[lvs-users] source hashing some times land on wrong server (with FTP)

Phillip Moore pdm at pobox.com
Fri Nov 1 18:40:20 GMT 2019


On Fri, Nov 1, 2019 at 12:39 PM Malcolm Turnbull <malcolm at loadbalancer.org>
wrote:

> Are you getting any health check failures?
> It's not a consistent hash - so any config changes would throw it


There do not seem to be any health check failures that correspond with the
incident. That was my thought to especially because of the sh-fallback
option in use.

>
>
Why not just use the standard persistence table?
> The stick table for that is completely reliable, as long as you have
> enough memory.
>

Can you elaborate on what you mean by this?  I was using source hash (sh)
on :0 to ensure that when a client connects back on the data port, it would
be hashed to the same place it was sent for the ftp control port. This
seems to work most of the time as most ftp sessions work. More than chance
anyway since we have 4 ftp servers behind the VIP and if the hashing was
not working I would expect it to only succeed randomly 25% of the time.

 So I'm not sure how this would work or what you mean by the standard
persistence table.

Thank you for the suggestions.

Phillip


>
>
> On Fri, 1 Nov 2019 at 15:49, Phillip Moore <pdm at pobox.com> wrote:
> >
> > Hello!
> > We have FTP setup with on its own VIP and just map all ports (:0) and use
> > source hashing. Sometimes when the FTP client opens the data channel it
> > will land on the wrong real server causing a reset. I stress sometimes
> > because mostly FTP seems to work but we do see this behavior of requests
> > landing on the wrong server.
> >
> > FTP client makes connection to VIP:0 on ftp port, is asked to open data
> > channel on VIP:0 on alternate port. FTP client sends SYN packet but that
> > packet doesn't land on the correct real FTP server, so connection is
> > reset.  That SYN packet likely came through a different IPVS server but
> > should have sync connection state by this time.
> >
> > Example of our config:
> >
> > -A -t x.y.z.220:0 -s sh -p 600 -b sh-fallback
> > -a -t x.y.z.220:0 -r a.b.c.4:0 -i -w 1
> > -a -t x.y.z.220:0 -r a.b.c.5:0 -i -w 1
> > -a -t x.y.z.220:0 -r a.b.c.6:0 -i -w 1
> > -a -t x.y.z.220:0 -r a.b.c.7:0 -i -w 1
> >
> > 3.10.0-1062.1.1.el7.x86_64
> >
> > We have this config running on multiple active IPVS servers all running
> > active/backup sync processes .
> >
> > We've also tried a non 1 weight (1000) to see if it was the overload
> logic
> > kicking in and sending requests to alt server, but that did not seem to
> be
> > it.
> >
> > Is there any reason why subsequent connections from the same source IP
> > would land on a different server?
> >
> > Thanks,
> > Phillip Moore
> > _______________________________________________
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>
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