1:socat
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socat - Multipurpose relay (SOcket CAT)
Contents |
SYNOPSIS
socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan
DESCRIPTION
Socat is a command line based utility that establishes two bidirectional 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 options may be applied to the streams, socat can be used for many
different purposes. It might be one of the tools that one `has already needed�.
Filan is a utility that prints information about its active file descriptors 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 purposes 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 conver-
sions 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 begins. 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 depen-
dend, 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 generated; applying this option also prints warning
messages. See DIAGNOSTICS 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[<facility>]
Writes messages to syslog instead of stderr; severity as defined with -d option. With optional <facility>,
the syslog type can be selected, default is "daemon".
-lf <logfile>
Writes messages to <logfile> [filename] instead of stderr.
-ls Writes messages to stderr (this is the default).
-lp<progname>
Overrides the program name printed in error messages.
-lu Extends the timestamp of error messages to microsecond resolution. Does not work when logging to syslog.
-lm[<facility>]
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 sock-
ets with fork option, before the first accept call), it switches logging to syslog. With optional <facil-
ity>, 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 HOSTNAME 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 conversions 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.
-b<size>
Sets the data transfer block <size> [size_t]. At most <size> 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 continue. Even with this
option, socat will exit on fatals, and will abort connection attempts when security checks failed.
-t<timeout>
When one channel has reached EOF, the write part of the other channel is shut down. Then, socat waits
<timeout> [timeval] seconds 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 intervall the
read part gives EOF, socat terminates without awaiting the timeout.
-T<timeout>
Total inactivity timeout: when socat is already in the transfer loop and nothing has happened for <time-
out> [timeval] seconds (no data arrived, no interrupt occurred...) then it terminates. Useful with proto-
cols 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.
-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 considered useful in the given address envi-
ronment. Use it if you want to force, e.g., appliance of a socket option to a serial device.
-L<lockfile>
If lockfile exists, exits with error. If lockfile does not exist, creates it and continues, unlinks lock-
file on exit.
-W<lockfile>
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 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 omit-
ted: 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.
The required number and type of address parameters depend on the address type. E.g., TCP4 requires a server spec-
ification (name or address), and a port specification (number or service name).
Zero or more address options may be given with each address. They influence the address in some ways. Options
consist of an option keyword or an option keyword and a value, separated by �=�. Option keywords are case insen-
sitive. For filtering the options that are useful with an address type, each option is member of one option
group. For each address type there is a set of option groups allowed. Only options belonging to one of these
address groups may be used (except with option -g).
Address specifications following the above schema are also called single address specifications. Two single
addresses can be combined with "!!" to form a dual type address for one channel. Here, the first address 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 handle quotes and parenthesis meaningfully and
allows escaping of special characters. If one of the characters ( { [ � is found, the corresponding 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:<filename>
Opens <filename> 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. <filename> must be a valid existing
or not existing path. If <filename> is a named pipe, creat() might block; if <filename> 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:<command-line>
Forks a sub process that establishes communication with its parent process and invokes the specified pro-
gram 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]. 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 gen-
erated by socketpair() per default.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, login, sig-
int, sigquit
See also: SYSTEM
FD:<fdnum>
Uses the file descriptor <fdnum>. It must already exist as valid UN*X file descriptor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR
GOPEN:<filename>
(Generic open) This address type tries to handle any file system entry except directories usefully. <file-
name> 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_APPEND flag. If it does not exist, it
is opened with flag O_CREAT as a regular file.
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 procotol 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, broadcast
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
IP-RECVFROM:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf, IP procotol 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 han-
dled by its own sub process. This allows a behaviour similar to typical UDP based servers like ntpd or
named. This address works well with IP-SENDTO address peers (see above). 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 procotol 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 rarly 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.
NOTE: The server certificate is only checked for validity against cafile or capath, but not for match with
the server�s name or its IP address!
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: cipher, method, verify, cafile, capath, certificate, bind, pf, connect-timeout, source-
port, 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.
Note: You probably want to use the certificate option with this address.
NOTE: Without verify option, the client certificate is not checked. Even with verify option, 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,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, cipher, method, verify, cafile, capath, certificate, fork, bind, range, tcpwrap, su,
reuseaddr, retry
See also: OPENSSL, TCP
PIPE:<filename>
If <filename> already exists, it is opened. If is does not exist, 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 socat tries to write more bytes than the pipe
can buffer (Linux 2.4: 2048 bytes), socat might block. Consider using socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode, unlink-early
See also: unnamed pipe
PIPE Creates an unnamed pipe and uses it for reading and writing. It works as an echo, because everything writ-
ten to it appeares immediately 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:<proxy>:<hostname>:<port>
Connects to an HTTP proxy server on port 8080 using TCP/IP version 4 or 6 depending on address specifica-
tion, 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. Another process may open the pty�s 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,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. This requires the GNU
readline and history libraries. Note that stdio should be a (pseudo) terminal device, otherwise readline
does not seem to work.
Option groups: FD,READLINE,TERMIOS
Useful options: history, noecho
See also: STDIO
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.
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 (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:<shell-command>
Forks a sub process that establishes communication with its parent process and invokes the specified pro-
gram with system(). Please note that <shell-command> [string] 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,TERMIOS
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, 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 pf. 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, backlog, mss, su, reuseaddr, retry, retry
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, UNIX-LISTEN, OPENSSL-LISTEN
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.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY
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.
Please note that, due to UDP protocol properties, no real connection 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:<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-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 is effectively 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 han-
dled 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 SENDTO address peers.
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 can be,
e.g., addressed by socat UDP-SENDTO address peers. This address works well with socat SENDTO address
peers; it behaves similar to a syslog server.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: fork, 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
) 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
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. It works well with socat
UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED
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
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
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV, IP-RECV
UNIX:<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
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 (but see option -g).
Address options have data types that their values must conform to. Every address option consists of just a key-
word 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 internal 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
chroot() call.
If multiple options are given to an address, their sequence in the address specification has (almost) no effect
on the sequence of their execution/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 specification, 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 required 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 operating system lacks
a feature, the related option is simply not available on this platform.
The following paragraphs introduce just the more common address options. 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 gener-
ated. 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 an other, 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.
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 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.
setlkw Tries to set a discretionary waiting lock to the whole file using the fcntl(fd, F_SETLKW, ...) system
call. If the file is already locked, this call blocks.
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) 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 using 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 mechanism depending on availability on the par-
ticular platform. If both are available, the POSIX variant (setlkw) is selected.
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!). With-
out 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!). With-
out 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> [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 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> [mode_t] 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() 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 often abort the connection.
This option is experimental.
NAMED option group
These options work on file system entries.
See also options user, group, and mode.
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> [mode_t] 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> [mode_t] 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, listening unix domain
sockets, and the symbolic links of pty addresses, the default is 1; for created files, opened files,
generic opened files, and client unix domain sockets the default is 0.
OPEN option group
The OPEN group options allow to set flags with the open() system call. E.g., option `creat� sets the O_CREAT
flag.
See also options append and nonblock.
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 to share 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 UN*X file descriptor, but their semantics make sense only on a file sup-
porting random access.
seek=<offset>
Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64) system call, thus positioning the file pointer
absolutely to <offset> [off_t or off64_t].
seek-cur=<offset>
Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64) system call, thus positioning the file pointer
<offset> [off_t or off64_t] bytes relatively to its current position (which is usually 0).
seek-end=<offset>
Applies the lseek(fd, <offset>, SEEK_END) (or lseek64) system call, thus positioning the file pointer
<offset> [off_t or off64_t] bytes relatively to the files current end.
ftruncate=<offset>
Applies the ftruncate(fd, <offset>) (or ftruncate64 if available) system call, thus truncating the file at
the position <offset> [off_t or off64_t].
secrm=<bool>
unrm=<bool>
compr=<bool>
ext2-sync=<bool>
immutable=<bool>
ext2-append=<bool>
nodump=<bool>
ext2-noatime=<bool>
journal-data=<bool>
notail=<bool>
dirsync=<bool>
These options change non standard file attributes on operating systems and file systems that support these
features, like Linux with ext2fs, ext3fs, 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 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.
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.
setgid-early=<group>
Changes the primary <group> of the process before opening the address. This call might require root privi-
lege.
setuid=<user>
Changes the <user> (owner) of the process after processing the address. This call might require root priv-
ilege.
setuid-early=<user>
Changes the <user> (owner) of the process before opening the address. This call might require root privi-
lege.
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.
READLINE option group
These options apply to the readline address type.
history=<filename>
Reads and writes history from/to <filename>.
noprompt
Since version 1.4.0, socat per default tries to determine a prompt - that is then passed to the readline
call - by remembering 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=<pattern>
Specifies a regular pattern for a prompt that prevents the following 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 newline character and before an input character was typed. The pattern
is a regular expression, e.g. "^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex(7) for details.
prompt=<string>
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 program on the other socat
address, consistent look and feel can be archieved.
APPLICATION option group
This group contains options that work at data level. Note that these options only apply to the "raw" data trans-
ferred 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/read-
ing on this channel.
crnl Converts the default line termination character NL (�\n�, 0x0a) to/from CRNL ("\r\n", 0x0d0a) when writ-
ing/reading on this channel. 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").
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 lock-
file on exit.
waitlock=<filename>
If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues,
unlinks lockfile on exit.
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=<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|hostaddress][:(service|port)], UNIX domain
sockets require <filename>.
connect-timeout=<seconds>
Abort the connection attempt after <seconds> [timeval] with error status.
interface=<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.
bsdcompat
Emulates some (old?) bugs of the BSD socket implementation.
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 [int] expired.
oobinline
Places out-of-band data in the input data stream.
priority=<priority>
Sets the protocol defined <priority> [<int>] for outgoing packets.
rcvbuf=<bytes>
Sets the size of the receive buffer after the socket() call to <bytes> [int]. 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> [int]. With TCP sock-
ets, this value corresponds to the socket�s maximal window size.
rcvlowat=<bytes>
Specifies the minimum number of received bytes [int] until the socket layer will pass the buffered data to
socat.
rcvtimeo=<seconds>
Sets the receive timeout [timeval].
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> [int].
sndbuf-late=<bytes>
Sets the size of the send buffer when the socket is connected to <bytes> [int].
sndlowat=<bytes>
Specifies the minimum number of bytes in the send buffer until the socket layer will send the data to
<bytes> [int].
sndtimeo=<seconds>
Sets the send timeout to seconds [timeval].
type=<type>
Sets the type of the socket, usually as argument to the socket() or socketpair() call, to <type> [int].
Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3 means raw socket.
pf=<string>
Forces the use of the specified IP version. <string> can be something like "ip4" or "ip6".
IP4 and IP6 option groups
These options can be used with IPv4 and IPv6 based sockets.
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> [byte].
ipoptions=<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 ipoptions=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.
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=<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.
TCP option group
These options may be applied to TCP sockets. They work by invoking setsockopt() 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=<count>
Sets the number of keepalives before shutting down the socket to <count> [int].
keepidle=<seconds>
Sets the idle time before sending the first keepalive to <seconds> [int].
keepintvl=<seconds>
Sets the intervall between two keepalives to <seconds> [int].
linger2=<seconds>
Sets the time to keep the socket in FIN-WAIT-2 state to <seconds> [int].
mss=<bytes>
Sets the MSS (maximum segment size) after the socket() call to <bytes> [int]. 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> [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=<count>
Sets the maximal number of SYN retransmits during connect to <count> [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=<milliseconds>
Sets the time to wait for an answer of the peer on an established connection (HP-UX).
conn-abort-threshold=<milliseconds>
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 connect() before giving up. Value in half sec-
onds, 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 and TCP option groups
Here we find options that are related to the network port mechanism and that thus can be used with UDP and TCP,
client and server addresses.
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.
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 lim-
ited authorization under some circumstances.
SOCKS option group
When using SOCKS type addresses, some socks specific options can be set.
socksport=<tcp service>
Overrides the default "socks" service or port 1080 for the socks server port with <TCP service>.
socksuser=<user>
Sends the <user> [string] 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-con-
nect.
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 stan-
dard, socat might not understand its answer. This option directs socat to interprete NL as line termina-
tor and to ignore CR in the answer. Nevertheless, socat sends CR+NL to the proxy.
proxyauth=<username>:<password>
Provide "basic" authentication to the proxy server. The argument to the option is used with a "Proxy-
Authorization: Base" 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.
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.
RANGE option group
These options check if a connecting client is granted access. They can be applied to listening and receiving net-
work sockets. tcp-wrappers options fall into this group.
range=<address-range>
After accepting a connection, tests if the peer is within range. For IPv4 addresses, address-range takes
the form ww.xx.yy.zz/bits, e.g. 10.0.0.0/8; for IPv6, it is [ip6-address/bits], e.g. [::1/128]. If the
client address does not match, socat issues a warning and keeps listening/receiving.
tcpwrap[=<name>]
Uses Wietse Venema�s libwrap (tcpd) library to determine if the client is allowed to connect. The configu-
ration 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.
backlog=<count>
Sets the backlog value passed with the listen() system call to <count> [int]. Default is 5.
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.
SSL-CONNECT and SSL-LISTEN differ in when they actually fork off the child: SSL-LISTEN forks before the
SSL handshake, while SSL-CONNECT forks afterwards. RETRY and FOREVER options are not inherited by the
child process.
EXEC option group
Options for addresses that invoke a program.
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 socket-
pair() 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 instance. 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 useless
o for the second address (the one with option nofork), options append, 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 (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 an eventual signal 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 effective 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 available. Use ispeed or ospeed instead.
echo=<bool>
Enables or disables local echo.
icanon=<bool>
Sets or clears canonical mode, enabling line buffering and some special characters.
raw Sets raw mode, thus passing input and output almost unprocessed.
ignbrk=<bool>
Ignores or interpretes the BREAK character (e.g., ^C)
brkint=<bool>
bs0
bs1
bsdly=<0|1>
clocal=<bool>
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=<bool>
crtscts=<bool>
cs5
cs6
cs7
cs8
Sets the character size to 5, 6, 7, or 8 bits, respectively.
csize=<0|1|2|3>
cstopb=<bool>
Sets two stop bits, rather than one.
dsusp=<byte>
Sets the value for the VDSUSP character that suspends the current foreground process and reactivates the
shell (all except Linux).
echoctl=<bool>
Echos control characters in hat notation (e.g. ^A)
echoe=<bool>
echok=<bool>
echoke=<bool>
echonl=<bool>
echoprt=<bool>
eof=<byte>
eol=<byte>
eol2=<byte>
erase=<byte>
discard=<byte>
ff0
ff1
ffdly=<bool>
flusho=<bool>
hupcl=<bool>
icrnl=<bool>
iexten=<bool>
igncr=<bool>
ignpar=<bool>
imaxbel=<bool>
inlcr=<bool>
inpck=<bool>
intr=<byte>
isig=<bool>
ispeed=<unsigned-int>
Set the baud rate for incoming data on this line.
See also: ospeed, b19200 dif(istrip=<bool>)
iuclc=<bool>
ixany=<bool>
ixoff=<bool>
ixon=<bool>
kill=<byte>
lnext=<byte>
min=<byte>
nl0 Sets the newline delay to 0.
nl1
nldly=<bool>
noflsh=<bool>
ocrnl=<bool>
ofdel=<bool>
ofill=<bool>
olcuc=<bool>
onlcr=<bool>
onlret=<bool>
onocr=<bool>
opost=<bool>
Enables or disables output processing; e.g., converts NL to CR-NL.
ospeed=<unsigned-int>
Set the baud rate for outgoing data on this line.
See also: ispeed, b19200
parenb=<bool>
Enable parity generation on output and parity checking for input.
parmrk=<bool>
parodd=<bool>
pendin=<bool>
quit=<byte>
reprint=<byte>
sane Brings the terminal to something like a useful default state.
start=<byte>
stop=<byte>
susp=<byte>
swtc=<byte>
tab0
tab1
tab2
tab3
tabdly=<unsigned-int>
time=<byte>
tostop=<bool>
vt0
vt1
vtdly=<bool>
werase=<byte>
xcase=<bool>
xtabs
PTY option group
These options are intended for use with the pty address type.
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. 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-intervall=<seconds>
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 intervall is 1s. Use the pty-intervall option
[timeval] to change this value.
OPENSSL option group
These options apply to the openssl and openssl-listen address types.
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.
method=<ssl-method>
Sets the protocol version to be used. Valid strings (not case sensitive) are:
SSLv2 Select SSL protocol version 2.
SSLv3 Select SSL protocol version 3.
SSLv23 Select SSL protocol version 2 or 3. This is the default when this option is not provided.
TLSv1 Select TLS protocol version 1.
verify=<bool>
Controls check of the peer�s certificate. Default is 1 (true) for client and 0 (false) for server
addresses. Disabling verify might open your socket for everyone!
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.
pseudo On systems where openssl cannot find an entropy source and where no entropy gathering daemon can be uti-
lized, this option activates a mechanism for providing pseudo entropy. This is archieved 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!
fips Enables FIPS mode if compiled in. For info about the FIPS encryption implementation standard see
http://oss-institute.org/fips-faq.html. This mode might require that the involved certificates are gener-
ated 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=<num>
Number of retries before the connection or listen attempt is aborted. Default is 0, which means just one
attempt.
intervall=<timespec>
Time between consecutive attempts (seconds, [timespec]). Default is 1 second.
forever
Performs an unlimited number of retry attempts.
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-option `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 A raw data specification following dalan syntax. The only documented form is a string starting with �x�
followed by an even number of hex digits.
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. Otherwise, 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, 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 (permission) 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 command 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 unsigned int.
user If the first character is a decimal digit, the value is read with strtoul() as unsigned integer specifying
a user id. Otherwise, it must be an existing user name.
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 interactive 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 (READ-
LINE) and use the history file .http_history; socat prints messages about progress (-d -d). The port is
specified by service name (www), and correct 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 listens 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,su=nobody,fork,range=10.0.0.0/8,reuseaddr \
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 parallel 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
private 10 network (range); due to reuseaddr, it allows immediate restart after master process�s termina-
tion, even if some child sockets are not completely shut down. With -lmlocal2, socat logs to stderr until
successfully reaching the accept loop. Further 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 implements a simple SMTP client. It is programmed
to "speak" SMTP on its FDs 3 (in) and 4 (out). The fdin and fdout options tell socat 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 - /dev/ttyS0,raw,echo=0,crnl
Opens an interactive connection via the serial line, e.g. for talking with a modem. raw and echo set
ttyS0�s terminal parameters to practicable values, crnl converts to correct newline characters. Consider
using READLINE instead of `-�.
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 display :1 on your machine, so XWindow client connections to DISPLAY=:1 are accepted. Socat
then speaks with the SOCKS4 server host.victim.org that might permit sourceport 20 based connections 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 configurations will allow this). So we get
a connection to the victims XWindow server and, if it does not require MIT cookies or Kerberos authentica-
tion, 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 READLINE,noecho=�[Pp]assword:� EXEC:�ftp ftp.server.com�,pty,setsid,ctty
Wraps a command line history (READLINE) around the EXEC�uted ftp client utility. This allows editing and
reuse of FTP commands for relatively comfortable browsing through the ftp directory hierarchy. The pass-
word is echoed! pty is required to have ftp issue a prompt. Nevertheless, there may occur some confusion
with the password and FTP prompts.
socat PTY,link=$HOME/dev/vmodem0,raw,echo=0,waitslave exec:�
Generates a pseudo terminal device (PTY) on the client that can be reached under the symbolic link
$HOME/dev/vmodem0. An application that expects a serial line or modem can be configured to use
$HOME/dev/vmodem0; its traffic will be directed to a modemserver via ssh where another socat instance
links it with /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 lis-
tening 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.
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 accepted 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 -e
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 executable from Cygwin
run under Windows, actual per May 2004).
socat - tcp:www.blackhat.org:31337,readbytes=1000
connect to an unknown service and prevent being flooded.
DIAGNOSTICS
Socat uses a logging mechanism that allows to filter messages by severity. The severities provided are more or
less compatible to the appropriate 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 termination.
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 correct further processing cannot be guaran-
teed, but might be possible.
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 to monitor 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
SOCAT_DEFAULT_LISTEN_IP
(Values 4 or 6) Sets the IP version to be used for listen, recv, and recvfrom addresses if no pf (proto-
col-family) option is given. Is overridden by socat options -4 or -6.
SOCAT_PREFERRED_RESOLVE_IP
(Values 0, 4, or 6) Sets the IP version to be used when resolving 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
Specifies the time (seconds) to sleep the parent and child processes after successful fork(). Useful for
debugging.
HOSTNAME
Is used to determine the hostname for logging (see -lh).
LOGNAME
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 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 With options su and su-d, SHELL is set to the login shell of the given user.
PATH Can be set with option path for exec and system addresses.
HOME 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.
Sourceforge (http://www.sourceforge.net/) for providing a compile farm with Solaris, FreeBSD, and MacOS X
machines, making these ports possible.
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 1.5.0 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 inconsistently when address options cr or crnl
are used: They show the data after conversion in either direction.
The data transfer blocksize setting (-b) is ignored with address readline.
Send bug reports to <socat@dest-unreach.org>
RELATED
nc(1), netcat6(1), sock(1), rinetd(8), cage(1), socks.conf(5), openssl(1), stunnel(8), pty(1), rlwrap(1), setsid(1), Socat home page http://www.dest-unreach.org/socat/
CATEGORY
Categories: Executable programs or shell commands | POSIX | BSD | Linux | UNIX | ISO | Solaris | AIX | HP-UX | Tru64 | FreeBSD | The Open Group | Secure Socket Library (SSL) | OpenSSL | ReiserFS | Ext2 | Ext3