dbclient and detecting broken connections

[Matt Johnston]

On Tue, Dec 08, 2009 at 03:47:02PM -0800, Ahilan Anantha wrote:
> Hi List,
>
> I plan to use "dbclient" as a low memory footprint alternative to  
> OpenSSH's "ssh" for SSH tunnels.
>
> On the client I have software that creates SSH tunnels to many systems.  
> Sometimes the connection to these remote systems will break, at which  
> point "ssh" will exit. The exit gets detected and the connection gets  
> reestablished. But this works in "ssh" because I'm using the  
> ServerAliveInterval and ServerAliveCountMax options. Without them, ssh  
> would never check that the connection was up and I'd have to wait an  
> eternity for a TCP timeout. Or implement my own heartbeat on top of the  
> tunnel.

dbclient sends an "ignore" packet every N seconds, but I
don't think that elicits a server response. It will
generally time out after a minute or so when the client OS
gives up on receiving an ACK, though SIGSTOP is a funny
case since the remote OS is probably still sending TCP ACKs.
I'll take a look at implementing something closer to what
ServerAliveInterval does (sending something that will fail
and checking for a reply, iirc).

OpenSSH's "tcpkeepalive" just sets the TCP keepalive option
on the socket with setsockopt(), but won't probe the
connection itself.

Cheers,
Matt