net: Interfaces returns no interfaces on DragonFly BSD #81503

Go version

go1.27.1 dragonfly/amd64

Output of go env in your module/workspace:

GOOS=dragonfly
GOARCH=amd64
GOVERSION=go1.27.1

What did you do?

Call net.Interfaces() on DragonFly BSD:

package main

import (
    "fmt"
    "net"
)

func main() {
    ifs, err := net.Interfaces()
    fmt.Println("count:", len(ifs), "err:", err)
}

What did you see happen?

count: 0

What did you expect to see?

The system's interfaces, e.g. em0, lo0, pflog0, pfsync0, tailscale0.

Additional context

internal/routebsd/interface_classic.go is compiled for darwin, dragonfly and
netbsd, and reads the if_msghdr/ifa_msghdr fields at the FreeBSD/darwin
offsets (addrs at offset 4, index at offset 12). DragonFly orders these
differently: ifm_index comes right after the message type (offset 4) and
ifm_addrs after the flags (offset 12). So the parser reads the index from the
addrs field and drops messages that lack the RTA_IFP bit, and net.Interfaces()
reports nothing.

This appears to have regressed when internal/routebsd was split out of
x/net/route (Go 1.26). Fix CL: https://go-review.googlesource.com/c/go/+/831784