package dns // A client implementation. import ( "bytes" "context" "crypto/tls" "encoding/binary" "io" "net" "strings" "time" ) const dnsTimeout time.Duration = 2 * time.Second const tcpIdleTimeout time.Duration = 8 * time.Second // A Conn represents a connection to a DNS server. type Conn struct { net.Conn // a net.Conn holding the connection UDPSize uint16 // minimum receive buffer for UDP messages TsigSecret map[string]string // secret(s) for Tsig map[], zonename must be in canonical form (lowercase, fqdn, see RFC 4034 Section 6.2) tsigRequestMAC string } // A Client defines parameters for a DNS client. type Client struct { Net string // if "tcp" or "tcp-tls" (DNS over TLS) a TCP query will be initiated, otherwise an UDP one (default is "" for UDP) UDPSize uint16 // minimum receive buffer for UDP messages TLSConfig *tls.Config // TLS connection configuration Dialer *net.Dialer // a net.Dialer used to set local address, timeouts and more // Timeout is a cumulative timeout for dial, write and read, defaults to 0 (disabled) - overrides DialTimeout, ReadTimeout, // WriteTimeout when non-zero. Can be overridden with net.Dialer.Timeout (see Client.ExchangeWithDialer and // Client.Dialer) or context.Context.Deadline (see the deprecated ExchangeContext) Timeout time.Duration DialTimeout time.Duration // net.DialTimeout, defaults to 2 seconds, or net.Dialer.Timeout if expiring earlier - overridden by Timeout when that value is non-zero ReadTimeout time.Duration // net.Conn.SetReadTimeout value for connections, defaults to 2 seconds - overridden by Timeout when that value is non-zero WriteTimeout time.Duration // net.Conn.SetWriteTimeout value for connections, defaults to 2 seconds - overridden by Timeout when that value is non-zero TsigSecret map[string]string // secret(s) for Tsig map[], zonename must be in canonical form (lowercase, fqdn, see RFC 4034 Section 6.2) SingleInflight bool // if true suppress multiple outstanding queries for the same Qname, Qtype and Qclass group singleflight } // Exchange performs a synchronous UDP query. It sends the message m to the address // contained in a and waits for a reply. Exchange does not retry a failed query, nor // will it fall back to TCP in case of truncation. // See client.Exchange for more information on setting larger buffer sizes. func Exchange(m *Msg, a string) (r *Msg, err error) { client := Client{Net: "udp"} r, _, err = client.Exchange(m, a) return r, err } func (c *Client) dialTimeout() time.Duration { if c.Timeout != 0 { return c.Timeout } if c.DialTimeout != 0 { return c.DialTimeout } return dnsTimeout } func (c *Client) readTimeout() time.Duration { if c.ReadTimeout != 0 { return c.ReadTimeout } return dnsTimeout } func (c *Client) writeTimeout() time.Duration { if c.WriteTimeout != 0 { return c.WriteTimeout } return dnsTimeout } // Dial connects to the address on the named network. func (c *Client) Dial(address string) (conn *Conn, err error) { // create a new dialer with the appropriate timeout var d net.Dialer if c.Dialer == nil { d = net.Dialer{} } else { d = net.Dialer(*c.Dialer) } d.Timeout = c.getTimeoutForRequest(c.writeTimeout()) network := "udp" useTLS := false switch c.Net { case "tcp-tls": network = "tcp" useTLS = true case "tcp4-tls": network = "tcp4" useTLS = true case "tcp6-tls": network = "tcp6" useTLS = true default: if c.Net != "" { network = c.Net } } conn = new(Conn) if useTLS { conn.Conn, err = tls.DialWithDialer(&d, network, address, c.TLSConfig) } else { conn.Conn, err = d.Dial(network, address) } if err != nil { return nil, err } return conn, nil } // Exchange performs a synchronous query. It sends the message m to the address // contained in a and waits for a reply. Basic use pattern with a *dns.Client: // // c := new(dns.Client) // in, rtt, err := c.Exchange(message, "127.0.0.1:53") // // Exchange does not retry a failed query, nor will it fall back to TCP in // case of truncation. // It is up to the caller to create a message that allows for larger responses to be // returned. Specifically this means adding an EDNS0 OPT RR that will advertise a larger // buffer, see SetEdns0. Messages without an OPT RR will fallback to the historic limit // of 512 bytes // To specify a local address or a timeout, the caller has to set the `Client.Dialer` // attribute appropriately func (c *Client) Exchange(m *Msg, address string) (r *Msg, rtt time.Duration, err error) { if !c.SingleInflight { return c.exchange(m, address) } t := "nop" if t1, ok := TypeToString[m.Question[0].Qtype]; ok { t = t1 } cl := "nop" if cl1, ok := ClassToString[m.Question[0].Qclass]; ok { cl = cl1 } r, rtt, err, shared := c.group.Do(m.Question[0].Name+t+cl, func() (*Msg, time.Duration, error) { return c.exchange(m, address) }) if r != nil && shared { r = r.Copy() } return r, rtt, err } func (c *Client) exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err error) { var co *Conn co, err = c.Dial(a) if err != nil { return nil, 0, err } defer co.Close() opt := m.IsEdns0() // If EDNS0 is used use that for size. if opt != nil && opt.UDPSize() >= MinMsgSize { co.UDPSize = opt.UDPSize() } // Otherwise use the client's configured UDP size. if opt == nil && c.UDPSize >= MinMsgSize { co.UDPSize = c.UDPSize } co.TsigSecret = c.TsigSecret t := time.Now() // write with the appropriate write timeout co.SetWriteDeadline(t.Add(c.getTimeoutForRequest(c.writeTimeout()))) if err = co.WriteMsg(m); err != nil { return nil, 0, err } co.SetReadDeadline(time.Now().Add(c.getTimeoutForRequest(c.readTimeout()))) r, err = co.ReadMsg() if err == nil && r.Id != m.Id { err = ErrId } rtt = time.Since(t) return r, rtt, err } // ReadMsg reads a message from the connection co. // If the received message contains a TSIG record the transaction signature // is verified. This method always tries to return the message, however if an // error is returned there are no guarantees that the returned message is a // valid representation of the packet read. func (co *Conn) ReadMsg() (*Msg, error) { p, err := co.ReadMsgHeader(nil) if err != nil { return nil, err } m := new(Msg) if err := m.Unpack(p); err != nil { // If an error was returned, we still want to allow the user to use // the message, but naively they can just check err if they don't want // to use an erroneous message return m, err } if t := m.IsTsig(); t != nil { if _, ok := co.TsigSecret[t.Hdr.Name]; !ok { return m, ErrSecret } // Need to work on the original message p, as that was used to calculate the tsig. err = TsigVerify(p, co.TsigSecret[t.Hdr.Name], co.tsigRequestMAC, false) } return m, err } // ReadMsgHeader reads a DNS message, parses and populates hdr (when hdr is not nil). // Returns message as a byte slice to be parsed with Msg.Unpack later on. // Note that error handling on the message body is not possible as only the header is parsed. func (co *Conn) ReadMsgHeader(hdr *Header) ([]byte, error) { var ( p []byte n int err error ) switch t := co.Conn.(type) { case *net.TCPConn, *tls.Conn: r := t.(io.Reader) // First two bytes specify the length of the entire message. l, err := tcpMsgLen(r) if err != nil { return nil, err } p = make([]byte, l) n, err = tcpRead(r, p) default: if co.UDPSize > MinMsgSize { p = make([]byte, co.UDPSize) } else { p = make([]byte, MinMsgSize) } n, err = co.Read(p) } if err != nil { return nil, err } else if n < headerSize { return nil, ErrShortRead } p = p[:n] if hdr != nil { dh, _, err := unpackMsgHdr(p, 0) if err != nil { return nil, err } *hdr = dh } return p, err } // tcpMsgLen is a helper func to read first two bytes of stream as uint16 packet length. func tcpMsgLen(t io.Reader) (int, error) { p := []byte{0, 0} n, err := t.Read(p) if err != nil { return 0, err } // As seen with my local router/switch, returns 1 byte on the above read, // resulting a a ShortRead. Just write it out (instead of loop) and read the // other byte. if n == 1 { n1, err := t.Read(p[1:]) if err != nil { return 0, err } n += n1 } if n != 2 { return 0, ErrShortRead } l := binary.BigEndian.Uint16(p) if l == 0 { return 0, ErrShortRead } return int(l), nil } // tcpRead calls TCPConn.Read enough times to fill allocated buffer. func tcpRead(t io.Reader, p []byte) (int, error) { n, err := t.Read(p) if err != nil { return n, err } for n < len(p) { j, err := t.Read(p[n:]) if err != nil { return n, err } n += j } return n, err } // Read implements the net.Conn read method. func (co *Conn) Read(p []byte) (n int, err error) { if co.Conn == nil { return 0, ErrConnEmpty } if len(p) < 2 { return 0, io.ErrShortBuffer } switch t := co.Conn.(type) { case *net.TCPConn, *tls.Conn: r := t.(io.Reader) l, err := tcpMsgLen(r) if err != nil { return 0, err } if l > len(p) { return int(l), io.ErrShortBuffer } return tcpRead(r, p[:l]) } // UDP connection n, err = co.Conn.Read(p) if err != nil { return n, err } return n, err } // WriteMsg sends a message through the connection co. // If the message m contains a TSIG record the transaction // signature is calculated. func (co *Conn) WriteMsg(m *Msg) (err error) { var out []byte if t := m.IsTsig(); t != nil { mac := "" if _, ok := co.TsigSecret[t.Hdr.Name]; !ok { return ErrSecret } out, mac, err = TsigGenerate(m, co.TsigSecret[t.Hdr.Name], co.tsigRequestMAC, false) // Set for the next read, although only used in zone transfers co.tsigRequestMAC = mac } else { out, err = m.Pack() } if err != nil { return err } if _, err = co.Write(out); err != nil { return err } return nil } // Write implements the net.Conn Write method. func (co *Conn) Write(p []byte) (n int, err error) { switch t := co.Conn.(type) { case *net.TCPConn, *tls.Conn: w := t.(io.Writer) lp := len(p) if lp < 2 { return 0, io.ErrShortBuffer } if lp > MaxMsgSize { return 0, &Error{err: "message too large"} } l := make([]byte, 2, lp+2) binary.BigEndian.PutUint16(l, uint16(lp)) p = append(l, p...) n, err := io.Copy(w, bytes.NewReader(p)) return int(n), err } n, err = co.Conn.Write(p) return n, err } // Return the appropriate timeout for a specific request func (c *Client) getTimeoutForRequest(timeout time.Duration) time.Duration { var requestTimeout time.Duration if c.Timeout != 0 { requestTimeout = c.Timeout } else { requestTimeout = timeout } // net.Dialer.Timeout has priority if smaller than the timeouts computed so // far if c.Dialer != nil && c.Dialer.Timeout != 0 { if c.Dialer.Timeout < requestTimeout { requestTimeout = c.Dialer.Timeout } } return requestTimeout } // Dial connects to the address on the named network. func Dial(network, address string) (conn *Conn, err error) { conn = new(Conn) conn.Conn, err = net.Dial(network, address) if err != nil { return nil, err } return conn, nil } // ExchangeContext performs a synchronous UDP query, like Exchange. It // additionally obeys deadlines from the passed Context. func ExchangeContext(ctx context.Context, m *Msg, a string) (r *Msg, err error) { client := Client{Net: "udp"} r, _, err = client.ExchangeContext(ctx, m, a) // ignorint rtt to leave the original ExchangeContext API unchanged, but // this function will go away return r, err } // ExchangeConn performs a synchronous query. It sends the message m via the connection // c and waits for a reply. The connection c is not closed by ExchangeConn. // This function is going away, but can easily be mimicked: // // co := &dns.Conn{Conn: c} // c is your net.Conn // co.WriteMsg(m) // in, _ := co.ReadMsg() // co.Close() // func ExchangeConn(c net.Conn, m *Msg) (r *Msg, err error) { println("dns: ExchangeConn: this function is deprecated") co := new(Conn) co.Conn = c if err = co.WriteMsg(m); err != nil { return nil, err } r, err = co.ReadMsg() if err == nil && r.Id != m.Id { err = ErrId } return r, err } // DialTimeout acts like Dial but takes a timeout. func DialTimeout(network, address string, timeout time.Duration) (conn *Conn, err error) { client := Client{Net: network, Dialer: &net.Dialer{Timeout: timeout}} conn, err = client.Dial(address) if err != nil { return nil, err } return conn, nil } // DialWithTLS connects to the address on the named network with TLS. func DialWithTLS(network, address string, tlsConfig *tls.Config) (conn *Conn, err error) { if !strings.HasSuffix(network, "-tls") { network += "-tls" } client := Client{Net: network, TLSConfig: tlsConfig} conn, err = client.Dial(address) if err != nil { return nil, err } return conn, nil } // DialTimeoutWithTLS acts like DialWithTLS but takes a timeout. func DialTimeoutWithTLS(network, address string, tlsConfig *tls.Config, timeout time.Duration) (conn *Conn, err error) { if !strings.HasSuffix(network, "-tls") { network += "-tls" } client := Client{Net: network, Dialer: &net.Dialer{Timeout: timeout}, TLSConfig: tlsConfig} conn, err = client.Dial(address) if err != nil { return nil, err } return conn, nil } // ExchangeContext acts like Exchange, but honors the deadline on the provided // context, if present. If there is both a context deadline and a configured // timeout on the client, the earliest of the two takes effect. func (c *Client) ExchangeContext(ctx context.Context, m *Msg, a string) (r *Msg, rtt time.Duration, err error) { var timeout time.Duration if deadline, ok := ctx.Deadline(); !ok { timeout = 0 } else { timeout = deadline.Sub(time.Now()) } // not passing the context to the underlying calls, as the API does not support // context. For timeouts you should set up Client.Dialer and call Client.Exchange. c.Dialer = &net.Dialer{Timeout: timeout} return c.Exchange(m, a) }