How can I keep my SSH sessions from freezing?

Question

I have ServerAliveInterval and in case of few machines also ClientAliveInterval set to 540 in SSH client/server configuration files (I suppose setting it to more than that would not be a good idea). I work with many SSH sessions which currently freeze after a few minutes.

How can I fix it? What I want is to have a session to not freeze at all, so that if I open a session at 8 and don't use it for 4 hours, for example, to still use it again at 12 without having to log-in again.

Answer 1

The changes you've made in /etc/ssh/ssh_config and /etc/ssh/sshd_config are correct but will still not have any effect.

To get your configuration working, make these configuration changes on the client:

/etc/ssh/ssh_config

Host *
ServerAliveInterval 100

ServerAliveInterval The client will send a null packet to the server every 100 seconds to keep the connection alive

NULL packet Is sent by the server to the client. The same packet is sent by the client to the server. A TCP NULL packet does not contain any controlling flag like SYN, ACK, FIN etc. because the server does not require a reply from the client. The NULL packet is described here: https://www.rfc-editor.org/rfc/rfc6592

Then configuring the sshd part on the server.

/etc/ssh/sshd_config

ClientAliveInterval 60
TCPKeepAlive yes
ClientAliveCountMax 10000

ClientAliveInterval The server will wait 60 seconds before sending a null packet to the client to keep the connection alive

TCPKeepAlive Is there to ensure that certain firewalls don't drop idle connections.

ClientAliveCountMax Server will send alive messages to the client even though it has not received any message back from the client.

Finally restart the ssh server

service ssh restart or service sshd restart depending on what system you are on.

Answer 2

Personal suggestion: use screen on the remote host; it will manage to keep your connection alive for as long as it stays active in a terminal.

Here's what I typically add to /etc/screenrc for quick identification of my screen sessions:

hardstatus alwayslastline
hardstatus string "%{= kG}[ %{G}%H %{g}][ %{=kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+Lw%?%?%= %{g}][%{B}%Y-%m-%d %{W}%c %{g}]"
defscrollback 8192

---

EDIT : Hints.

The hardstatus string will show a bottom status line such as this one:

The scrollback buffer is also extended to 8192 lines instead of the usual 1000-1500 (depending on the distribution).

Answer 3

If the problem is a hibernated laptop or a less-than-prefect network connection, I'd recommend using mosh which runs over ssh and allows automatic reconnecting.

From the website:

Mosh (mobile shell)

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes.

Mosh is a replacement for SSH. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

Mosh is free software, available for GNU/Linux, BSD, macOS, Solaris, Android, Chrome, and iOS.

In combination with tmux (or the older screen), this allows me to connect via ssh to a server from my laptop and stay connected for days even when changing wifi connections and surviving mobile data drop-outs.

Answer 4

With OpenSSH:

You need to enable

TCPKeepAlive yes

both in your client ssh_config (e.g. /etc/ssh/ssh_config or in ~/.ssh/config) and your destination SSH server running OpenSSH (e.g. /etc/ssh/sshd_config).

So every time your connection idles, OpenSSH sends some dummy packet to your destination host...

Answer 5

Check your sshd configuration on the host machine /etc/sshd_config for the IdleTimeout setting.

IdleTimeout time
Sets idle timeout limit to time in seconds (s or nothing afternumber), 
in minutes (m), in hours (h), in days (d), or in weeks (w).If the 
connection have been idle (all channels) for that long time thechild 
process is killed with SIGHUP, and connection is closed down.

Answer 6

A little hack I use if it's not possible to use screen, or edit the hosts ssh server settings, or any of the good and proper suggestions above...

Just run top whenever you are going to leave the session alone.

Generally that'll keep it alive indefinitely, although, as others have mentioned, that might not always be wise vis-a-vis security!

Answer 7

For me, the SSH connection used to hang up.

The below solution worked for me. Now the SSH connection always stays live.

From the server:

---

1. Edit the file at /etc/ssh/sshd_config

$ sudo vi /etc/ssh/sshd_config

2. Add or enable this line in the file.

ClientAliveInterval 60

3. Save the file and restart the sshd service

$ sudo service sshd restart

From the local machine/client:

---

1. Edit the file at /etc/ssh/ssh_config

$ sudo vi /etc/ssh/ssh_config

2. Add this line to the file.

ServerAliveInterval 60

3. Save the file

About these Parameters :

ServerAliveInterval: number of seconds that the client will wait before sending a null packet to the server (to keep the connection alive).

ClientAliveInterval: number of seconds that the server will wait before sending a null packet to the client (to keep the connection alive).

Important: Whenever any modification is performed on a Production instance, please ensure to create a backup.

Answer 8

I had a similar problem:

• ssh connection remained ESTABLISHED
• terminal freezes and un-freezes repeatedly
• packets are visible in SEND-Q while terminal freezes
• freeze time about 2-4 minutes
• un-frozen for about 30 seconds

Turns out the firmware of my router was outdated!

• fixed issue with firmware update

Answer 9

In my case problem was in large MTU size. You can change MTU on router if you using NAT, but I change MTU on server:

sudo /sbin/ifconfig eth0 mtu 1036
sudo /etc/init.d/networking restart

Answer 10

My SSH started closing consistently after a minute of being open and I think it was because of a network adapter setting I changed in order to stop my PC from waking up from sleep. My PC had been waking up from sleep for no reason so I found this article on how to stop it and it worked. The article said to go into the Device Manager, find the Ethernet or Wi-Fi adapter in question under Network Adapters, right-click on it, choose Properties, and open the Power Management tab and uncheck the Allow This Device to Wake the Computer.

Alternatively, it said you can keep that box checked, and check Only Allow a Magic Packet to Wake the Computer. I did that and later my SSH would consistently close down after a minute so I believe that was the culprit. So make sure that you don't have that setting on your network adapter changed from the default. Might have to restart your computer for any changes you make to take effect.