Tuning and hardening Squid

Tuning and hardening Squid will be the topic of this post, where tuning means making it a little bit faster and hardening means less vulnerable to malicious use. The default installation of Squid on a Debian box has a lot of features enabled which most likely aren’t used: we want to turn these off. Then there might be situations where you probably want to use Squid but don’t want it to function as a cache: we’ll investigate this too.

This post is geared towards my next post: it’s about a tiny router which hasn’t got a great disk or plenty of RAM. So I’m not going to discuss the pros and cons of various filesystems, using a RAID or having enough filedescriptors. If you’d like to read about that go here, here or here.

Tuning

Tuning Squid will speed things up a little bit. So without further ado lets first take a look a the directives for the squid.conf:

pipeline_prefetch on
shutdown_lifetime 1 second

While pipeline_prefetch will boost the performance of pipelined requests to closer match that of a non-proxied environment, the second directive shutdown_lifetime saves you a lot of time waiting for Squid to shut down. The latter comes in very handy if you’re tweaking Squid and need to restart it a lot.

Even though Squid is meant as a cache there are reasons running it without a cache, i.e. as a pure forwarding proxy: you might want to use it as a load balancer with some parent proxies, simply as a transparent proxy or you don’t have particularly fast hardware. There are two methods to circumvent caching:

  1. Deny caching for all connections:
    acl all src 0.0.0.0/0.0.0.0
    no_cache deny all

    This way neither a request will be satisfied from the cache nor the reply will be cached. Note that the first line might already be in your configuration.

  2. If you use a parent proxy you can specify the proxy-only option to prevent that retrieved data from the remote cache is stored locally. An example:
    cache_peer proxy.isp.com parent 8080 0 proxy-only

Finally you might want to turn off logging. On a Debian based system it’s sufficient to turn of cache_access_log and cache_store_log:

cache_access_log none
cache_store_log none

Hardening

When talking about hardening I think about turning off features that aren’t used and restricting access to the proxy. Features that aren’t used might be ICP and HTCP: they are used to communicate with other caches in a hierarchy. In most cases we don’t need this:

icp_port 0
htcp_port 0
icp_access deny all
htcp_access deny all

If you don’t wish to use SNMP we can disable this too. This is already the default for systems running Debian.

snmp_port 0
snmp_access deny all

At last you definitely want to restrict access to your proxy: define an access control list (acl) and either allow or deny access with http_access. Lets say your LAN is 172.16.0.0/24 and 172.16.1.0/24. Then you would put the following into squid.conf:

acl LAN src 172.16.0.0/24 172.16.1.0/24
http_access allow LAN

If somebody outside your network tries to access your proxy he’ll get an error message that he isn’t allowed to do so.

Conclusion

Of course, this post just outlined the tip of the iceberg. There’s so much more to explore when it comes to tuning and hardening. Some ideas might include evaluating your efforts – is the cache really faster now? – or looking at other points in your network regarding security like a firewall. Anyway, I hope you got an overview of all the possibilities.

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