fix(mdns): filter addresses to reduce packet size #3434
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Filter mDNS advertised addresses to help stay within 1500-byte MTU: - Skip browser transports (WebTransport, WebRTC, WebSocket) - Skip circuit relay addresses - Skip non-.local DNS names (require unicast DNS) - Allow IP addresses and .local DNS names (RFC 6762) Fixes: #3415
This was referenced Dec 1, 2025
MarcoPolo
reviewed
Dec 2, 2025
| switch first.Protocol().Code { | ||
| case ma.P_IP4, ma.P_IP6: | ||
| // Direct IP addresses are always suitable for LAN discovery | ||
| case ma.P_DNS, ma.P_DNS4, ma.P_DNS6, ma.P_DNSADDR: |
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Does DNSADDR make sense here?
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Added it mostly for completeness, but yes, technically someone could have TXT record on _dnsaddr.foo.local and use it for signaling peerid similar to how one can ipfs swarm connect /dnsaddr/bootstrap.libp2p.io without knowing peerid upfront (or connecting to more than one peer)
MarcoPolo
reviewed
Dec 2, 2025
p2p/discovery/mdns/mdns.go
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| // WebRTC, WebSocket) because these are not useful for direct LAN discovery. | ||
| // | ||
| // Filtering reduces mDNS packet size, helping stay within the 1500-byte MTU | ||
| // limit per RFC 6762. See: https://github.com/libp2p/go-libp2p/issues/3415 |
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A slight correction in this comment. The RFC states the packet "MUST NOT exceed 9000 bytes". Below 1500 bytes is recommended.
MarcoPolo
approved these changes
Dec 2, 2025
per review feedback: RFC 6762 states packets MUST NOT exceed 9000 bytes, with 1500 bytes being recommended
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This PR filters mDNS advertised addresses to help stay within 1500-byte MTU:
.localDNS names (require unicast DNS).localDNS names (RFC 6762)This PR aims to help IPFS (Kubo) users with local discovery problems on Windows and macOS, as noted in #3415