An asynchronous Internet in GNOME

squid-deb-proxy does this, with squid-deb-proxy-client for mDNS-based autoconfiguration.

It is full of pain[*][+], so I'm not surprised nobody installs it by default.

[*] like shipping a default config file that causes the nigthly cron script to send errors to root because the default list of exceptions happens to be empty
[+] like rejecting all 3rd-party repositories (including PPAs) by default with an unhelpful 403 Forbidden error that apt shows as originating from the real upstream mirror and don't let me start counting the hours I lost while debugging this