unix: add functions to encode Inet4Pktinfo and Inet6Pktinfo
It's possible to control the source address of a UDP packet by
passing a socket control message of type IP_PKTINFO or IPV6_PKTINFO.
This is a somewhat esoteric feature of the network stack, but it's
extremely useful feature when you really need it.
Change-Id: I8300575f975679f6689d6f1282af253ba62e8f9d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/355610
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
diff --git a/unix/creds_test.go b/unix/creds_test.go
index 483bfb1..1d4c091 100644
--- a/unix/creds_test.go
+++ b/unix/creds_test.go
@@ -9,9 +9,11 @@
import (
"bytes"
+ "errors"
"net"
"os"
"testing"
+ "time"
"golang.org/x/sys/unix"
)
@@ -118,3 +120,78 @@
}
}
}
+
+func TestPktInfo(t *testing.T) {
+ testcases := []struct {
+ network string
+ address *net.UDPAddr
+ }{
+ {"udp4", &net.UDPAddr{IP: net.ParseIP("127.0.0.1")}},
+ {"udp6", &net.UDPAddr{IP: net.ParseIP("::1")}},
+ }
+ for _, test := range testcases {
+ t.Run(test.network, func(t *testing.T) {
+ conn, err := net.ListenUDP(test.network, test.address)
+ if errors.Is(err, unix.EADDRNOTAVAIL) {
+ t.Skipf("%v is not available", test.address)
+ }
+ if err != nil {
+ t.Fatal("Listen:", err)
+ }
+ defer conn.Close()
+
+ var pktInfo []byte
+ var src net.IP
+ switch test.network {
+ case "udp4":
+ var info4 unix.Inet4Pktinfo
+ src = net.ParseIP("127.0.0.2").To4()
+ copy(info4.Spec_dst[:], src)
+ pktInfo = unix.PktInfo4(&info4)
+
+ case "udp6":
+ var info6 unix.Inet6Pktinfo
+ src = net.ParseIP("2001:0DB8::1")
+ copy(info6.Addr[:], src)
+ pktInfo = unix.PktInfo6(&info6)
+
+ raw, err := conn.SyscallConn()
+ if err != nil {
+ t.Fatal("SyscallConn:", err)
+ }
+ var opErr error
+ err = raw.Control(func(fd uintptr) {
+ opErr = unix.SetsockoptInt(int(fd), unix.SOL_IPV6, unix.IPV6_FREEBIND, 1)
+ })
+ if err != nil {
+ t.Fatal("Control:", err)
+ }
+ if errors.Is(opErr, unix.ENOPROTOOPT) {
+ // Happens on android-amd64-emu, maybe Android has disabled
+ // IPV6_FREEBIND?
+ t.Skip("IPV6_FREEBIND not supported")
+ }
+ if opErr != nil {
+ t.Fatal("Can't enable IPV6_FREEBIND:", opErr)
+ }
+ }
+
+ msg := []byte{1}
+ addr := conn.LocalAddr().(*net.UDPAddr)
+ _, _, err = conn.WriteMsgUDP(msg, pktInfo, addr)
+ if err != nil {
+ t.Fatal("WriteMsgUDP:", err)
+ }
+
+ conn.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+ _, _, _, remote, err := conn.ReadMsgUDP(msg, nil)
+ if err != nil {
+ t.Fatal("ReadMsgUDP:", err)
+ }
+
+ if !remote.IP.Equal(src) {
+ t.Errorf("Got packet from %v, want %v", remote.IP, src)
+ }
+ })
+ }
+}
diff --git a/unix/sockcmsg_linux.go b/unix/sockcmsg_linux.go
index 8bf4570..e86d543 100644
--- a/unix/sockcmsg_linux.go
+++ b/unix/sockcmsg_linux.go
@@ -34,3 +34,25 @@
ucred := *(*Ucred)(unsafe.Pointer(&m.Data[0]))
return &ucred, nil
}
+
+// PktInfo4 encodes Inet4Pktinfo into a socket control message of type IP_PKTINFO.
+func PktInfo4(info *Inet4Pktinfo) []byte {
+ b := make([]byte, CmsgSpace(SizeofInet4Pktinfo))
+ h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
+ h.Level = SOL_IP
+ h.Type = IP_PKTINFO
+ h.SetLen(CmsgLen(SizeofInet4Pktinfo))
+ *(*Inet4Pktinfo)(h.data(0)) = *info
+ return b
+}
+
+// PktInfo6 encodes Inet6Pktinfo into a socket control message of type IPV6_PKTINFO.
+func PktInfo6(info *Inet6Pktinfo) []byte {
+ b := make([]byte, CmsgSpace(SizeofInet6Pktinfo))
+ h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
+ h.Level = SOL_IPV6
+ h.Type = IPV6_PKTINFO
+ h.SetLen(CmsgLen(SizeofInet6Pktinfo))
+ *(*Inet6Pktinfo)(h.data(0)) = *info
+ return b
+}
