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tailscale/net/packet/tsmp.go

197 lines
5.7 KiB
Go

// 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.
// TSMP is our ICMP-like "Tailscale Message Protocol" for signaling
// Tailscale-specific messages between nodes. It uses IP protocol 99
// (reserved for "any private encryption scheme") within
// Wireguard's normal encryption between peers and never hits the host
// network stack.
package packet
import (
"encoding/binary"
"errors"
"fmt"
"inet.af/netaddr"
"tailscale.com/net/flowtrack"
)
// TailscaleRejectedHeader is a TSMP message that says that one
// Tailscale node has rejected the connection from another. Unlike a
// TCP RST, this includes a reason.
//
// On the wire, after the IP header, it's currently 7 or 8 bytes:
// * '!'
// * IPProto byte (IANA protocol number: TCP or UDP)
// * 'A' or 'S' (RejectedDueToACLs, RejectedDueToShieldsUp)
// * srcPort big endian uint16
// * dstPort big endian uint16
// * [optional] byte of flag bits:
// lowest bit (0x1): MaybeBroken
//
// In the future it might also accept 16 byte IP flow src/dst IPs
// after the header, if they're different than the IP-level ones.
type TailscaleRejectedHeader struct {
IPSrc netaddr.IP // IPv4 or IPv6 header's src IP
IPDst netaddr.IP // IPv4 or IPv6 header's dst IP
Src netaddr.IPPort // rejected flow's src
Dst netaddr.IPPort // rejected flow's dst
Proto IPProto // proto that was rejected (TCP or UDP)
Reason TailscaleRejectReason // why the connection was rejected
// MaybeBroken is whether the rejection is non-terminal (the
// client should not fail immediately). This is sent by a
// target when it's not sure whether it's totally broken, but
// it might be. For example, the target tailscaled might think
// its host firewall or IP forwarding aren't configured
// properly, but tailscaled might be wrong (not having enough
// visibility into what the OS is doing). When true, the
// message is simply an FYI as a potential reason to use for
// later when the pendopen connection tracking timer expires.
MaybeBroken bool
}
const rejectFlagBitMaybeBroken = 0x1
func (rh TailscaleRejectedHeader) Flow() flowtrack.Tuple {
return flowtrack.Tuple{Src: rh.Src, Dst: rh.Dst}
}
func (rh TailscaleRejectedHeader) String() string {
return fmt.Sprintf("TSMP-reject-flow{%s %s > %s}: %s", rh.Proto, rh.Src, rh.Dst, rh.Reason)
}
type TSMPType uint8
const (
// TSMPTypeRejectedConn is the type byte for a TailscaleRejectedHeader.
TSMPTypeRejectedConn TSMPType = '!'
)
type TailscaleRejectReason byte
// IsZero reports whether r is the zero value, representing no rejection.
func (r TailscaleRejectReason) IsZero() bool { return r == TailscaleRejectReasonNone }
const (
// TailscaleRejectReasonNone is the TailscaleRejectReason zero value.
TailscaleRejectReasonNone TailscaleRejectReason = 0
// RejectedDueToACLs means that the host rejected the connection due to ACLs.
RejectedDueToACLs TailscaleRejectReason = 'A'
// RejectedDueToShieldsUp means that the host rejected the connection due to shields being up.
RejectedDueToShieldsUp TailscaleRejectReason = 'S'
// RejectedDueToIPForwarding means that the relay node's IP
// forwarding is disabled.
RejectedDueToIPForwarding TailscaleRejectReason = 'F'
// RejectedDueToHostFirewall means that the target host's
// firewall is blocking the traffic.
RejectedDueToHostFirewall TailscaleRejectReason = 'W'
)
func (r TailscaleRejectReason) String() string {
switch r {
case RejectedDueToACLs:
return "acl"
case RejectedDueToShieldsUp:
return "shields"
case RejectedDueToIPForwarding:
return "host-ip-forwarding-unavailable"
case RejectedDueToHostFirewall:
return "host-firewall"
}
return fmt.Sprintf("0x%02x", byte(r))
}
func (h TailscaleRejectedHeader) hasFlags() bool {
return h.MaybeBroken // the only one currently
}
func (h TailscaleRejectedHeader) Len() int {
v := 1 + // TSMPType byte
1 + // IPProto byte
1 + // TailscaleRejectReason byte
2*2 // 2 uint16 ports
if h.IPSrc.Is4() {
v += ip4HeaderLength
} else if h.IPSrc.Is6() {
v += ip6HeaderLength
}
if h.hasFlags() {
v++
}
return v
}
func (h TailscaleRejectedHeader) Marshal(buf []byte) error {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
if h.Src.IP.Is4() {
iph := IP4Header{
IPProto: TSMP,
Src: h.IPSrc,
Dst: h.IPDst,
}
iph.Marshal(buf)
buf = buf[ip4HeaderLength:]
} else if h.Src.IP.Is6() {
iph := IP6Header{
IPProto: TSMP,
Src: h.IPSrc,
Dst: h.IPDst,
}
iph.Marshal(buf)
buf = buf[ip6HeaderLength:]
} else {
return errors.New("bogus src IP")
}
buf[0] = byte(TSMPTypeRejectedConn)
buf[1] = byte(h.Proto)
buf[2] = byte(h.Reason)
binary.BigEndian.PutUint16(buf[3:5], h.Src.Port)
binary.BigEndian.PutUint16(buf[5:7], h.Dst.Port)
if h.hasFlags() {
var flags byte
if h.MaybeBroken {
flags |= rejectFlagBitMaybeBroken
}
buf[7] = flags
}
return nil
}
// AsTailscaleRejectedHeader parses pp as an incoming rejection
// connection TSMP message.
//
// ok reports whether pp was a valid TSMP rejection packet.
func (pp *Parsed) AsTailscaleRejectedHeader() (h TailscaleRejectedHeader, ok bool) {
p := pp.Payload()
if len(p) < 7 || p[0] != byte(TSMPTypeRejectedConn) {
return
}
h = TailscaleRejectedHeader{
Proto: IPProto(p[1]),
Reason: TailscaleRejectReason(p[2]),
IPSrc: pp.Src.IP,
IPDst: pp.Dst.IP,
Src: netaddr.IPPort{IP: pp.Dst.IP, Port: binary.BigEndian.Uint16(p[3:5])},
Dst: netaddr.IPPort{IP: pp.Src.IP, Port: binary.BigEndian.Uint16(p[5:7])},
}
if len(p) > 7 {
flags := p[7]
h.MaybeBroken = (flags & rejectFlagBitMaybeBroken) != 0
}
return h, true
}