How to maintain a persistent SSH connection?

martin f krafft
Wed, 15 Nov 2017 14:04:51 -0000

also sprach martin f krafft <madduck at madduck.net> [2017-11-15 14:08
+0100]:
> The sshd(8) process stays around for an unspecified time (>90min)
> after the connection went down.

For the record, I've attached strace and tcpdump to the various
processes to see what happens server-side after the connection goes
down, but there is *nothing*:

  1. strace on the privileged sshd process has it sitting at
       restart_syscall(<? resuming interrupted poll ?>) = 1

     until at some indefinite point, the unprivileged child dies, at
     which point it tears down after catching SIGCHLD.

  2. strace on the unprivileged sshd process, the one that bound the
     remote port just stays at
       select(10, [3 6 8 9], [], NULL, NULL?
     indefinitely.

     Those FDs are (3) main SSH connection, (6) a pipe, (8) IPv6
     listening socket for port 2217, (9) IPv4 listening socket for
     port 2217.

  3. tcpdump on the main SSH connection has not seen a single packet
     in the last 20 minutes. Curiously, netstat/lsof seem to think
     that the connection is ESTABLISHED.

This isn't good, but maybe it's also not a problem of SSH, but the
OS/kernel (4.9.0).

Best regards,

--
@martinkrafft | http://madduck.net/ | http://two.sentenc.es/

"in just seven days, i can make you a man!"
                                      -- the rocky horror picture show

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