NAT devices not translating privileged ports

Fri Jun 4 16:13:59 UTC 2021

Current gen Cisco ASA firewalls have logic so that if the connection 
from a private host originated from a privileged source port, the NAT 
translation to public IP also uses an unprivileged source port (not 
necessarily the same source port though).

I found out that this behavior can cause issues when you have devices on 
your network that implement older DNS libraries or configs using UDP 53 
as a source and destination port for their DNS lookups. Occasionally the 
source port gets translated to one that ISC BIND servers have in a 
blocklist (chargen, echo, time, and a few others) and the query is 
ignored. As I recall, this behavior is hard coded so patching and 
recompiling BIND is required to work around it.

I forget what the older ASA behavior was. It may have been to leave the 
source port unchanged through the NAT process (I think this is what you 
mean by "not translated"). In that case the client doesn't implement 
source port randomization and the NAT doesn't "upgrade" the connection 
to a random source port so I don't really see it as an issue. Ideally 
the client would implement source port randomization itself so it would 
be using source ports within its ephemeral port range for outgoing 
connections.

--Blake

On 6/4/2021 7:36 AM, Jean St-Laurent via NANOG wrote:
> I believe all devices will translate a privileged ports, but it won't translate to the same number on the other side. It will translate to an unprivileged port. Is it what you meant or really there are some devices that will not translate at all a privileged port?
>
> What are you trying to achieve?
>
> Jean
>
> -----Original Message-----
> From: NANOG <nanog-bounces+jean=ddostest.me at nanog.org> On Behalf Of Fernando Gont
> Sent: June 4, 2021 3:00 AM
> To: nanog at nanog.org
> Subject: NAT devices not translating privileged ports
>
> Folks,
>
> While discussing port randomization (in the context of https://www.ietf.org/archive/id/draft-ietf-ntp-port-randomization-06.txt
> ), it has been raised to us that some NAT devices do not translate the source port if the source port is a privileged port (<1024).
>
> Any clues/examples of this type of NATs?
>
> Thanks!
>
> Regards,
> --
> Fernando Gont
> Director of Information Security
> EdgeUno, Inc.
> PGP Fingerprint: DFBD 63E3 B248 AE79 C598 AF23 EBAE DA03 0644 1531
>
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