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

Phillip Moore pdm at pobox.com
Fri Nov 1 15:48:50 GMT 2019


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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