IPFW redirect to another port?

[John Levine]

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.