mirror of
https://github.com/slackhq/nebula.git
synced 2026-07-01 02:50:29 +02:00
add IPv6 reject packet generation (#1766)
* add IPv6 reject packet generation (ICMPv6 Destination Unreachable and TCP RST) * use ICMPv6 code 1 (administratively prohibited) and cap body at 1000 bytes * cleanup, use ICMP error code 13 for ipv4 * better docs * cleanup
This commit is contained in:
Wade Simmons
+246
-19
@@ -4,26 +4,50 @@ import (
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"encoding/binary"
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"golang.org/x/net/ipv4"
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"golang.org/x/net/ipv6"
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)
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const (
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// Need 96 bytes for the largest reject packet:
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// MaxIPv4RejectPacketSize is the largest IPv4 reject packet:
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// - 20 byte ipv4 header
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// - 8 byte icmpv4 header
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// - 68 byte body (60 byte max orig ipv4 header + 8 byte orig icmpv4 header)
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MaxRejectPacketSize = ipv4.HeaderLen + 8 + 60 + 8
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maxIPv4RejectPacketSize = ipv4.HeaderLen + 8 + 60 + 8
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// MaxRejectPacketSize is sized for the largest possible reject packet (IPv6):
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// - 40 byte ipv6 header
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// - 8 byte icmpv6 header
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// - up to 1000 byte body (original packet, possibly truncated. We want to stay
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// under the MTU with Nebula overhead included)
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maxIPv6RejectPacketSize = ipv6.HeaderLen + 8 + 1000
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MaxRejectPacketSize = maxIPv6RejectPacketSize
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)
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func CreateRejectPacket(packet []byte, out []byte) []byte {
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if len(packet) < ipv4.HeaderLen || int(packet[0]>>4) != ipv4.Version {
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if len(packet) < 1 {
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return nil
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}
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switch packet[9] {
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case 6: // tcp
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return ipv4CreateRejectTCPPacket(packet, out)
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version := int(packet[0] >> 4)
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switch version {
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case ipv4.Version:
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if len(packet) < ipv4.HeaderLen {
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return nil
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}
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switch packet[9] {
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case 6: // tcp
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return ipv4CreateRejectTCPPacket(packet, out)
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default:
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return ipv4CreateRejectICMPPacket(packet, out)
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}
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case ipv6.Version:
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if len(packet) < ipv6.HeaderLen {
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return nil
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}
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return ipv6CreateRejectPacket(packet, out)
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default:
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return ipv4CreateRejectICMPPacket(packet, out)
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return nil
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}
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}
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@@ -36,10 +60,7 @@ func ipv4CreateRejectICMPPacket(packet []byte, out []byte) []byte {
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}
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// ICMP reply includes original header and first 8 bytes of the packet
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packetLen := len(packet)
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if packetLen > ihl+8 {
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packetLen = ihl + 8
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}
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packetLen := min(len(packet), ihl+8)
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outLen := ipv4.HeaderLen + 8 + packetLen
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if outLen > cap(out) {
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@@ -71,14 +92,14 @@ func ipv4CreateRejectICMPPacket(packet []byte, out []byte) []byte {
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// ICMP Destination Unreachable
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icmpOut := out[ipv4.HeaderLen:]
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icmpOut[0] = 3 // type (Destination unreachable)
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icmpOut[1] = 3 // code (Port unreachable error)
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icmpOut[2] = 0 // checksum
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icmpOut[3] = 0 // .
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icmpOut[4] = 0 // unused
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icmpOut[5] = 0 // .
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icmpOut[6] = 0 // .
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icmpOut[7] = 0 // .
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icmpOut[0] = 3 // type (Destination unreachable)
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icmpOut[1] = 13 // code (Communication administratively prohibited)
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icmpOut[2] = 0 // checksum
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icmpOut[3] = 0 // .
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icmpOut[4] = 0 // unused
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icmpOut[5] = 0 // .
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icmpOut[6] = 0 // .
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icmpOut[7] = 0 // .
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// Copy original IP header and first 8 bytes as body
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copy(icmpOut[8:], packet[:packetLen])
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@@ -165,6 +186,197 @@ func ipv4CreateRejectTCPPacket(packet []byte, out []byte) []byte {
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return out
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}
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func ipv6CreateRejectPacket(packet []byte, out []byte) []byte {
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proto := ipv6FindUpperProtocol(packet)
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switch proto {
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case 6: // tcp
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return ipv6CreateRejectTCPPacket(packet, out)
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default:
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return ipv6CreateRejectICMPPacket(packet, out)
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}
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}
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func ipv6FindUpperProtocol(packet []byte) uint8 {
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nextHeader := packet[6]
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offset := ipv6.HeaderLen
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for {
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switch nextHeader {
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case 0, 43, 60: // Hop-by-Hop, Routing, Destination
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if len(packet) < offset+2 {
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return nextHeader
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}
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nextHeader = packet[offset]
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offset += int(packet[offset+1]+1) << 3
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case 44: // Fragment
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if len(packet) < offset+8 {
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return nextHeader
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}
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nextHeader = packet[offset]
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offset += 8
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case 51: // AH
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if len(packet) < offset+2 {
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return nextHeader
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}
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nextHeader = packet[offset]
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offset += int(packet[offset+1]+2) << 2
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default:
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return nextHeader
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}
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}
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}
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func ipv6CreateRejectICMPPacket(packet []byte, out []byte) []byte {
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// Include as much of the original packet as possible, up to 1000 bytes,
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// so the response fits comfortably within any tunnel MTU.
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packetLen := min(len(packet), 1000)
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outLen := ipv6.HeaderLen + 8 + packetLen
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if outLen > cap(out) {
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return nil
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}
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out = out[:outLen]
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// IPv6 header
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ipHdr := out[0:ipv6.HeaderLen]
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ipHdr[0] = ipv6.Version << 4 // version, traffic class (high bits)
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ipHdr[1] = 0 // traffic class (low bits), flow label (high bits)
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ipHdr[2] = 0 // flow label
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ipHdr[3] = 0 // flow label
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payloadLen := uint16(outLen - ipv6.HeaderLen)
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binary.BigEndian.PutUint16(ipHdr[4:], payloadLen) // payload length
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ipHdr[6] = 58 // next header (ICMPv6)
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ipHdr[7] = 64 // hop limit
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// Swap dest / src IPs (each 16 bytes, src at 8, dst at 24)
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copy(ipHdr[8:24], packet[24:40])
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copy(ipHdr[24:40], packet[8:24])
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// ICMPv6 Destination Unreachable
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icmpOut := out[ipv6.HeaderLen:]
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icmpOut[0] = 1 // type (Destination Unreachable)
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icmpOut[1] = 1 // code (Communication with destination administratively prohibited)
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icmpOut[2] = 0 // checksum
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icmpOut[3] = 0 // .
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icmpOut[4] = 0 // unused
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icmpOut[5] = 0 // .
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icmpOut[6] = 0 // .
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icmpOut[7] = 0 // .
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copy(icmpOut[8:], packet[:packetLen])
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// ICMPv6 checksum uses a pseudo-header
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csum := ipv6PseudoheaderChecksum(ipHdr[8:24], ipHdr[24:40], 58, uint32(payloadLen))
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binary.BigEndian.PutUint16(icmpOut[2:], tcpipChecksum(icmpOut, csum))
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return out
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}
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func ipv6CreateRejectTCPPacket(packet []byte, out []byte) []byte {
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const tcpLen = 20
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offset := ipv6FindUpperProtocolOffset(packet)
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if len(packet) < offset+tcpLen {
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return nil
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}
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outLen := ipv6.HeaderLen + tcpLen
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if outLen > cap(out) {
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return nil
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}
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out = out[:outLen]
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// IPv6 header
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ipHdr := out[0:ipv6.HeaderLen]
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ipHdr[0] = ipv6.Version << 4 // version, traffic class (high bits)
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ipHdr[1] = 0 // traffic class (low bits), flow label (high bits)
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ipHdr[2] = 0 // flow label
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ipHdr[3] = 0 // flow label
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binary.BigEndian.PutUint16(ipHdr[4:], tcpLen) // payload length
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ipHdr[6] = 6 // next header (TCP)
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ipHdr[7] = 64 // hop limit
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// Swap dest / src IPs
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copy(ipHdr[8:24], packet[24:40])
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copy(ipHdr[24:40], packet[8:24])
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// TCP RST
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tcpIn := packet[offset:]
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var ackSeq, seq uint32
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outFlags := byte(0b00000100) // RST
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inAck := tcpIn[13]&0b00010000 != 0
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if inAck {
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seq = binary.BigEndian.Uint32(tcpIn[8:])
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} else {
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inSyn := uint32((tcpIn[13] & 0b00000010) >> 1)
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inFin := uint32(tcpIn[13] & 0b00000001)
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ackSeq = binary.BigEndian.Uint32(tcpIn[4:]) + inSyn + inFin + uint32(len(tcpIn)) - uint32(tcpIn[12]>>4)<<2
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outFlags |= 0b00010000 // ACK
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}
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tcpOut := out[ipv6.HeaderLen:]
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// Swap dest / src ports
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copy(tcpOut[0:2], tcpIn[2:4])
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copy(tcpOut[2:4], tcpIn[0:2])
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binary.BigEndian.PutUint32(tcpOut[4:], seq)
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binary.BigEndian.PutUint32(tcpOut[8:], ackSeq)
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tcpOut[12] = (tcpLen >> 2) << 4 // data offset, reserved, NS
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tcpOut[13] = outFlags // CWR, ECE, URG, ACK, PSH, RST, SYN, FIN
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tcpOut[14] = 0 // window size
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tcpOut[15] = 0 // .
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tcpOut[16] = 0 // checksum
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tcpOut[17] = 0 // .
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tcpOut[18] = 0 // URG Pointer
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tcpOut[19] = 0 // .
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// Calculate checksum with IPv6 pseudo-header
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csum := ipv6PseudoheaderChecksum(ipHdr[8:24], ipHdr[24:40], 6, tcpLen)
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binary.BigEndian.PutUint16(tcpOut[16:], tcpipChecksum(tcpOut, csum))
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return out
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}
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func ipv6FindUpperProtocolOffset(packet []byte) int {
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nextHeader := packet[6]
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offset := ipv6.HeaderLen
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for {
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switch nextHeader {
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case 0, 43, 60: // Hop-by-Hop, Routing, Destination
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if len(packet) < offset+2 {
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return offset
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}
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nextHeader = packet[offset]
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offset += int(packet[offset+1]+1) << 3
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case 44: // Fragment
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if len(packet) < offset+8 {
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return offset
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}
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nextHeader = packet[offset]
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offset += 8
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case 51: // AH
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if len(packet) < offset+2 {
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return offset
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}
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nextHeader = packet[offset]
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offset += int(packet[offset+1]+2) << 2
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default:
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return offset
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}
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}
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}
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func CreateICMPEchoResponse(packet, out []byte) []byte {
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// Return early if this is not a simple ICMP Echo Request
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//TODO: make constants out of these
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@@ -236,3 +448,18 @@ func ipv4PseudoheaderChecksum(src, dst []byte, proto, length uint32) (csum uint3
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csum += length >> 16
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return csum
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}
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// based on:
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// - https://github.com/google/gopacket/blob/v1.1.19/layers/tcpip.go#L37-L48
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func ipv6PseudoheaderChecksum(src, dst []byte, proto, length uint32) (csum uint32) {
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for i := 0; i < 16; i += 2 {
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csum += uint32(src[i]) << 8
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csum += uint32(src[i+1])
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csum += uint32(dst[i]) << 8
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csum += uint32(dst[i+1])
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}
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csum += proto
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csum += length & 0xffff
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csum += length >> 16
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return csum
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}
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