Hotline

One of the oldest file sharing programs on the Net, Hotline is still in use today, and I recently had the opportunity to confirm that. Even though the parent company (who originally intended to sell the product as a BBS/collaboration tool) seems to have gone under, there has been a resurgence in its use that

Hotline uses (at least) TCP 5500, but recent versions (official 1.8.x and some third-party clones) have implemented both SOCKS and HTTP tunnelling support, so it is harder to spot and block (but you can configure Squid to only proxy "normal" HTTP requests).

Generally, accesses to Tracker-Tracker in your proxy log files are a good hint that someone is using either Hotline or Carracho (reminder: set up a Snort signature for this).

Hotline was traditionally a single-sourced application, but there have been recent efforts in cloning the protocol (fortunately dampened by the emergence of other file-sharing tools). A simple Google search revealed the following:

• Pitbull Pro
• Fidelio is a Gnome/GTK implementation of Hotline, and its sources include a nearly complete protocol stack.
• KDX, the program I originally spotted
• hxd (another OpenSource implementation)
• VIVAHX! (RDF) - an apparently thriving news site that covers current Hotline efforts