Howto force Nginx resolver to use new source UDP port

Maxim Dounin mdounin at
Thu Nov 17 20:20:56 UTC 2022


On Thu, Nov 17, 2022 at 05:17:06AM -0500, jeanpaul wrote:

> We're using Nginx as ingress controller in a Kubernetes environment. Nginx
> uses a DNS service inside k8s with 1 single service-IP and multiple DNS PODs
> behind that.
> The translation from service-IP to any of the multiple DNS PODs is done via
> DNAT rules.
> The problem:
> - Nginx establishes an UDP 'connection' on for example localIP
> source-port 12345 towards serviceIP
> - DNAT translates this as sourceport 12345 towards any DNS-POD; for example
> - If DNS-POD terminates Nginx is not aware of this because of the
> 'connection' towards serviceIP
> - Nginx will send all future DNS request using the same tuple of
> serviceIP:sourcePort, so ->
> - This will result in a timeout because the destination POD isn't there
> anymore
> Is it possible to force Nginx to use different source UDP ports every X
> seconds, of after a timeout?

No, nginx will continue to use the same source port unless there 
will be an error when sending a UDP packet, which is not going to 
happen in the described setup.

Note that even if there will be a way to force nginx to use 
difference source ports every X seconds, this will anyway result 
in downtime till the next source port change if the DNS POD 
terminates.  This suggests that it might be a good idea to rethink 
how DNS balancing works in your setup.

Maxim Dounin

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