Port 17500 TCP UDP

Dropbox LanSync Protocol

Dropbox LanSync (TCP/UDP 17500), used for LAN discovery and file Sync between Dropbox Clients.

Dropbox is a client application (software) installed on a machine, that allows files to be easily synced between two or more devices, this can be either locally on the LAN or via the internet and dropbox servers, it also keeps a backup of your files off-site within Dropbox.

The application uses port 17500 on both TCP and UDP, this is to perform two different functions.

17500 - UDP - Discovery

First UDP packets are sent to the local networks broadcast address on UDP 17500, any other DropBox clients on the LAN (part of the same Dropbox account) should listen and respond to this probe traffic.

The dropbox client should then be aware of all clients on the local network that have Dropbox installed, as a result when syncing files between these clients the files can pass locally, using less internet bandwidth and speeding up the whole process.

The nature of the broadcast traffic means that this is typically spotted within firewall logs.

17500 - TCP - File Transfer

When trasfering files, either to the Dropbox servers on the internet or two other local clients disovered using the above probe's, Dropbox clients will use TCP and port 17500 to transfer the raw data.

As this is TCP the traffic will be direct between two endpoints, as this is a file transfer the protocol may use excessive bandwidth.

HTTP/HTTPs - 80/443

Dropbox also have a web based interface that can be used to download a users files or admin the user account.


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External Links: Dropbox

Top Comments

Tom 5 months ago

Thank's James, I've updated the post with more detail on the difference between TCP/UDP

James 5 months ago

This TCP traffic is the actual data transfer for dropbox on a LAN, however they also use the same port on UDP to discover other clients.

James 5 months ago

This UDP traffic is normally just Dropbox trying to discover other clients on the network, you'll normally see that the destination address is the local broadcast address, hence why it shows up so often in firewall logs. The TCP traffic is files passing between two computers, and is therefore direct.

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