AdGuardHome/internal/dhcpd/conn_linux.go

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//go:build linux
package dhcpd
import (
"fmt"
"net"
"os"
"time"
"github.com/AdguardTeam/golibs/errors"
"github.com/AdguardTeam/golibs/log"
"github.com/AdguardTeam/golibs/netutil"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/insomniacslk/dhcp/dhcpv4"
"github.com/insomniacslk/dhcp/dhcpv4/server4"
"github.com/mdlayher/ethernet"
"github.com/mdlayher/packet"
)
// dhcpUnicastAddr is the combination of MAC and IP addresses for responding to
// the unconfigured host.
type dhcpUnicastAddr struct {
// packet.Addr is embedded here to make *dhcpUcastAddr a net.Addr without
// actually implementing all methods. It also contains the client's
// hardware address.
packet.Addr
// yiaddr is an IP address just allocated by server for the host.
yiaddr net.IP
}
// dhcpConn is the net.PacketConn capable of handling both net.UDPAddr and
// net.HardwareAddr.
type dhcpConn struct {
// udpConn is the connection for UDP addresses.
udpConn net.PacketConn
// bcastIP is the broadcast address specific for the configured
// interface's subnet.
bcastIP net.IP
// rawConn is the connection for MAC addresses.
rawConn net.PacketConn
// srcMAC is the hardware address of the configured network interface.
srcMAC net.HardwareAddr
// srcIP is the IP address of the configured network interface.
srcIP net.IP
}
// newDHCPConn creates the special connection for DHCP server.
func (s *v4Server) newDHCPConn(iface *net.Interface) (c net.PacketConn, err error) {
var ucast net.PacketConn
if ucast, err = packet.Listen(iface, packet.Raw, int(ethernet.EtherTypeIPv4), nil); err != nil {
return nil, fmt.Errorf("creating raw udp connection: %w", err)
}
// Create the UDP connection.
var bcast net.PacketConn
bcast, err = server4.NewIPv4UDPConn(iface.Name, &net.UDPAddr{
// TODO(e.burkov): Listening on zeroes makes the server handle
// requests from all the interfaces. Inspect the ways to
// specify the interface-specific listening addresses.
//
// See https://github.com/AdguardTeam/AdGuardHome/issues/3539.
IP: net.IP{0, 0, 0, 0},
Port: dhcpv4.ServerPort,
})
if err != nil {
return nil, fmt.Errorf("creating ipv4 udp connection: %w", err)
}
return &dhcpConn{
udpConn: bcast,
bcastIP: s.conf.broadcastIP.AsSlice(),
rawConn: ucast,
srcMAC: iface.HardwareAddr,
srcIP: s.conf.dnsIPAddrs[0].AsSlice(),
}, nil
}
// wrapErrs is a helper to wrap the errors from two independent underlying
// connections.
func (*dhcpConn) wrapErrs(action string, udpConnErr, rawConnErr error) (err error) {
switch {
case udpConnErr != nil && rawConnErr != nil:
return errors.List(fmt.Sprintf("%s both connections", action), udpConnErr, rawConnErr)
case udpConnErr != nil:
return fmt.Errorf("%s udp connection: %w", action, udpConnErr)
case rawConnErr != nil:
return fmt.Errorf("%s raw connection: %w", action, rawConnErr)
default:
return nil
}
}
// WriteTo implements net.PacketConn for *dhcpConn. It selects the underlying
// connection to write to based on the type of addr.
func (c *dhcpConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
switch addr := addr.(type) {
case *dhcpUnicastAddr:
// Unicast the message to the client's MAC address. Use the raw
// connection.
//
// Note: unicasting is performed on the only network interface
// that is configured. For now it may be not what users expect
// so additionally broadcast the message via UDP connection.
//
// See https://github.com/AdguardTeam/AdGuardHome/issues/3539.
var rerr error
n, rerr = c.unicast(p, addr)
_, uerr := c.broadcast(p, &net.UDPAddr{
IP: netutil.IPv4bcast(),
Port: dhcpv4.ClientPort,
})
return n, c.wrapErrs("writing to", uerr, rerr)
case *net.UDPAddr:
if addr.IP.Equal(net.IPv4bcast) {
// Broadcast the message for the client which supports
// it. Use the UDP connection.
return c.broadcast(p, addr)
}
// Unicast the message to the client's IP address. Use the UDP
// connection.
return c.udpConn.WriteTo(p, addr)
default:
return 0, fmt.Errorf("addr has an unexpected type %T", addr)
}
}
// ReadFrom implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
return c.udpConn.ReadFrom(p)
}
// unicast wraps respData with required frames and writes it to the peer.
func (c *dhcpConn) unicast(respData []byte, peer *dhcpUnicastAddr) (n int, err error) {
var data []byte
data, err = c.buildEtherPkt(respData, peer)
if err != nil {
return 0, err
}
return c.rawConn.WriteTo(data, &peer.Addr)
}
// Close implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) Close() (err error) {
rerr := c.rawConn.Close()
if errors.Is(rerr, os.ErrClosed) {
// Ignore the error since the actual file is closed already.
rerr = nil
}
return c.wrapErrs("closing", c.udpConn.Close(), rerr)
}
// LocalAddr implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) LocalAddr() (a net.Addr) {
return c.udpConn.LocalAddr()
}
// SetDeadline implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) SetDeadline(t time.Time) (err error) {
return c.wrapErrs("setting deadline on", c.udpConn.SetDeadline(t), c.rawConn.SetDeadline(t))
}
// SetReadDeadline implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) SetReadDeadline(t time.Time) error {
return c.wrapErrs(
"setting reading deadline on",
c.udpConn.SetReadDeadline(t),
c.rawConn.SetReadDeadline(t),
)
}
// SetWriteDeadline implements net.PacketConn for *dhcpConn.
func (c *dhcpConn) SetWriteDeadline(t time.Time) error {
return c.wrapErrs(
"setting writing deadline on",
c.udpConn.SetWriteDeadline(t),
c.rawConn.SetWriteDeadline(t),
)
}
// ipv4DefaultTTL is the default Time to Live value in seconds as recommended by
// RFC-1700.
//
// See https://datatracker.ietf.org/doc/html/rfc1700.
const ipv4DefaultTTL = 64
// buildEtherPkt wraps the payload with IPv4, UDP and Ethernet frames.
// Validation of the payload is a caller's responsibility.
func (c *dhcpConn) buildEtherPkt(payload []byte, peer *dhcpUnicastAddr) (pkt []byte, err error) {
udpLayer := &layers.UDP{
SrcPort: dhcpv4.ServerPort,
DstPort: dhcpv4.ClientPort,
}
ipv4Layer := &layers.IPv4{
Version: uint8(layers.IPProtocolIPv4),
Flags: layers.IPv4DontFragment,
TTL: ipv4DefaultTTL,
Protocol: layers.IPProtocolUDP,
SrcIP: c.srcIP,
DstIP: peer.yiaddr,
}
// Ignore the error since it's only returned for invalid network layer's
// type.
_ = udpLayer.SetNetworkLayerForChecksum(ipv4Layer)
ethLayer := &layers.Ethernet{
SrcMAC: c.srcMAC,
DstMAC: peer.HardwareAddr,
EthernetType: layers.EthernetTypeIPv4,
}
buf := gopacket.NewSerializeBuffer()
setts := gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
}
err = gopacket.SerializeLayers(
buf,
setts,
ethLayer,
ipv4Layer,
udpLayer,
gopacket.Payload(payload),
)
if err != nil {
return nil, fmt.Errorf("serializing layers: %w", err)
}
return buf.Bytes(), nil
}
// send writes resp for peer to conn considering the req's parameters according
// to RFC-2131.
//
// See https://datatracker.ietf.org/doc/html/rfc2131#section-4.1.
func (s *v4Server) send(peer net.Addr, conn net.PacketConn, req, resp *dhcpv4.DHCPv4) {
switch giaddr, ciaddr, mtype := req.GatewayIPAddr, req.ClientIPAddr, resp.MessageType(); {
case giaddr != nil && !giaddr.IsUnspecified():
// Send any return messages to the server port on the BOOTP relay agent
// whose address appears in giaddr.
peer = &net.UDPAddr{
IP: giaddr,
Port: dhcpv4.ServerPort,
}
if mtype == dhcpv4.MessageTypeNak {
// Set the broadcast bit in the DHCPNAK, so that the relay agent
// broadcasts it to the client, because the client may not have a
// correct network address or subnet mask, and the client may not be
// answering ARP requests.
resp.SetBroadcast()
}
case mtype == dhcpv4.MessageTypeNak:
// Broadcast any DHCPNAK messages to 0xffffffff.
case ciaddr != nil && !ciaddr.IsUnspecified():
// Unicast DHCPOFFER and DHCPACK messages to the address in ciaddr.
peer = &net.UDPAddr{
IP: ciaddr,
Port: dhcpv4.ClientPort,
}
case !req.IsBroadcast() && req.ClientHWAddr != nil:
// Unicast DHCPOFFER and DHCPACK messages to the client's hardware
// address and yiaddr.
peer = &dhcpUnicastAddr{
Addr: packet.Addr{HardwareAddr: req.ClientHWAddr},
yiaddr: resp.YourIPAddr,
}
default:
// Go on since peer is already set to broadcast.
}
pktData := resp.ToBytes()
log.Debug("dhcpv4: sending %d bytes to %s: %s", len(pktData), peer, resp.Summary())
_, err := conn.WriteTo(pktData, peer)
if err != nil {
log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
}
}