hotfix(pmtud/tcp): fix code for IPv6 destinations

internal/pmtud/ip/ipv6_darwin.go

@@ -1,5 +0,0 @@
-package ip
-
-func SetIPv6HeaderIncluded(fd int) error {
-	panic("darwin does not allow an application to build IPv6 headers")
-}

internal/pmtud/ip/ipv6_linux.go

@@ -1,7 +0,0 @@
-package ip
-
-import "golang.org/x/sys/unix"
-
-func SetIPv6HeaderIncluded(fd int) error {
-	return unix.SetsockoptInt(fd, unix.IPPROTO_IPV6, unix.IPV6_HDRINCL, 1)
-}

internal/pmtud/ip/ipv6_unspecified.go

@@ -1,7 +0,0 @@
-//go:build !linux && !windows && !darwin
-
-package ip
-
-func SetIPv6HeaderIncluded(fd int) error {
-	panic("not implemented")
-}

internal/pmtud/ip/ipv6_windows.go

@@ -1,7 +0,0 @@
-package ip
-
-import "golang.org/x/sys/windows"
-
-func SetIPv6HeaderIncluded(fd windows.Handle) error {
-	panic("windows does not allow an application to build IPv6 headers")
-}

internal/pmtud/ip/source_unix.go

@@ -29,7 +29,7 @@ func makeSockAddr(ip netip.Addr, port uint16) unix.Sockaddr {
 		}
 	}
 	return &unix.SockaddrInet6{
-		Port: int(port),
+		Port: 0,
 		Addr: ip.As16(),
 	}
 }

internal/pmtud/tcp/packet.go

@@ -65,13 +65,15 @@ func createPacket(src, dst netip.AddrPort,
 	if dst.Addr().Is4() {
 		ipHeader = ip.HeaderV4(src.Addr(), dst.Addr(), payloadLength)
 	} else {
+		// Pseudo-header, this is actually not part of the packet since
+		// the kernel will calculate and add it itself to the packet;
+		// it is only used for calculating the TCP checksum.
 		ipHeader = ip.HeaderV6(src.Addr(), dst.Addr(),
 			uint16(payloadLength), byte(constants.IPPROTO_TCP)) //nolint:gosec
 	}
 
 	tcpHeader := makeTCPHeader(src.Port(), dst.Port(), seq, ack, flags)
 
-	// data is just zeroes
 	dataLength := int(payloadLength - constants.BaseTCPHeaderLength)
 	var data []byte
 	if dataLength > 0 {
@@ -81,10 +83,18 @@ func createPacket(src, dst netip.AddrPort,
 	tcpHeader[16] = byte(checksum >> 8)   //nolint:mnd
 	tcpHeader[17] = byte(checksum & 0xff) //nolint:mnd
 
-	packet := make([]byte, len(ipHeader)+int(constants.BaseTCPHeaderLength)+dataLength)
+	var packet []byte
+	i := 0
+	if dst.Addr().Is4() {
+		packet = make([]byte, len(ipHeader)+int(constants.BaseTCPHeaderLength)+dataLength)
 		copy(packet, ipHeader)
-	copy(packet[len(ipHeader):], tcpHeader)
+		i += len(ipHeader)
-	copy(packet[len(ipHeader)+int(constants.BaseTCPHeaderLength):], data)
+	} else {
+		packet = make([]byte, int(constants.BaseTCPHeaderLength)+dataLength)
+	}
+	copy(packet[i:], tcpHeader)
+	i += int(constants.BaseTCPHeaderLength)
+	copy(packet[i:], data)
 	return packet
 }
 

internal/pmtud/tcp/tcp.go

@@ -47,13 +47,11 @@ func startRawSocket(family, excludeMark int) (fd fileDescriptor, stop func(), er
 
 	if family == constants.AF_INET {
 		err = ip.SetIPv4HeaderIncluded(fdPlatform)
-	} else {
-		err = ip.SetIPv6HeaderIncluded(fdPlatform)
-	}
 		if err != nil {
 			_ = closeSocket(fdPlatform)
 			return 0, nil, fmt.Errorf("setting header option on raw socket: %w", err)
 		}
+	}
 
 	// Allow sending packets larger than cached PMTU (for PMTUD probing)
 	err = setMTUDiscovery(fdPlatform, family == constants.AF_INET)

internal/pmtud/tcp/tcp_unix.go

@@ -45,7 +45,7 @@ func makeSockAddr(addr netip.AddrPort) unix.Sockaddr {
 		}
 	}
 	return &unix.SockaddrInet6{
-		Port: int(addr.Port()),
+		Port: 0,
 		Addr: addr.Addr().As16(),
 	}
 }