client.go

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>]<base64 secret>, 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>]<base64 secret>, 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
	}