slayers: unmap IPv4-mapped IPv6 addresses (#4377)

The Go standard library can produce IPv4-mapped IPv6 addresses when
resolving IP addresses. These IP addresses need to be unmapped before
putting them on the wire.

Before this patch, we could observe the following with tshark:

    Len=1304 SCION 1-ff00:0:110,[::ffff:172.20.2.2] -> 1-ff00:0:111,[::ffff:172.20.3.2] UDP 32769 -> 32768 1208

The regression was introduced in #4346, which removed the unmapping behavior
in slayers.PackAddr. This patch restores the behavior to ensure only
unmapped IPv4 addresses make it on the wire.


Handling the unmapping in the code that generates the addresses and only
checking this in slayers would seem ideal, but these calls are often
very far away from the place that would then trigger an error. Thus
handling this in slayers seems like a compromise that saves us and the
users of the slayers package a lot of trouble.

pkg/slayers/scion.go

@@ -394,15 +394,16 @@ func ParseAddr(addrType AddrType, raw []byte) (addr.Host, error) {
 func PackAddr(host addr.Host) (AddrType, []byte, error) {
 	switch host.Type() {
 	case addr.HostTypeIP:
-		ip := host.IP()
+		// The IP is potentially IPv4-in-IPv6. We need to unmap it to ensure
+		// we only have true IPv4 or IPv6 addresses.
+		ip := host.IP().Unmap()
 		if !ip.IsValid() {
 			break
 		}
-		t := T4Ip
 		if ip.Is6() {
-			t = T16Ip
+			return T16Ip, ip.AsSlice(), nil
 		}
-		return t, ip.AsSlice(), nil
+		return T4Ip, ip.AsSlice(), nil
 	case addr.HostTypeSVC:
 		raw := make([]byte, 4)
 		binary.BigEndian.PutUint16(raw, uint16(host.SVC()))

pkg/slayers/scion_test.go

@@ -33,10 +33,11 @@ import (
 )
 
 var (
-	ip6Addr = addr.MustParseHost("2001:db8::68")
-	ip4Addr = addr.MustParseHost("10.0.0.100")
-	svcAddr = addr.MustParseHost("Wildcard")
-	rawPath = func() []byte {
+	ip6Addr    = addr.MustParseHost("2001:db8::68")
+	ip4Addr    = addr.MustParseHost("10.0.0.100")
+	ip4in6Addr = addr.MustParseHost("::ffff:10.0.0.100")
+	svcAddr    = addr.MustParseHost("Wildcard")
+	rawPath    = func() []byte {
 		return []byte("\x00\x00\x20\x80\x00\x00\x01\x11\x00\x00\x01\x00\x01\x00\x02\x22\x00" +
 			"\x00\x01\x00\x00\x3f\x00\x01\x00\x00\x01\x02\x03\x04\x05\x06\x00\x3f\x00\x03\x00" +
 			"\x02\x01\x02\x03\x04\x05\x06\x00\x3f\x00\x00\x00\x02\x01\x02\x03\x04\x05\x06\x00" +
@@ -311,6 +312,12 @@ func TestPackAddr(t *testing.T) {
 			rawAddr:   ip4Addr.IP().AsSlice(),
 			errorFunc: assert.NoError,
 		},
+		"pack IPv4-mapped IPv6": {
+			addr:      ip4in6Addr,
+			addrType:  slayers.T4Ip,
+			rawAddr:   []byte{0xa, 0x0, 0x0, 0x64},
+			errorFunc: assert.NoError,
+		},
 		"pack IPv6": {
 			addr:      ip6Addr,
 			addrType:  slayers.T16Ip,