mDNS: LabelIsNotAscii

[thearossman]

Summary

mDNS discovery sometimes fails with "parsing MDNS packet failed" message; may be an old mdns_parser issue in general?

Expected behaviour

I'd assume that all mDNS packets sent/received should be correctly parsed? And that I shouldn't see this error message?

Actual behaviour

mDNS sometimes works, and sometimes prints the debug message: DEBUG libp2p_mdns::behaviour::iface::query] Parsing mdns packet failed: LabelIsNotAscii.

After a threshold of errors, mDNS seems to begin behaving inconsistently -- e.g., expiring peers and erroneously disconnecting. (So, for example, if I'm using mDNS to do peer discovery for a GossipSub network, the network will eventually fail w/ insufficient peers or uptime will suffer.)

This first showed up on a separate project, but I was able to observe the same behavior running the chat-tokio example, which uses mDNS. I've reproduced this on multiple libp2p versions, including the most recent (0.44.0) and older (e.g., 0.39.1).

[2022-05-28T01:49:34Z DEBUG libp2p_mdns::behaviour::iface::query] Parsing mdns packet failed: LabelIsNotAscii
[2022-05-28T01:49:35Z DEBUG libp2p_mdns::behaviour::iface::query] Parsing mdns packet failed: LabelIsNotAscii
[2022-05-28T01:49:36Z DEBUG libp2p_mdns::behaviour::iface::query] Parsing mdns packet failed: LabelIsNotAscii

Possible Solution

This is an error from dns_parser here. Seems like others have encountered it before.

I've been working around with a high TTL and query interval.

Version

0.44.0, 0.39.1 (I've been able to reproduce this on multiple versions)

[mxinden]

Do these mdns packets come from libp2p hosts? If no, this should not affect libp2p-mdns.

[thearossman]

I believe they come from libp2p hosts -- we're not using anything else, and we were able to get the same error just by running the chat-tokio example with nothing else (https://github.com/libp2p/rust-libp2p/blob/master/examples/chat-tokio.rs).

[mxinden]

@thearossman can you use wireshark to post a dump of the original packet here?

[mxinden]

Friendly ping @thearossman.

[thearossman]

Thanks so much for the ping; apologies for dropping this.

This is from running the chat-tokio example with two hosts, running for about 30 seconds. You can see the MDNS error message; in this case, the hosts didn't disconnect from each other.

Partial log from one of the hosts (you can see the mdns error message): debuglog-libp2p-mdns.txt

Here's a pcap -- can't upload to Github; so it's on Google Drive here