[dnsdist] dnsdist using loopback address as source address for queries

[Adam Bishop]

Good Evening,

After running for some amount of time (seems to be days), our dnsdist instances suddenly start trying to talk to the backends using the loopback address as the source:

    # tcpdump -i ens192 -nn port 53
    dropped privs to tcpdump
    tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
    listening on ens192, link-type EN10MB (Ethernet), capture size 262144 bytes
    22:39:07.014963 IP6 <snip>:ac10:0:ac10:2e.64975 > <snip>::197.53: 35980+ [1au] SOA? lbdn.domain. (45)
    22:39:07.015390 IP6 ::1.38717 > <snip>::195.53: 43034 [1au] SOA? lbdn.domain. (69)

Note this is not the loopback interface - packets are being placed on the wire and fired off into the network with ::1 as the source address. This is affecting all our instances, but they don't fail simultaneously.

Bizarrely, this only affects queries made by clients - the backend health check still uses the correct source address while this is going on. Restarting dnsdist brings them back into service.

I think I can work around this by setting an explicit source IP for each backend - I'm suspecting that trying to talk to backends with the return address set to ::1 is probably a bug though!

We're running the current master branch builds on RHEL8.

Adam Bishop

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