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

Nicolas Williams Nicolas.Williams at
Fri Sep 3 13:41:18 GMT 1999

On Fri, Sep 03, 1999 at 09:08:04AM +0200, Mattias Gronlund wrote:
> Nicolas Williams wrote:
> > 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.
> But how do you set the keepalive-parameter per connection?

With the 'keepalive' smb.conf parameter. I don't know how well it works
by itself however.

NOTE: This is NOT the SO_KEEPALIVE option. You cannot rely on
      SO_KEEPALIVE achieving what you want.

> Is the script tested with multiuser Windows NT systems?

The script runs on the Samba server. I quoted the URL for my original
e-mail about this to the list; that e-mail includes the root
preexec/postexec commands I used and the script in question (I should
post a newer version of it). Read that. Basically, the script is called
with a number of arguments (% token substitutions) about the share
connection and it creates a pid/lock file named after those arguments.
These pid/lock files are used to detect stale smbd processes; it works
because the script's arguments identify the session/share connection in
question accurately.

> Your analysis sounds right, but I think that because we think that the NT-
> client is aggressive in reconnecting (we should check the exact timeout) we
> could just say that it is just hopeless to wait for longer than that in recv.

NT clients reconnect after a 45 second timeout.

> If the PC do not alvays reconnect after a short timeout, we should either
> process oplock-requests when waiting and give up if someone needs one
> of our locks. But I think we will find a timeout and not have to do any
> more advanced oplock-handling...

It's not just oplocks though. Share modes and plain locks are involved
as well. SMB relies on the client to keep lock state across failures; it
is up to SMB clients to attempt to restablish any locks they may have
been holding prior to any [network or server] failure. I like this
approach, actually.

Samba needs a way to deal with these stale smbd processes. I'm still not
exactly clear on what goes on that causes Samba to block on a socket,
that ought to be dead, waiting for input; I've not spent enough time
tracing the packets or the smbd processes so my analysis is partly based
on guessing (I had no idea about the FIN-FIN/ACK bug when I sent my very
first e-mail about this to the list); it could even be that there's a
bug in the way the NT clients abandon the old connection (i.e., maybe
they don't explicitly close it) or maybe there's a bug in NT's TCP/IP
stack that causes TCP shutdown to not be reliable.

Whatever the case I do know this: the solution I use works.

> At the moment I have a fix only in one-place that timeouts after 5 minutes
> that seems to keep us from getting hurds of smbd:s for a user...

Yeah. Five minutes works. When I foudn this problem I determined, during
testing, that with no workarounds the system recovers withing 12-15
minutes (i.e., the stale smbd processes take that long to figure out
that they ought to exit).

> /Mattias

We've only got experience with Samba running on Solaris, so the above
might only apply to Solaris. I wonder what others' experiences on other
platforms have been.

Nicolas Williams	(x5220, Stamford, CT)
Stamford SysAdmin

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