Problems with NSD on multihome FreeBSD host

Erik Rozendaal erik at
Mon Apr 26 20:03:51 UTC 2004

Sebastian Castro wrote:
> Hi All:
> I'm still working in my FreeBSD server to make it work with NSD.
> I've already updated my ports with NSD 2.0.2 and it was pretty easy
> (thanks to Peter Hessler).
> My host has two network interfaces, with two IP addresses belonging to
> two different IP networks (one for national traffic, one for
> international traffic). Let's suppose first IP is X.X.X.X and second is
> Y.Y.Y.Y
> The default route is through first network interface.
> When I query to first interface, I got the expected answer.
> When I query to second interface, I got a query time out.
> Checking out with tcpdump, I've found that queries coming through 2nd
> interface (dst address Y.Y.Y.Y) went out through first interface (probably due to the default
> router) but using as IP source address X.X.X.X, so any firewall will not
> match the answer because is not "related".

This is the way UDP works on most (all?) systems when a single UDP 
socket is used for multiple interface.  To get correct multihoming 
source addresses from the server make sure NSD uses multiple UDP 
sockets, one for each interface, using the -a flag:

nsd -a X.X.X.X -a Y.Y.Y.Y

This way queries to the Y.Y.Y.Y interface will be received on the UDP 
socket for interface Y.Y.Y.Y and will also be responded to the Y.Y.Y.Y 
source address.


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