Installing from PPAs behind a proxy

You are building a Dockerfile or trying to run apt-get at work, but the network policy is blocking both direct access to the keyserver and through a correctly-configured proxy.

Let's say we're trying to install mono, which has the following instruction - but it hangs inexplicably.

sudo apt-key adv --keyserver \
hkp:// \
--recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF

Take note of the key specified in --recv-keys, this is the part we need to copy/paste into the next steps.

The work-around (tl;dr):

  • Step 1: Log into a computer in the free world (VPN/tunnel/mobile-hotspot/open WiFi network etc).

  • Step 2: Export the key to a text file.

  • Step 3: Copy the contents of the file across to the machine, and import it.

Step 1:

$ export keyid=3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
$ gpg --keyserver --recv $keyid && gpg --export --armor $keyid > key.txt

gpg: requesting key D3D831EF from hkp server
gpg: key D3D831EF: "Xamarin Public Jenkins (auto-signing) <>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

Alter the value of keyid as per your needs

Step 2

The file key.txt can now be copied across to your locked-down machine by email, copy/paste, etc into a file named key.txt.

Step 3

cat key_file | sudo apt-key add -

If you are doing this inside a Dockerfile, then you could use the ADD command to put the key in /tmp/.

Now continue with the rest of the instructions and the apt-repository will now be accepted by the system.