Server process not closed after disconnect, problems with reconnect

[Matt Johnston]

Hi,

The keepalive option makes sure that traffic is transmitted every N seconds. It can be used to avoid routers closing idle (keeping them "alive").

If the connection is gone then the connection will be terminated after a timeout (a minute perhaps?) when the traffic is transmitted. A program doesn't get notified when a network interface goes down (at least with standard Unix programming interfaces), so it only finds out when traffic is sent.

An alternative might be to "killall dropbear", "/etc/init.d/dropbear start" in /etc/ppp/ip-down.d or similar, since that part knows that the interface has gone away.

Cheers,
Matt

On 3 February 2014 8:34:15 pm AWST, Alexander Kriegisch <alexander at kriegisch.name> wrote:
>Hi Matt.
>
>Thanks for your swift reply. Could you please elaborate on what "-K" is
>actually meant to do (what is kept alive and how?) and how it might
>help me solve my problem? Documentation on Dropbear is sparse and I
>fail to see the connection.
>
>Kind regards
>-- 
>Alexander Kriegisch
>http://scrum-master.de
>
>
>Matt Johnston schrieb am 03.02.2014 13:02:
>
>> Running with -K 30 for keepalive might sort it out?
>> 
>> 
>> On 3 February 2014 4:25:49 pm AWST, Alexander Kriegisch
>> <alexander at kriegisch.name> wrote:
>>>My setup:
>>>  - Server (DSL/WLAN router running Dropbear sshd v2012.55)
>>>
>>>      $ uname -mrsp
>>>      Linux 2.6.19.2 mips unknown
>>>
>>>      $ ps | grep dropbear | grep -v grep
>>>       1673 root      1352 S    dropbear -i -R -a
>>>      14826 root      1396 S    dropbear -i -R -a
>>>
>>>  - Client
>>>
>>>      OS: Windows XP Pro 64-bit
>>>      SSH Client: Bitvise
>>>
>>>As you can see, my Dropbear runs as root in inetd mode, permitting
>root
>>>logins (which is what I use) and accepting connections to forwarded
>>>ports from other hosts. I need this because my connection basically
>>>creates a few forward tunnels (client to server) to other machines
>>>behind the router as well as backward tunnels (server to client) to a
>>>few services on the client's network. So far, so good.
>>>
>>>The DSL router gets disconnected once every night, reconnects within
>>>seconds and gets a new IP address, which is the usual thing in
>Germany
>>>for consumer-type ISP connections. What I expect to happen is that
>the
>>>dropbear process goes down, but in ca. 4 out of 7 days this does not
>>>happen. The main symptom is that the auto-reconnect for the SSH
>>>connection to the dynamic host name fails because ports on the router
>>>cannot be bound because they are already in use. When I check with
>>>netstat I can see that indeed all the listening ports for the reverse
>>>tunnels are still in use by the old Dropbear process which has not
>>>terminated. On a few days a week it works, but I do not know the
>>>circumstances or race conditions which cause this behaviour. So what
>I
>>>end up doing most of the time is log on to the router without the
>>>tunnels and kill the non-terminated Dropbear process blocking the
>>>listening ports. A few seconds later, the full-blown SSH connection
>>>with forward and reverse tunnels automatical
>>> ly reconnects and everything is fine for another 24 hours.
>>>
>>>Now this obviously is ugly and unstable. Is there a way to make
>>>Dropbear understand it should terminate when the DSL connection is
>>>gone? Or is there at least a workaround by which I can check if the
>SSH
>>>process ist still alive? I thought that maybe I could try and connect
>>>to one of the stale reverse tunnels (localhost:someport on the
>router)
>>>in order to see if it is still functional, then kill the process
>>>otherwise, but I had difficulty doing so because a wget test does not
>>>work (I only have Busybox wget which does not have a time-out
>>>parameter).
>>>
>>>Please ask for more information if this was too unspecific.
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