[tor-project] Ongoing DDoS on the Network - Status

[David Goulet]

Greetings,

Earlier this month, many relay operators started noticing huge loads on their
relays both in terms of traffic and memory consumption leading to relays
malfunctionning or even dying in some cases.

We've started looking at this in depth in the last few days. It turns out that
many relays (not all) are under a distributed denial of service (DDoS) attack
which makes them use a lot of memory ultimately making the operating system
stop the process or becoming unreliable because of the resource pressure.

This has lead to some relays to restart, being shutdown or becoming so
unstable that they would fall in and out of the network. You can see here on
the Metrics portal the consequences of this ongoing attack:

https://metrics.torproject.org/relayflags.html?start=2017-09-20&end=2017-12-20&flag=Running&flag=Exit&flag=Fast&flag=Guard&flag=Stable&flag=HSDir

Among other things, it is badly affecting relays with the HSDir flag because
once they restart, it takes 96 hours before they get the flag back. This
affects the reachability of hidden services and thus the UX of .onions.

We've been analyzing some relays being flooded to understand what is going on
and how to fix it. The good news is that we are fairly confident that we know
what is happening and we are currently testing some fixes to address the
situation.

In the meantime, if your relay is under heavy memory pressure that is tor is
taking a huge amount of RAM making your machine fail to operate properly, you
can set the MaxMemInQueues option in your torrc file to a reasonable upper
limit which limits the amount of memory used by tor. At least 2GB if you can
for a fast relay is usually a good value for tor to operate properly and not
degrading performance too much.

With this, if the memory usage reaches that limit, tor's OOM (Out Of Memory
handler) will kick in and cleanup what it can. It is still possible that your
relay goes above the limit, it is one of the thing we are currently
investigating. However, it should not grow indefinitely.

Thanks everyone and we'll hopefully resolve the situation soon!
David

-- 
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