go binaries use bind on startup, requiring network-bind

[Zygmunt Krynicki]

This bug is here to investigate why spread, our test suite runner and a go binary uses bind() on startup, failing as it does not use the network-bind interface.

The strace log of "spread -list" is attached to the bug. The key parts seem to be:

socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
close(3) = 0

Hmm? ^^ Perhaps just a test if "Can I haz socket?"

socket(PF_INET6, SOCK_STREAM, IPPROTO_TCP) = 3
setsockopt(3, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
bind(3, {sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0
futex(0xb58cd0, FUTEX_WAKE, 1) = 1
futex(0xb58c10, FUTEX_WAKE, 1) = 1

We have a IPv6 TCP socket bound to ::1 as the fd 3

socket(PF_INET6, SOCK_STREAM, IPPROTO_TCP) = 4
setsockopt(4, SOL_IPV6, IPV6_V6ONLY, [0], 4) = 0
bind(4, {sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=
futex(0xb58cd0, FUTEX_WAKE, 1) = 1
futex(0xb58c10, FUTEX_WAKE, 1) = 1

We have a IPv6 TCP socket bound to ::ffff:127.0.0.1 (aka IPv4 in IPv4 version of the ::1 above) as fd 4

close(4) = 0
close(3) = 0

Both sockets are closed here.

[Zygmunt Krynicki (zyga)]

This seems to be go runtime startup:

// Should we try to use the IPv4 socket interface if we're
// only dealing with IPv4 sockets? As long as the host system
// understands IPv6, it's okay to pass IPv4 addresses to the IPv6
// interface. That simplifies our code and is most general.
// Unfortunately, we need to run on kernels built without IPv6
// support too. So probe the kernel to figure it out.
//
// probeIPv6Stack probes both basic IPv6 capability and IPv6 IPv4-
// mapping capability which is controlled by IPV6_V6ONLY socket
// option and/or kernel state "net.inet6.ip6.v6only".
// It returns two boolean values. If the first boolean value is
// true, kernel supports basic IPv6 functionality. If the second
// boolean value is true, kernel supports IPv6 IPv4-mapping.
func probeIPv6Stack() (supportsIPv6, supportsIPv4map bool) {
...
}

[Zygmunt Krynicki (zyga)]

And in addition this:

// BUG(rsc,mikio): On DragonFly BSD and OpenBSD, listening on the
// "tcp" and "udp" networks does not listen for both IPv4 and IPv6
// connections. This is due to the fact that IPv4 traffic will not be
// routed to an IPv6 socket - two separate sockets are required if
// both address families are to be supported.
// See inet6(4) for details.

func probeIPv4Stack() bool {
...
}

[Zygmunt Krynicki (zyga)]

Both functions are in golang-1.6-1.6.2/src/net/ipsock_posix.go

I'm renaming this bug to indicate that this is specific to go

[Jamie Strandboge (jdstrand)]

This was addressed in https://github.com/snapcore/snapd/pull/1251