• Client : OpenSSH_5.1p1 Debian-5ubuntu1 (Ubuntu 9.04)
• Server : OpenSSH_5.1p1 Debian-5 (Proxmox 2.6.24-7-pve)

I use SSH to execute commands remotely on the server (module check_by_ssh of Nagios). But SSH hangs from time to time when trying to execute commands. I can log to the server via SSH but not executing a simple 'ls'. And it seems to block from all clients from the same IP address. Authentication is not the problem, may it be made by SSH keys or password.

ssh -l root -p 2222 server.domain.tld 'ls'

Here the client debug info

debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug1: Sending environment.
debug3: Ignored env ORBIT_SOCKETDIR
*** skipping approx 40 env var ignored
debug1: Sending command: ls
debug2: channel 0: request exec confirm 1

It hangs there. Then after a random time, it works again (without doing anything). Killing all sshd process on the server seems to work too. It works from a Putty. I saw that some people had trouble like this due to ISP reverse DNS problem, but it does not seem to be the case here.

It can work for hours and then not work for half an hour or so.

What could explain this behaviour ?