Connecting to a dual-stack (ipv4 & ipv6 available) endpoint can return EADDRNOTAVAIL

Version tokio v1.41.1 (but master is affected too)

Platform Linux meeting-data 6.8.0-1029-aws #31~22.04.1-Ubuntu SMP Thu Apr 24 21:16:18 UTC 2025 x86_64 GNU/Linux

Description The issue is TcpStream::connect may sometimes return EADDRNOTAVAIL (errno 99) hiding the actual error that occurred in an ipv4 connection.

When running in an ipv4-only environment (i.e. no external ipv6), getaddrinfo will happily return ipv6 addresses as well. The problem is that the code https://github.com/tokio-rs/tokio/blob/5efb1c3b16496f51f2ec959b68a1b97efd7ee234/tokio/src/net/tcp/stream.rs#L116-L132 returns the LAST error encountered. So to trigger EADDRNOTAVAIL, you need:

• getaddrinfo returns [IPV4, IPv6] in that order
• Attempt to connect to IPV4 yields an actual network error (e.g. timeout, connection refused)
• The code attempts an IPV6 connection.
• The IPV6 connection fails with EADDRNOTAVAIL since the host doesn't have external IPv6 support enabled.
• The last error wins, so instead of a "connection refused" message, you get a "cannot assign requested address"

For reference for others, one fix for this is to disable returning ipv6 addresses - add options no-aaaa to /etc/resolv.conf.