[nsd-users] Multi-homed server UDP packets wrong interface

Robert Blayzor rblayzor.bulk at inoc.net
Thu Jun 23 14:05:16 UTC 2022

We have hosts that are multi-homed on IPv6 networks and having an issue 
with NSD sourcing out UDP reply packets via a different interface/IP 
than the query was received on.

This obviously will cause issues with clients behind firewalls, etc. 
Looks as though NSD just sources the packet on the egress interface 
based on the host routing table. So the reply is coming back from a 
different IP address.

With TCP this isn't an issue....

Coincidentally Unbound has a option to toggle this behavior....

         # enable this feature to copy the source address of queries to 
         # Socket options are not supported on all platforms. experimental.
         interface-automatic: yes

Is there such a knob in NSD? If not, can there be?

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