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pcp.go
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pcp.go
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// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package portmapper
import (
"context"
"crypto/rand"
"encoding/binary"
"fmt"
"net/netip"
"time"
)
// References:
//
// https://www.rfc-editor.org/rfc/pdfrfc/rfc6887.txt.pdf
// https://tools.ietf.org/html/rfc6887
//go:generate go run tailscale.com/cmd/addlicense -year 2021 -file pcpresultcode_string.go go run golang.org/x/tools/cmd/stringer -type=pcpResultCode -trimprefix=pcpCode
type pcpResultCode uint8
// PCP constants
const (
pcpVersion = 2
pcpDefaultPort = 5351
pcpMapLifetimeSec = 7200 // TODO does the RFC recommend anything? This is taken from PMP.
pcpCodeOK pcpResultCode = 0
pcpCodeNotAuthorized pcpResultCode = 2
// From RFC 6887:
// ADDRESS_MISMATCH: The source IP address of the request packet does
// not match the contents of the PCP Client's IP Address field, due
// to an unexpected NAT on the path between the PCP client and the
// PCP-controlled NAT or firewall.
pcpCodeAddressMismatch pcpResultCode = 12
pcpOpReply = 0x80 // OR'd into request's op code on response
pcpOpAnnounce = 0
pcpOpMap = 1
pcpUDPMapping = 17 // portmap UDP
pcpTCPMapping = 6 // portmap TCP
)
type pcpMapping struct {
c *Client
gw netip.AddrPort
internal netip.AddrPort
external netip.AddrPort
renewAfter time.Time
goodUntil time.Time
// TODO should this also contain an epoch?
// Doesn't seem to be used elsewhere, but can use it for validation at some point.
}
func (p *pcpMapping) GoodUntil() time.Time { return p.goodUntil }
func (p *pcpMapping) RenewAfter() time.Time { return p.renewAfter }
func (p *pcpMapping) External() netip.AddrPort { return p.external }
func (p *pcpMapping) Release(ctx context.Context) {
uc, err := p.c.listenPacket(ctx, "udp4", ":0")
if err != nil {
return
}
defer uc.Close()
pkt := buildPCPRequestMappingPacket(p.internal.Addr(), p.internal.Port(), p.external.Port(), 0, p.external.Addr())
uc.WriteToUDPAddrPort(pkt, p.gw)
}
// buildPCPRequestMappingPacket generates a PCP packet with a MAP opcode.
// To create a packet which deletes a mapping, lifetimeSec should be set to 0.
// If prevPort is not known, it should be set to 0.
// If prevExternalIP is not known, it should be set to 0.0.0.0.
func buildPCPRequestMappingPacket(
myIP netip.Addr,
localPort, prevPort uint16,
lifetimeSec uint32,
prevExternalIP netip.Addr,
) (pkt []byte) {
// 24 byte common PCP header + 36 bytes of MAP-specific fields
pkt = make([]byte, 24+36)
pkt[0] = pcpVersion
pkt[1] = pcpOpMap
binary.BigEndian.PutUint32(pkt[4:8], lifetimeSec)
myIP16 := myIP.As16()
copy(pkt[8:24], myIP16[:])
mapOp := pkt[24:]
rand.Read(mapOp[:12]) // 96 bit mapping nonce
// TODO: should this be a UDP mapping? It looks like it supports "all protocols" with 0, but
// also doesn't support a local port then.
mapOp[12] = pcpUDPMapping
binary.BigEndian.PutUint16(mapOp[16:18], localPort)
binary.BigEndian.PutUint16(mapOp[18:20], prevPort)
prevExternalIP16 := prevExternalIP.As16()
copy(mapOp[20:], prevExternalIP16[:])
return pkt
}
// parsePCPMapResponse parses resp into a partially populated pcpMapping.
// In particular, its Client is not populated.
func parsePCPMapResponse(resp []byte) (*pcpMapping, error) {
if len(resp) < 60 {
return nil, fmt.Errorf("Does not appear to be PCP MAP response")
}
res, ok := parsePCPResponse(resp[:24])
if !ok {
return nil, fmt.Errorf("Invalid PCP common header")
}
if res.ResultCode == pcpCodeNotAuthorized {
return nil, fmt.Errorf("PCP is implemented but not enabled in the router")
}
if res.ResultCode != pcpCodeOK {
return nil, fmt.Errorf("PCP response not ok, code %d", res.ResultCode)
}
// TODO: don't ignore the nonce and make sure it's the same?
externalPort := binary.BigEndian.Uint16(resp[42:44])
externalIPBytes := [16]byte{}
copy(externalIPBytes[:], resp[44:])
externalIP := netip.AddrFrom16(externalIPBytes).Unmap()
external := netip.AddrPortFrom(externalIP, externalPort)
lifetime := time.Second * time.Duration(res.Lifetime)
now := time.Now()
mapping := &pcpMapping{
external: external,
renewAfter: now.Add(lifetime / 2),
goodUntil: now.Add(lifetime),
}
return mapping, nil
}
// pcpAnnounceRequest generates a PCP packet with an ANNOUNCE opcode.
func pcpAnnounceRequest(myIP netip.Addr) []byte {
// See https://tools.ietf.org/html/rfc6887#section-7.1
pkt := make([]byte, 24)
pkt[0] = pcpVersion
pkt[1] = pcpOpAnnounce
myIP16 := myIP.As16()
copy(pkt[8:], myIP16[:])
return pkt
}
type pcpResponse struct {
OpCode uint8
ResultCode pcpResultCode
Lifetime uint32
Epoch uint32
}
func parsePCPResponse(b []byte) (res pcpResponse, ok bool) {
if len(b) < 24 || b[0] != pcpVersion {
return
}
res.OpCode = b[1]
res.ResultCode = pcpResultCode(b[3])
res.Lifetime = binary.BigEndian.Uint32(b[4:])
res.Epoch = binary.BigEndian.Uint32(b[8:])
return res, true
}