SSH connection freezes after bigger output when inactive for a while

arch linuxopensshssh

I have a problem with an SSH session freezing after using it again when it was idle for a while.

The problem is that the session doesn't freeze immediately, I can still use it. But as soon as a command sends more data, it freezes before any output arrives, and I have to restart the terminal.

If have read earlier solutions about MTU, but there, the session freezes everytime. Here it only happens after a period of inactivity.

Note that I have set ServerAliveInterval 120 in .ssh/ssh_config

Any clues about how I can solve this?

Best Answer

Change ~/.ssh/ssh_config to ~/.ssh/config. Make sure the permissions on it are 700.

This discussion has a lot of good information. You can also follow the tag for ssh (just click on /ssh under your question) to go to a tag wiki for more information and trouble shooting guidance.

Related Solutions

Ssh – Running vim on a remote Linux machine “freezes” OS X SSH connection

I second @Gilles answer. But easier to diagnose, just set the MTU on your ethernet or wireless interface to something really small and see what happens. On OS X:

$ sudo ifconfig en0 mtu 1100  # (or en1 for wifi, unless on an Air)

Then test scp and see what happens. If it still stalls at 1100 you probably need to fix a really broken network device somewhere.

SSH tunnel to router periodically hangs. TCP problem

Two guesses:

Packet loss. Do not trust the "ping" you ran in parallel since the packet loss may be affecting only those TCP streams, for whatever reason. One simple way to detect it is to run $ netstat -s -p|grep "segments retransmited" frequently on both sides (ssh client and server) during the SSH session. See if the counter increases, in which case you experienced packet loss on TCP sessions in this machine. That said, the best way is to use tcpdump or wireshark to record what is happening and confirm there are re-transmissions occurring (they are usually marked with red by wireshark, but YMMV).
MTU problems. If you're trying to list the content of a large directory via SSH, for example, it may occur that the amount of data being transmitted would require it to be fragmented. The endpoints will use path MTU detection to determine the amount of data to send at a time. But in some cases, a firewall in the path may be blocking all ICMP packets and therefore breaking the path MTU detection, causing apparent freezes. This can be hard to confirm and diagnose. The easiest workaround that always works is to change the MTU of the network interface in either side to a low value, such as 1200 or 1000. This may hurt your performance so you'd better not use this forever.

Best Answer

Related Solutions

Ssh – Running vim on a remote Linux machine “freezes” OS X SSH connection

SSH tunnel to router periodically hangs. TCP problem

Related Question