EXAMPLES - socat - Git at Google



            // Examples for using socat (and filan)


            //"$" means normal user, "#" requires privileges, "//" starts a comment


            ///////////////////////////////////////////////////////////////////////////////

            
            // similar to netcat


            // connect to 10.1.1.1 on port 80 and relay to and from stdio

            
            $ socat - TCP:10.1.1.1:80	# similar to "netcat 10.1.1.1 80"


            // listen on port 25, wait for an incoming connection, use CR+NL on this

            
            // connection, relay data to and from stdio;

            
            // then emulate a mailserver by hand :-)

            
            # socat - TCP-LISTEN:25,crlf


            // listen on port 25, wait for an incoming connection, use CR+NL on this

            
            // connection, relay data to and from stdio, but have line editing and history;

            
            // then emulate a mailserver by hand :-)

            
            # socat readline TCP-LISTEN:25,crlf


            // provide a transient history enabled front end to stupid line based

            
            // interactive programs

            
            $ socat readline exec:"nslookup",pty,ctty,setsid,echo=0

            
            // same works for ftp (but password is not hidden)


            // you may also use a file based history list

            
            $ socat readline,history=.nslookup_hist exec:"nslookup",pty,ctty,setsid,echo=0

            
            // using ~ as abbreviation for $HOME does not work!


            // poor mans 'telnetd' replacement

            
            # socat tcp-l:2023,reuseaddr,fork exec:/bin/login,pty,setsid,setpgid,stderr,ctty

            
            // and here an appropriate client:

            
            $ socat -,raw,echo=0 tcp:172.16.181.130:2023

            
            // use ssl with client and server certificate for improved security;

            
            // replace /bin/login by /bin/bash when using SSL client authentication, can be

            
            // run without root then


            // this is a cool trick, proposed by Christophe Lohr, to dump communications to

            
            // two files; it would also work for other manipulations (recode, compress...)

            
            // and it might also work with netcat ;-)

            
            $ socat TCP-LISTEN:5555 SYSTEM:'tee l2r | socat - "TCP:remote:5555"  | tee r2l'


            ///////////////////////////////////////////////////////////////////////////////

            
            // emergence solution because usleep(1) is not always available

            
            // this will "sleep" for 0.1s

            
            $ socat -T 0.1 pipe pipe


            ///////////////////////////////////////////////////////////////////////////////

            
            // a very primitive HTTP/1.0 echo server (problems: sends reply headers before

            
            // request; hangs if client does not shutdown - HTTP keep-alive)

            
            // wait for a connection on port 8000; do not wait for request, but immediately

            
            // start a shell that sends reply headers and an empty line; then echo all

            
            // incoming data back to client

            
            $ socat TCP-LISTEN:8000,crlf SYSTEM:"echo HTTP/1.0 200; echo Content-Type\: text/plain; echo; cat"


            // a less primitive HTTP echo server that sends back not only the reqest but

            
            // also server and client address and port. Might have portability issues with

            
            // echo

            
            ./socat -T 1 -d -d tcp-l:10081,reuseaddr,fork,crlf system:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/html\\\n\\\n<html>date: \$\(date\)<br>server:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT<br>client: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n<pre>\\\"\"; cat; echo -e \"\\\"\\\n</pre></html>\\\"\""


            ///////////////////////////////////////////////////////////////////////////////

            
            // for communicating with an attached modem, I had reasonable results with

            
            // following command line. Required privileges depend on device mode.

            
            // after leaving socat, type "sane".

            
            // replace /dev/ttyS0 by the correct serial line or with /dev/modem

            
            $ socat readline /dev/ttyS0,raw,echo=0,crlf

            
            // or

            
            $ socat readline /dev/ttyS0,raw,echo=0,crlf,nonblock

            
            // then enter "at$"


            ///////////////////////////////////////////////////////////////////////////////

            
            // relay TCP port 80 from everywhere (internet, intranet, dmz) through your

            
            // firewall to your DMZ webserver (like plug-gw)

            
            // listen on port 80; whenever a connection is made, fork a new process (parent

            
            // process keeps accepting connections), su to nobody, and connect to

            
            // www.dmz.mydomain.org on port 80.

            
            // attention: this is a substitute for a reverse proxy without providing

            
            // application level security.

            
            # socat TCP-LISTEN:80,reuseaddr,fork,su=nobody TCP:www.dmz.mydomain.org:80

            
            // Note: parent process keeps running as root, su after forking


            ///////////////////////////////////////////////////////////////////////////////

            
            // relay mail from your DMZ server through your firewall.

            
            // accept connections only on dmz interface and allow connections only from

            
            // smtp.dmz.mydomain.org.

            
            // the advantages over plug-gw and other relays are:

            
            // * you can bind to an IP address (even an alias), therefore enhance security

            
            // * in your OS you can create several IP aliases and bind another socat daemon

            
            //   to each, making several application servers addressable

            
            // * lots of options, like switching user, chroot, IP performance tuning

            
            // * no need for inetd

            
            # socat -lm -d -d TCP-LISTEN:25,bind=fw.dmz.mydomain.org,fork,su=nobody,range=smtp.dmz.mydomain.org/32 TCP:smtp.intra.mydomain.org:25


            ///////////////////////////////////////////////////////////////////////////////

            
            // convert line terminator in ascii streams, stdin to stdout

            
            // use unidirectional mode, convert nl to crnl

            
            $ socat -u - -,crlf

            
            // or cr to nl

            
            $ socat -u -,cr -


            // save piped data similar to 'tee':

            
            // copies stdin to stdout, but writes everything to the file too

            
            $ socat -,echo=0 open:/tmp/myfile,create,trunc,ignoreeof!!/tmp/myfile


            ///////////////////////////////////////////////////////////////////////////////

            
            // intrusion testing


            // found an XWindow Server behind IP filters with FTP data hole? (you are

            
            // lucky!)

            
            // prepare your host:

            
            # rm -f /tmp/.X11-unix/X1

            
            // relay a pseudo display :1 on your machine to victim:0

            
            # socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork TCP:host.victim.org:6000,sp=20 &

            
            // and try to take a screendump (must be very lucky - when server has not even

            
            // host based authentication!)

            
            # xwd -root -display :1 -silent >victim.xwd


            // you sit behind a socks firewall that has IP filters but lazily allows socks

            
            // connections to loopback and has only host based X11 security.

            
            // like above, but from your inside client:

            
            # socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork SOCKS4:firewall:loopback:6000

            
            // or for the HTTP proxy:

            
            # socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork PROXY:firewall:loopback:6000


            ///////////////////////////////////////////////////////////////////////////////

            
            // forms of stdin with stdout, all equivalent

            
            $ socat echo -

            
            $ socat echo STDIO

            
            $ socat echo STDIN!!STDOUT

            
            $ socat echo STDIO!!STDIO

            
            $ socat echo -!!-

            
            $ socat echo FD:0!!FD:1

            
            $ socat echo 0!!1

            
            $ socat echo /dev/stdin!!/dev/stdout	// if your OS provides these


            ///////////////////////////////////////////////////////////////////////////////

            
            // some echo address examples

            
            $ socat - PIPE

            
            $ socat - PIPE:/tmp/pipi		// other version of echo

            
            $ socat - PIPE:/tmp/pipi,nonblock!!/tmp/pipi	// other version of echo

            
            $ socat - EXEC:/bin/cat		// another echo

            
            $ socat - SYSTEM:/bin/cat		// another echo

            
            $ socat - TCP:loopback:7	// if inetd echo/TCP service activated

            
            $ socat - UDP:loopback:7	// if inetd echo/UDP service activated

            
            $ socat - /tmp/hugo,trunc,ignoreeof!!/tmp/hugo	// with delay

            
            $ socat - UDP:loopback:2000,bind=:2000	// self "connection"

            
            $ socat - TCP:loopback:2000,bind=:2000	// Linux bug?

            
            # socat - IP:loopback:222	// raw protocol, self "connected" (attention,

            
            // Linux might drop packets with less than 8 bytes payload)


            ///////////////////////////////////////////////////////////////////////////////

            
            // unidirectional data transfer

            
            $ socat -u - -

            
            // like "tail -f", but start with showing all file contents

            
            $ socat -u FILE:/var/log/syslog.debug,ignoreeof -

            
            // like "tail -f", but do not show existing file contents

            
            $ socat -u FILE:/var/log/syslog.debug,ignoreeof,seek-end -

            
            // write to new file, create with given permission and group (must be member) - race condition with group!!!

            
            $ socat -u - CREATE:/tmp/outfile1,group=floppy,perm=0640

            
            //

            
            // for an existing file /tmp/outfile1

            
            # socat -u - FILE:/tmp/outfile1,group=floppy,perm=0700,user=4321


            ///////////////////////////////////////////////////////////////////////////////

            
            // file handling

            
            $ socat - FILE:/tmp/outfile1,ignoreeof!!FILE:/tmp/outfile1,append	// prints outfile1, then echoes input and protocols into file (appends to old data)


            ///////////////////////////////////////////////////////////////////////////////

            
            // unix socket handling


            // create a listening unix socket

            
            $ rm -f /tmp/mysocket; socat UNIX-LISTEN:/tmp/mysocket -

            
            // from another terminal, connect to this socket

            
            $ socat UNIX:/tmp/mysocket -

            
            // then transfer data bidirectionally


            ///////////////////////////////////////////////////////////////////////////////

            
            // transport examples


            // socks relay (externally socksify applications);

            
            // your ssh client and OS are not socksified, but you want to pass a socks

            
            // server with ssh:

            
            $ socat TCP-LISTEN:10022,fork SOCKS4:socks.mydomain.org:ssh-serv:22

            
            $ ssh -p 10022 loopback

            
            // or better define a ProxyCommand in ~/.ssh/config:

            
            ProxyCommand socat - SOCKS:socks.mydomain.org:%h:%p

            
            // and with proxy:

            
            ProxyCommand socat - PROXY:proxy.mydomain.org:%h:%p,proxyport=8000


            ///////////////////////////////////////////////////////////////////////////////

            
            // application examples


            // run sendmail daemon with your favorite network options

            
            # socat TCP-LISTEN:25,fork,ip-ttl=4,ip-tos=7,tcp-maxseg=576 EXEC:"/usr/sbin/sendmail -bs",nofork


            // local mail delivery over UNIX socket - no SUID program required

            
            # socat UNIX-LISTEN:/tmp/postoffice,fork,perm-early=0666 EXEC:"/usr/sbin/sendmail -bs"

            
            $ socat - /tmp/postoffice


            ///////////////////////////////////////////////////////////////////////////////

            
            // uses of filan

            
            // see what your operating system opens for you

            
            $ filan

            
            // or if that was too detailled

            
            $ filan -s

            
            // see what file descriptors are passed via exec function

            
            $ socat - EXEC:filan,nofork

            
            $ socat - EXEC:filan

            
            $ socat - EXEC:filan,pipes,stderr

            
            $ socat - EXEC:filan,pipes

            
            $ socat - EXEC:filan,pty

            
            // see what's done by your shell and with option "pipes"

            
            $ socat - SYSTEM:filan,pipes

            
            // see if gdb gives you an equivalent environment or opens some files for your program

            
            $ gdb ./filan

            
            (gdb) r

            
            (gdb) r -s


            ///////////////////////////////////////////////////////////////////////////////

            
            // want to use chat from the ppp package?

            
            // note: some OS's do not need "-e" for echo to print control characters

            
            // note: chat might send bytes one by one

            
            // with AIX, a similar program is available under the name "pppdial"

            
            $ socat -d -d tcp:localhost:25,crlf,nodelay exec:'/usr/sbin/chat -v -s "\"220 \"" "\"HELO loopback\"" "\"250 \"" "\"MAIL FROM: <hugo@localhost>\"" "\"250 \"" "\"RCPT TO: root\"" "\"250 \"" "\"DATA\"" "\"354 \"" "\"test1'$(echo -e "\r.")'\"" "\"250 \"" "\"QUIT\"" "\"221 \""',pty,echo=0,cr


            //////////////////////////////////////////////////////////////////////////////

            
            // IP6


            # socat readline TCP6:[::1]:21	# if your inetd/ftp is listening on ip6


            //////////////////////////////////////////////////////////////////////////////

            
            // VSOCK

            
            # start a linux VM with cid=21

            
            #    qemu-system-x86_64 -m 1G -smp 2 -cpu host -M accel=kvm \

            
            #     -drive if=virtio,file=/path/to/fedora.img,format=qcow2  \

            
            #     -device vhost-vsock-pci,guest-cid=21


            # guest listens on port 1234 and host connects to it

            
            guest$ socat - vsock-listen:1234

            
            host$ socat - vsock-connect:21:1234


            # host (well know CID_HOST = 2) listens on port 4321 and guest connects to it

            
            host$ socat - vsock-listen:4321

            
            guest$ socat - vsock-connect:2:4321


            ///////////////////////////////////////////////////////////////////////////////

            
            // application server solutions

            
            // run a program (here: /bin/sh) chrooted, unprivileged;

            
            // parent process stays in real / running as root

            
            # socat -d -d - EXEC:/bin/sh,chroot=/home/sandbox,su=sandbox,pty


            // make a program available on the network chrooted, unprivileged;

            
            // parent process stays in / running as root

            
            // script path is already chrooted

            
            # ./socat -lm -d -d TCP-LISTEN:5555,fork EXEC:/bin/myscript,chroot=/home/sandbox,su=sandbox,pty,stderr

            
            // to avoid terminal problems, you might - instead of telnet - connect using

            
            $ socat -,icanon=0,echo=0 tcp:target:5555; reset


            // access local display from ssh server, when ssh port forwarding is disabled

            
            // socat must be installed on ssh server host

            
            // might have to use xauth...

            
            // this example is one-shot because ssh can handle only one channel

            
            xterm1$ socat -d -d exec:"ssh www.dest-unreach.org rm -f /tmp/.X11-unix/X9; ~/bin/socat -d -d unix-l\:/tmp/.X11-unix/X9\,fork -" unix:/tmp/.X11-unix/X0

            
            xterm2$ ssh target

            
            target$ DISPLAY=:9 myxapplication


            // touch with perms:

            
            // no race condition for perms (applied with creat() call)

            
            $ socat -u /dev/null creat:/tmp/tempfile,perm=0600


            // touch with owner and perms:

            
            // race condition before changing owner, but who cares - only root may access

            
            # socat -u /dev/null creat:/tmp/tempfile,user=user1,perm=0600


            // invoke an interactive ssh with exec

            
            // first example passes control chars (^C etc.) to remote server as usual

            
            socat -,echo=0,raw exec:'ssh server',pty,setsid,ctty

            
            // second example interprets control chars on local command line

            
            socat -,echo=0,icanon=0 exec:'ssh server',pty,setsid,ctty

            
            // afterwards, type "reset"!


            // convince ssh to provide an "interactive" shell to your script

            
            // three main versions for entering password:

            
            // 1) from your TTY; have 10 seconds to enter password:

            
            (sleep 10; echo "ls"; sleep 1) |socat - exec:'ssh server',pty

            
            // 2) from XWindows (DISPLAY !); again 10 seconds

            
            (sleep 10; echo "ls"; sleep 1) |socat - exec:'ssh server',pty,setsid

            
            // 3) from script

            
            (sleep 5; echo PASSWORD; echo ls; sleep 1) |./socat - exec:'ssh server',pty,setsid,ctty


            // download with proxy CONNECT

            
            // use echo -e if required for \n

            
            $ (echo -e "CONNECT 128.129.130.131:80 HTTP/1.0\n"; sleep 5; echo -e "GET

            
            /download/file HTTP/1.0\n"; sleep 10) |socat -d -d -t 3600 - tcp:proxy:8080,crlf


            // retrieve a file from an sshd site with sourceforge style entry menu;

            
            // fill in your personal values; cat lets you enter your password (will be

            
            // visible on screen)

            
            $ (sleep 10; read pass; echo $pass; sleep 10; echo M; sleep 5; echo cat FILENAME; sleep 10) |./socat -d -d -ly - EXEC:'ssh -c 3des -l USER cf.sourceforge.net',pty,setsid,ctty |tee FILENAME


            // multicast community on local network: start the following command on all

            
            // participating hosts; like a conference call:

            
            # socat -d -d -d -d - udp-datagram:224.0.0.2:6666,bind=:6666,ip-add-membership=224.0.0.2:eth0,bindtodevice=eth0

            
            // or

            
            $ socat -d -d -d -d - udp-datagram:224.0.0.2:6666,bind=:6666,ip-add-membership=224.0.0.2:eth0

            
            // possible reasons for failure:

            
            // iptables or other filters (open your filters as required)

            
            // packets leave via wrong interface (set route: ...)

            
            // socket bound to specific address


            //=============================================================================

            
            // GENERIC FUNCTION CALLS


            // ioctl(): open CD drive (given value valid on Linux)

            
            // on my Linux system I find in /usr/include/linux/cdrom.h the define:

            
            // #define CDROMEJECT           0x5309 /* Ejects the cdrom media */

            
            // the following command makes something like ioctl(fd, CDROMEJECT, NULL)

            
            // (don't care about the read error):

            
            $ socat /dev/cdrom,o-nonblock,ioctl-void=0x5309 -


            // setsockopt(): SO_REUSEADDR

            
            // the following command performs - beyond lots of overhead - something like:

            
            // myint=1; setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &myint, sizeof(myint))

            
            $ socat -u udp-recv:7777,setsockopt-int=1:2:1 -

            
            // setsockopt(): SO_BINDTODEVICE


            // ways to apply SO_BINDTODEVICE without using the special socat address option

            
            // so-bindtodevice:

            
            // with string argument:

            
            $ sudo ./socat tcp-l:7777,setsockopt-string=1:25:eth0 pipe

            
            // with binary argument:

            
            $ sudo ./socat tcp-l:7777,setsockopt-bin=1:25:x6574683000 pipe


            ===============================================================================


            // not tested, just ideas, or have problems


            // traverse firewall for making internal telnet server accessible for outside

            
            // telnet client, when only outbound traffic (syn-filter) is allowed:

            
            //   on external client run "double server". this process waits for a

            
            // connection from localhost on port 10023, and, when it is established, waits

            
            // for a connection from anywhere to port 20023:

            
            ext$ socat -d TCP-LISTEN:10023,range=localhost TCP-LISTEN:20023

            
            //   on internal server run double client:

            
            int$ socat -d TCP:localhost:23 TCP:extclient:10023

            
            //   or, with socks firewall:

            
            int$ socat -d TCP:localhost:23 SOCKS:socksserver:extclient:10023

            
            //   login with:

            
            ext$ telnet localhost 20023


            // you can make a double server capable of handling multiple instances:

            
            ext$ socat -d TCP-LISTEN:10023,range=localhost,fork TCP-LISTEN:20023,reuseaddr


            // access remote display via ssh, when ssh port forwarding is disabled

            
            $ socat -d -d EXEC:"ssh target socat - UNIX:/tmp/.X11-unix/X0" TCP-LISTEN:6030

            
            $ xclock -display localhost:30


            // relay multiple webserver addresses through your firewall into your DMZ:

            
            // make IP aliases on your firewall, and then:

            
            # socat -d -d TCP-L:80,bind=fw-addr1,fork TCP:dmz-www1:80

            
            # socat -d -d TCP-L:80,bind=fw-addr2,fork TCP:dmz-www2:80

            
            // and for improved security:

            
            # socat -d -d TCP-L:80,bind=fw-addr3,su=nobody,fork TCP:dmz-www3:80


            // proxy an arbitrary IP protocol over your firewall (answers won't work)

            
            # socat -d -d IP:0.0.0.0:150,bind=fwnonsec IP:sec-host:150,bind=fwsec


            // proxy an unsupported IP protocol over your firewall, point to point

            
            // end points see firewall interfaces as IP peers!

            
            # socat -d -d IP:nonsec-host:150,bind=fwnonsec IP:sec-host:150,bind=fwsec

            
            // note that, for IPsec, you might face problems that are known with NAT

	// Examples for using socat (and filan)


	//"$" means normal user, "#" requires privileges, "//" starts a comment

	///////////////////////////////////////////////////////////////////////////////
	// similar to netcat

	// connect to 10.1.1.1 on port 80 and relay to and from stdio
	$ socat - TCP:10.1.1.1:80 # similar to "netcat 10.1.1.1 80"

	// listen on port 25, wait for an incoming connection, use CR+NL on this
	// connection, relay data to and from stdio;
	// then emulate a mailserver by hand :-)
	# socat - TCP-LISTEN:25,crlf

	// listen on port 25, wait for an incoming connection, use CR+NL on this
	// connection, relay data to and from stdio, but have line editing and history;
	// then emulate a mailserver by hand :-)
	# socat readline TCP-LISTEN:25,crlf

	// provide a transient history enabled front end to stupid line based
	// interactive programs
	$ socat readline exec:"nslookup",pty,ctty,setsid,echo=0
	// same works for ftp (but password is not hidden)

	// you may also use a file based history list
	$ socat readline,history=.nslookup_hist exec:"nslookup",pty,ctty,setsid,echo=0
	// using ~ as abbreviation for $HOME does not work!

	// poor mans 'telnetd' replacement
	# socat tcp-l:2023,reuseaddr,fork exec:/bin/login,pty,setsid,setpgid,stderr,ctty
	// and here an appropriate client:
	$ socat -,raw,echo=0 tcp:172.16.181.130:2023
	// use ssl with client and server certificate for improved security;
	// replace /bin/login by /bin/bash when using SSL client authentication, can be
	// run without root then

	// this is a cool trick, proposed by Christophe Lohr, to dump communications to
	// two files; it would also work for other manipulations (recode, compress...)
	// and it might also work with netcat ;-)
	$ socat TCP-LISTEN:5555 SYSTEM:'tee l2r \| socat - "TCP:remote:5555" \| tee r2l'

	///////////////////////////////////////////////////////////////////////////////
	// emergence solution because usleep(1) is not always available
	// this will "sleep" for 0.1s
	$ socat -T 0.1 pipe pipe

	///////////////////////////////////////////////////////////////////////////////
	// a very primitive HTTP/1.0 echo server (problems: sends reply headers before
	// request; hangs if client does not shutdown - HTTP keep-alive)
	// wait for a connection on port 8000; do not wait for request, but immediately
	// start a shell that sends reply headers and an empty line; then echo all
	// incoming data back to client
	$ socat TCP-LISTEN:8000,crlf SYSTEM:"echo HTTP/1.0 200; echo Content-Type\: text/plain; echo; cat"

	// a less primitive HTTP echo server that sends back not only the reqest but
	// also server and client address and port. Might have portability issues with
	// echo
	./socat -T 1 -d -d tcp-l:10081,reuseaddr,fork,crlf system:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/html\\\n\\\n<html>date: \$\(date\)<br>server:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT<br>client: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n<pre>\\\"\"; cat; echo -e \"\\\"\\\n</pre></html>\\\"\""

	///////////////////////////////////////////////////////////////////////////////
	// for communicating with an attached modem, I had reasonable results with
	// following command line. Required privileges depend on device mode.
	// after leaving socat, type "sane".
	// replace /dev/ttyS0 by the correct serial line or with /dev/modem
	$ socat readline /dev/ttyS0,raw,echo=0,crlf
	// or
	$ socat readline /dev/ttyS0,raw,echo=0,crlf,nonblock
	// then enter "at$"

	///////////////////////////////////////////////////////////////////////////////
	// relay TCP port 80 from everywhere (internet, intranet, dmz) through your
	// firewall to your DMZ webserver (like plug-gw)
	// listen on port 80; whenever a connection is made, fork a new process (parent
	// process keeps accepting connections), su to nobody, and connect to
	// www.dmz.mydomain.org on port 80.
	// attention: this is a substitute for a reverse proxy without providing
	// application level security.
	# socat TCP-LISTEN:80,reuseaddr,fork,su=nobody TCP:www.dmz.mydomain.org:80
	// Note: parent process keeps running as root, su after forking

	///////////////////////////////////////////////////////////////////////////////
	// relay mail from your DMZ server through your firewall.
	// accept connections only on dmz interface and allow connections only from
	// smtp.dmz.mydomain.org.
	// the advantages over plug-gw and other relays are:
	// * you can bind to an IP address (even an alias), therefore enhance security
	// * in your OS you can create several IP aliases and bind another socat daemon
	// to each, making several application servers addressable
	// * lots of options, like switching user, chroot, IP performance tuning
	// * no need for inetd
	# socat -lm -d -d TCP-LISTEN:25,bind=fw.dmz.mydomain.org,fork,su=nobody,range=smtp.dmz.mydomain.org/32 TCP:smtp.intra.mydomain.org:25

	///////////////////////////////////////////////////////////////////////////////
	// convert line terminator in ascii streams, stdin to stdout
	// use unidirectional mode, convert nl to crnl
	$ socat -u - -,crlf
	// or cr to nl
	$ socat -u -,cr -

	// save piped data similar to 'tee':
	// copies stdin to stdout, but writes everything to the file too
	$ socat -,echo=0 open:/tmp/myfile,create,trunc,ignoreeof!!/tmp/myfile

	///////////////////////////////////////////////////////////////////////////////
	// intrusion testing

	// found an XWindow Server behind IP filters with FTP data hole? (you are
	// lucky!)
	// prepare your host:
	# rm -f /tmp/.X11-unix/X1
	// relay a pseudo display :1 on your machine to victim:0
	# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork TCP:host.victim.org:6000,sp=20 &
	// and try to take a screendump (must be very lucky - when server has not even
	// host based authentication!)
	# xwd -root -display :1 -silent >victim.xwd

	// you sit behind a socks firewall that has IP filters but lazily allows socks
	// connections to loopback and has only host based X11 security.
	// like above, but from your inside client:
	# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork SOCKS4:firewall:loopback:6000
	// or for the HTTP proxy:
	# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork PROXY:firewall:loopback:6000

	///////////////////////////////////////////////////////////////////////////////
	// forms of stdin with stdout, all equivalent
	$ socat echo -
	$ socat echo STDIO
	$ socat echo STDIN!!STDOUT
	$ socat echo STDIO!!STDIO
	$ socat echo -!!-
	$ socat echo FD:0!!FD:1
	$ socat echo 0!!1
	$ socat echo /dev/stdin!!/dev/stdout // if your OS provides these

	///////////////////////////////////////////////////////////////////////////////
	// some echo address examples
	$ socat - PIPE
	$ socat - PIPE:/tmp/pipi // other version of echo
	$ socat - PIPE:/tmp/pipi,nonblock!!/tmp/pipi // other version of echo
	$ socat - EXEC:/bin/cat // another echo
	$ socat - SYSTEM:/bin/cat // another echo
	$ socat - TCP:loopback:7 // if inetd echo/TCP service activated
	$ socat - UDP:loopback:7 // if inetd echo/UDP service activated
	$ socat - /tmp/hugo,trunc,ignoreeof!!/tmp/hugo // with delay
	$ socat - UDP:loopback:2000,bind=:2000 // self "connection"
	$ socat - TCP:loopback:2000,bind=:2000 // Linux bug?
	# socat - IP:loopback:222 // raw protocol, self "connected" (attention,
	// Linux might drop packets with less than 8 bytes payload)

	///////////////////////////////////////////////////////////////////////////////
	// unidirectional data transfer
	$ socat -u - -
	// like "tail -f", but start with showing all file contents
	$ socat -u FILE:/var/log/syslog.debug,ignoreeof -
	// like "tail -f", but do not show existing file contents
	$ socat -u FILE:/var/log/syslog.debug,ignoreeof,seek-end -
	// write to new file, create with given permission and group (must be member) - race condition with group!!!
	$ socat -u - CREATE:/tmp/outfile1,group=floppy,perm=0640
	//
	// for an existing file /tmp/outfile1
	# socat -u - FILE:/tmp/outfile1,group=floppy,perm=0700,user=4321


	///////////////////////////////////////////////////////////////////////////////
	// file handling
	$ socat - FILE:/tmp/outfile1,ignoreeof!!FILE:/tmp/outfile1,append // prints outfile1, then echoes input and protocols into file (appends to old data)

	///////////////////////////////////////////////////////////////////////////////
	// unix socket handling

	// create a listening unix socket
	$ rm -f /tmp/mysocket; socat UNIX-LISTEN:/tmp/mysocket -
	// from another terminal, connect to this socket
	$ socat UNIX:/tmp/mysocket -
	// then transfer data bidirectionally


	///////////////////////////////////////////////////////////////////////////////
	// transport examples

	// socks relay (externally socksify applications);
	// your ssh client and OS are not socksified, but you want to pass a socks
	// server with ssh:
	$ socat TCP-LISTEN:10022,fork SOCKS4:socks.mydomain.org:ssh-serv:22
	$ ssh -p 10022 loopback
	// or better define a ProxyCommand in ~/.ssh/config:
	ProxyCommand socat - SOCKS:socks.mydomain.org:%h:%p
	// and with proxy:
	ProxyCommand socat - PROXY:proxy.mydomain.org:%h:%p,proxyport=8000

	///////////////////////////////////////////////////////////////////////////////
	// application examples

	// run sendmail daemon with your favorite network options
	# socat TCP-LISTEN:25,fork,ip-ttl=4,ip-tos=7,tcp-maxseg=576 EXEC:"/usr/sbin/sendmail -bs",nofork

	// local mail delivery over UNIX socket - no SUID program required
	# socat UNIX-LISTEN:/tmp/postoffice,fork,perm-early=0666 EXEC:"/usr/sbin/sendmail -bs"
	$ socat - /tmp/postoffice

	///////////////////////////////////////////////////////////////////////////////
	// uses of filan
	// see what your operating system opens for you
	$ filan
	// or if that was too detailled
	$ filan -s
	// see what file descriptors are passed via exec function
	$ socat - EXEC:filan,nofork
	$ socat - EXEC:filan
	$ socat - EXEC:filan,pipes,stderr
	$ socat - EXEC:filan,pipes
	$ socat - EXEC:filan,pty
	// see what's done by your shell and with option "pipes"
	$ socat - SYSTEM:filan,pipes
	// see if gdb gives you an equivalent environment or opens some files for your program
	$ gdb ./filan
	(gdb) r
	(gdb) r -s

	///////////////////////////////////////////////////////////////////////////////
	// want to use chat from the ppp package?
	// note: some OS's do not need "-e" for echo to print control characters
	// note: chat might send bytes one by one
	// with AIX, a similar program is available under the name "pppdial"
	$ socat -d -d tcp:localhost:25,crlf,nodelay exec:'/usr/sbin/chat -v -s "\"220 \"" "\"HELO loopback\"" "\"250 \"" "\"MAIL FROM: <hugo@localhost>\"" "\"250 \"" "\"RCPT TO: root\"" "\"250 \"" "\"DATA\"" "\"354 \"" "\"test1'$(echo -e "\r.")'\"" "\"250 \"" "\"QUIT\"" "\"221 \""',pty,echo=0,cr

	//////////////////////////////////////////////////////////////////////////////
	// IP6

	# socat readline TCP6:[::1]:21 # if your inetd/ftp is listening on ip6

	//////////////////////////////////////////////////////////////////////////////
	// VSOCK
	# start a linux VM with cid=21
	# qemu-system-x86_64 -m 1G -smp 2 -cpu host -M accel=kvm \
	# -drive if=virtio,file=/path/to/fedora.img,format=qcow2 \
	# -device vhost-vsock-pci,guest-cid=21

	# guest listens on port 1234 and host connects to it
	guest$ socat - vsock-listen:1234
	host$ socat - vsock-connect:21:1234

	# host (well know CID_HOST = 2) listens on port 4321 and guest connects to it
	host$ socat - vsock-listen:4321
	guest$ socat - vsock-connect:2:4321

	///////////////////////////////////////////////////////////////////////////////
	// application server solutions
	// run a program (here: /bin/sh) chrooted, unprivileged;
	// parent process stays in real / running as root
	# socat -d -d - EXEC:/bin/sh,chroot=/home/sandbox,su=sandbox,pty

	// make a program available on the network chrooted, unprivileged;
	// parent process stays in / running as root
	// script path is already chrooted
	# ./socat -lm -d -d TCP-LISTEN:5555,fork EXEC:/bin/myscript,chroot=/home/sandbox,su=sandbox,pty,stderr
	// to avoid terminal problems, you might - instead of telnet - connect using
	$ socat -,icanon=0,echo=0 tcp:target:5555; reset


	// access local display from ssh server, when ssh port forwarding is disabled
	// socat must be installed on ssh server host
	// might have to use xauth...
	// this example is one-shot because ssh can handle only one channel
	xterm1$ socat -d -d exec:"ssh www.dest-unreach.org rm -f /tmp/.X11-unix/X9; ~/bin/socat -d -d unix-l\:/tmp/.X11-unix/X9\,fork -" unix:/tmp/.X11-unix/X0
	xterm2$ ssh target
	target$ DISPLAY=:9 myxapplication

	// touch with perms:
	// no race condition for perms (applied with creat() call)
	$ socat -u /dev/null creat:/tmp/tempfile,perm=0600

	// touch with owner and perms:
	// race condition before changing owner, but who cares - only root may access
	# socat -u /dev/null creat:/tmp/tempfile,user=user1,perm=0600

	// invoke an interactive ssh with exec
	// first example passes control chars (^C etc.) to remote server as usual
	socat -,echo=0,raw exec:'ssh server',pty,setsid,ctty
	// second example interprets control chars on local command line
	socat -,echo=0,icanon=0 exec:'ssh server',pty,setsid,ctty
	// afterwards, type "reset"!

	// convince ssh to provide an "interactive" shell to your script
	// three main versions for entering password:
	// 1) from your TTY; have 10 seconds to enter password:
	(sleep 10; echo "ls"; sleep 1) \|socat - exec:'ssh server',pty
	// 2) from XWindows (DISPLAY !); again 10 seconds
	(sleep 10; echo "ls"; sleep 1) \|socat - exec:'ssh server',pty,setsid
	// 3) from script
	(sleep 5; echo PASSWORD; echo ls; sleep 1) \|./socat - exec:'ssh server',pty,setsid,ctty


	// download with proxy CONNECT
	// use echo -e if required for \n
	$ (echo -e "CONNECT 128.129.130.131:80 HTTP/1.0\n"; sleep 5; echo -e "GET
	/download/file HTTP/1.0\n"; sleep 10) \|socat -d -d -t 3600 - tcp:proxy:8080,crlf

	// retrieve a file from an sshd site with sourceforge style entry menu;
	// fill in your personal values; cat lets you enter your password (will be
	// visible on screen)
	$ (sleep 10; read pass; echo $pass; sleep 10; echo M; sleep 5; echo cat FILENAME; sleep 10) \|./socat -d -d -ly - EXEC:'ssh -c 3des -l USER cf.sourceforge.net',pty,setsid,ctty \|tee FILENAME

	// multicast community on local network: start the following command on all
	// participating hosts; like a conference call:
	# socat -d -d -d -d - udp-datagram:224.0.0.2:6666,bind=:6666,ip-add-membership=224.0.0.2:eth0,bindtodevice=eth0
	// or
	$ socat -d -d -d -d - udp-datagram:224.0.0.2:6666,bind=:6666,ip-add-membership=224.0.0.2:eth0
	// possible reasons for failure:
	// iptables or other filters (open your filters as required)
	// packets leave via wrong interface (set route: ...)
	// socket bound to specific address

	//=============================================================================
	// GENERIC FUNCTION CALLS

	// ioctl(): open CD drive (given value valid on Linux)
	// on my Linux system I find in /usr/include/linux/cdrom.h the define:
	// #define CDROMEJECT 0x5309 /* Ejects the cdrom media */
	// the following command makes something like ioctl(fd, CDROMEJECT, NULL)
	// (don't care about the read error):
	$ socat /dev/cdrom,o-nonblock,ioctl-void=0x5309 -

	// setsockopt(): SO_REUSEADDR
	// the following command performs - beyond lots of overhead - something like:
	// myint=1; setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &myint, sizeof(myint))
	$ socat -u udp-recv:7777,setsockopt-int=1:2:1 -
	// setsockopt(): SO_BINDTODEVICE

	// ways to apply SO_BINDTODEVICE without using the special socat address option
	// so-bindtodevice:
	// with string argument:
	$ sudo ./socat tcp-l:7777,setsockopt-string=1:25:eth0 pipe
	// with binary argument:
	$ sudo ./socat tcp-l:7777,setsockopt-bin=1:25:x6574683000 pipe

	===============================================================================

	// not tested, just ideas, or have problems


	// traverse firewall for making internal telnet server accessible for outside
	// telnet client, when only outbound traffic (syn-filter) is allowed:
	// on external client run "double server". this process waits for a
	// connection from localhost on port 10023, and, when it is established, waits
	// for a connection from anywhere to port 20023:
	ext$ socat -d TCP-LISTEN:10023,range=localhost TCP-LISTEN:20023
	// on internal server run double client:
	int$ socat -d TCP:localhost:23 TCP:extclient:10023
	// or, with socks firewall:
	int$ socat -d TCP:localhost:23 SOCKS:socksserver:extclient:10023
	// login with:
	ext$ telnet localhost 20023

	// you can make a double server capable of handling multiple instances:
	ext$ socat -d TCP-LISTEN:10023,range=localhost,fork TCP-LISTEN:20023,reuseaddr

	// access remote display via ssh, when ssh port forwarding is disabled
	$ socat -d -d EXEC:"ssh target socat - UNIX:/tmp/.X11-unix/X0" TCP-LISTEN:6030
	$ xclock -display localhost:30

	// relay multiple webserver addresses through your firewall into your DMZ:
	// make IP aliases on your firewall, and then:
	# socat -d -d TCP-L:80,bind=fw-addr1,fork TCP:dmz-www1:80
	# socat -d -d TCP-L:80,bind=fw-addr2,fork TCP:dmz-www2:80
	// and for improved security:
	# socat -d -d TCP-L:80,bind=fw-addr3,su=nobody,fork TCP:dmz-www3:80

	// proxy an arbitrary IP protocol over your firewall (answers won't work)
	# socat -d -d IP:0.0.0.0:150,bind=fwnonsec IP:sec-host:150,bind=fwsec

	// proxy an unsupported IP protocol over your firewall, point to point
	// end points see firewall interfaces as IP peers!
	# socat -d -d IP:nonsec-host:150,bind=fwnonsec IP:sec-host:150,bind=fwsec
	// note that, for IPsec, you might face problems that are known with NAT