Playing with the sockets: socat and netcat

Connect two netcats to each other

mkfifo backpipe
nc localhost 55545 0backpipe

Telnet with readline and a history:

socat -d -d READLINE,history=$HOME/.http_history \
TCP4:www.domain.org:www,crnl

Talk to your modem in raw mode:

socat - /dev/ttyS0,raw,echo=0,crnl

Simulate tail -f:

socat -u /var/log/messages,seek-end=0,ignoreeof -

Give a random interactive program, such as nslookup, a history:

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

Use your modem with a non-persistent history:

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

Use your modem with a persistent history:

socat READLINE,history:/tmp/serial.cmds \
  OPEN:/dev/ttyS0,ispeed=9600,ospeed=9600,crnl,raw,sane,echo=false

To dos (as in tofrodos):

socat -u - -,crlf

From dos (as in tofrodos):

socat -u -,cr -

Run sendmail daemon with your favorite network options

Warning: using this wrong may result in becoming an open relay!

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

Send a mail using chat (from ppp package):

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

Connect remote X :1 to local X :0

socat exec:'ssh root@troas socat unix-l\:/tmp/.X11-unix/X1 -' unix:/tmp/.X11-unix/X0

Note the escaping of the colon in the remote command.

Sending a file - Server sending the file

server$ socat -u FILE:test.dat TCP-LISTEN:9876,reuseaddr
client$ socat -u TCP:127.0.0.1:9876 OPEN:out.dat,creat

Sending a file - Server receiving the file

server$ socat -u TCP-LISTEN:9876,reuseaddr OPEN:out.txt,creat
client$ socat -u FILE:test.txt TCP:127.0.0.1:9876

Be a syslog server:

socat -u UDP4-LISTEN:5140,reuseaddr,fork OPEN:/tmp/syslog.msg,creat,append

I can't figure out how to put a newline after each message...

Send syslog messages to screen:

socat -t0 -T0 -u UDP4-LISTEN:514,reuseaddr,fork -

To get time from time server:

socat TCP:time.nist.gov:13 -

Really sick - use socat as a VPN solution:

socat -d -d  \
    TUN:192.168.99.2/24,up \
    SYSTEM:"ssh root@remote-server socat -d -d  - 'TUN:192.168.99.1/24,up'"

This must be run as a user that can modify tap/tun devices on both sides of the tunnel.

Use a remote modem

On the side with the modem

socat /dev/ttyS0,raw,echo=0 tcp4-listen:3334

On the side where you want the modem transferred

socat PTY,link=$HOME/vmodem0,raw,echo=0 TCP:servername:3334

You can now access remote /dev/ttyS0 through local $HOME/vmodem0

Use a remote modem over SSH

socat PTY,link=$HOME/vmodem0,waitslave \
 EXEC:"ssh root@remote-server socat - /dev/ttyS0"

You can now access remote /dev/ttyS0 through local $HOME/vmodem0. Remove waitslave to keep alive after local client disconnect.

Using OpenSSL over UDP

This uses a chaining method I believe to be only available in socat2.
On the listening side:

socat2 - "OPENSSL-SERVER,cert=client.pem,cafile=server.crt|UDP4-LISTEN:4430,fork"

On the connecting side:

socat2 exec:ls "OPENSSL-CLIENT,cert=server.pem,cafile=client.crt|UDP4:localhost:4430"

OpenSSL Tunnel

First, generate certificates and distribute them to either side:

FILENAME=server
openssl genrsa -out $FILENAME.key 1024 
openssl req -new -key $FILENAME.key -x509 -days 3653 -out $FILENAME.crt 
cat $FILENAME.key $FILENAME.crt >$FILENAME.pem 
chmod 600 $FILENAME.key $FILENAME.pem 
 
FILENAME=client
openssl genrsa -out $FILENAME.key 1024 
openssl req -new -key $FILENAME.key -x509 -days 3653 -out $FILENAME.crt 
cat $FILENAME.key $FILENAME.crt >$FILENAME.pem 
chmod 600 $FILENAME.key $FILENAME.pem

On the listening side:

socat openssl-listen:4433,reuseaddr,cert=server.pem,cafile=client.crt tcp-connect:localhost

On the connecting side:

socat - openssl-connect:server.domain.org:4433,cert=client.pem,cafile=server.crt

More information:
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