I have installed Dropbox on 2 Ubuntu machines and enabled LAN sync. I was curious how does one Dropbox instance detect another.
It looks like a UDP broadcast packet is sent over the LAN.
I found these links to support:
"we've experimented with various service discovery protocols, both standardized and our own. right now we're using UDP broadcast, but we're testing a few different ideas to see what'll work best for most people. the server is only involved in lansync negotiation to enforce our security model."
For example, if I look at a packet capture for my LAN I see "[source IP] 255.255.255.255 DB-LSP-Dropbox LAN Sync Discovery Protocol" every few seconds.
It does by sending UDP packets every 30 seconds. Check the post below ...
Dropbox LAN Sync Protocol (archived)