[squid-users] Recommended squid settings when using IPS-based domain blocking

[Alex Rousskov]

On 2024-03-06 09:48, Jason Marshall wrote:

> We have been using squid (version squid-5.5-6.el9_3.5) under RHEL9 as a 
> simple pass-through proxy without issue for the past month or so. 
> Recently our security team implemented an IPS product that intercepts 
> domain names known to be associated with malware and ransomware command 
> and control. Once this was in place, we started having issues with the 
> behavior of squid.
> 
> Through some troubleshooting, it appears that what is happening is that 
> that when a user's machine make a request through squid for one of these 
> bad domains, the request is dropped by the IPS, squid waits for the DNS 
> timeout, and then all requests made to squid after that result 
> in NONE_NONE/500 errors, and it never seems to recover until we do a 
> restart or reload of the service.


DNS errors, including DNS query timeouts, are common, and Squid is 
supposed to handle them well. Assuming the DNS server is operational, 
what you describe sounds like a Squid bug. Lots of bugs were fixed since 
Squid v5.5, but I do not recall any single bug that would have such a 
drastic outcome.

Squid v5 is not supported by the Squid Project. I recommend upgrading to 
the latest Squid v6 and retesting.


HTH,

Alex.


> Initially the dns_timeout was set for 30 seconds. I reduced this, 
> thinking that perhaps requests were building up or something along those 
> lines. I set it to 5 seconds, but that just got us to a failure state 
> faster.
> 
> I also found the negative_dns_ttl setting and thought it might be having 
> an effect, but setting this to 0 seconds resulted in no change to the 
> behavior.
> 
> Are there any configuration tips that anyone can provide that might work 
> better with dropped/intercepted DNS requests? My current configuration 
> is included here:
> 
> acl localnet src 0.0.0.1-0.255.255.255  # RFC 1122 "this" network (LAN)
> acl localnet src 10.0.0.0/8 <http://10.0.0.0/8>             # RFC 1918 
> local private network (LAN)
> acl localnet src 100.64.0.0/10 <http://100.64.0.0/10>          # RFC 
> 6598 shared address space (CGN)
> acl localnet src 169.254.0.0/16 <http://169.254.0.0/16>         # RFC 
> 3927 link-local (directly plugged) machines
> acl localnet src 172.16.0.0/12 <http://172.16.0.0/12>          # RFC 
> 1918 local private network (LAN)
> acl localnet src 192.168.0.0/16 <http://192.168.0.0/16>         # RFC 
> 1918 local private network (LAN)
> 
> acl localnet src fc00::/7               # RFC 4193 local private network 
> range
> acl localnet src fe80::/10              # RFC 4291 link-local (directly 
> plugged) machines
> 
> acl SSL_ports port 443
> acl Safe_ports port 80          # http
> acl Safe_ports port 443         # https
> acl Safe_ports port 9191        # papercut
> http_access deny !Safe_ports
> http_access allow localhost manager
> http_access deny manager
> 
> http_access allow localnet
> http_access allow localhost
> http_access deny all
> http_port 0.0.0.0:3128 <http://0.0.0.0:3128>
> http_port 0.0.0.0:3129 <http://0.0.0.0:3129>
> cache deny all
> coredump_dir /var/spool/squid
> refresh_pattern ^ftp:           1440    20%     10080
> refresh_pattern ^gopher:        1440    0%      1440
> refresh_pattern -i (/cgi-bin/|\?) 0     0%      0
> refresh_pattern .               0       20%     4320
> debug_options rotate=1 ALL,2
> negative_dns_ttl 0 seconds
> dns_timeout 5 seconds
> 
> Thank you for any help that you can provide.
> 
> Jason Marshall
> 
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