Stale smbd processes (was: DOS: Clients can freeze other clients smbd)

[Nicolas Williams]

I've posted about this once before.

http://us1.samba.org/listproc/samba/January1999/0355.html

Basically, NT SMB clients are very aggressive in reconnecting to SMB
servers in the face of network timeouts all the while smbd may fail to
notice that the TCP socket in question is dead.

This shouldn't happen as TCP should recover from any loss of FIN or
FIN/ACK packets. However I remember a Solaris bug where Solaris would
respond with two packets (ACK, then FIN/ACK) to FIN packets that have
more than 0 bytes of data; though this is technically legal behaviour,
some stacks respond by resending the FIN + data packet (as opposed to
just a FIN), thus entering an infinite retransmission loop and the
socket never closes.

Meantime, the NT clients that reconnect cause a new smbd process to be
spawned for them. When the reconnecting client attempts to obtain locks
it already had, smbd blocks while waiting for the stale smbd to give up
the locks in question, and since the stale smbd is waiting for the
client (which considers the connection closed) to respond to oplock
break requests (or what have you) you get a DEADLOCK.

There's two ways around this problem:

 - set the 'keepalive' parameter to a sufficiently small value

 - run a script via 'root preexec' and 'root postexec' to check for and
   kill smbd process[es] that were serving the same client.

I use both approaches (they are not mutually exclusive).

My analysis of the problem may not be correct or very accurate and I'm
writing from memory here, but the solution works fine for me.

Enjoy,

Nico
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