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// DNS server implementation.
package dns
import (
"context"
"crypto/tls"
"encoding/binary"
"errors"
"io"
"net"
"strings"
"sync"
"time"
)
// Default maximum number of TCP queries before we close the socket.
const maxTCPQueries = 128
// aLongTimeAgo is a non-zero time, far in the past, used for
// immediate cancelation of network operations.
var aLongTimeAgo = time.Unix(1, 0)
// Handler is implemented by any value that implements ServeDNS.
type Handler interface {
ServeDNS(w ResponseWriter, r *Msg)
}
// The HandlerFunc type is an adapter to allow the use of
// ordinary functions as DNS handlers. If f is a function
// with the appropriate signature, HandlerFunc(f) is a
// Handler object that calls f.
type HandlerFunc func(ResponseWriter, *Msg)
// ServeDNS calls f(w, r).
func (f HandlerFunc) ServeDNS(w ResponseWriter, r *Msg) {
f(w, r)
}
// A ResponseWriter interface is used by an DNS handler to
// construct an DNS response.
type ResponseWriter interface {
// LocalAddr returns the net.Addr of the server
LocalAddr() net.Addr
// RemoteAddr returns the net.Addr of the client that sent the current request.
RemoteAddr() net.Addr
// WriteMsg writes a reply back to the client.
WriteMsg(*Msg) error
// Write writes a raw buffer back to the client.
Write([]byte) (int, error)
// Close closes the connection.
Close() error
// TsigStatus returns the status of the Tsig.
TsigStatus() error
// TsigTimersOnly sets the tsig timers only boolean.
TsigTimersOnly(bool)
// Hijack lets the caller take over the connection.
// After a call to Hijack(), the DNS package will not do anything with the connection.
Hijack()
}
// A ConnectionStater interface is used by a DNS Handler to access TLS connection state
// when available.
type ConnectionStater interface {
ConnectionState() *tls.ConnectionState
}
type response struct {
closed bool // connection has been closed
hijacked bool // connection has been hijacked by handler
tsigTimersOnly bool
tsigStatus error
tsigRequestMAC string
tsigSecret map[string]string // the tsig secrets
udp *net.UDPConn // i/o connection if UDP was used
tcp net.Conn // i/o connection if TCP was used
udpSession *SessionUDP // oob data to get egress interface right
writer Writer // writer to output the raw DNS bits
}
// HandleFailed returns a HandlerFunc that returns SERVFAIL for every request it gets.
func HandleFailed(w ResponseWriter, r *Msg) {
m := new(Msg)
m.SetRcode(r, RcodeServerFailure)
// does not matter if this write fails
w.WriteMsg(m)
}
// ListenAndServe Starts a server on address and network specified Invoke handler
// for incoming queries.
func ListenAndServe(addr string, network string, handler Handler) error {
server := &Server{Addr: addr, Net: network, Handler: handler}
return server.ListenAndServe()
}
// ListenAndServeTLS acts like http.ListenAndServeTLS, more information in
// http://golang.org/pkg/net/http/#ListenAndServeTLS
func ListenAndServeTLS(addr, certFile, keyFile string, handler Handler) error {
cert, err := tls.LoadX509KeyPair(certFile, keyFile)
if err != nil {
return err
}
config := tls.Config{
Certificates: []tls.Certificate{cert},
}
server := &Server{
Addr: addr,
Net: "tcp-tls",
TLSConfig: &config,
Handler: handler,
}
return server.ListenAndServe()
}
// ActivateAndServe activates a server with a listener from systemd,
// l and p should not both be non-nil.
// If both l and p are not nil only p will be used.
// Invoke handler for incoming queries.
func ActivateAndServe(l net.Listener, p net.PacketConn, handler Handler) error {
server := &Server{Listener: l, PacketConn: p, Handler: handler}
return server.ActivateAndServe()
}
// Writer writes raw DNS messages; each call to Write should send an entire message.
type Writer interface {
io.Writer
}
// Reader reads raw DNS messages; each call to ReadTCP or ReadUDP should return an entire message.
type Reader interface {
// ReadTCP reads a raw message from a TCP connection. Implementations may alter
// connection properties, for example the read-deadline.
ReadTCP(conn net.Conn, timeout time.Duration) ([]byte, error)
// ReadUDP reads a raw message from a UDP connection. Implementations may alter
// connection properties, for example the read-deadline.
ReadUDP(conn *net.UDPConn, timeout time.Duration) ([]byte, *SessionUDP, error)
}
// defaultReader is an adapter for the Server struct that implements the Reader interface
// using the readTCP and readUDP func of the embedded Server.
type defaultReader struct {
*Server
}
func (dr defaultReader) ReadTCP(conn net.Conn, timeout time.Duration) ([]byte, error) {
return dr.readTCP(conn, timeout)
}
func (dr defaultReader) ReadUDP(conn *net.UDPConn, timeout time.Duration) ([]byte, *SessionUDP, error) {
return dr.readUDP(conn, timeout)
}
// DecorateReader is a decorator hook for extending or supplanting the functionality of a Reader.
// Implementations should never return a nil Reader.
type DecorateReader func(Reader) Reader
// DecorateWriter is a decorator hook for extending or supplanting the functionality of a Writer.
// Implementations should never return a nil Writer.
type DecorateWriter func(Writer) Writer
// A Server defines parameters for running an DNS server.
type Server struct {
// Address to listen on, ":dns" if empty.
Addr string
// if "tcp" or "tcp-tls" (DNS over TLS) it will invoke a TCP listener, otherwise an UDP one
Net string
// TCP Listener to use, this is to aid in systemd's socket activation.
Listener net.Listener
// TLS connection configuration
TLSConfig *tls.Config
// UDP "Listener" to use, this is to aid in systemd's socket activation.
PacketConn net.PacketConn
// Handler to invoke, dns.DefaultServeMux if nil.
Handler Handler
// Default buffer size to use to read incoming UDP messages. If not set
// it defaults to MinMsgSize (512 B).
UDPSize int
// The net.Conn.SetReadTimeout value for new connections, defaults to 2 * time.Second.
ReadTimeout time.Duration
// The net.Conn.SetWriteTimeout value for new connections, defaults to 2 * time.Second.
WriteTimeout time.Duration
// TCP idle timeout for multiple queries, if nil, defaults to 8 * time.Second (RFC 5966).
IdleTimeout func() time.Duration
// Secret(s) for Tsig map[<zonename>]<base64 secret>. The zonename must be in canonical form (lowercase, fqdn, see RFC 4034 Section 6.2).
TsigSecret map[string]string
// If NotifyStartedFunc is set it is called once the server has started listening.
NotifyStartedFunc func()
// DecorateReader is optional, allows customization of the process that reads raw DNS messages.
DecorateReader DecorateReader
// DecorateWriter is optional, allows customization of the process that writes raw DNS messages.
DecorateWriter DecorateWriter
// Maximum number of TCP queries before we close the socket. Default is maxTCPQueries (unlimited if -1).
MaxTCPQueries int
// Whether to set the SO_REUSEPORT socket option, allowing multiple listeners to be bound to a single address.
// It is only supported on go1.11+ and when using ListenAndServe.
ReusePort bool
// AcceptMsgFunc will check the incoming message and will reject it early in the process.
// By default DefaultMsgAcceptFunc will be used.
MsgAcceptFunc MsgAcceptFunc
// Shutdown handling
lock sync.RWMutex
started bool
shutdown chan struct{}
conns map[net.Conn]struct{}
// A pool for UDP message buffers.
udpPool sync.Pool
}
func (srv *Server) isStarted() bool {
srv.lock.RLock()
started := srv.started
srv.lock.RUnlock()
return started
}
func makeUDPBuffer(size int) func() interface{} {
return func() interface{} {
return make([]byte, size)
}
}
func (srv *Server) init() {
srv.shutdown = make(chan struct{})
srv.conns = make(map[net.Conn]struct{})
if srv.UDPSize == 0 {
srv.UDPSize = MinMsgSize
}
if srv.MsgAcceptFunc == nil {
srv.MsgAcceptFunc = DefaultMsgAcceptFunc
}
if srv.Handler == nil {
srv.Handler = DefaultServeMux
}
srv.udpPool.New = makeUDPBuffer(srv.UDPSize)
}
func unlockOnce(l sync.Locker) func() {
var once sync.Once
return func() { once.Do(l.Unlock) }
}
// ListenAndServe starts a nameserver on the configured address in *Server.
func (srv *Server) ListenAndServe() error {
unlock := unlockOnce(&srv.lock)
srv.lock.Lock()
defer unlock()
if srv.started {
return &Error{err: "server already started"}
}
addr := srv.Addr
if addr == "" {
addr = ":domain"
}
srv.init()
switch srv.Net {
case "tcp", "tcp4", "tcp6":
l, err := listenTCP(srv.Net, addr, srv.ReusePort)
if err != nil {
return err
}
srv.Listener = l
srv.started = true
unlock()
return srv.serveTCP(l)
case "tcp-tls", "tcp4-tls", "tcp6-tls":
if srv.TLSConfig == nil || (len(srv.TLSConfig.Certificates) == 0 && srv.TLSConfig.GetCertificate == nil) {
return errors.New("dns: neither Certificates nor GetCertificate set in Config")
}
network := strings.TrimSuffix(srv.Net, "-tls")
l, err := listenTCP(network, addr, srv.ReusePort)
if err != nil {
return err
}
l = tls.NewListener(l, srv.TLSConfig)
srv.Listener = l
srv.started = true
unlock()
return srv.serveTCP(l)
case "udp", "udp4", "udp6":
l, err := listenUDP(srv.Net, addr, srv.ReusePort)
if err != nil {
return err
}
u := l.(*net.UDPConn)
if e := setUDPSocketOptions(u); e != nil {
return e
}
srv.PacketConn = l
srv.started = true
unlock()
return srv.serveUDP(u)
}
return &Error{err: "bad network"}
}
// ActivateAndServe starts a nameserver with the PacketConn or Listener
// configured in *Server. Its main use is to start a server from systemd.
func (srv *Server) ActivateAndServe() error {
unlock := unlockOnce(&srv.lock)
srv.lock.Lock()
defer unlock()
if srv.started {
return &Error{err: "server already started"}
}
srv.init()
pConn := srv.PacketConn
l := srv.Listener
if pConn != nil {
// Check PacketConn interface's type is valid and value
// is not nil
if t, ok := pConn.(*net.UDPConn); ok && t != nil {
if e := setUDPSocketOptions(t); e != nil {
return e
}
srv.started = true
unlock()
return srv.serveUDP(t)
}
}
if l != nil {
srv.started = true
unlock()
return srv.serveTCP(l)
}
return &Error{err: "bad listeners"}
}
// Shutdown shuts down a server. After a call to Shutdown, ListenAndServe and
// ActivateAndServe will return.
func (srv *Server) Shutdown() error {
return srv.ShutdownContext(context.Background())
}
// ShutdownContext shuts down a server. After a call to ShutdownContext,
// ListenAndServe and ActivateAndServe will return.
//
// A context.Context may be passed to limit how long to wait for connections
// to terminate.
func (srv *Server) ShutdownContext(ctx context.Context) error {
srv.lock.Lock()
if !srv.started {
srv.lock.Unlock()
return &Error{err: "server not started"}
}
srv.started = false
if srv.PacketConn != nil {
srv.PacketConn.SetReadDeadline(aLongTimeAgo) // Unblock reads
}
if srv.Listener != nil {
srv.Listener.Close()
}
for rw := range srv.conns {
rw.SetReadDeadline(aLongTimeAgo) // Unblock reads
}
srv.lock.Unlock()
if testShutdownNotify != nil {
testShutdownNotify.Broadcast()
}
var ctxErr error
select {
case <-srv.shutdown:
case <-ctx.Done():
ctxErr = ctx.Err()
}
if srv.PacketConn != nil {
srv.PacketConn.Close()
}
return ctxErr
}
var testShutdownNotify *sync.Cond
// getReadTimeout is a helper func to use system timeout if server did not intend to change it.
func (srv *Server) getReadTimeout() time.Duration {
if srv.ReadTimeout != 0 {
return srv.ReadTimeout
}
return dnsTimeout
}
// serveTCP starts a TCP listener for the server.
func (srv *Server) serveTCP(l net.Listener) error {
defer l.Close()
if srv.NotifyStartedFunc != nil {
srv.NotifyStartedFunc()
}
var wg sync.WaitGroup
defer func() {
wg.Wait()
close(srv.shutdown)
}()
for srv.isStarted() {
rw, err := l.Accept()
if err != nil {
if !srv.isStarted() {
return nil
}
if neterr, ok := err.(net.Error); ok && neterr.Temporary() {
continue
}
return err
}
srv.lock.Lock()
// Track the connection to allow unblocking reads on shutdown.
srv.conns[rw] = struct{}{}
srv.lock.Unlock()
wg.Add(1)
go srv.serveTCPConn(&wg, rw)
}
return nil
}
// serveUDP starts a UDP listener for the server.
func (srv *Server) serveUDP(l *net.UDPConn) error {
defer l.Close()
if srv.NotifyStartedFunc != nil {
srv.NotifyStartedFunc()
}
reader := Reader(defaultReader{srv})
if srv.DecorateReader != nil {
reader = srv.DecorateReader(reader)
}
var wg sync.WaitGroup
defer func() {
wg.Wait()
close(srv.shutdown)
}()
rtimeout := srv.getReadTimeout()
// deadline is not used here
for srv.isStarted() {
m, s, err := reader.ReadUDP(l, rtimeout)
if err != nil {
if !srv.isStarted() {
return nil
}
if netErr, ok := err.(net.Error); ok && netErr.Temporary() {
continue
}
return err
}
if len(m) < headerSize {
if cap(m) == srv.UDPSize {
srv.udpPool.Put(m[:srv.UDPSize])
}
continue
}
wg.Add(1)
go srv.serveUDPPacket(&wg, m, l, s)
}
return nil
}
// Serve a new TCP connection.
func (srv *Server) serveTCPConn(wg *sync.WaitGroup, rw net.Conn) {
w := &response{tsigSecret: srv.TsigSecret, tcp: rw}
if srv.DecorateWriter != nil {
w.writer = srv.DecorateWriter(w)
} else {
w.writer = w
}
reader := Reader(defaultReader{srv})
if srv.DecorateReader != nil {
reader = srv.DecorateReader(reader)
}
idleTimeout := tcpIdleTimeout
if srv.IdleTimeout != nil {
idleTimeout = srv.IdleTimeout()
}
timeout := srv.getReadTimeout()
limit := srv.MaxTCPQueries
if limit == 0 {
limit = maxTCPQueries
}
for q := 0; (q < limit || limit == -1) && srv.isStarted(); q++ {
m, err := reader.ReadTCP(w.tcp, timeout)
if err != nil {
// TODO(tmthrgd): handle error
break
}
srv.serveDNS(m, w)
if w.closed {
break // Close() was called
}
if w.hijacked {
break // client will call Close() themselves
}
// The first read uses the read timeout, the rest use the
// idle timeout.
timeout = idleTimeout
}
if !w.hijacked {
w.Close()
}
srv.lock.Lock()
delete(srv.conns, w.tcp)
srv.lock.Unlock()
wg.Done()
}
// Serve a new UDP request.
func (srv *Server) serveUDPPacket(wg *sync.WaitGroup, m []byte, u *net.UDPConn, s *SessionUDP) {
w := &response{tsigSecret: srv.TsigSecret, udp: u, udpSession: s}
if srv.DecorateWriter != nil {
w.writer = srv.DecorateWriter(w)
} else {
w.writer = w
}
srv.serveDNS(m, w)
wg.Done()
}
func (srv *Server) serveDNS(m []byte, w *response) {
dh, off, err := unpackMsgHdr(m, 0)
if err != nil {
// Let client hang, they are sending crap; any reply can be used to amplify.
return
}
req := new(Msg)
req.setHdr(dh)
switch action := srv.MsgAcceptFunc(dh); action {
case MsgAccept:
if req.unpack(dh, m, off) == nil {
break
}
fallthrough
case MsgReject, MsgRejectNotImplemented:
opcode := req.Opcode
req.SetRcodeFormatError(req)
req.Zero = false
if action == MsgRejectNotImplemented {
req.Opcode = opcode
req.Rcode = RcodeNotImplemented
}
// Are we allowed to delete any OPT records here?
req.Ns, req.Answer, req.Extra = nil, nil, nil
w.WriteMsg(req)
fallthrough
case MsgIgnore:
if w.udp != nil && cap(m) == srv.UDPSize {
srv.udpPool.Put(m[:srv.UDPSize])
}
return
}
w.tsigStatus = nil
if w.tsigSecret != nil {
if t := req.IsTsig(); t != nil {
if secret, ok := w.tsigSecret[t.Hdr.Name]; ok {
w.tsigStatus = TsigVerify(m, secret, "", false)
} else {
w.tsigStatus = ErrSecret
}
w.tsigTimersOnly = false
w.tsigRequestMAC = req.Extra[len(req.Extra)-1].(*TSIG).MAC
}
}
if w.udp != nil && cap(m) == srv.UDPSize {
srv.udpPool.Put(m[:srv.UDPSize])
}
srv.Handler.ServeDNS(w, req) // Writes back to the client
}
func (srv *Server) readTCP(conn net.Conn, timeout time.Duration) ([]byte, error) {
// If we race with ShutdownContext, the read deadline may
// have been set in the distant past to unblock the read
// below. We must not override it, otherwise we may block
// ShutdownContext.
srv.lock.RLock()
if srv.started {
conn.SetReadDeadline(time.Now().Add(timeout))
}
srv.lock.RUnlock()
var length uint16
if err := binary.Read(conn, binary.BigEndian, &length); err != nil {
return nil, err
}
m := make([]byte, length)
if _, err := io.ReadFull(conn, m); err != nil {
return nil, err
}
return m, nil
}
func (srv *Server) readUDP(conn *net.UDPConn, timeout time.Duration) ([]byte, *SessionUDP, error) {
srv.lock.RLock()
if srv.started {
// See the comment in readTCP above.
conn.SetReadDeadline(time.Now().Add(timeout))
}
srv.lock.RUnlock()
m := srv.udpPool.Get().([]byte)
n, s, err := ReadFromSessionUDP(conn, m)
if err != nil {
srv.udpPool.Put(m)
return nil, nil, err
}
m = m[:n]
return m, s, nil
}
// WriteMsg implements the ResponseWriter.WriteMsg method.
func (w *response) WriteMsg(m *Msg) (err error) {
if w.closed {
return &Error{err: "WriteMsg called after Close"}
}
var data []byte
if w.tsigSecret != nil { // if no secrets, dont check for the tsig (which is a longer check)
if t := m.IsTsig(); t != nil {
data, w.tsigRequestMAC, err = TsigGenerate(m, w.tsigSecret[t.Hdr.Name], w.tsigRequestMAC, w.tsigTimersOnly)
if err != nil {
return err
}
_, err = w.writer.Write(data)
return err
}
}
data, err = m.Pack()
if err != nil {
return err
}
_, err = w.writer.Write(data)
return err
}
// Write implements the ResponseWriter.Write method.
func (w *response) Write(m []byte) (int, error) {
if w.closed {
return 0, &Error{err: "Write called after Close"}
}
switch {
case w.udp != nil:
return WriteToSessionUDP(w.udp, m, w.udpSession)
case w.tcp != nil:
if len(m) > MaxMsgSize {
return 0, &Error{err: "message too large"}
}
l := make([]byte, 2)
binary.BigEndian.PutUint16(l, uint16(len(m)))
n, err := (&net.Buffers{l, m}).WriteTo(w.tcp)
return int(n), err
default:
panic("dns: internal error: udp and tcp both nil")
}
}
// LocalAddr implements the ResponseWriter.LocalAddr method.
func (w *response) LocalAddr() net.Addr {
switch {
case w.udp != nil:
return w.udp.LocalAddr()
case w.tcp != nil:
return w.tcp.LocalAddr()
default:
panic("dns: internal error: udp and tcp both nil")
}
}
// RemoteAddr implements the ResponseWriter.RemoteAddr method.
func (w *response) RemoteAddr() net.Addr {
switch {
case w.udpSession != nil:
return w.udpSession.RemoteAddr()
case w.tcp != nil:
return w.tcp.RemoteAddr()
default:
panic("dns: internal error: udpSession and tcp both nil")
}
}
// TsigStatus implements the ResponseWriter.TsigStatus method.
func (w *response) TsigStatus() error { return w.tsigStatus }
// TsigTimersOnly implements the ResponseWriter.TsigTimersOnly method.
func (w *response) TsigTimersOnly(b bool) { w.tsigTimersOnly = b }
// Hijack implements the ResponseWriter.Hijack method.
func (w *response) Hijack() { w.hijacked = true }
// Close implements the ResponseWriter.Close method
func (w *response) Close() error {
if w.closed {
return &Error{err: "connection already closed"}
}
w.closed = true
switch {
case w.udp != nil:
// Can't close the udp conn, as that is actually the listener.
return nil
case w.tcp != nil:
return w.tcp.Close()
default:
panic("dns: internal error: udp and tcp both nil")
}
}
// ConnectionState() implements the ConnectionStater.ConnectionState() interface.
func (w *response) ConnectionState() *tls.ConnectionState {
type tlsConnectionStater interface {
ConnectionState() tls.ConnectionState
}
if v, ok := w.tcp.(tlsConnectionStater); ok {
t := v.ConnectionState()
return &t
}
return nil
}
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