[tor-dev] Detecting if a IP address belongs to a Tor Exit node.

Karsten Loesing karsten at torproject.org
Thu Dec 6 07:39:09 UTC 2012

>>>> Why not just run and query an Onionoo server?
>>> Onionoo isn't really optimised in regards to giving out lists of
>>> exits, the parsing of the JSON sounds like a duplicate effort to me.
>>> Also, shipping Onionoo with every facilitator seems a bit overkill.

It's not Onionoo's primary purpose to give out lists of exit addresses,
but it provides that information, too.  It just doesn't offer good query
parameters for that use case.  But I think you should do okay
downloading the full set of relay summaries once per hour and cache that
data locally.  The URL is:


The protocol specification is here:


I wouldn't recommend running your own Onionoo server, particularly not
on every facilitator.  But if you cache results, you don't really have
to do that.

> Is running a Tor client really so heavyweight? Let me explain more about
> what we're trying to do. The facilitator needs to know, for each
> request, whether the requestor is a Tor exit.

A Tor client won't tell you that, or at least not very reliably.  The
reason is that some relays are multi-homed, using different IP addresses
for registering in the network (which is what the Tor client would tell
you) and for exiting to the Internet.

If you want to learn about both network-internal and external IP
addresses, you want to download TorDNSEL's exit list.  The URL is:


Metrics archives these exit lists and has a format description here:


> I read that Onionoo reads its information
> from metrics; where does metrics get the data from?

Metrics aggregates data from network statuses, relay descriptors, and
TorDNSEL exit lists, among others.

So, in summary, I could see you using either TorDNSEL's original data or
Onionoo's summaries.

If you decide to use Onionoo, we should add Flashproxy to the list of
known clients on Onionoo's project page, so that we who we have to
contact when we're planning changes to Onionoo's protocol.


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