IPFW redirect to another port?

Karl Denninger karl at denninger.net
Tue Jun 18 17:56:12 UTC 2019

On 6/18/2019 11:50 AM, John Levine wrote:
> I would like to use ipfw to redirect incoming port 53 requests from a
> few IP addresses to a different port, so I can use a custom DNS server
> to answer them.  I can figure out how to redirect their traffic to,
> say, port 5553, but the responses come from 5553 which of course
> doesn't work.
> Any suggestions about how to adjust the port numbers going both ways?  It's
> fine if they screw up other traffic to those IPs.  TIA.
The easiest way to do this is likely with an in-kernel NAT entry with a 
"redirect_port" stanza; that will "twist" packets going in both 
directions.  The most-common use for this is to take something on the 
external interface (e.g. and direct it at an internal host 
on port 80 (e.g.; the "redirect_port" stanza allows for 
both TCP and UDP redirection with both sides being translated.
-- Karl Denninger
/The Market-Ticker/
S/MIME Email accepted and preferred
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