As part of the process of migrating as many instances as possible from public IPv4 addresses, because it's going to get expensive I have to use a proxy from the backend interface to github in order to pull updates from the repos. This is part of the distribution of the whole system apart from configuration not in the repos. To begin with I planned on adding a forward Proxy on the existing HAProxy on the front of the production web servers, it's in the same backend subnet and it's always going to have an IPv4 interface. But that got very ugly. I decided instead on using ssh and proxy through the sshd already running on the HAProxy instance. After a lot of complexity I went back to the simplest.
Host github.com
HostName github.com
AddressFamily inet
IdentityFile ~/.ssh/github_rsa
IdentityFile ~/.ssh/keyforproxyinstance
User git