TCP and UDP connections, it sets the source using an extra bind() call. With TCP or UDP lis‐ ten addresses, socat immediately shuts down the connection if the client does not use this sourceport. UDP-RECV, UDP-RECVFROM, UDP-SENDTO, and UDP-DATAGRAM addresses ignore the packet when it does not match. (example). lowport Outgoing (client) TCP and UDP connections with this option use an unused random source port between 640 and 1023 incl. On UNIX class operating systems, this requires root privilege, and thus indicates that the client process is authorized by local root. TCP and UDP listen addresses with this option immediately shut down the connection if the client does not use a sourceport <= 1023. This mechanism can provide limited authorization under some circumstances. SOCKS option group When using SOCKS type addresses, some socks specific options can be set. socksport= Overrides the default "socks" service or port 1080 for the socks server port with . socksuser= Sends the [string] in the username field to the socks server. Default is the actual user name ($LOGNAME or $USER) (ex‐ ample). HTTP option group Options that can be provided with HTTP type addresses. The only HTTP address currently implemented is proxy-connect. proxyport= Overrides the default HTTP proxy port 8080 with . ignorecr The HTTP protocol requires the use of CR+NL as line terminator. When a proxy server violates this standard, socat might not un‐ derstand its answer. This option directs socat to interprete NL as line terminator and to ignore CR in the answer. Nevertheless, socat sends CR+NL to the proxy. proxy-authorization=: Provide "basic" authentication to the proxy server. The argument to the option is used with a "Proxy-Authorization: Basic" header in base64 encoded form. Note: username and password are visible for every user on the local machine in the process list; username and password are transferred to the proxy server unencrypted (base64 encoded) and might be sniffed. proxy-authorization-file= Like option proxy-authorization, but the credentials are read from the file and therefore not visible in the process list. resolve Per default, socat sends to the proxy a CONNECT request contain‐ ing the target hostname. With this option, socat resolves the hostname locally and sends the IP address. Please note that, ac‐ cording to RFC 2396, only name resolution to IPv4 addresses is implemented. RANGE option group These options check if a connecting client should be granted access. They can be applied to listening and receiving network sockets. tcp-wrappers options fall into this group. range= After accepting a connection, tests if the peer is within range. For IPv4 addresses, address-range takes the form address/bits, e.g. 10.0.0.0/8, or address:mask, e.g. 10.0.0.0:255.0.0.0 (ex‐ ample); for IPv6, it is [ip6-address]/bits, e.g. [::1]/128. If the client address does not match, socat refuses the connection attempt, issues a warning, and keeps listening/receiving. tcpwrap[=] Uses Wietse Venema’s libwrap (tcpd) library to determine if the client is allowed to connect. The configuration files are /etc/hosts.allow and /etc/hosts.deny per default, see "man 5 hosts_access" for more information. The optional (type string) is passed to the wrapper functions as daemon process name (example). If omitted, the basename of socats invocation (argv[0]) is passed. If both tcpwrap and range options are ap‐ plied to an address, both conditions must be fulfilled to allow the connection. allow-table= Takes the specified file instead of /etc/hosts.allow. deny-table= Takes the specified file instead of /etc/hosts.deny. tcpwrap-etc= Looks for hosts.allow and hosts.deny in the specified directory. Is overridden by options hosts-allow and hosts-deny. LISTEN option group Options specific to listening sockets. backlog= Sets the backlog value passed with the listen() system call to [int]. Default is 5. accept-timeout= End waiting for a connection after [timeval] with er‐ ror status. max-children= Limits the number of concurrent child processes [int]. Default is no limit. CHILD option group Options for addresses with multiple connections via child processes. fork After establishing a connection, handles its channel in a child process and keeps the parent process attempting to produce more connections, either by listening or by connecting in a loop (ex‐ ample). OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they actually fork off the child: OPENSSL-LISTEN forks before the SSL hand‐ shake, while OPENSSL-CONNECT forks afterwards. retry and for‐ ever options are not inherited by the child process. On some operating systems (e.g. FreeBSD) this option does not work for UDP-LISTEN addresses. EXEC option group Options for addresses that invoke a program. path= Overrides the PATH environment variable for searching the pro‐ gram with . This $PATH value is effective in the child process too. login Prefixes argv[0] for the execvp() call with ’-’, thus making a shell behave as login shell. FORK option group EXEC or SYSTEM addresses invoke a program using a child process and transfer data between socat and the program. The interprocess communi‐ cation mechanism can be influenced with the following options. Per de‐ fault, a socketpair() is created and assigned to stdin and stdout of the child process, while stderr is inherited from the socat process, and the child process uses file descriptors 0 and 1 for communicating with the main socat process. nofork Does not fork a subprocess for executing the program, instead calls execvp() or system() directly from the actual socat in‐ stance. This avoids the overhead of another process between the program and its peer, but introduces a lot of restrictions: o this option can only be applied to the second socat address. o it cannot be applied to a part of a dual address. o the first socat address cannot be OPENSSL or READLINE o socat options -b, -t, -D, -l, -v, -x become useless o for both addresses, options ignoreeof, cr, and crnl become use‐ less o for the second address (the one with option nofork), options ap‐ pend, cloexec, flock, user, group, mode, nonblock, perm-late, setlk, and setpgid cannot be applied. Some of these could be used on the first address though. pipes Creates a pair of unnamed pipes for interprocess communication instead of a socket pair. openpty Establishes communication with the sub process using a pseudo terminal created with openpty() instead of the default (socket‐ pair or ptmx). ptmx Establishes communication with the sub process using a pseudo terminal created by opening /dev/ptmx or /dev/ptc instead of the default (socketpair). pty Establishes communication with the sub process using a pseudo terminal instead of a socket pair. Creates the pty with an available mechanism. If openpty and ptmx are both available, it uses ptmx because this is POSIX compliant (example). ctty Makes the pty the controlling tty of the sub process (example). stderr Directs stderr of the sub process to its output channel by mak‐ ing stderr a dup() of stdout (example). fdin= Assigns the sub processes input channel to its file descriptor instead of stdin (0). The program started from the sub‐ process has to use this fd for reading data from socat (exam‐ ple). fdout= Assigns the sub processes output channel to its file descriptor instead of stdout (1). The program started from the sub‐ process has to use this fd for writing data to socat (example). sighup, sigint, sigquit Has socat pass signals of this type to the sub process. If no address has this option, socat terminates on these signals. TERMIOS option group For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the terminal parameters defined in the UN*X termios mechanism are made available as address option parameters. Please note that changes of the parameters of your interactive terminal remain ef‐ fective after socat’s termination, so you might have to enter "reset" or "stty sane" in your shell afterwards. For EXEC and SYSTEM addresses with option PTY, these options apply to the pty by the child processes. b0 Disconnects the terminal. b19200 Sets the serial line speed to 19200 baud. Some other rates are possible; use something like socat -hh |grep ’ b[1-9]’ to find all speeds supported by your implementation. Note: On some operating systems, these options may not be avail‐ able. Use ispeed or ospeed instead. echo= Enables or disables local echo. icanon= Sets or clears canonical mode, enabling line buffering and some special characters. raw Sets raw mode, thus passing input and output almost unprocessed. This option is obsolete, use option rawer or cfmakeraw instead. rawer Makes terminal rawer than raw option. This option implicitly turns off echo. (example). cfmakeraw Sets raw mode by invoking cfmakeraw() or by simulating this call. This option implicitly turns off echo. ignbrk= Ignores or interpretes the BREAK character (e.g., ^C) brkint= bs0 bs1 bsdly=<0|1> clocal= cr0 cr1 cr2 cr3 Sets the carriage return delay to 0, 1, 2, or 3, respectively. 0 means no delay, the other values are terminal dependent. crdly=<0|1|2|3> cread= crtscts= cs5 cs6 cs7 cs8 Sets the character size to 5, 6, 7, or 8 bits, respectively. csize=<0|1|2|3> cstopb= Sets two stop bits, rather than one. dsusp= Sets the value for the VDSUSP character that suspends the cur‐ rent foreground process and reactivates the shell (all except Linux). echoctl= Echos control characters in hat notation (e.g. ^A) echoe= echok= echoke= echonl= echoprt= eof= eol= eol2= erase= discard= ff0 ff1 ffdly= flusho= hupcl= icrnl= iexten= igncr= ignpar= imaxbel= inlcr= inpck= intr= isig= ispeed= Set the baud rate for incoming data on this line. See also: ospeed, b19200 istrip= iuclc= ixany= ixoff= ixon= kill= lnext= min= nl0 Sets the newline delay to 0. nl1 nldly= noflsh= ocrnl= ofdel= ofill= olcuc= onlcr= onlret= onocr= opost= Enables or disables output processing; e.g., converts NL to CR-NL. ospeed= Set the baud rate for outgoing data on this line. See also: ispeed, b19200 parenb= Enable parity generation on output and parity checking for in‐ put. parmrk= parodd= pendin= quit= reprint= sane Brings the terminal to something like a useful default state. start= stop= susp= swtc= tab0 tab1 tab2 tab3 tabdly= time= tostop= vt0 vt1 vtdly= werase= xcase= xtabs i-pop-all With UNIX System V STREAMS, removes all drivers from the stack. i-push= With UNIX System V STREAMS, pushes the driver (module) with the given name (string) onto the stack. For example, to make sure that a character device on Solaris supports termios etc, use the following options: i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat PTY option group These options are intended for use with the pty address type. link= Generates a symbolic link that points to the actual pseudo ter‐ minal (pty). This might help to solve the problem that ptys are generated with more or less unpredictable names, making it dif‐ ficult to directly access the socat generated pty automatically. With this option, the user can specify a "fix" point in the file hierarchy that helps him to access the actual pty (example). Beginning with socat version 1.4.3, the symbolic link is removed when the address is closed (but see option unlink-close). wait-slave Blocks the open phase until a process opens the slave side of the pty. Usually, socat continues after generating the pty with opening the next address or with entering the transfer loop. With the wait-slave option, socat waits until some process opens the slave side of the pty before continuing. This option only works if the operating system provides the poll() system call. And it depends on an undocumented behaviour of pty’s, so it does not work on all operating systems. It has successfully been tested on Linux, FreeBSD, NetBSD, and on Tru64 with openpty. pty-interval= When the wait-slave option is set, socat periodically checks the HUP condition using poll() to find if the pty’s slave side has been opened. The default polling interval is 1s. Use the pty-in‐ terval option [timeval] to change this value. OPENSSL option group These options apply to the openssl and openssl-listen address types. cipher= Selects the list of ciphers that may be used for the connection. See the man page of ciphers , section CIPHER LIST FORMAT, for detailed information about syntax, values, and default of . Several cipher strings may be given, separated by ’:’. Some simple cipher strings: 3DES Uses a cipher suite with triple DES. MD5 Uses a cipher suite with MD5. aNULL Uses a cipher suite without authentication. NULL Does not use encryption. HIGH Uses a cipher suite with "high" encryption. Note that the peer must support the selected property, or the negotiation will fail. method= This option is based on deprecated functions and is only avail‐ able when socat was build with option --with-openssl-method. Sets the protocol version to be used. Valid strings (not case sensitive) are: SSL2 Select SSL protocol version 2. SSL3 Select SSL protocol version 3. SSL23 Select the best available SSL or TLS protocol. TLS1 Select TLS protocol version 1. TLS1.1 Select TLS protocol version 1.1. TLS1.2 Select TLS protocol version 1.2. When this option is not pro‐ vided OpenSSL negotiates the mothod with its peer. verify= Controls check of the peer’s certificate. Default is 1 (true). Disabling verify might open your socket for everyone, making the encryption useless! cert= Specifies the file with the certificate and private key for au‐ thentication. The certificate must be in OpenSSL format (*.pem). With openssl-listen, use of this option is strongly recommended. Except with cipher aNULL, "no shared ciphers" error will occur when no certificate is given. key= Specifies the file with the private key. The private key may be in this file or in the file given with the cert option. The party that has to proof that it is the owner of a certificate needs the private key. dhparams= Specifies the file with the Diffie Hellman parameters. These pa‐ rameters may also be in the file given with the cert option in which case the dhparams option is not needed. cafile= Specifies the file with the trusted (root) authority certifi‐ cates. The file must be in PEM format and should contain one or more certificates. The party that checks the authentication of its peer trusts only certificates that are in this file. capath= Specifies the directory with the trusted (root) certificates. The directory must contain certificates in PEM format and their hashes (see OpenSSL documentation) egd= On some systems, openssl requires an explicit source of random data. Specify the socket name where an entropy gathering daemon like egd provides random data, e.g. /dev/egd-pool. pseudo On systems where openssl cannot find an entropy source and where no entropy gathering daemon can be utilized, this option acti‐ vates a mechanism for providing pseudo entropy. This is achieved by taking the current time in microseconds for feeding the libc pseudo random number generator with an initial value. openssl is then feeded with output from random() calls. NOTE:This mechanism is not sufficient for generation of secure keys! compress Enable or disable the use of compression for a connection. Set‐ ting this to "none" disables compression, setting it to "auto" lets OpenSSL choose the best available algorithm supported by both parties. The default is to not touch any compression-re‐ lated settings. NOTE: Requires OpenSSL 0.9.8 or higher and dis‐ abling compression with OpenSSL 0.9.8 affects all new connec‐ tions in the process. commonname= Specify the commonname that the peer certificate must match. With OPENSSL-CONNECT address this overrides the given hostname or IP target address; with OPENSSL-LISTEN this turns on check of peer certificates commonname. This option has only meaning when option verify is not disabled and the chosen cipher provides a peer certificate. no-sni= Do not use the client side Server Name Indication (SNI) feature that selects the desired server certificate. Note: SNI is automatically used since socat version 1.7.4.0 and uses commonname or the given host name. snihost= Set the client side Server Name Indication (SNI) host name dif‐ ferent from the addressed server name or common name. This might be useful when the server certificate has multiple host names or wildcard names because the SNI host name is passed in cleartext to the server and might be eavesdropped; with this option a mock name of the desired certificate may be transferred. fips Enables FIPS mode if compiled in. For info about the FIPS en‐ cryption implementation standard see http://oss-insti‐ tute.org/fips-faq.html. This mode might require that the in‐ volved certificates are generated with a FIPS enabled version of openssl. Setting or clearing this option on one socat address affects all OpenSSL addresses of this process. RETRY option group Options that control retry of some system calls, especially connection attempts. retry= Number of retries before the connection or listen attempt is aborted. Default is 0, which means just one attempt. interval= Time between consecutive attempts (seconds, [timespec]). Default is 1 second. forever Performs an unlimited number of retry attempts. TUN option group Options that control Linux TUN/TAP interface device addresses. tun-device= Instructs socat to take another path for the TUN clone device. Default is /dev/net/tun. tun-name= Gives the resulting network interface a specific name instead of the system generated (tun0, tun1, etc.) tun-type=[tun|tap] Sets the type of the TUN device; use this option to generate a TAP device. See the Linux docu for the difference between these types. When you try to establish a tunnel between two TUN de‐ vices, their types should be the same. iff-no-pi Sets the IFF_NO_PI flag which controls if the device includes additional packet information in the tunnel. When you try to establish a tunnel between two TUN devices, these flags should have the same values. iff-up Sets the TUN network interface status UP. Strongly recommended. iff-broadcast Sets the BROADCAST flag of the TUN network interface. iff-debug Sets the DEBUG flag of the TUN network interface. iff-loopback Sets the LOOPBACK flag of the TUN network interface. iff-pointopoint Sets the POINTOPOINT flag of the TUN device. iff-notrailers Sets the NOTRAILERS flag of the TUN device. iff-running Sets the RUNNING flag of the TUN device. iff-noarp Sets the NOARP flag of the TUN device. iff-promisc Sets the PROMISC flag of the TUN device. iff-allmulti Sets the ALLMULTI flag of the TUN device. iff-master Sets the MASTER flag of the TUN device. iff-slave Sets the SLAVE flag of the TUN device. iff-multicast Sets the MULTICAST flag of the TUN device. iff-portsel Sets the PORTSEL flag of the TUN device. iff-automedia Sets the AUTOMEDIA flag of the TUN device. iff-dynamic Sets the DYNAMIC flag of the TUN device. DATA VALUES This section explains the different data types that address parameters and address options can take. address-range Is currently only implemented for IPv4 and IPv6. See address-op‐ tion `range’ bool "0" or "1"; if value is omitted, "1" is taken. byte An unsigned int number, read with strtoul() , lower or equal to UCHAR_MAX . command-line A string specifying a program name and its arguments, separated by single spaces. data This is a more general data specification. The given text string contains information about the target data type and value. Gen‐ erally a leading character specifies the type of the following data item. In its specific context a default data type may ex‐ ist. Currently only the following specifications are implemented: i A signed integer number, stored in host byte order. Example: i-1000 (Integer number -1000) I An unsigned integer number, stored in host byte order. l A signed long integer number, stored in host byte order. L An unsigned long integer number, stored in host byte order. s A signed short integer number, stored in host byte order. S An unsigned short integer number, stored in host byte order. b A signed byte (signed char). B An unsigned byte (unsigned char). x Following is an even number of hex digits, stored as sequence of bytes. Example: x7f000001 (IP address 127.0.0.1) " Following is a string that is used with the common conversions \n \r \t \f \b \a \e \0; the string must be closed with ’"’. Please note that the quotes and backslashes need to be escaped from shell and socat conversion. Example: "Hello world!\n" ’ A single char, with the usual conversions. Please note that the quotes and backslashes need to be escaped from shell and socat conversion. Example: ’a’ Data items may be separated with white space without need to repeat the type specifier again. directory A string with usual UN*X directory name semantics. facility The name of a syslog facility in lower case characters. fdnum An unsigned int type, read with strtoul() , specifying a UN*X file descriptor. filename A string with usual UN*X filename semantics. group If the first character is a decimal digit, the value is read with strtoul() as unsigned integer specifying a group id. Other‐ wise, it must be an existing group name. int A number following the rules of the strtol() function with base "0", i.e. decimal number, octal number with leading "0", or hexadecimal number with leading "0x". The value must fit into a C int. interface A string specifying the device name of a network interface as shown by ifconfig or procan, e.g. "eth0". IP address An IPv4 address in numbers-and-dots notation, an IPv6 address in hex notation enclosed in brackets, or a hostname that resolves to an IPv4 or an IPv6 address. Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1 IPv4 address An IPv4 address in numbers-and-dots notation or a hostname that resolves to an IPv4 address. Examples: 127.0.0.1, www.dest-unreach.org, dns2 IPv6 address An IPv6 address in hexnumbers-and-colons notation enclosed in brackets, or a hostname that resolves to an IPv6 address. Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0], ip6name.domain.org long A number read with strtol() . The value must fit into a C long. long long A number read with strtoll() . The value must fit into a C long long. off_t An implementation dependend signed number, usually 32 bits, read with strtol or strtoll. off64_t An implementation dependend signed number, usually 64 bits, read with strtol or strtoll. mode_t An unsigned integer, read with strtoul() , specifying mode (per‐ mission) bits. pid_t A number, read with strtol() , specifying a process id. port A uint16_t (16 bit unsigned number) specifying a TCP or UDP port, read with strtoul() . protocol An unsigned 8 bit number, read with strtoul() . size_t An unsigned number with size_t limitations, read with strtoul . sockname A socket address. See address-option `bind’ string A sequence of characters, not containing ’\0’ and, depending on the position within the command line, ’:’, ’,’, or "!!". Note that you might have to escape shell meta characters in the com‐ mand line. TCP service A service name, not starting with a digit, that is resolved by getservbyname() , or an unsigned int 16 bit number read with strtoul() . timeval A double float specifying seconds; the number is mapped into a struct timeval, consisting of seconds and microseconds. timespec A double float specifying seconds; the number is mapped into a struct timespec, consisting of seconds and nanoseconds. UDP service A service name, not starting with a digit, that is resolved by getservbyname() , or an unsigned int 16 bit number read with strtoul() . unsigned int A number read with strtoul() . The value must fit into a C un‐ signed int. user If the first character is a decimal digit, the value is read with strtoul() as unsigned integer specifying a user id. Other‐ wise, it must be an existing user name. VSOCK cid A uint32_t (32 bit unsigned number) specifying a VSOCK Context Identifier (CID), read with strtoul() . There are several spe‐ cial addresses: VMADDR_CID_ANY (-1U) means any address for bind‐ ing; VMADDR_CID_HOST (2) is the well-known address of the host. VSOCK port A uint32_t (32 bit unsigned number) specifying a VSOCK port, read with strtoul() . EXAMPLES socat - TCP4:www.domain.org:80 transfers data between STDIO (-) and a TCP4 connection to port 80 of host www.domain.org. This example results in an interac‐ tive connection similar to telnet or netcat. The stdin terminal parameters are not changed, so you may close the relay with ^D or abort it with ^C. socat -d -d READLINE,history=$HOME/.http_history \ TCP4:www.domain.org:www,crnl this is similar to the previous example, but you can edit the current line in a bash like manner (READLINE) and use the his‐ tory file .http_history; socat prints messages about progress (-d -d). The port is specified by service name (www), and cor‐ rect network line termination characters (crnl) instead of NL are used. socat TCP4-LISTEN:www TCP4:www.domain.org:www installs a simple TCP port forwarder. With TCP4-LISTEN it lis‐ tens on local port "www" until a connection comes in, accepts it, then connects to the remote host (TCP4) and starts data transfer. It will not accept a second connection. socat -d -d -lmlocal2 \ TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \ TCP4:www.domain.org:80,bind=myaddr2 TCP port forwarder, each side bound to another local IP address (bind). This example handles an almost arbitrary number of par‐ allel or consecutive connections by fork’ing a new process after each accept() . It provides a little security by su’ing to user nobody after forking; it only permits connections from the pri‐ vate 10 network (range); due to reuseaddr, it allows immediate restart after master process’s termination, even if some child sockets are not completely shut down. With -lmlocal2, socat logs to stderr until successfully reaching the accept loop. Fur‐ ther logging is directed to syslog with facility local2. socat TCP4-LISTEN:5555,fork,tcpwrap=script \ EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr a simple server that accepts connections (TCP4-LISTEN) and fork’s a new child process for each connection; every child acts as single relay. The client must match the rules for daemon process name "script" in /etc/hosts.allow and /etc/hosts.deny, otherwise it is refused access (see "man 5 hosts_access"). For EXEC’uting the program, the child process chroot’s to /home/sandbox, su’s to user sandbox, and then starts the program /home/sandbox/bin/myscript. Socat and myscript communicate via a pseudo tty (pty); myscript’s stderr is redirected to stdout, so its error messages are transferred via socat to the connected client. socat EXEC:"mail.sh target@domain.com",fdin=3,fdout=4 \ TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512 mail.sh is a shell script, distributed with socat, that imple‐ ments a simple SMTP client. It is programmed to "speak" SMTP on its FDs 3 (in) and 4 (out). The fdin and fdout options tell so‐ cat to use these FDs for communication with the program. Because mail.sh inherits stdin and stdout while socat does not use them, the script can read a mail body from stdin. Socat makes alias1 your local source address (bind), cares for correct network line termination (crnl) and sends at most 512 data bytes per packet (mss). socat -,escape=0x0f /dev/ttyS0,rawer,crnl opens an interactive connection via the serial line, e.g. for talking with a modem. rawer sets the console’s and ttyS0’s ter‐ minal parameters to practicable values, crnl converts to correct newline characters. escape allows terminating the socat process with character control-O. socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork \ SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20 with UNIX-LISTEN, socat opens a listening UNIX domain socket /tmp/.X11-unix/X1. This path corresponds to local XWindow dis‐ play :1 on your machine, so XWindow client connections to DIS‐ PLAY=:1 are accepted. Socat then speaks with the SOCKS4 server host.victim.org that might permit sourceport 20 based connec‐ tions due to an FTP related weakness in its static IP filters. Socat pretends to be invoked by socksuser nobody, and requests to be connected to loopback port 6000 (only weak sockd configu‐ rations will allow this). So we get a connection to the victims XWindow server and, if it does not require MIT cookies or Ker‐ beros authentication, we can start work. Please note that there can only be one connection at a time, because TCP can establish only one session with a given set of addresses and ports. socat -u /tmp/readdata,seek-end=0,ignoreeof - this is an example for unidirectional data transfer (-u). Socat transfers data from file /tmp/readdata (implicit address GOPEN), starting at its current end (seek-end=0 lets socat start reading at current end of file; use seek=0 or no seek option to first read the existing data) in a "tail -f" like mode (ignoreeof). The "file" might also be a listening UNIX domain socket (do not use a seek option then). (sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) | socat - EXEC:'ssh -l user server',pty,setsid,ctty EXEC’utes an ssh session to server. Uses a pty for communication between socat and ssh, makes it ssh’s controlling tty (ctty), and makes this pty the owner of a new process group (setsid), so ssh accepts the password from socat. socat -u TCP4-LISTEN:3334,reuseaddr,fork \ OPEN:/tmp/in.log,creat,append implements a simple network based message collector. For each client connecting to port 3334, a new child process is generated (option fork). All data sent by the clients are append’ed to the file /tmp/in.log. If the file does not exist, socat creat’s it. Option reuseaddr allows immediate restart of the server process. socat PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \ EXEC:"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer" generates a pseudo terminal device (PTY) on the client that can be reached under the symbolic link $HOME/dev/vmodem0. An appli‐ cation that expects a serial line or modem can be configured to use $HOME/dev/vmodem0; its traffic will be directed to a modem‐ server via ssh where another socat instance links it to /dev/ttyS0. socat TCP4-LISTEN:2022,reuseaddr,fork \ PROXY:proxy:www.domain.org:22,proxyport=3128,proxyauth=user:pass starts a forwarder that accepts connections on port 2022, and directs them through the proxy daemon listening on port 3128 (proxyport) on host proxy, using the CONNECT method, where they are authenticated as "user" with "pass" (proxyauth). The proxy should establish connections to host www.domain.org on port 22 then. socat - OPENSSL:server:4443,cafile=server.crt,cert=client.pem is an OpenSSL client that tries to establish a secure connection to an SSL server. Option cafile specifies a file that contains trust certificates: we trust the server only when it presents one of these certificates and proofs that it owns the related private key. Otherwise the connection is terminated. With cert a file containing the client certificate and the associated pri‐ vate key is specified. This is required in case the server wishes a client authentication; many Internet servers do not. The first address (’-’) can be replaced by almost any other so‐ cat address. socat OPENSSL-LISTEN:4443,reuse‐ addr,pf=ip4,fork,cert=server.pem,cafile=client.crt PIPE is an OpenSSL server that accepts TCP connections, presents the certificate from the file server.pem and forces the client to present a certificate that is verified against cafile.crt. The second address (’PIPE’) can be replaced by almost any other socat address. For instructions on generating and distributing OpenSSL keys and certificates see the additional socat docu socat-openssl.txt. echo |socat -u - file:/tmp/bigfile,create,largefile,seek=100000000000 creates a 100GB sparse file; this requires a file system type that supports this (ext2, ext3, reiserfs, jfs; not minix, vfat). The operation of writing 1 byte might take long (reiserfs: some minutes; ext2: "no" time), and the resulting file can consume some disk space with just its inodes (reiserfs: 2MB; ext2: 16KB). socat tcp-l:7777,reuseaddr,fork system:’filan -i 0 -s >&2’,nofork listens for incoming TCP connections on port 7777. For each ac‐ cepted connection, invokes a shell. This shell has its stdin and stdout directly connected to the TCP socket (nofork). The shell starts filan and lets it print the socket addresses to stderr (your terminal window). echo -en "\0\14\0\0\c" |socat -u - file:/usr/bin/squid.exe,seek=0x00074420 functions as primitive binary editor: it writes the 4 bytes 000 014 000 000 to the executable /usr/bin/squid at offset 0x00074420 (this is a real world patch to make the squid exe‐ cutable from Cygwin run under Windows, actual per May 2004). socat - tcp:www.blackhat.org:31337,readbytes=1000 connects to an unknown service and prevents being flooded. socat -U TCP:target:9999,end-close TCP-L:8888,reuseaddr,fork merges data arriving from different TCP streams on port 8888 to just one stream to target:9999. The end-close option prevents the child processes forked off by the second address from termi‐ nating the shared connection to 9999 (close(2) just unlinks the inode which stays active as long as the parent process lives; shutdown(2) would actively terminate the connection). socat - UDP4-DATAGRAM:192.168.1.0:123,sp=123,broad‐ cast,range=192.168.1.0/24 sends a broadcast to the network 192.168.1.0/24 and receives the replies of the timeservers there. Ignores NTP packets from hosts outside this network. socat - SOCKET-DATAGRAM:2:2:17:x007bxc0a80100x0000000000000000,b‐ ind=x007bx00000000x0000000000000000,setsockopt-int=1:6:1,r‐ ange=x0000xc0a80100x0000000000000000:x0000xffffff00x0000000000000000 is semantically equivalent to the previous example, but all pa‐ rameters are specified in generic form. the value 6 of setsock‐ opt-int is the Linux value for SO_BROADCAST. socat - IP4-DATAGRAM:255.255.255.255:44,broadcast,range=10.0.0.0/8 sends a broadcast to the local network(s) using protocol 44. Ac‐ cepts replies from the private address range only. socat - UDP4-DATAGRAM:224.255.0.1:6666,bind=:6666,ip-add-member‐ ship=224.255.0.1:eth0 transfers data from stdin to the specified multicast address us‐ ing UDP. Both local and remote ports are 6666. Tells the inter‐ face eth0 to also accept multicast packets of the given group. Multiple hosts on the local network can run this command, so all data sent by any of the hosts will be received by all the other ones. Note that there are many possible reasons for failure, in‐ cluding IP-filters, routing issues, wrong interface selection by the operating system, bridges, or a badly configured switch. socat UDP:host2:4443 TUN:192.168.255.1/24,up establishes one side of a virtual (but not private!) network with host2 where a similar process might run, with UDP-L and tun address 192.168.255.2. They can reach each other using the ad‐ dresses 192.168.255.1 and 192.168.255.2. Note that streaming eg.via TCP or SSL does not guarantee to retain packet boundaries and might thus cause packet loss. socat - VSOCK-CONNECT:2:1234 establishes a VSOCK connection with the host (host is always reachable with the well-know CID=2) on 1234 port. socat - VSOCK-LISTEN:1234 listens for a VSOCK connection on 1234 port. socat - VSOCK-CONNECT:31:4321,bind:5555 establishes a VSOCK connection with the guest that have CID=31 on 1234 port, binding the local socket to the 5555 port. socat VSOCK-LISTEN:3333,reuseaddr,fork VSOCK-CONNECT:42,3333 starts a forwarder that accepts VSOCK connections on port 3333, and directs them to the guest with CID=42 on the same port. socat VSOCK-LISTEN:22,reuseaddr,fork TCP:localhost:22 forwards VSOCK connections from 22 port to the local SSH server. Running this in a VM allows you to connect via SSH from the host using VSOCK, as in the example below. socat TCP4-LISTEN:22222,reuseaddr,fork VSOCK-CONNECT:33:22 forwards TCP connections from 22222 port to the guest with CID=33 listening on VSOCK port 22. Running this in the host, allows you to connect via SSH running "ssh -p 22222 user@local‐ host", if the guest runs the example above. socat PTY,link=/var/run/ppp,rawer INTERFACE:hdlc0 circumvents the problem that pppd requires a serial device and thus might not be able to work on a synchronous line that is represented by a network device. socat creates a PTY to make pppd happy, binds to the network interface hdlc0, and can trans‐ fer data between both devices. Use pppd on device /var/run/ppp then. socat -T 1 -d -d TCP-L:10081,reuseaddr,fork,crlf SYSTEM:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/plain\\\n\\\ndate: \$\(date\)\\\nserver:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT\\\nclient: \$SO‐ CAT_PEERADDR:\$SOCAT_PEERPORT\\\n\\\"\"; cat; echo -e \"\\\"\\\n\\\"\"" creates a simple HTTP echo server: each HTTP client that con‐ nects gets a valid HTTP reply that contains information about the client address and port as it is seen by the server host, the host address (which might vary on multihomed servers), and the original client request. socat -d -d UDP4-RECVFROM:9999,so-broadcast,so-timestamp,ip-pkt‐ info,ip-recverr,ip-recvopts,ip-recvtos,ip-recvttl!!- SYSTEM:’export; sleep 1’ |grep SOCAT waits for an incoming UDP packet on port 9999 and prints the en‐ vironment variables provided by socat. On BSD based systems you have to replace ip-pktinfo with ip-recvdstaddr,ip-recvif. Espe‐ cially interesting is SOCAT_IP_DSTADDR: it contains the target address of the packet which may be a unicast, multicast, or broadcast address. echo -e "M-SEARCH * HTTP/1.1\nHOST: 239.255.255.250:1900\nMAN: \"ssdp:discover\"\nMX: 4\nST: \"ssdp:all\"\n" |./socat - UDP-DATA‐ GRAM:239.255.255.250:1900,crlf sends an SSDP (Simple Service Discovery Protocol) query to the local network and collects and outputs the answers received. DIAGNOSTICS Socat uses a logging mechanism that allows filtering messages by sever‐ ity. The severities provided are more or less compatible to the appro‐ priate syslog priority. With one or up to four occurrences of the -d command line option, the lowest priority of messages that are issued can be selected. Each message contains a single uppercase character specifying the messages severity (one of F, E, W, N, I, or D) FATAL: Conditions that require unconditional and immediate program ter‐ mination. ERROR: Conditions that prevent proper program processing. Usually the program is terminated (see option -s). WARNING: Something did not function correctly or is in a state where cor‐ rect further processing cannot be guaranteed, but might be pos‐ sible. NOTICE: Interesting actions of the program, e.g. for supervising socat in some kind of server mode. INFO: Description of what the program does, and maybe why it happens. Allows monitoring the lifecycles of file descriptors. DEBUG: Description of how the program works, all system or library calls and their results. Log messages can be written to stderr, to a file, or to syslog. On exit, socat gives status 0 if it terminated due to EOF or inactivity timeout, with a positive value on error, and with a negative value on fatal error. FILES /usr/bin/socat /usr/bin/filan /usr/bin/procan ENVIRONMENT VARIABLES Input variables carry information from the environment to socat, output variables are set by socat for use in executed scripts and programs. In the output variables beginning with "SOCAT" this prefix is actually replaced by the upper case name of the executable or the value of op‐ tion -lp. SOCAT_DEFAULT_LISTEN_IP (input) (Values 4 or 6) Sets the IP version to be used for listen, recv, and recvfrom addresses if no pf (protocol-family) option is given. Is overridden by socat options -4 or -6. SOCAT_PREFERRED_RESOLVE_IP (input) (Values 0, 4, or 6) Sets the IP version to be used when resolv‐ ing target host names when version is not specified by address type, option pf (protocol-family), or address format. If name resolution does not return a matching entry, the first result (with differing IP version) is taken. With value 0, socat always selects the first record and its IP version. SOCAT_FORK_WAIT (input) Specifies the time (seconds) to sleep the parent and child pro‐ cesses after successful fork(). Useful for debugging. SOCAT_VERSION (output) Socat sets this variable to its version string, e.g. "1.7.0.0" for released versions or e.g. "1.6.0.1+envvar" for temporary versions; can be used in scripts invoked by socat. SOCAT_PID (output) Socat sets this variable to its process id. In case of fork ad‐ dress option, SOCAT_PID gets the child processes id. Forking for exec and system does not change SOCAT_PID. SOCAT_PPID (output) Socat sets this variable to its process id. In case of fork, SO‐ CAT_PPID keeps the pid of the master process. SOCAT_PEERADDR (output) With passive socket addresses (all LISTEN and RECVFROM ad‐ dresses), this variable is set to a string describing the peers socket address. Port information is not included. SOCAT_PEERPORT (output) With appropriate passive socket addresses (TCP, UDP, and SCTP - LISTEN and RECVFROM), this variable is set to a string contain‐ ing the number of the peer port. SOCAT_SOCKADDR (output) With all LISTEN addresses, this variable is set to a string de‐ scribing the local socket address. Port information is not in‐ cluded example SOCAT_SOCKPORT (output) With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses, this variable is set to the local port. SOCAT_TIMESTAMP (output) With all RECVFROM addresses where address option so-timestamp is applied, socat sets this variable to the resulting timestamp. SOCAT_IP_OPTIONS (output) With all IPv4 based RECVFROM addresses where address option ip-recvopts is applied, socat fills this variable with the IP options of the received packet. SOCAT_IP_DSTADDR (output) With all IPv4 based RECVFROM addresses where address option ip-recvdstaddr (BSD) or ip-pktinfo (other platforms) is applied, socat sets this variable to the destination address of the re‐ ceived packet. This is particularly useful to identify broadcast and multicast addressed packets. SOCAT_IP_IF (output) With all IPv4 based RECVFROM addresses where address option ip-recvif (BSD) or ip-pktinfo (other platforms) is applied, so‐ cat sets this variable to the name of the interface where the packet was received. SOCAT_IP_LOCADDR (output) With all IPv4 based RECVFROM addresses where address option ip-pktinfo is applied, socat sets this variable to the address of the interface where the packet was received. SOCAT_IP_TOS (output) With all IPv4 based RECVFROM addresses where address option ip-recvtos is applied, socat sets this variable to the TOS (type of service) of the received packet. SOCAT_IP_TTL (output) With all IPv4 based RECVFROM addresses where address option ip-recvttl is applied, socat sets this variable to the TTL (time to live) of the received packet. SOCAT_IPV6_HOPLIMIT (output) With all IPv6 based RECVFROM addresses where address option ipv6-recvhoplimit is applied, socat sets this variable to the hoplimit value of the received packet. SOCAT_IPV6_DSTADDR (output) With all IPv6 based RECVFROM addresses where address option ipv6-recvpktinfo is applied, socat sets this variable to the destination address of the received packet. SOCAT_IPV6_TCLASS (output) With all IPv6 based RECVFROM addresses where address option ipv6-recvtclass is applied, socat sets this variable to the transfer class of the received packet. SOCAT_OPENSSL_X509_ISSUER (output) Issuer field from peer certificate SOCAT_OPENSSL_X509_SUBJECT (output) Subject field from peer certificate SOCAT_OPENSSL_X509_COMMONNAME (output) commonName entries from peer certificates subject. Multiple val‐ ues are separated by " // ". SOCAT_OPENSSL_X509_* (output) all other entries from peer certificates subject SOCAT_OPENSSL_X509V3_DNS (output) DNS entries from peer certificates extensions - subjectAltName field. Multiple values are separated by " // ". HOSTNAME (input) Is used to determine the hostname for logging (see -lh). LOGNAME (input) Is used as name for the socks client user name if no socksuser is given. With options su and su-d, LOGNAME is set to the given user name. USER (input) Is used as name for the socks client user name if no socksuser is given and LOGNAME is empty. With options su and su-d, USER is set to the given user name. SHELL (output) With options su and su-d, SHELL is set to the login shell of the given user. PATH (output) Can be set with option path for exec and system addresses. HOME (output) With options su and su-d, HOME is set to the home directory of the given user. CREDITS The work of the following groups and organizations was invaluable for this project: The FSF (GNU, http://www.fsf.org/ project with their free and portable development software and lots of other useful tools and libraries. The Linux developers community (http://www.linux.org/) for providing a free, open source operating system. The Open Group (http://www.unix-systems.org/) for making their standard specifications available on the Internet for free. VERSION This man page describes version 1.7.4 of socat. BUGS Addresses cannot be nested, so a single socat process cannot, e.g., drive ssl over socks. Address option ftruncate without value uses default 1 instead of 0. Verbose modes (-x and/or -v) display line termination characters incon‐ sistently when address options cr or crnl are used: They show the data after conversion in either direction. The licenses of OpenSSL and GNU Readline are incompatible. Therefore readline support is disabled in Debian. Send bug reports to SEE ALSO nc(1), rinetd(8), openssl(1), stunnel(8), rlwrap(1), setsid(1) Socat home page http://www.dest-unreach.org/socat/ AUTHOR Gerhard Rieger and contributors socat(1)