Re: [tor-relays] General overload -> DNS timeouts

Hi Ibex

There's some discussion about this issue on #tor-relays. First of all, I actually don't think there's any problem with your Exit node. IMO 1.5% is expected and the current threshold is just too low (or the timeout too low).

We're hosting some fairly high capacity exit nodes (around 100mbit/s each), generating about 100 DNS queries/sec in total. We're also seeing around 1.5% failed DNS queries.

Here's what I think is going on:

For some queries (1.5%?), the authoritative DNS servers for the requested domain are not responding. Recursive DNS resolvers are by default more patient than the Tor software. Eventually the recursive resolver will return a 'SERVFAIL' to Tor, but by then, Tor has already given up, and counts it as a timeout.

One example of a domain that is currently (2021-11-08T23:40+01:00) failing to resolve because of its authoritative DNS servers being down is mzfgbh.com. Try running `dig +trace +additional mzfgbh.com` (for some reason, dig ignores the 'additional' section of one of the answers. Essentially the problem is that this query (and a few others) times out: `dig mzfgbh.com @1.1.1.20`).

On a related note, when this metric was introduced, we were seeing around 5-6% failed queries. The recursive DNS resolver we were using was hosted on the same IP as a guard node. Apparently many authoritative DNS servers block traffic from all Tor relays!

The most extreme example of this I found, is that the authoritative DNS servers for the entire .by ccTLD (Belarus) are blocking DNS requests from guard and exit nodes. This resulted in all <domain>.by queries timing out!

By moving our recursive DNS resolver to an IP not used by any Tor relays, the DNS timeouts fraction reported by Tor dropped from 5-6% to 1.5%.

The Tor relay guide should recommend running your recursive resolver (unbound) on a different IP than your exit: https://community.torproject.org/relay/setup/exit/

- Anders Trier Olesen

On Sat, Nov 6, 2021 at 5:53 PM Intrepid Ibex via tor-relays <tor-relays@xxxxxxxxxxxxxxxxxxxx> wrote:

Hi everybody,

I am new to tor (as server operator) and try to support the network by operating an exit node.

Machine is running Ubuntu 20.04 - Tor 0.4.6.8.

nyx is giving me a notive every 10 minutes or so: [NOTICE] General overload -> DNS timeouts (6) fraction 1.4742% is above threshold of 1.0000%

DNS on this machine however works perfectly. I told my tor browser to use my specific exit node, everything works fine.

Node is running since 4 days now. Load is as expected.

Thanks in advance for any help!

Ibex

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