nsd users - Jun 2022 - Multi-homed server UDP packets wrong interface

Hi Robert,

Setting the outgoing IP address depends on the IP_PKTINFO socket
option. Currently NSD does not implement it, but given some time it can
certainly be made to do so.

NSD currently does support the bindtodevice socket option, which should
get you the desired result(?) The option causes NSD to bind the socket
to the device that has it assigned. It's an attribute of the ip-address
configuration option and you can set it like so:

    ip-address: 1.2.3.4  bindtodevice=yes

Let me know if it works for you. As for the IP_PKTINFO option, I've
created a GitHub issue: https://github.com/NLnetLabs/nsd/issues/217.

Regards,
Jeroen


On Thu, 2022-06-23 at 10:05 -0400, Robert Blayzor via nsd-users
wrote:
> 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 
> reply.
> ???????? # Socket options are not supported on all platforms.
> experimental.
> ???????? interface-automatic: yes
> 
> 
> 
> Is there such a knob in NSD? If not, can there be?
>

On 2022/06/28 09:50, Jeroen Koekkoek via nsd-users
wrote:
> Hi Robert,
> 
> Setting the outgoing IP address depends on the IP_PKTINFO socket
> option. Currently NSD does not implement it, but given some time it can
> certainly be made to do so.
> 
> NSD currently does support the bindtodevice socket option, which should
> get you the desired result(?) The option causes NSD to bind the socket
> to the device that has it assigned. It's an attribute of the ip-address
> configuration option and you can set it like so:
> 
>     ip-address: 1.2.3.4  bindtodevice=yes
Normally just listing the various ip-addresses separately is enough

The issue we had was some asymmetric routing where the packet coming 
into the server didn't always match the interface going out. So the 
reply would be sent from a different source address.

I think with Unbound it will source the IP on the interface it received 
the request on, or at least that knob exists. With NSD it's possible the 
UDP reply comes from the wrong source address. TCP works fine.

I did work around this by limiting which interfaces NSD was bound to, 
but that's only part of the solution. It would use the correct SOURCE 
IP, but the OS would still try to use the routing table and send it out 
the wrong interface.

I know, it's a unique case. This was on a FreeBSD server, so the final 
solution was to use IPFW to set the next-hop manually rather than rely 
on the default route.


On 6/28/22 03:50, Jeroen Koekkoek wrote:
> NSD currently does support the bindtodevice socket option, which should
> get you the desired result(?) The option causes NSD to bind the socket
> to the device that has it assigned. It's an attribute of the ip-address
> configuration option and you can set it like so:
> 
>      ip-address: 1.2.3.4  bindtodevice=yes
> 
> Let me know if it works for you. As for the IP_PKTINFO option, I've
> created a GitHub issue:https://github.com/NLnetLabs/nsd/issues/217.