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45578b47f3
The new interface lets implementors more precisely distinguish local traffic from forwarded traffic, and applies different forwarding logic within Machines for each type. This allows Machines to be packet forwarders, which didn't quite work with the implementation of Inject. Signed-off-by: David Anderson <danderson@tailscale.com>
258 lines
6.9 KiB
Go
258 lines
6.9 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package natlab
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import (
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"context"
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"fmt"
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"net"
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"sync"
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"time"
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"inet.af/netaddr"
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)
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// mapping is the state of an allocated NAT session.
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type mapping struct {
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lanSrc netaddr.IPPort
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lanDst netaddr.IPPort
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wanSrc netaddr.IPPort
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deadline time.Time
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// pc is a PacketConn that reserves an outbound port on the NAT's
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// WAN interface. We do this because ListenPacket already has
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// random port selection logic built in. Additionally this means
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// that concurrent use of ListenPacket for connections originating
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// from the NAT box won't conflict with NAT mappings, since both
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// use PacketConn to reserve ports on the machine.
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pc net.PacketConn
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}
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// NATType is the mapping behavior of a NAT device. Values express
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// different modes defined by RFC 4787.
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type NATType int
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const (
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// EndpointIndependentNAT specifies a destination endpoint
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// independent NAT. All traffic from a source ip:port gets mapped
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// to a single WAN ip:port.
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EndpointIndependentNAT NATType = iota
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// AddressDependentNAT specifies a destination address dependent
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// NAT. Every distinct destination IP gets its own WAN ip:port
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// allocation.
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AddressDependentNAT
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// AddressAndPortDependentNAT specifies a destination
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// address-and-port dependent NAT. Every distinct destination
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// ip:port gets its own WAN ip:port allocation.
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AddressAndPortDependentNAT
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)
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// natKey is the lookup key for a NAT session. While it contains a
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// 4-tuple ({src,dst} {ip,port}), some NATTypes will zero out some
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// fields, so in practice the key is either a 2-tuple (src only),
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// 3-tuple (src ip+port and dst ip) or 4-tuple (src+dst ip+port).
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type natKey struct {
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src, dst netaddr.IPPort
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}
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func (t NATType) key(src, dst netaddr.IPPort) natKey {
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k := natKey{src: src}
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switch t {
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case EndpointIndependentNAT:
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case AddressDependentNAT:
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k.dst.IP = dst.IP
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case AddressAndPortDependentNAT:
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k.dst = dst
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default:
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panic(fmt.Sprintf("unknown NAT type %v", t))
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}
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return k
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}
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// DefaultMappingTimeout is the default timeout for a NAT mapping.
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const DefaultMappingTimeout = 30 * time.Second
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// SNAT44 implements an IPv4-to-IPv4 source NAT (SNAT) translator, with
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// optional builtin firewall.
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type SNAT44 struct {
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// Machine is the machine to which this NAT is attached. Altered
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// packets are injected back into this Machine for processing.
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Machine *Machine
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// ExternalInterface is the "WAN" interface of Machine. Packets
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// from other sources get NATed onto this interface.
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ExternalInterface *Interface
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// Type specifies the mapping allocation behavior for this NAT.
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Type NATType
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// MappingTimeout is the lifetime of individual NAT sessions. Once
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// a session expires, the mapped port effectively "closes" to new
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// traffic. If MappingTimeout is 0, DefaultMappingTimeout is used.
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MappingTimeout time.Duration
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// Firewall is an optional packet handler that will be invoked as
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// a firewall during NAT translation. The firewall always sees
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// packets in their "LAN form", i.e. before translation in the
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// outbound direction and after translation in the inbound
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// direction.
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Firewall PacketHandler
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// TimeNow is a function that returns the current time. If
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// nil, time.Now is used.
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TimeNow func() time.Time
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mu sync.Mutex
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byLAN map[natKey]*mapping // lookup by outbound packet tuple
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byWAN map[netaddr.IPPort]*mapping // lookup by wan ip:port only
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}
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func (n *SNAT44) timeNow() time.Time {
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if n.TimeNow != nil {
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return n.TimeNow()
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}
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return time.Now()
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}
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func (n *SNAT44) mappingTimeout() time.Duration {
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if n.MappingTimeout == 0 {
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return DefaultMappingTimeout
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}
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return n.MappingTimeout
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}
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func (n *SNAT44) initLocked() {
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if n.byLAN == nil {
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n.byLAN = map[natKey]*mapping{}
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n.byWAN = map[netaddr.IPPort]*mapping{}
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}
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if n.ExternalInterface.Machine() != n.Machine {
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panic(fmt.Sprintf("NAT given interface %s that is not part of given machine %s", n.ExternalInterface, n.Machine.Name))
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}
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}
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func (n *SNAT44) HandleOut(p *Packet, oif *Interface) *Packet {
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// NATs don't affect locally originated packets.
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if n.Firewall != nil {
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return n.Firewall.HandleOut(p, oif)
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}
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return p
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}
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func (n *SNAT44) HandleIn(p *Packet, iif *Interface) *Packet {
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if iif != n.ExternalInterface {
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// NAT can't apply, defer to firewall.
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if n.Firewall != nil {
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return n.Firewall.HandleIn(p, iif)
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}
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return p
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}
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n.mu.Lock()
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defer n.mu.Unlock()
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n.initLocked()
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now := n.timeNow()
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mapping := n.byWAN[p.Dst]
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if mapping == nil || now.After(mapping.deadline) {
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// NAT didn't hit, defer to firewall or allow in for local
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// socket handling.
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if n.Firewall != nil {
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return n.Firewall.HandleIn(p, iif)
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}
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return p
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}
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p.Dst = mapping.lanSrc
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p.Trace("dnat to %v", p.Dst)
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// Don't process firewall here. We mutated the packet such that
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// it's no longer destined locally, so we'll get reinvoked as
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// HandleForward and need to process the altered packet there.
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return p
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}
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func (n *SNAT44) HandleForward(p *Packet, iif, oif *Interface) *Packet {
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switch {
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case oif == n.ExternalInterface:
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if p.Src.IP == oif.V4() {
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// Packet already NATed and is just retraversing Forward,
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// don't touch it again.
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return p
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}
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if n.Firewall != nil {
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p2 := n.Firewall.HandleForward(p, iif, oif)
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if p2 == nil {
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// firewall dropped, done
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return nil
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}
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if !p.Equivalent(p2) {
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// firewall mutated packet? Weird, but okay.
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return p2
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}
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}
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n.mu.Lock()
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defer n.mu.Unlock()
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n.initLocked()
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k := n.Type.key(p.Src, p.Dst)
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now := n.timeNow()
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m := n.byLAN[k]
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if m == nil || now.After(m.deadline) {
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pc, wanAddr := n.allocateMappedPort()
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m = &mapping{
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lanSrc: p.Src,
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lanDst: p.Dst,
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wanSrc: wanAddr,
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pc: pc,
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}
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n.byLAN[k] = m
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n.byWAN[wanAddr] = m
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}
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m.deadline = now.Add(n.mappingTimeout())
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p.Src = m.wanSrc
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p.Trace("snat from %v", p.Src)
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return p
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case iif == n.ExternalInterface:
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// Packet was already un-NAT-ed, we just need to either
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// firewall it or let it through.
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if n.Firewall != nil {
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return n.Firewall.HandleForward(p, iif, oif)
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}
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return p
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default:
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// No NAT applies, invoke firewall or drop.
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if n.Firewall != nil {
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return n.Firewall.HandleForward(p, iif, oif)
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}
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return nil
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}
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}
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func (n *SNAT44) allocateMappedPort() (net.PacketConn, netaddr.IPPort) {
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// Clean up old entries before trying to allocate, to free up any
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// expired ports.
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n.gc()
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ip := n.ExternalInterface.V4()
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pc, err := n.Machine.ListenPacket(context.Background(), "udp", net.JoinHostPort(ip.String(), "0"))
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if err != nil {
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panic(fmt.Sprintf("ran out of NAT ports: %v", err))
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}
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addr := netaddr.IPPort{
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IP: ip,
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Port: uint16(pc.LocalAddr().(*net.UDPAddr).Port),
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}
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return pc, addr
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}
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func (n *SNAT44) gc() {
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now := n.timeNow()
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for _, m := range n.byLAN {
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if !now.After(m.deadline) {
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continue
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}
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m.pc.Close()
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delete(n.byLAN, n.Type.key(m.lanSrc, m.lanDst))
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delete(n.byWAN, m.wanSrc)
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}
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}
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