ssh keep-alive tip
July 10th, 2005Whenever I login remotely to my machine in Canada, the ssh session dies after a while if I don’t actively type something. This is not the server timing out, but rather the TCP connection hanging. Figuring out a way to keep sending it keystrokes automatically is beyond my ability, but I found out that I could just have it send me data continually, and that works just as well to keep the terminal session alive. The following Bourne Shell loop works:
> while date ; do sleep 10 ; done
This just prints the date and time to my terminal every 10 seconds. At any time, I can interrupt it with a CTRL-C and continue to use the session. So now you know. 
August 4th, 2005 at 1657 UTC
Well, you just gotta add these two lines to the
/etc/ssh/sshd_configfile on your server:KeepAlive yes
ClientAliveInterval 60
Now just restart sshd and your connection should never be interrupted again.
August 21st, 2005 at 0218 UTC
Max: But remember that most people have no root access on most machines. In addition, keep alives are a big security threat. If somebody walks away from their shell and somebody else drops in, sshd will have actually kept the shell open for them.
So I took the command from above, wrapped it in a “sh” call and it works just fine. I use this to keep my various ssh tunnel alive. Every 20 sec I get an echo to the shell but I can still type commands as usual:
sh -c 'while echo "Tunnel..." ; do sleep 20 ; done'&
Thanks for the tip, Mad Philosopher
September 24th, 2005 at 0405 UTC
> Every 20 sec I get an echo to the shell but I can still type commands as
> usual:
>
> sh -c ‘while echo “Tunnel…†; do sleep 20 ; done’&
So do this:
sh -c 'while echo "Tunnel…" > /dev/null; do sleep 20 ; done' &
November 26th, 2005 at 0759 UTC
You could also do something clever with port forwards and netcat.
Like this little keep-alive ssh script:
--- begin kassh --- #!/bin/bash # Pick a random high numbered port. port=$[$RANDOM+32000] ## Keepalive process.. (sleep 300; while netcat -z -v localhost $port; do sleep 300 done) >/dev/null 2>&1 & # Initiate SSH connection with whatever argument we were given. ssh -L $port:localhost:7 $*November 26th, 2005 at 0832 UTC
Oh, and in OpenSSH 3.8 and above, there’s the “ServerAliveInterval”.
February 4th, 2006 at 2229 UTC
Redirecting the echo to /dev/null probably will not work, because sshd would have no reason to send bytes to the client. Personally I do something like ‘ping -i 10 ‘ or ‘tail -f /var/log/httpd-access.log’ when I need such a hack; as long as you are sending junk packets you might as well have some information in them.
March 6th, 2006 at 1019 UTC
When SSH’ing, I always use “screen”. An excellent tutorial is located here: http://jmcpherson.org/screen.html
Screen can also display a little clock running. Put the following line in a file called “.screenrc” and put it in the home directory.
caption always '%c:%s'
April 13th, 2006 at 0605 UTC
A friend of mine wrote this for me awhile back. He calls it
spin.cCompile this and run it… it’ll give you a nice spinny thing to keep your ssh session busy.
#include <stdio.h>
int i;
char spin[4] = { '|', '/', '-', '\\' };
void main() {
for(i=0;;usleep(300000), fflush(stdout))
printf("%c\x08", spin[++i%4]);
}
May 9th, 2006 at 1837 UTC
Do as Bart says, use screen if you do not have root access on the remote machine. It is really usefull, much easier than the other sollutions, and it is not a hack like most of them. If you do have root access you can fix the source of the problem, but you might still want to have a look at screen.
June 2nd, 2006 at 1656 UTC
why not just run ‘top’?
June 3rd, 2006 at 1148 UTC
Read the earlier post about ServerAliveInterval. You don’t need root access. Just create a file:
$HOME/.ssh/config
and put:
ServerAliveInterval 60
in it.
June 8th, 2006 at 1336 UTC
DO NOT RUN TOP – it requires too much overhead(processor cycles) and your sysadmin will kill it if he notices anyway.
July 28th, 2006 at 1612 UTC
Thanks Cam, yours was the simplest to implement, worked great.
All the others & madphilo: thanks for your inputs !
No wonder google lists this page at the top when searching for ssh keepalive.
August 7th, 2006 at 1305 UTC
Cam’s response was top-notch, thank you much!
September 9th, 2006 at 1806 UTC
Cam’s response seems easiest. I just wanted to note that I had to create the ~/.ssh/config file, and ‘chmod g-w’ on it before I could slogin somewhere (note: this may be a SELinux feature).
September 29th, 2006 at 1558 UTC
There’s a program called autossh that is especially useful for keeping SSH tunnels alive. It can also be used to keep a shell login session active over a long period.
You configure the autossh process, which runs in the background to maintain various SSH connections, and then rather than connect to the remote host, you connect to a port on your local machine. The underlying SSH session can break many times, but your client programs never detect an actual break in the connection; they are always presented with an active connection. autossh handles all the reconnections behind the scenes. I’ve used it successfully on intermittant wireless and dialup connections.
http://freshmeat.net/projects/autossh
October 27th, 2006 at 1452 UTC
The suggestions involving tunneling sound nice as they wouldn’t interfere with your console. However, they are a bit more work to set up. If you don’t want to do all that you could do what I do. I usually just start up top. That’s usually good enough to keep the session going as there is almost always some kind of movement going as various processes’ priorities are moved up and down. Even if there isn’t, it has a clock n it which is constantly updating.
I also highly recommend the screen utility. It is good for two reasons. 1) if you are running something long that doesn’t have any output to keep your session alive you can open a new terminal within screen and run top or something else there. 2) If your connection does close whatever you were running should continue running and you only need connect again and re-attach to the old screen session.
Of course, if you do have root, setting the KeepAlive in sshd_config is the most convenient option.
November 14th, 2006 at 2229 UTC
Thanks Cam
ServerAliveInterval 60
is exactly what I was looking for. I will also check out autossh and yay for google
.
December 7th, 2006 at 1859 UTC
watch date
or
watch uptime
December 8th, 2006 at 1404 UTC
now for some reason, cam’s suggestion is not working anymore…ssh term keeps timing out on me…I’ll try autossh …
January 5th, 2007 at 1056 UTC
autossh is bs…but
Thanks to Ron at http://www.jaguarpc.com/forums/showthread.php?p=101123
I was informed there is a simple setting on the client to enable the keepalive.
Now putty doesnt timeout anymore ! Thanks Ron !
January 16th, 2007 at 1504 UTC
or you could just do a
ssh -o TCPKeepAlive mark@localhost
which will do the same and handy if bouncing between boxes.
January 29th, 2007 at 1615 UTC
Here is the script I wrote to overcome this issue. This script scrolls a text message on screen randomly as “Keep Alive On…”
#!/bin/ksh x=0 y=0 flip_flop=0 while [ 1=1 ] do clear if [[ $x -ge ${LINES} ]]; then x=1 else x=`expr $x + 2` fi if [[ $y -ge ${COLUMNS} ]]; then y=1 else y=`expr $y + 2` fi tput cup $x $y if [[ $flip_flop -eq 0 ]]; then echo "Keep Alive On" flip_flop=1 else flip_flop=0 echo "`tput smso`Keep Alive On`tput rmso`" fi sleep 2 doneFebruary 14th, 2007 at 2033 UTC
Does the
“ServerAliveInterval 60″
go on the local machine or on the foreign host?
February 20th, 2007 at 0925 UTC
Mister Confused Person:
The “ServerAliveInterval 60″ is a client-side option, so it goes on the local machine, as you say.
Here’s the pertinent paragraph from the ssh_config man page on my Ubuntu system:
May 1st, 2007 at 1208 UTC
It’s worth noting that many SSH timeouts come from firewalls and no amount of tinkering with timeout settings (other than on the firewall) will fix it. The only way in these cases is to simulate traffic (which is what the client’s keepalive settings should do, but don’t always in my experience).
I find this works very well (although possibly it’s overkill being in perl):
Run it as soon as you SSH in, and remember the pid so you can kill it before you leave. It squirts a null every 59 seconds. If you change the \x00 for a printable character you will see it appear on your terminal but it still won’t impact what you are doing.
–
May 6th, 2007 at 0303 UTC
Bruce:
You can automate this by putting the following in your .kshrc (or its equivalent for your shell in its rc file, or put it in .profile (or .login)):
case $- in i*s*) perl -e 'sleep 59 && print STDERR "\x00" while 1' & trap "kill ${!}" 0 ;; esacBecause, in working with computers, being lazy is a great virtue…
July 12th, 2007 at 1813 UTC
Both Cam and Max’s solutions (ie ClientAliveInterval and ServerAliveInterval) resolve the timeout issue, even when a firewall is timing out connections which transit it.
We just installed a new firewall here and it’s default behavior (which is also unchangeable without a firmware upgrade) is to timeout sessions after 5 minutes. Having a bunch of pissed off engineers in your cube is never a good thing, so thanks to Cam and Max.
I guess this is what I get for buying a firewall off of ebay.
October 16th, 2007 at 1243 UTC
Another dirty hack that keeps an ssh connection alive without printing annoying messages to the console:
This “prints” an alert, which probably produces a beep every 20 seconds but since my workstation has no sounds, its a neat variant of the above keep-alive command.
September 11th, 2008 at 2307 UTC
dude, echo -ne “\a” would beep on the server not the client. the admin at the bunker is probably going nuts trying to figure out what all that beeping is. (….the server probably doesn’t have sounds, either, though)
October 24th, 2008 at 1054 UTC
“some other dude”, that’s not correct. Don’t forget the difference between console and terminal.
When using SSH, you’re on a terminal, not the console. If you sent the \a to the console then the beep would be at the server, otherwise you’re printing it to your (pseudo) terminal and your terminal emulator (putty or whatever) will receive it. It’s then up to the terminal emulator to actually do something with it. putty can be configured to do various things on receiving the alarm, such as beep or flash the window.
February 20th, 2009 at 1803 UTC
The suggestion
is perfect and elegant. I used it with success. Thank you.
June 17th, 2009 at 1004 UTC
Remember that
is limited by
and that
ServerAliveCountMaxis set to 3 by default!That is, if you set
ServerAliveIntervalto 60 andServerAliveCountMaxis left to default value (3), after the client has sent 3 keep alive packets it will disconnect. That makes 60 x 3 = 180 seconds.So if you want more time away, you set
ServerAliveIntervalto 60 andServerAliveCountMaxto the numer you want (for example: 100). The client will send one keep alive packet every 0 seconds for 100 times or, if you prefer, 100 keep alive packets, one every 60 seconds.You set this up in your
$HOME/.ssh/configfile.Most of these options are explained in
man ssh_config.Thanks a lot to Cam for pointing us all in the right direction!!!!
August 18th, 2009 at 2349 UTC
Todd Says:
Absolutely not true.
1> It only uses many cycles if you have the interval set way too short, like under .5 seconds
2> if your sysadmin is obsessed with you not using top, he/she should simply disable your ability to run it, not ‘kill it’ once you do.
September 24th, 2009 at 0057 UTC
Thanks for all the help