The code was using a C "int", which is a signed 32-bit integer.
That means some valid IP addresses were negative numbers.
(In particular, the default router address handed out by AT&T
fiber: 192.168.1.254. No I don't know why they do that.)
A negative number is < 255, and so was treated by the Go code
as an error.
This fixes the unit test failure:
$ go test -v -run=TestLikelyHomeRouterIPSyscallExec ./net/interfaces
=== RUN TestLikelyHomeRouterIPSyscallExec
interfaces_darwin_cgo_test.go:15: syscall() = invalid IP, false, netstat = 192.168.1.254, true
--- FAIL: TestLikelyHomeRouterIPSyscallExec (0.00s)
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
iOS doesn't let you run subprocesses,
which means we can't use netstat to get routing information.
Instead, use syscalls and grub around in the results.
We keep the old netstat version around,
both for use in non-cgo builds,
and for use testing the syscall-based version.
Note that iOS doesn't ship route.h,
so we include a copy here from the macOS 10.15 SDK
(which is itself unchanged from the 10.14 SDK).
I have tested manually that this yields the correct
gateway IP address on my own macOS and iOS devices.
More coverage would be most welcome.
Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>
