// 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 tstun import ( "fmt" "net" "os" "os/exec" "github.com/insomniacslk/dhcp/dhcpv4" "golang.org/x/sys/unix" "golang.zx2c4.com/wireguard/tun" "inet.af/netaddr" "inet.af/netstack/tcpip" "inet.af/netstack/tcpip/buffer" "inet.af/netstack/tcpip/header" "inet.af/netstack/tcpip/network/ipv4" "inet.af/netstack/tcpip/transport/udp" "tailscale.com/net/packet" "tailscale.com/types/ipproto" ) // TODO: this was randomly generated once. Maybe do it per process start? But // then an upgraded tailscaled would be visible to devices behind it. So // maybe instead make it a function of the tailscaled's wireguard public key? // For now just hard code it. var ourMAC = net.HardwareAddr{0x30, 0x2D, 0x66, 0xEC, 0x7A, 0x93} func init() { createTAP = createTAPLinux } func createTAPLinux(tapName, bridgeName string) (tun.Device, error) { fd, err := unix.Open("/dev/net/tun", unix.O_RDWR, 0) if err != nil { return nil, err } dev, err := openDevice(fd, tapName, bridgeName) if err != nil { unix.Close(fd) return nil, err } return dev, nil } func openDevice(fd int, tapName, bridgeName string) (tun.Device, error) { ifr, err := unix.NewIfreq(tapName) if err != nil { return nil, err } // Flags are stored as a uint16 in the ifreq union. ifr.SetUint16(unix.IFF_TAP | unix.IFF_NO_PI) if err := unix.IoctlIfreq(fd, unix.TUNSETIFF, ifr); err != nil { return nil, err } if err := run("ip", "link", "set", "dev", tapName, "up"); err != nil { return nil, err } if bridgeName != "" { if err := run("brctl", "addif", bridgeName, tapName); err != nil { return nil, err } } // Also sets non-blocking I/O on fd when creating tun.Device. dev, _, err := tun.CreateUnmonitoredTUNFromFD(fd) // TODO: MTU if err != nil { return nil, err } return dev, nil } type etherType [2]byte var ( etherTypeARP = etherType{0x08, 0x06} etherTypeIPv4 = etherType{0x08, 0x00} etherTypeIPv6 = etherType{0x86, 0xDD} ) const ipv4HeaderLen = 20 const ( consumePacket = true passOnPacket = false ) // handleTAPFrame handles receiving a raw TAP ethernet frame and reports whether // it's been handled (that is, whether it should NOT be passed to wireguard). func (t *Wrapper) handleTAPFrame(ethBuf []byte) bool { if len(ethBuf) < ethernetFrameSize { // Corrupt. Ignore. if tapDebug { t.logf("tap: short TAP frame") } return consumePacket } ethDstMAC, ethSrcMAC := ethBuf[:6], ethBuf[6:12] _ = ethDstMAC et := etherType{ethBuf[12], ethBuf[13]} switch et { default: if tapDebug { t.logf("tap: ignoring etherType %v", et) } return consumePacket // filter out packet we should ignore case etherTypeIPv6: // TODO: support DHCPv6/ND/etc later. For now pass all to WireGuard. if tapDebug { t.logf("tap: ignoring IPv6 %v", et) } return passOnPacket case etherTypeIPv4: if len(ethBuf) < ethernetFrameSize+ipv4HeaderLen { // Bogus IPv4. Eat. if tapDebug { t.logf("tap: short ipv4") } return consumePacket } return t.handleDHCPRequest(ethBuf) case etherTypeARP: arpPacket := header.ARP(ethBuf[ethernetFrameSize:]) if !arpPacket.IsValid() { // Bogus ARP. Eat. return consumePacket } switch arpPacket.Op() { case header.ARPRequest: req := arpPacket // better name at this point buf := make([]byte, header.EthernetMinimumSize+header.ARPSize) // Our ARP "Table" of one: var srcMAC [6]byte copy(srcMAC[:], ethSrcMAC) if old := t.destMAC(); old != srcMAC { t.destMACAtomic.Store(srcMAC) } eth := header.Ethernet(buf) eth.Encode(&header.EthernetFields{ SrcAddr: tcpip.LinkAddress(ourMAC[:]), DstAddr: tcpip.LinkAddress(ethSrcMAC), Type: 0x0806, // arp }) res := header.ARP(buf[header.EthernetMinimumSize:]) res.SetIPv4OverEthernet() res.SetOp(header.ARPReply) // If the client's asking about their own IP, tell them it's // their own MAC. TODO(bradfitz): remove String allocs. if net.IP(req.ProtocolAddressTarget()).String() == theClientIP { copy(res.HardwareAddressSender(), ethSrcMAC) } else { copy(res.HardwareAddressSender(), ourMAC[:]) } copy(res.ProtocolAddressSender(), req.ProtocolAddressTarget()) copy(res.HardwareAddressTarget(), req.HardwareAddressSender()) copy(res.ProtocolAddressTarget(), req.ProtocolAddressSender()) n, err := t.tdev.Write(buf, 0) if tapDebug { t.logf("tap: wrote ARP reply %v, %v", n, err) } } return consumePacket } } // TODO(bradfitz): remove these hard-coded values and move from a /24 to a /10 CGNAT as the range. const theClientIP = "100.70.145.3" // TODO: make dynamic from netmap const routerIP = "100.70.145.1" // must be in same netmask (currently hack at /24) as theClientIP // handleDHCPRequest handles receiving a raw TAP ethernet frame and reports whether // it's been handled as a DHCP request. That is, it reports whether the frame should // be ignored by the caller and not passed on. func (t *Wrapper) handleDHCPRequest(ethBuf []byte) bool { const udpHeader = 8 if len(ethBuf) < ethernetFrameSize+ipv4HeaderLen+udpHeader { if tapDebug { t.logf("tap: DHCP short") } return passOnPacket } ethDstMAC, ethSrcMAC := ethBuf[:6], ethBuf[6:12] if string(ethDstMAC) != "\xff\xff\xff\xff\xff\xff" { // Not a broadcast if tapDebug { t.logf("tap: dhcp no broadcast") } return passOnPacket } p := parsedPacketPool.Get().(*packet.Parsed) defer parsedPacketPool.Put(p) p.Decode(ethBuf[ethernetFrameSize:]) if p.IPProto != ipproto.UDP || p.Src.Port() != 68 || p.Dst.Port() != 67 { // Not a DHCP request. if tapDebug { t.logf("tap: DHCP wrong meta") } return passOnPacket } dp, err := dhcpv4.FromBytes(ethBuf[ethernetFrameSize+ipv4HeaderLen+udpHeader:]) if err != nil { // Bogus. Trash it. if tapDebug { t.logf("tap: DHCP FromBytes bad") } return consumePacket } if tapDebug { t.logf("tap: DHCP request: %+v", dp) } switch dp.MessageType() { case dhcpv4.MessageTypeDiscover: offer, err := dhcpv4.New( dhcpv4.WithReply(dp), dhcpv4.WithMessageType(dhcpv4.MessageTypeOffer), dhcpv4.WithRouter(net.ParseIP(routerIP)), // the default route dhcpv4.WithDNS(net.ParseIP("100.100.100.100")), dhcpv4.WithServerIP(net.ParseIP("100.100.100.100")), // TODO: what is this? dhcpv4.WithOption(dhcpv4.OptServerIdentifier(net.ParseIP("100.100.100.100"))), dhcpv4.WithYourIP(net.ParseIP(theClientIP)), dhcpv4.WithLeaseTime(3600), // hour works //dhcpv4.WithHwAddr(ethSrcMAC), dhcpv4.WithNetmask(net.IPMask(net.ParseIP("255.255.255.0").To4())), // TODO: wrong //dhcpv4.WithTransactionID(dp.TransactionID), ) if err != nil { t.logf("error building DHCP offer: %v", err) return consumePacket } // Make a layer 2 packet to write out: pkt := packLayer2UDP( offer.ToBytes(), ourMAC, ethSrcMAC, netaddr.IPPortFrom(netaddr.IPv4(100, 100, 100, 100), 67), // src netaddr.IPPortFrom(netaddr.IPv4(255, 255, 255, 255), 68), // dst ) n, err := t.tdev.Write(pkt, 0) if tapDebug { t.logf("tap: wrote DHCP OFFER %v, %v", n, err) } case dhcpv4.MessageTypeRequest: ack, err := dhcpv4.New( dhcpv4.WithReply(dp), dhcpv4.WithMessageType(dhcpv4.MessageTypeAck), dhcpv4.WithDNS(net.ParseIP("100.100.100.100")), dhcpv4.WithRouter(net.ParseIP(routerIP)), // the default route dhcpv4.WithServerIP(net.ParseIP("100.100.100.100")), // TODO: what is this? dhcpv4.WithOption(dhcpv4.OptServerIdentifier(net.ParseIP("100.100.100.100"))), dhcpv4.WithYourIP(net.ParseIP(theClientIP)), // Hello world dhcpv4.WithLeaseTime(3600), // hour works dhcpv4.WithNetmask(net.IPMask(net.ParseIP("255.255.255.0").To4())), ) if err != nil { t.logf("error building DHCP ack: %v", err) return consumePacket } // Make a layer 2 packet to write out: pkt := packLayer2UDP( ack.ToBytes(), ourMAC, ethSrcMAC, netaddr.IPPortFrom(netaddr.IPv4(100, 100, 100, 100), 67), // src netaddr.IPPortFrom(netaddr.IPv4(255, 255, 255, 255), 68), // dst ) n, err := t.tdev.Write(pkt, 0) if tapDebug { t.logf("tap: wrote DHCP ACK %v, %v", n, err) } default: if tapDebug { t.logf("tap: unknown DHCP type") } } return consumePacket } func packLayer2UDP(payload []byte, srcMAC, dstMAC net.HardwareAddr, src, dst netaddr.IPPort) []byte { buf := buffer.NewView(header.EthernetMinimumSize + header.UDPMinimumSize + header.IPv4MinimumSize + len(payload)) payloadStart := len(buf) - len(payload) copy(buf[payloadStart:], payload) srcB := src.IP().As4() srcIP := tcpip.Address(srcB[:]) dstB := dst.IP().As4() dstIP := tcpip.Address(dstB[:]) // Ethernet header eth := header.Ethernet(buf) eth.Encode(&header.EthernetFields{ SrcAddr: tcpip.LinkAddress(srcMAC), DstAddr: tcpip.LinkAddress(dstMAC), Type: ipv4.ProtocolNumber, }) // IP header ipbuf := buf[header.EthernetMinimumSize:] ip := header.IPv4(ipbuf) ip.Encode(&header.IPv4Fields{ TotalLength: uint16(len(ipbuf)), TTL: 65, Protocol: uint8(udp.ProtocolNumber), SrcAddr: srcIP, DstAddr: dstIP, }) ip.SetChecksum(^ip.CalculateChecksum()) // UDP header u := header.UDP(buf[header.EthernetMinimumSize+header.IPv4MinimumSize:]) u.Encode(&header.UDPFields{ SrcPort: src.Port(), DstPort: dst.Port(), Length: uint16(header.UDPMinimumSize + len(payload)), }) // Calculate the UDP pseudo-header checksum. xsum := header.PseudoHeaderChecksum(udp.ProtocolNumber, srcIP, dstIP, uint16(len(u))) // Calculate the UDP checksum and set it. xsum = header.Checksum(payload, xsum) u.SetChecksum(^u.CalculateChecksum(xsum)) return []byte(buf) } func run(prog string, args ...string) error { cmd := exec.Command(prog, args...) cmd.Stdout = os.Stdout cmd.Stderr = os.Stderr if err := cmd.Run(); err != nil { return fmt.Errorf("error running %v: %v", cmd, err) } return nil } func (t *Wrapper) destMAC() [6]byte { mac, _ := t.destMACAtomic.Load().([6]byte) return mac } func (t *Wrapper) tapWrite(buf []byte, offset int) (int, error) { if offset < ethernetFrameSize { return 0, fmt.Errorf("[unexpected] weird offset %d for TAP write", offset) } eth := buf[offset-ethernetFrameSize:] dst := t.destMAC() copy(eth[:6], dst[:]) copy(eth[6:12], ourMAC[:]) et := etherTypeIPv4 if buf[offset]>>4 == 6 { et = etherTypeIPv6 } eth[12], eth[13] = et[0], et[1] if tapDebug { t.logf("tap: tapWrite off=%v % x", offset, buf) } return t.tdev.Write(buf, offset-ethernetFrameSize) }