Caching APT updates

It’s bothered me for a while that all these VM’s are pulling down a lot of the same updates. As well as needlessly using some bandwidth, I’m hammering the update servers (that I don’t pay for) with the same requests over and over. I did briefly consider running my own mirror, but that’s not simple, plus I’d then be mirroring a heap of files in a complete repository that I’d never use. What I really needed was some sort of cache so once I’ll pulled down an update, it would hang around for a few days being available to other machines on the local network. Luckily, that exact thing exists – APT Cacher NG.

It works pretty much as described above – all of the machines on the LAN have their APT calls proxied through a little server. If the server doesn’t have a copy of the appropriate package, it pulls it down and delivers it. If it’s got a good copy already, it just provides that.

Installing the server

I decided an unprivileged LXC container would be the perfect base for this service. I created one from the Debian 12 image with 1MB RAM but a largish 30GB drive. I don’t really have any feel for how big the cache will get under normal use so I erred on the large side. It’s not doing any computationally expensive work, so one CPU is plenty.

Then we just install it, and start and enable it as a service.

apt install apt-cacher-ng
systemctl start apt-cacher-ng
systemctl enable apt-cacher-ng

During the install, it asked me about https:

I said no, but then to enable it, I had to (after the installation) edit the config file at /etc/apt-cacher-ng/acng.conf to uncomment the line

PassThroughPattern: .* # this would allow CONNECT to everything

With that done, and the service restarted, we’re now serving proxies at localhost:3142, and also a little web page with some advice.

Installing the client

The two bits of information I’ve put red boxes around are the things we need to do to enable apt on the client machines to use the cache. We need to create a file called /etc/apt/apt.conf.d/00aptproxy and add the single line to it of:

Acquire::http::Proxy "http://192.168.100.37:3142";

Note that the ip address will be different on yours, just copy it off the little web page. Since I’ve got a heap of machines to do this do, I made the conf file once and pushed out out with Ansible.

The hosts: local in the pkaybook refers to the local: children group in my hosts ini file.

Statistics

If you’re curious to see what the savings are, there’s another web page served by the cache at <server ip>:3142/acng-report.html

Further down on that page are some options that can be changed as well.

Resources

I learned most of this from this video by RickMakes, and this Linux Help page.