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I have installed Dropbox on 2 Ubuntu machines and enabled LAN sync. I was curious how does one Dropbox instance detect another.

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Probably a hello, handshake, and a package. Just like your UPS driver. –  surfasb Aug 18 '11 at 5:19

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up vote 4 down vote accepted

It looks like a UDP broadcast packet is sent over the LAN.

I found these links to support:

http://forums.dropbox.com/topic.php?id=12180

"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."

http://p0stmaster.blogspot.com/2011/01/dropbox-lan-sync.html

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.

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It does by sending UDP packets every 30 seconds. Check the post below ...

Dropbox LAN Sync Protocol (archived)

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The link you gave is dead. –  Garrett Fogerlie May 30 '13 at 8:44

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