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Transparent Squid in a container

This is a trivial Dockerfile to build a proxy container.
It will use the famous Squid proxy, configured to work in transparent mode.

Why?

If you build a lot of containers, and have a not-so-fast internet link,
you might be spending a lot of time waiting for packages to download.
It would be nice if all those downloads could be automatically cached,
without tweaking your Dockerfiles, right?

Or, maybe your corporate network forbids direct outside access, and require
you to use a proxy. Then you can edit this recipe so that it cascades to the
corporate proxy. Your containers will use the transparent proxy, which itself
will pass along to the corporate proxy.

How?

docker run --net host jpetazzo/squid-in-a-can
iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to 3129

That's it. Now all HTTP requests going through your Docker host will be
transparently routed through the proxy running in the container.

Note: it will only affect HTTP traffic on port 80.

Note: traffic originating from the host will not be affected, because
the PREROUTING chain is not traversed by packets originating from the
host.

Note: if your Docker host is also a router for other things (e.g. if it
runs various virtual machines, or is a VPN server, etc), those things
will also see their HTTP traffic routed through the proxy. They have to
use internal IP addresses, though.

Note: if you plan to run this on EC2 (or any kind of infrastructure
where the machine has an internal IP address), you should probably
tweak the ACLs, or make sure that outside machines cannot access ports
3128 and 3129 on your host.

What?

The jpetazzo/squid-in-a-can container runs a really basic Squid3 proxy.
Rather than writing my own configuration file, I patch the default Debian
configuration. The main thing is to enable intercept on another port
(here, 3129).

Then, this container should be started using the network namespace of the
host
(that's what the --net host option is for).
Another strategy would be to start the container with its own namespace.
Then, the HTTP traffic can be directed to it with a DNAT rule.
The problem with this approach, is that Squid will "see" the traffic as
being directed to its own IP address, instead of the destination HTTP
server IP address; and since Squid 3.3, it refuses to honor such requests.

(The reasoning is, that it would then have to trust the HTTP Host:
header to know where to send the request. You can check CVE-2009-0801
for details.)

Notes

Ideas for improvement:

  • persistent caching (with, obviously, a volume!)
  • easy chaining to an upstream proxy
  • setup the iptables automatically if the container runs in privileged mode
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