[squid-users] squid 6.3: client internal ip address PTR DNS query

[Alex Rousskov]

On 2025-03-18 06:25, David Touzeau wrote:
> 
> We note that Squid performs a client DNS PTR query each time client 
> sends query.
> 
> We have taken care to ensure that
> 
>   * that the log model does not use machine names
>   * No acls concerning workstation hostnames are added.

FWIW, the phrase "workstation hostnames" is a red flag for me, 
especially when the other bullet uses "machine names" terminology. In my 
experience, it is easy to overlook a logformat %code or ACL that 
requires Squid to do a reverse DNS lookup.

N.B. In modern Squids (including your v6.3), default ICAP logformat 
triggers reverse DNS lookups if icap_log is enabled.


> We use kerberos authentication with Squid: is 
> negotiate_kerberos_auth/process plugin is able to perform PTR requests?

I am not a Kerberos expert, but I believe that plugin can trigger DNS 
requests at startup (at least). I do not know whether it can trigger DNS 
requests at runtime. You should be able to check that theory by 
disabling authentication for a test client/transaction.


> Is there another option that denies squid to perform such requests?

I do not think so. You have to figure out what triggers those queries 
and adjust the corresponding configuration accordingly. I can offer a 
free private review of your cache.log file collected while reproducing 
the problem using as few transactions as possible and enabling full 
debugging (e.g., setting debug_options to ALL,9). More hints are 
available at 
https://wiki.squid-cache.org/SquidFaq/BugReporting#debugging-a-single-transaction

If you would like to proceed with the above analysis, please email me a 
link to the corresponding compressed cache.log.


HTH,

Alex.