I recently ran into the following problem when trying to ssh into my Ubuntu 11.04 server:
ssh greg@site.com
PTY allocation request failed on channel 0
I don't know for sure, but I think this happened after one of the latest Ubuntu updates I installed. In case it's of relevance, I'm remoting in from Terminal (Mac OS X 10.7 – Lion).
EDIT: It turns out that I can remote in as another user (i.e. matt) on my server… seems to me like that would point to SSH key problems. If I remote in from another account on the same client machine, it works fine for all users on the server.
SOLUTION:
Turns out that I had gitolite configurations in my ~/.ssh/allowedkeys on the server side. I accidentally ran a configuration script some time ago as my user instead of as the git user. This was kicking off configurations upon remote (keys matched up) that disabled needed settings (see below). I removed the relevant fields from allowedkeys, and I was golden.
debug1: Remote: Forced command.
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
debug1: Remote: Forced command.
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
PTY allocation request failed on channel 0
Best Answer
It appears like the server operator has adjusted the sshd configuration or setup options in the authroized_keys files to prevent you from getting an interactive shell. Server operators that want to provide sftp access only frequently do this.
Try connecting without using your keys for authentication. If you can get a shell when you don't use a key the problem is in your authorized_keys file. If you cannot login when trying to use password authentication, and password authentication isn't disabled, then the problem is going to be in the sshd_config.