"PTY allocation request failed" error when trying to connect using SSH with ControlMaster

Question

I'm trying to log into Ubuntu desktop. I get the following error message:

PTY allocation request failed

What does it mean and how to connect to my desktop?

This is the output with -v:

debug1: Reading configuration data /home/evag/.ssh/config  
debug1: /home/evag/.ssh/config line 1: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config 
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: auto-mux: Trying existing master
debug1: mux_client_request_session: master session id: 2
PTY allocation request failed

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Given the output presented above with the verbose option, I think I understand where the problem comes from: I have two computers, A and B. I'm using a controlmaster in A to keep persistent connections to B. A logs into B in a safe way, with command=/bin/false to limit the usage A has on B (it has only forwarding capabilities).

However I would want to connect to ssh from A to B without using this identity (-i), but the problem is that since there is a persistent connection, it uses the existing connection regardless of the option I put in -i, so I end up with not being able to allocate a PTY.

So given that there is an existing controlmaster and protected connection from A to B, how can I get another connection from A to B, but this time with full rights?

Answer 1

I'm assuming that you have specified the ControlPath and ControlMaster in ~/.ssh/config ? In which case ssh will always check for the ControlPath when attempting to access that server. As I understand it this is the default if a ControlPath exists even if ControlMaster=no is set (because it is automatically set to 'no' if the ControlPath exists so as not to attempt to create a new ControlMaster).

You will need to specify a new ControlPath:

ssh -o ControlPath=~/.ssh/%r@%h:%p-newsession server

However: I do the same thing but do not need to tell it to run 'false', I simply use the available options in SSH. See the man page for a way to create forwarding connections without starting a shell. When done properly the PTY is only allocated as needed, even over the same control connection.

-f      Requests ssh to go to background just before command execution.
-N      Do not execute a remote command.  This is useful for just forwarding ports.
-T      Disable pseudo-tty allocation.

So in my case (along with other, unrelated options):

ssh -fNT server

where ~/.ssh/config already specifies the ControlPath.