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net/netns: document Windows socket binding a bit more

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Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick 2020-09-22 12:51:43 -07:00 committed by Brad Fitzpatrick
parent 5d8b88be88
commit fda9dc8815
1 changed files with 14 additions and 4 deletions
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18
net/netns/netns_windows.go
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@ -82,17 +82,27 @@ func getDefaultInterface(family winipcfg.AddressFamily) (ifidx uint32, err error
return index, nil return index, nil
} }
// sockoptBoundInterface is the value of IP_UNICAST_IF and IPV6_UNICAST_IF.
//
// See https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options
// and https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ipv6-socket-options
const sockoptBoundInterface = 31 const sockoptBoundInterface = 31
// bindSocket4 binds the given RawConn to the network interface with // bindSocket4 binds the given RawConn to the network interface with
// index ifidx, for IPv4 traffic only. // index ifidx, for IPv4 traffic only.
func bindSocket4(c syscall.RawConn, ifidx uint32) error { func bindSocket4(c syscall.RawConn, ifidx uint32) error {
// For v4 the interface index must be passed as a big-endian // For IPv4 (but NOT IPv6) the interface index must be passed
// integer, regardless of platform endianness. // as a big-endian integer (regardless of platform endianness)
index := nativeToBigEndian(ifidx) // because the underlying sockopt takes either an IPv4 address
// or an index shoved into IPv4 address representation (an IP
// in 0.0.0.0/8 means it's actually an index).
//
// See https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options
// and IP_UNICAST_IF.
indexAsAddr := nativeToBigEndian(ifidx)
var controlErr error var controlErr error
err := c.Control(func(fd uintptr) { err := c.Control(func(fd uintptr) {
controlErr = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, sockoptBoundInterface, int(index)) controlErr = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, sockoptBoundInterface, int(indexAsAddr))
}) })
if err != nil { if err != nil {
return err return err