Pseudo-terminal will not be allocated because stdin is not a terminal

Question

I'm trying to set up automatic SSH hopping through a server which doesn't have nc.

This works from the command line:

ssh -A gateway ssh steve@target

(I have added my public key to the SSH agent).

However, adding it to ~/.ssh/config doesn't:

Host target
  User steveb
  ProxyCommand ssh -A gateway ssh steve@targetip

$ ssh target
Pseudo-terminal will not be allocated because stdin is not a terminal.


^CKilled by signal 2.

Attempting to force the issue with -t is amusing but unhelpful.

ProxyCommand ssh -A -t gateway ssh steve@targetip

$ ssh target
Pseudo-terminal will not be allocated because stdin is not a terminal.
Pseudo-terminal will not be allocated because stdin is not a terminal.


^CKilled by signal 2.

More -t's? No good.

ProxyCommand ssh -A -t -t gateway ssh steve@targetip

$ ssh target
tcgetattr: Inappropriate ioctl for device


^CKilled by signal 2.

Is this possible? Most tutorials (eg http://www.arrfab.net/blog/?p=246 ) suggest using nc.

Looks like it. In this instance, I was able to get it installed, solving my problem - but I don't always have that luxury. — Steve Bennett, Jul 09 '13 at 06:28
See my answer below for two ways I was able to do this without netcat. — MountainX, Jul 27 '13 at 03:52

score 14 · Accepted Answer · edited Apr 13 '17 at 12:37

SSH ProxyCommand without netcat

The ProxyCommand is very useful when hosts are only indirectly accessible. With netcat it is relative strait forward:

ProxyCommand ssh {gw} netcat -w 1 {host} 22

Here {gw }and {host} are placeholders for the gateway and the host.

But it is also possible when netcat is not installed on the gateway:

ProxyCommand ssh {gw} 'exec 3<>/dev/tcp/{host}/22; cat <&3 & cat >&3;kill $!'

The /dev/tcp is a built-in feature of standard bash. The files don't exist. To check whether bash has this feature built-in use run:

cat < /dev/tcp/google.com/80

...on the gateway.

To make sure that bash is used, use:

ProxyCommand ssh {gw} "/bin/bash -c 'exec 3<>/dev/tcp/{host}/22; cat <&3 & cat >&3;kill $!'"

And it even works together with ControlMaster.

(Updated on Oct 22 to include kill to clean up background cat) (Updated on Mar 3 2011 to make placeholders more clear and explain /dev/tcp)

100% credit to roland schulz. Here's the source:
http://www.rschulz.eu/2008/09/ssh-proxycommand-without-netcat.html
see more useful info in the comments there.

There is also more here:
http://www.linuxjournal.com/content/tech-tip-tcpip-access-using-bash
http://securityreliks.securegossip.com/2010/08/enabling-devtcp-on-backtrack-4r1ubuntu/

UPDATE: here's something new from Marco

In reference to a ProxyCommand in ~/.ssh/config where one has a line like this:

ProxyCommand ssh gateway nc localhost %p

Marco says:

You don't need netcat if you use a recent version of OpenSSH. You can replace nc localhost %p with -W localhost:%p.

The result would look like this:

ProxyCommand ssh gateway -W localhost:%p

score 9 · Answer 2 · edited Aug 28 '13 at 23:21

9

Big T, not little t.

-T' Disable pseudo-tty allocation.
-t' Force pseudo-tty allocation.

My script used to return that message, and does no longer.

/usr/bin/ssh -T -q -i $HOME/.ssh/one_command other_system

I use the authorized_key on the other_system to cause this to run a command:

from="my.mydomain.com",command="bin/remotely-run" ssh-rsa ...

edited Aug 28 '13 at 23:21

slm

369,824

answered Aug 28 '13 at 23:02

user46083

91

Hauke Laging · Answer 3 · 2013-07-08T06:47:45.900

3

Give this a try:

ProxyCommand ssh -A -t gateway ssh -t steve@targetip

edited Jul 08 '13 at 06:47

answered Jul 08 '13 at 04:22

Hauke Laging

90,279

Wait, how's that different from what I've tried? – Steve Bennett Jul 08 '13 at 04:26
@SteveBennett The difference is that this does not only try to allocate a TTY on the second system but on the first, too. – Hauke Laging Jul 08 '13 at 04:33
it's exactly the same command I mentioned with the "amusing but unhelpful" result? – Steve Bennett Jul 08 '13 at 06:24
@SteveBennett I misread that, indeed. My aim was to have the -t in both connections and I saw it in the wrong one. I have edited my answer. – Hauke Laging Jul 08 '13 at 06:48
Ah. Well, still no good. Tried all the combinations. – Steve Bennett Jul 08 '13 at 07:41

score -3 · Answer 4 · edited Sep 10 '14 at 12:56

-3

You could try the following technique of ssh'ing into server1 followed by ssh'ing into server2.

$ ssh -t user1@server1 ssh -t user2@server2

Doing it like this works for me.

edited Sep 10 '14 at 12:56

slm

369,824

answered Sep 10 '14 at 11:46

Jan Marius Evang

26

1

Please explain more... What this command does and how it is useful to solve the answer. – TPS Sep 10 '14 at 12:00

Pseudo-terminal will not be allocated because stdin is not a terminal

4 Answers4

SSH ProxyCommand without netcat

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