IPv4 client is extremely slow but IPv6 seems fine

It’s my understanding that the Tor network is currently being hit by another DDoS attack of some kind. My tor connection has been slow for the past few weeks, and today it was unusable almost the whole day. I wonder if it is related to the surge in relay bandwidth followed by the Ator ban? Either way, it reminds me of the DDoS from last year, although that was before PoW defenses were in place.

Anyway, I did some playing around today and I noticed something interesting. My Tor clients configured with “ClientUseIPv4 1” and “ClientUseIPv6 0” are terribly slow, but when I swap them (i.e. configure for IPv6 only), the problem suddenly goes away. I’m talking connection timeouts on IPv4, and up to 1.5MB/sec on IPv6. I tried it on two different machines and got the same result.

It’s worth noting I was using the same internet connection throughout, and regular (non-Tor) IPv4 traffic doesn’t seem impeded on my network, so I’m inclined to rule out any WiFi- or ISP-related explanations.

I was originally thinking maybe the DDoS is only targeting IPv4 relay addresses, but there can’t be more than a few IPv6-only relays (that is, relays with no IPv4 ORPort), if any, right?

Can anyone explain this, or reproduce it?

(PS: if you’re using the Tor Browser Bundle, you can edit the torrc located at tor-browser_xx-XX/Browser/TorBrowser/Data/Tor/torrc)

I think I figured it out. It seems to be two-part problem.

Part 1: My network uses asymmetric routing, and due to a subtle misconfiguration, IPv4 reply packets with high round trip time were getting lost at a high rate. The longer it took for a reply to come in, the more likely it would be dropped. IPv6 traffic and normal-latency IPv4 traffic were unaffected however.

Part 2: The recent attacks on the Tor network caused a higher-than-usual delay in reply packets from Tor routers. Combined with part #1, this meant a higher than usual amount of packet loss on my network.

Luckily it was an easy fix once I found it. I’m now getting about 1.5MB/sec on both IPv4 and IPv6. Sorry for the false alarm everyone.

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