Sep 22, 2016

ssh_exchange_identification connection reset by peer problem

At Badoo we have a distributed system that uses ssh on start to connect to each node and exchange some data.

Sometimes I've seen ssh fail with error

ssh_exchange_identification: read: Connection reset by peer error

Usually, If you see this error, it means that server didn't like your certificate or you were banned from accessing this server.

The best way to debug this problem is to append -vvv while executing ssh client to increase verbosity and to do the same on server.

If your Linux distribution uses systemd, then the best way to look into sshd logs is journalctl -f command or specifically journalctl -u sshd.service -f command.

To increase ssd verbosity, edit /etc/ssh/sshd_config and add LogLevel DEBUG3 and then restart sshd with command sudo systemctl restart ssd.service.

Unfortunately, logs didn't show anything useful for me.

But I had a hunch that it is somehow connected to the fact that our distributed system makes a lot of ssh connections and very rapidly.

While browsing trough sshd_config man page I've found config variable that could be to blame. And I was right.

If you happen to have the same problem I did, try increasing MaxStartups variable and restarting sshd service.

I've increased it to 100:30:600 from default 10:30:60 and connection resets vanished.

It is unfortunate that ssh and sshd logs gave me no clue about the problem.