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ssh via multiple hosts

For connecting to server B I have to first ssh to server A. What's the command line to access server B?

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marked as duplicate by xenoterracide Jun 24 '12 at 14:54

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2 Answers 2

up vote 11 down vote accepted

If server B is reachable via ssh and you only need ssh (not direct scp or sftp), this also works very well:

ssh -t $SERVER_A ssh $SERVER_B

The -t option forces allocation of a pseudo-tty even when running a single command at the other end. This is helpful, since ssh needs a pseudo-tty.

Since you're using two nested instances of ssh, the escape character in the inner session is Enter ~ ~ (two tildes). One tilde will send the escape to the first shell.

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There isn't a built-in way in ssh to do this, other than to use port forwarding.

However, there is a way that works reasonably well - the ProxyCommand setting for ssh. You can specify that on a per-host basis in ~/.ssh/config and use it to specify the command to run to connect to the remote ssh port.

I use this on several hosts:

host serverB.example.com serverB
  ProxyCommand /usr/bin/ssh serverA.example.com /usr/bin/nc %h %p

See the ssh(1) manual page for the details, and nc(1) from the netcat package for the command I am using to forward on the connection. (You can use anything that makes a TCP connection and passes standard input and output through it, though.)

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I replaced /usr/bin/nc with ssh. then after it asks for password of serverA I get this error(s): Permission denied, please try again. Permission denied, please try again. Permission denied (publickey,password). ssh_exchange_identification: Connection closed by remote host –  Omid Jun 24 '12 at 7:20
    
There's no need to replace nc there. See, it's used for creating a TCP connection from serverA to serverB, which the ssh serverB command then uses to talk SSH. –  sr_ Jun 24 '12 at 9:22
5  
Just for the record newer versions of ssh support the -W option, you can do something like ProxyCommand ssh -W %h:%p gateway instead of depending on nc –  Ulrich Dangel Jun 24 '12 at 9:48
    
@sr_: You are right! I thought it was mc(midnight commander) –  Omid Jun 24 '12 at 19:39
    
@UlrichDangel you should add that as an answer to the other question: it is not included there. –  jasonwryan Jun 24 '12 at 20:05

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