[lug] ssh login error

D. Stimits stimits at comcast.net
Mon Jan 8 18:55:04 MST 2007

Steve Webb wrote:

> I had a similar experience on a hosted server (vmware-type situation) 
> where they limited the number of PTYs that I had (I was running GNU 
> 'screen' and ran out of PTYs pretty quickly).  I asked the hosting 
> company to adjust the number of PTYs so I could spawn more 'screen' 
> sessions and it kind of went away.
Everything seems to center on this, but I know the CentOS kernel is not 
a dumbed-down kernel, so I can't imagine I have to recompile for pty's. 
During the ssh hangs, there are only 2 or 3 logins of real users, and 
the public web and dns interfaces are not being hit. There are times 
when I've had a dozen or more logins going at once, with no trouble. Is 
there any kind of configuration setting (aside from kernel compile) that 
someone can think of which could limit pty availability? How would I 
monitor the existing use of pty's in order to see what's actually 
getting used at any moment (sort of like top for ptys)?

> I think that you can adjust the number of PTYs in the kernel config 
> somewhere, so if you can recompile the kernel, might want to poke 
> around in there a little (device drivers -> character devices -> 
> number of legacy PTY in use (mine's set to 256)) and I have tons of 
> /dev/pty* and /dev/tty* entries in my /dev dir.
So far as I can tell very few pty's were actually in use. When I ran the 
MAKEDEV to force all those ptys to exist, it had no effect. From what I 
can tell so far, it ran out with 2 ptys in use...both by root, as I 
monitored. Although a regular user could not log in, root could still 
log in many more instances (which would imply the limit was not a limit 
on the whole machine). You're probably thinking of permissions, but the 
problem has gone away today, and in the past, the problem had taken out 
root's login ability, so I used su from a regular user login (it had 
never touched the outside world at the time, other than for yum update 
behind a firewall). When I do look at the ptys created from MAKEDEV, 
they all appear to have proper permissions. In reality though the 
MAKEDEV probably does not matter, it is using /dev/pts/*, and MAKEDEV 
makes /dev/pty*. I'm stumped.

D. Stimits, stimits AT comcast DOT net

More information about the LUG mailing list