package dns // A client implementation. import ( "context" "crypto/tls" "encoding/binary" "fmt" "io" "net" "strings" "time" ) const ( dnsTimeout time.Duration = 2 * time.Second 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 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{Timeout: c.getTimeoutForRequest(c.dialTimeout())} } else { d = *c.Dialer } network := c.Net if network == "" { network = "udp" } useTLS := strings.HasPrefix(network, "tcp") && strings.HasSuffix(network, "-tls") conn = new(Conn) if useTLS { network = strings.TrimSuffix(network, "-tls") conn.Conn, err = tls.DialWithDialer(&d, network, address, c.TLSConfig) } else { conn.Conn, err = d.Dial(network, address) } if err != nil { return nil, err } conn.UDPSize = c.UDPSize 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) { co, err := c.Dial(address) if err != nil { return nil, 0, err } defer co.Close() return c.ExchangeWithConn(m, co) } // ExchangeWithConn has the same behavior as Exchange, just with a predetermined connection // that will be used instead of creating a new one. // Usage pattern with a *dns.Client: // c := new(dns.Client) // // connection management logic goes here // // conn := c.Dial(address) // in, rtt, err := c.ExchangeWithConn(message, conn) // // This allows users of the library to implement their own connection management, // as opposed to Exchange, which will always use new connections and incur the added overhead // that entails when using "tcp" and especially "tcp-tls" clients. func (c *Client) ExchangeWithConn(m *Msg, conn *Conn) (r *Msg, rtt time.Duration, err error) { if !c.SingleInflight { return c.exchange(m, conn) } q := m.Question[0] key := fmt.Sprintf("%s:%d:%d", q.Name, q.Qtype, q.Qclass) r, rtt, err, shared := c.group.Do(key, func() (*Msg, time.Duration, error) { return c.exchange(m, conn) }) if r != nil && shared { r = r.Copy() } return r, rtt, err } func (c *Client) exchange(m *Msg, co *Conn) (r *Msg, rtt time.Duration, err error) { 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()))) if _, ok := co.Conn.(net.PacketConn); ok { for { r, err = co.ReadMsg() // Ignore replies with mismatched IDs because they might be // responses to earlier queries that timed out. if err != nil || r.Id == m.Id { break } } } else { 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 ) if _, ok := co.Conn.(net.PacketConn); ok { if co.UDPSize > MinMsgSize { p = make([]byte, co.UDPSize) } else { p = make([]byte, MinMsgSize) } n, err = co.Read(p) } else { var length uint16 if err := binary.Read(co.Conn, binary.BigEndian, &length); err != nil { return nil, err } p = make([]byte, length) n, err = io.ReadFull(co.Conn, 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 } // 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 _, ok := co.Conn.(net.PacketConn); ok { // UDP connection return co.Conn.Read(p) } var length uint16 if err := binary.Read(co.Conn, binary.BigEndian, &length); err != nil { return 0, err } if int(length) > len(p) { return 0, io.ErrShortBuffer } return io.ReadFull(co.Conn, p[:length]) } // 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 } _, err = co.Write(out) return err } // Write implements the net.Conn Write method. func (co *Conn) Write(p []byte) (int, error) { if len(p) > MaxMsgSize { return 0, &Error{err: "message too large"} } if _, ok := co.Conn.(net.PacketConn); ok { return co.Conn.Write(p) } l := make([]byte, 2) binary.BigEndian.PutUint16(l, uint16(len(p))) n, err := (&net.Buffers{l, p}).WriteTo(co.Conn) return int(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. // Deprecated: 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}} return client.Dial(address) } // 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} return client.Dial(address) } // 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} return client.Dial(address) } // 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 = time.Until(deadline) } // 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. // TODO(tmthrgd,miekg): this is a race condition. c.Dialer = &net.Dialer{Timeout: timeout} return c.Exchange(m, a) }