dnsclient_unix.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build darwin freebsd linux netbsd openbsd

// DNS client: see RFC 1035.
// Has to be linked into package net for Dial.

// TODO(rsc):
//	Check periodically whether /etc/resolv.conf has changed.
//	Could potentially handle many outstanding lookups faster.
//	Could have a small cache.
//	Random UDP source port (net.Dial should do that for us).
//	Random request IDs.

package net

import (
	"math/rand"
	"sync"
	"time"
)

// Send a request on the connection and hope for a reply.
// Up to cfg.attempts attempts.
func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
	if len(name) >= 256 {
		return nil, &DNSError{Err: "name too long", Name: name}
	}
	out := new(dnsMsg)
	out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
	out.question = []dnsQuestion{
		{name, qtype, dnsClassINET},
	}
	out.recursion_desired = true
	msg, ok := out.Pack()
	if !ok {
		return nil, &DNSError{Err: "internal error - cannot pack message", Name: name}
	}

	for attempt := 0; attempt < cfg.attempts; attempt++ {
		n, err := c.Write(msg)
		if err != nil {
			return nil, err
		}

		if cfg.timeout == 0 {
			c.SetReadDeadline(noDeadline)
		} else {
			c.SetReadDeadline(time.Now().Add(time.Duration(cfg.timeout) * time.Second))
		}

		buf := make([]byte, 2000) // More than enough.
		n, err = c.Read(buf)
		if err != nil {
			if e, ok := err.(Error); ok && e.Timeout() {
				continue
			}
			return nil, err
		}
		buf = buf[0:n]
		in := new(dnsMsg)
		if !in.Unpack(buf) || in.id != out.id {
			continue
		}
		return in, nil
	}
	var server string
	if a := c.RemoteAddr(); a != nil {