knowledge/technology/applications/cli/network/socat.md

obj	repo	website	rev
application	https://repo.or.cz/socat.git	http://www.dest-unreach.org/socat	2024-09-01

socat

socat (SOcket CAT) is a powerful command-line utility designed for data transfer and networking. It functions similarly to the popular netcat (nc) tool but offers additional features and flexibility, making it suitable for a wide range of network and system administration tasks. Socat enables bidirectional data transfer between two independent data channels, which can be files, pipes, devices (e.g., serial lines), or network sockets.

Usage

Usage: socat [options] <address> <address>

Options

Option	Description
-r <file>	Dumps the raw (binary) data flowing from left to right address to the given file.
-R <file>	Dumps the raw (binary) data flowing from right to left address to the given file.
-b<size>	Sets the data transfer block <size>. At most <size> bytes are transferred per step. Default is 8192 bytes.
-u	Uses unidirectional mode. The first address is only used for reading, and the second address is only used for writing (example).
-U	Uses unidirectional mode in reverse direction. The first address is only used for writing, and the second address is only used for read

Address

With the address command line arguments, the user gives socat instructions 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 keyword 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). Keywords are case insensitive. For a few special address types, the keyword may be omitted: Address specifications starting with a number are assumed to be FD (raw file descriptor) addresses; if a / is found before the first : or ,, GOPEN (generic file open) is assumed.

Address Types

This section describes the available address types with their keywords, parameters, and semantics.

CREATE:<filename>
Opens <filename> and uses the file descriptor for writing. This address type requires write-only context.
Option groups: FD,REG,NAMED
Useful options: mode, user, group, unlink-early, unlink-late, append

EXEC:<command-line>
Forks a sub process that establishes communication with its parent process and invokes the specified program with execvp() . <command-line> is a simple command with arguments separated by single spaces. If the program name contains a /, the part after the last / is taken as ARGV[0].
Option groups: FD,SOCKET,EXEC,FORK Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, login, sigint, sigquit`

FD:<fdnum>
Uses the file descriptor <fdnum>. It must already exist as valid UNIX file descriptor.
Option groups: FD
See also: STDIO, STDIN, STDOUT, STDERR

GOPEN:<filename>
(Generic open) This address type tries to handle any file system entry except directories usefully. <filename> may be a relative or absolute path.
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT

IP-SENDTO:<host>:<protocol>
Opens a raw IP socket. Depending on host specification or option pf, IP protocol version 4 or 6 is used. It uses <protocol> to send packets to <host> [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

IP4-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4

IP6-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6

INTERFACE:<interface>
Communicates with a network connected on an interface using raw packets including link level data. <interface> is the name of the network interface. Currently only available on Linux.
Option groups: FD,SOCKET
Useful options: pf, type

IP-DATAGRAM:<address>:<protocol>
Sends outgoing data to the specified address which may in particular 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 multicast communications.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-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:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE

IP6-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv6. Please note that IPv6 does not know broadcasts.
Option groups: FD,SOCKET,IP6,RANGE

IP-RECVFROM:<protocol>
Opens a raw IP socket of <protocol>. 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 because there is no port number to distinguish the sockets. This address works well with IP-SENDTO address peers. Protocol 255 uses the raw socket with the IP header being part of the data.
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:<protocol>
Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE

IP6-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE

IP-RECV:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version 4 or 6 is used. It receives packets from multiple unspecified peers and merges the data. No replies are possible. It can be, e.g., addressed by socat IP-SENDTO address peers. Protocol 255 uses the raw socket with the IP header being 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:<protocol>
Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE

IP6-RECV:<protocol>
Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE

OPEN:<filename>
Opens <filename> using the open() system call. 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:<host>:<port>
Tries to establish a SSL connection to <port> [TCP service] on <host> [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,OPENSSL,RETRY
Useful options: cipher, verify, commonname, cafile, capath, certificate, key, compress, bind, pf, connect-timeout, sourceport, retry
See also: OPENSSL-LISTEN, TCP

OPENSSL-LISTEN:<port>
Listens on tcp <port> [TCP service]. The IP version is 4 or the one specified with pf. When a connection is accepted, this address behaves as SSL server.
Option groups: FD,SOCKET,IP4,IP6,TCP,LISTEN,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:<host>:<port>
Tries to establish a DTLS connection to <port> [UDP service] on <host> [IP address] using UDP/IP version 4 or 6 depending on address 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.
Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY
Useful options: cipher, verify, commonname, cafile, capath, certificate, key, compress, bind, pf, sourceport, retry
See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT

OPENSSL-DTLS-SERVER:<port>
Listens on UDP <port> [UDP service]. The IP version is 4 or the one specified with pf. When a connection is accepted, this address behaves as DTLS server.
Option groups: FD,SOCKET,IP4,IP6,LISTEN,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:<filename>
If <filename> already exists, it is opened. If it does not exist, a named pipe is created and opened.
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode, unlink-early

PIPE Creates an unnamed pipe and uses it for reading and writing. It works as an echo, because everything written to it appeares immediately as read data.

PROXY:<proxy>:<hostname>:<port>
Connects to an HTTP proxy server on port 8080 using TCP/IP version 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

PTY Generates a pseudo terminal (pty) and uses its master side. Another process may open the ptys slave side using it like a serial line or terminal. If both the ptmx and the openpty mechanisms are available, ptmx is used (POSIX). Option groups: FD,NAMED,PTYUseful options:link, openpty, wait-slave, mode, user, groupSee also:UNIX-LISTEN, PIPE, EXEC, SYSTEM`

SOCKET-CONNECT:<domain>:<protocol>:<remote-address>
Creates a stream socket using the first and second given socket parameters and SOCK_STREAM and connects to the remote-address. The two socket parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO

SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>
Creates a datagram socket using the first three given socket parameters and sends outgoing data to the remote-address. The three socket parameters have to be specified by int numbers.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV, SOCKET-RECVFROM

SOCKET-LISTEN:<domain>:<protocol>:<local-address>
Creates a stream socket using the first and second given socket parameters and SOCK_STREAM and waits for incoming connections
on local-address. The two socket parameters have to be specified by int numbers.
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:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters and binds it to <local-address>. Receives arriving data. The three parameters have to be specified by int numbers.
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:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters and binds it to <local-address>. Receives arriving data and sends replies back to the sender. The first three parameters have to be specified as int numbers.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt, setsockopt-listen
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM, SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV

SOCKET-SENDTO:<domain>:<type>:<protocol>:<remote-address>
Creates a socket using the three given socket parameters. Sends outgoing data to the given address and receives replies. The
three parameters have to be specified as int numbers.
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:<socks-server>:<host>:<port>
Connects via <socks-server> [IP address] to <host> [IPv4 address] on <port> [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:<socks-server>:<host>:<port>
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

STDIN Uses file descriptor 0.
Option groups: FD

STDIO Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD

STDOUT Uses file descriptor 1.
Option groups: FD

SYSTEM:<shell-command>
Forks a sub process that establishes communication with its parent process and invokes the specified program with system() . Please note that <shell-command> must not contain , or "!!", and that shell meta characters may have to be protected. After successful program start, socat writes data to stdin of the process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, sigint, sigquit
See also: EXEC

TCP:<host>:<port>
Connects to <port> [TCP service] on <host> [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, mtudiscover, mss, nodelay, nonblock, sourceport, retry, readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT, UNIX-CONNECT

TCP4:<host>:<port>
Like TCP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,TCP,RETRY

TCP6:<host>:<port>
Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY

TCP-LISTEN:<port>
Listens on <port> [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_DEFAULT_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-children, backlog, accept-timeout, mss, su, reuseaddr, retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, UNIX-LISTEN, OPENSSL-LISTEN, TCP-CONNECT

TCP4-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY

TCP6-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY

TUN[:<if-addr>/<bits>]
Creates a Linux TUN/TAP device and optionally assignes it the address and netmask given by the parameters. The resulting network interface is almost ready for use by other processes; socat serves its "wire side". This address requires read and write access to the tunnel cloning device, usually /dev/net/tun, as well as permission to set some ioctl()s. Option iff-up is required to immediately activate the interface!
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type, iff-no-pi

UDP:<host>:<port>
Connects to <port> [UDP service] on <host> [IP address] using UDP/IP version 4 or 6 depending on address specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP

UDP4:<host>:<port>
Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4

UDP6:<host>:<port>
Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6

UDP-DATAGRAM:<address>:<port>
Sends outgoing data to the specified address which may in particular 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 version 1.7.4.0) and if their source addresses match RANGE or TCPWRAP options. This address type can for example be used for implementing symmetric or asymmetric broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-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:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4, RANGE

UDP6-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE

UDP-LISTEN:<port>
Waits for a UDP/IP packet arriving on <port> [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 protocol 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:<port>
Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4

UDP6-LISTEN:<port>
Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6

UDP-SENDTO:<host>:<port>
Communicates with the specified peer socket, defined by <port> [UDP service] on <host> [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 address 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:<host>:<port>
Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4

UDP6-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6

UDP-RECVFROM:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP version 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 typical 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:<port>
Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE

UDP6-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE

UDP-RECV:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP version 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 unidirectional 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:<port>
Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE

UDP6-RECV:<port>
Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE

UNIX-CONNECT:<filename>
Connects to <filename> assuming it is a UNIX domain socket. If <filename> does not exist, this is an error; if <filename> is not a UNIX domain socket, this is an error; if <filename> is a UNIX domain socket, but no process is listening, this is an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP

UNIX-LISTEN:<filename>
Listens on <filename> using a UNIX domain stream socket and accepts a connection. If <filename> exists and is not a socket, this is an error. If <filename> 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 unlink-close).
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:<filename>
Communicates with the specified peer socket, defined by [<filename>] 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 (eg. /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:<filename>
Creates a UNIX domain datagram socket [<filename>]. Receives one packet 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 address works well with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
Useful options: fork
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM, IP-RECVFROM

UNIX-RECV:<filename>
Creates a UNIX domain datagram socket [<filename>]. 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:<filename>
Communicates with the specified peer socket, defined by [<filename>] 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

Address Options

Address options can be applied to address specifications to influence the process of opening the addresses and the properties of the resulting 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 concept 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.

FD option group

This option group contains options that are applied to a UNIX style file descriptor, no matter how it was generated. Because all current socat address types are file descriptor based, these options may be applied 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 mechanism is used. The second, non-fd based mechanism is prioritized.

Option	Description
cloexec=<bool>	Sets the FD_CLOEXEC flag with the fcntl() system call to value <bool>. 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 using 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
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 using the `flock(fd, LOCK_SH
lock	Sets a blocking lock on the file. Uses the setlk or flock mechanism depending on availability on the particular platform. If both are available, the POSIX variant (setlkw) is used.
user=<user>	Sets the <user> (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=<user>	Sets the owner of the fd to <user> with the fchown() system call after opening or connecting the channel. This is useful only on file system entries.
group=<group>	Sets the <group> 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 group of the stream with the fchown() system call. These calls might require group membership or root privilege.
group-late=<group>	Sets the group of the fd to <group> with the fchown() system call after opening or connecting the channel. This is useful only on file system entries.
mode=<mode>	Sets the <mode> (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() systemcall after opening the filesystem entry or binding to the UNIX domain socket (race condition!). Otherwise, socat sets the mode of the stream using fchmod(). These calls might require ownership or root privilege.
perm-late=<mode>	Sets the permissions of the fd to value <mode> using the fchmod() system call after opening or connecting the channel. This is useful only on file system entries.
append=<bool>	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. Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND) call.
nonblock=<bool>	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() systemcall. 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 often 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 connection is to be reused by or shared with other processes. Normally, socket connections will be ended with shutdown which terminates the socket even if it is shared by multiple processes. close "unlinks" the socket from the process but keeps it active as long as there are still links from other processes. Similarly, when an address of type EXEC or SYSTEM is ended, socat usually will explicitly kill the sub process. With this option, 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.
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 protocols. Has been tested against netcat and socat with option null-eof.
null-eof	Normally socat will ignore empty (zero size payload) packets arriving on datagram sockets, so it survives port scans. With this option socat interprets empty datagram packets as EOF indicator.
ioctl-void=<request>	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=<request>:<value>	Calls ioctl() with the request value as second argument and the integer value as third argument.
ioctl-intp=<request>:<value>	Calls ioctl() with the request value as second argument and a pointer to the integer value as third argument.
ioctl-bin=<request>:<value>	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 <dalan> form.
ioctl-string=<request>:<value>	Calls ioctl() with the request value as second argument and a pointer to the given string as third argument.

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.

Option	Description
user-early=<user>	Changes the <user> (owner) of the file system entry before accessing it, using the chown() system call. This call might require root privilege.
group-early=<group>	Changes the <group> of the file system entry before accessing it, using the chown() system call. This call might require group membership or root privilege.
perm-early=<mode>	Changes the <mode> of the file system entry before accessing it, using the chmod() system call. This call might require ownership or root privilege.
umask=<mode>	Sets the umask of the process to <mode> before accessing 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 applying 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 inaccessible for other processes after a short race condition.
unlink-close	Removes the addresses file system entry when closing the address. 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.

Option	Description
creat=<bool>	Creates the file if it does not exist.
dsync=<bool>	Blocks write() calls until metainfo is physically written to media.
excl=<bool>	With option creat, if file exists this is an error.
largefile=<bool>	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=<bool>	Does not make this file the controlling terminal.
nofollow=<bool>	Does not follow symbolic links.
nshare=<bool>	Does not allow sharing this file with other processes.
rshare=<bool>	Does not allow other processes to open this file for writing.
rsync=<bool>	Blocks write() until metainfo is physically written to media.
sync=<bool>	Blocks write() until data is physically written to media.
rdonly=<bool>	Opens the file for reading only.
wronly=<bool>	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 UNIX file descriptor, but their semantics make sense only on a file supporting random access.

Option	Description
seek=<offset>	Applies the lseek(fd, <offset>, SEEK_SET) system call, thus positioning the file pointer absolutely to <offset>. Please note that a missing value defaults to 1, not 0.
seek-cur=<offset>	Applies the lseek(fd, <offset>, SEEK_CUR) system call, thus positioning the file pointer at <offset> bytes relatively to its current position (which is usually 0). Please note that a missing value defaults to 1, not 0.
seek-end=<offset>	Applies the lseek(fd, <offset>, SEEK_END) system call, thus positioning the file pointer <offset> bytes relatively to the files current end. Please note that a missing value defaults to 1, not 0.
ftruncate=<offset>	Applies the ftruncate(fd, <offset>) system call, thus truncating the file at the position <offset>. Please note that a missing value defaults to 1, not 0.

PROCESS option group

Options of this group change the process properties instead of just affecting 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.

Option	Description
chroot=<directory>	Performs a chroot() operation to <directory> after processing the address. This call might require root privilege.
chroot-early=<directory>	Performs a chroot() operation to <directory> before opening the address. This call might require root privilege.
setgid=<group>	Changes the primary <group> 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=<group>	Like setgit but is performed before opening the address.
setuid=<user>	Changes the <user> (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=<user>	Like setuid but is performed before opening the address.
su=<user>	Changes the <user> (owner) and groups of the process after processing the address. This call might require root privilege.
su-d=<user>	Short name for substuser-delayed. Changes the <user> (owner) and groups of the process after processing the address. The user and his groups are retrieved before a possible chroot() . This call might require root privilege.
setpgid=<pid_t>	Makes the process a member of the specified process group <pid_t>. 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.

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.

Option	Description
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 channel.
ignoreeof	When EOF occurs on this channel, socat ignores it and tries to read more data (like "tail -f").
readbytes=<bytes>	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=<filename>	If lockfile exists, exits with error. If lockfile does not exist, creates it and continues, unlinks lockfile on exit.
waitlock=<filename>	If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues, unlinks lockfile on exit.
escape=<int>	Specifies the numeric code of a character that triggers EOF on the input stream. It is useful with a terminal in raw mode.

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.

Option	Description
bind=<sockname>	Binds the socket to the given socket address using the bind() system call. The form of <sockname> is socket domain dependent: IP4 and IP6 allow the form `[hostname
connect-timeout=<seconds>	Abort the connection attempt after <seconds> with error status.
so-bindtodevice=<interface>	Binds the socket to the given <interface>. 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=<seconds>	Blocks shutdown() or close() until data transfers have finished or the given timeout expired.
oobinline	Places out-of-band data in the input data stream.
priority=<priority>	Sets the protocol defined <priority> for outgoing packets.
rcvbuf=<bytes>	Sets the size of the receive buffer after the socket() call to <bytes>. With TCP sockets, this value corresponds to the socket`s maximal window size.
rcvbuf-late=<bytes>	Sets the size of the receive buffer when the socket is already connected to <bytes>. With TCP sockets, this value corresponds to the socket`s maximal window size.
rcvlowat=<bytes>	Specifies the minimum number of received bytes 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.
sndbuf=<bytes>	Sets the size of the send buffer after the socket() call to <bytes>.
sndbuf-late=<bytes>	Sets the size of the send buffer when the socket is connected to <bytes>.
sndlowat=<bytes>	Specifies the minimum number of bytes in the send buffer until the socket layer will send the data to <bytes>.
pf=<string>	Forces the use of the specified IP version or protocol. <string> 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=<type>	Sets the type of the socket, specified as second argument to the socket() or socketpair() calls, to <type>. Address resolution 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 <protocol>. 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=<level>:<optname>:<optval>	Invokes setsockopt() for the socket with the given parameters. level is used as second argument to setsockopt() and specifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or SOL_SOCKET for the socket layer (1 on Linux). optname 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 specifies an arbitrary sequence of bytes that are passed to the function per pointer, with the automatically derived length parameter.
setsockopt-int=<level>:<optname>:<optval>	Like setsockopt, but <optval> is a pointer to int
setsockopt-listen=<level>:<optname>:<optval>	Like setsockopt, but for listen type addresses it is applied to the listening socket instead of the connected socket.
setsockopt-string=<level>:<optname>:<optval>	Like setsockopt, but <optval> is a string. This string is passed to the function with trailing null character, and the length parameter is automatically derived from the data.

IP4 and IP6 option groups

These options can be used with IPv4 and IPv6 based sockets.

Option	Description
tos=<tos>	Sets the TOS (type of service) field of outgoing packets to <tos> [byte] (see RFC 791).
ttl=<ttl>	Sets the TTL (time to live) field of outgoing packets to <ttl>.
ip-options=<data>	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-options=x8307040a000001. IP options are defined in RFC 791.
ip-pktinfo	Sets the IP_PKTINFO socket option. This enables receiving and logging of ancillary messages containing destination address and interface (Linux).
ip-recverr	Sets the IP_RECVERR socket option. This enables receiving and logging of ancillary messages containing detailed error information.
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).
ip-recvif	Sets the IP_RECVIF socket option. This enables receiving and logging of interface ancillary messages (*BSD).
ip-multicast-if=<hostname>	Specifies hostname or address of the network interface to be used for multicast traffic.
ip-multicast-loop=<bool>	Specifies if outgoing multicast traffic should loop back to the interface.
ip-multicast-ttl=<byte>	Sets the TTL used for outgoing multicast traffic. Default is 1.
ip-transparent	Sets the IP_TRANSPARENT socket option. This option might require root privilege.

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.

Option	Description
ipv6only=<bool>	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 options.
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)(<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 information.
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 setsockopt() with the appropriate parameters.

Option	Description
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=<count>	Sets the number of keepalives before shutting down the socket to <count>.
keepidle=<seconds>	Sets the idle time before sending the first keepalive to <seconds>.
keepintvl=<seconds>	Sets the interval between two keepalives to <seconds>.
linger2=<seconds>	Sets the time to keep the socket in FIN-WAIT-2 state to <seconds>.
mss=<bytes>	Sets the MSS (maximum segment size) after the socket() call to <bytes>. This value is then proposed to the peer with the SYN or SYN/ACK packet.
mss-late=<bytes>	Sets the MSS of the socket after connection has been established to <bytes>.
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=<count>	Sets the maximal number of SYN retransmits during connect to <count>.
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).
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.

Option	Description
udp-ignore-peerport>	Address UDP-DATAGRAM expects incoming responses to come from the port specified in its second parameter. With this option, it accepts packets coming from any port.

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.

Option	Description
sourceport=<port>	For outgoing (client) TCP and UDP connections, it sets the source <port> using an extra bind() call. With TCP or UDP listen 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.
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.

Option	Description
socksport=<tcp service>	Overrides the default "socks" service or port 1080 for the socks server port with <TCP service>.
socksuser=<user>	Sends the <user> in the username field to the socks server. Default is the actual user name ($LOGNAME or $USER).

HTTP option group

Options that can be provided with HTTP type addresses. The only HTTP address currently implemented is proxy-connect.

Option	Description
proxyport=<TCP service>	Overrides the default HTTP proxy port 8080 with <TCP service>.
ignorecr	The HTTP protocol requires the use of CR+NL as line terminator. When a proxy server violates this standard, socat might not understand 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=<username>:<password>	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=<filename>	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 containing the target hostname. With this option, socat resolves the hostname locally and sends the IP address. Please note that, according 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.

Option	Description
range=<address-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; 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[=<name>]	Use 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 <name> (type string) is passed to the wrapper functions as daemon process name. If omitted, the basename of socats invocation (argv[0]) is passed. If both tcpwrap and range options are applied to an address, both conditions must be fulfilled to allow the connection.
allow-table=<filename>	Takes the specified file instead of /etc/hosts.allow.
deny-table=<filename>	Takes the specified file instead of /etc/hosts.deny.
tcpwrap-etc=<directoryname>	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.

Option	Description
backlog=<count>	Sets the backlog value passed with the listen() system call to <count>. Default is 5.
accept-timeout=<seconds>	End waiting for a connection after <seconds> with error status.
max-children=<count>	Limits the number of concurrent child processes. Default is no limit.

CHILD option group

Options for addresses with multiple connections via child processes.

Option	Description
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.

EXEC option group

Options for addresses that invoke a program.

Option	Description
path=<string>	Overrides the $PATH environment variable for searching the program with <string>. 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 communication mechanism can be influenced with the following options. Per default, 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.

Option	Description
nofork	Does not fork a subprocess for executing the program, instead calls execvp() or system() directly from the actual socat instance. This avoids the overhead of another process between the program and its peer, but introduces a lot of restrictions.
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 (socketpair 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.
ctty	Makes the pty the controlling tty of the sub process.
stderr	Directs stderr of the sub process to its output channel by making stderr a dup() of stdout.
fdin=<fdnum>	Assigns the sub processes input channel to its file descriptor <fdnum> instead of stdin (0). The program started from the subprocess has to use this fd for reading data from socat.
fdout=<fdnum>	Assigns the sub processes output channel to its file descriptor <fdnum> instead of stdout (1). The program started from the subprocess has to use this fd for writing data to socat.
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.

PTY option group

These options are intended for use with the pty address type.

Option	Description
link=<filename>	Generates a symbolic link that points to the actual pseudo terminal (pty). This might help to solve the problem that ptys are generated with more or less unpredictable names, making it difficult 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).
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.
pty-interval=<seconds>	When the wait-slave option is set, socat periodically checks the HUP condition using poll() to find if the ptys slave side has been opened. The default polling interval is 1s. Use the pty-interval option [timeval]` to change this value.

OPENSSL option group

These options apply to the openssl and openssl-listen address types.

Option	Description
cipher=<cipherlist>	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 <cipherlist>. 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.
verify=<bool>	Controls check of the peer`s certificate. Default is 1 (true). Disabling verify might open your socket for everyone, making the encryption useless!
cert=<filename>	Specifies the file with the certificate and private key for authentication. 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=<filename>	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=<filename>	Specifies the file with the Diffie Hellman parameters. These parameters may also be in the file given with the cert option in which case the dhparams option is not needed.
cafile=<filename>	Specifies the file with the trusted (root) authority certificates. 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=<dirname>	Specifies the directory with the trusted (root) certificates. The directory must contain certificates in PEM format and their hashes (see OpenSSL documentation)
egd=<filename>	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.
compress	Enable or disable the use of compression for a connection. Setting 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-related settings.
commonname=<string>	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=<bool>	Do not use the client side Server Name Indication (SNI) feature that selects the desired server certificate.
snihost=<string>	Set the client side Server Name Indication (SNI) host name different 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.

RETRY option group

Options that control retry of some system calls, especially connection attempts.

Option	Description
retry=<num>	Number of retries before the connection or listen attempt is aborted. Default is 0, which means just one attempt.
interval=<timespec>	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.

Option	Description
tun-device=<device-file>	Instructs socat to take another path for the TUN clone device. Default is /dev/net/tun.
tun-name=<if-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 devices, 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.

Examples

Connect to TCP port 80 on the local or remote system. In this case, socat transfers data between STDIO (-) and a TCP4 connection to port 80 on a host named www.example.com:

socat - TCP4:www.example.com:80

Use socat as a TCP port forwarder. This example listens on port 81, accepts connections, and forwards the connections to port 80 on the remote host:

socat TCP4-LISTEN:81,fork,reuseaddr TCP4:TCP4:192.168.1.10:80