ssh check if a tunnel is alive

Question

I have written a small bash script which needs an ssh tunnel to draw data from a remote server  so it prompts the user   echo  Please open an ssh tunnel using  ssh -L 6000 localhost 5432 example com     I would like to check whether the user had opened this tunnel  and exit with an error message if no tunnel exist  Is there any way to query the ssh tunnel  i e  check if the local port 6000 is really tunneled to that server

User · Answer

Netcat is your friend  nc -z localhost 6000    echo  quot no tunnel open quot

User · Answer

Autossh is best option - checking process is not working in all cases  e g  zombie process  network related problems   example   autossh -M 2323 -c arcfour -f -N -L 8088 localhost 80 host2

User · Answer

This is really more of a serverfault-type question  but you can use netstat   something like      netstat -lpnt   grep 6000   grep ssh   This will tell you if there s an ssh process listening on the specified port  it will also tell you the PID of the process    If you really want to double-check that the ssh process was started with the right options  you can then look up the process by PID in something like     ps aux   grep PID

User · Answer

If you are using ssh in background  use this   sudo lsof -i -n   egrep    lt ssh  gt

User · Answer

These are more detailed steps to test or troubleshoot an SSH tunnel  You can use some of them in a script  I m adding this answer because I had to troubleshoot the link between two applications after they stopped working  Just grepping for the ssh process wasn t enough  as it was still there  And I couldn t use nc -z because that option wasn t available on my incantation of netcat   Let s start from the beginning  Assume there is a machine  which will be called local with IP address 10 0 0 1 and another  called remote  at 10 0 3 12  I will prepend these hostnames  to the commands below  so it s obvious where they re being executed   The goal is to create a tunnel that will forward TCP traffic from the loopback address on the remote machine on port 123 to the local machine on port 456  This can be done with the following command  on the local machine   local    ssh -N -R 123 127 0 0 1 456 10 0 3 12   To check that the process is running  we can do   local    ps aux   grep ssh   If you see the command in the output  we can proceed  Otherwise  check that the SSH key is installed in the remote  Note that excluding the username before the remote IP  makes ssh use the current username   Next  we want to check that the tunnel is open on the remote   remote    netstat   grep 10 0 0 1   We should get an output similar to this   tcp  0  0  10 0 3 12 ssh  10 0 0 1 45988  ESTABLISHED   Would be nice to actually see some data going through from the remote to the host  This is where netcat comes in  On CentOS it can be installed with yum install nc   First  open a listening port on the local machine   local    nc -l 127 0 0 1 456   Then make a connection on the remote   remote    nc 127 0 0 1 123   If you open a second terminal to the local machine  you can see the connection  Something like this   local    netstat   grep 456 tcp  0  0 localhost localdom 456 localhost localdo 33826 ESTABLISHED tcp  0  0 localhost localdo 33826 localhost localdom 456 ESTABLISHED   Better still  go ahead and type something on the remote   remote    nc 127 0 0 1 8888 Hallo  anyone there    You should see this being mirrored on the local terminal   local    nc -l 127 0 0 1 456 Hallo  anyone there    The tunnel is working  But what if you have an application  called appname  which is supposed to be listening on port 456 on the local machine  Terminate nc on both sides then run your application  You can check that it s listening on the correct port with this   local    netstat -tulpn   grep LISTEN   grep appname tcp  0  0  127 0 0 1 456  0 0 0 0   LISTEN  2964 appname   By the way  running the same command on the remote should show sshd listening on port 127 0 0 1 123

User · Answer

This is my test  Hope it is useful      COMMAND is the command used to create the reverse ssh tunnel COMMAND  ssh -p  SSH PORT -q -N -R  REMOTE HOST  REMOTE HTTP PORT localhost 80  USER NAME  REMOTE HOST     Is the tunnel up  Perform two tests     1  Check for relevant process   COMMAND  pgrep -f -x   COMMAND   gt   dev null 2 gt  amp 1     COMMAND    2  Test tunnel by looking at  netstat  output on  REMOTE HOST ssh -p  SSH PORT  USER NAME  REMOTE HOST netstat -an   egrep  tcp    REMOTE HTTP PORT  LISTEN        gt   dev null 2 gt  amp 1 if      -ne 0     then    pkill -f -x   COMMAND      COMMAND fi

User · Answer

We can check using ps command    ps -aux   grep ssh   Will show all shh service running and we can find the tunnel service listed

User · Answer

bin bash    Check do we have tunnel to example com server lsof -i tcp localhost 6000  gt   dev null    If exit code wasn t 0 then tunnel doesn t exist  if      -eq 1   then   echo    gt  You missing ssh tunnel  Creating one      ssh -L 6000 localhost 5432 example com fi  echo    gt  DO YOUR STUFF  lt

User · Answer

Use autossh  It s the tool that s meant for monitoring the ssh connection

User · Answer

stunnel is a good tool to make semi-permanent connections between hosts   http   www stunnel org

[ssh] ssh: check if a tunnel is alive

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