// 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 dns

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

// Send a request on the connection and hope for a reply.
// Up to cfg.attempts attempts.
func exchange(cfg *dnsConfig, c net.Conn, name string, qtype uint16) (*Msg, error) {
	if len(name) >= 256 {
		return nil, &Error{Err: "name too long", Name: name}
	}
	out := new(Msg)
	out.ID = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
	out.Question = []Question{
		{name, qtype, ClassINET},
	}
	out.RecursionDesired = true
	msg, ok := out.Pack()
	if !ok {
		return nil, &Error{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(time.Time{})
		} 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.(net.Error); ok && e.Timeout() {
				continue
			}
			return nil, err
		}
		buf = buf[0:n]
		in := new(Msg)
		if !in.Unpack(buf) || in.ID != out.ID {
			continue
		}
		return in, nil
	}
	var server string
	if a := c.RemoteAddr(); a != nil {
		server = a.String()
	}
	return nil, &Error{Err: "no answer from server", Name: name, Server: server, IsTimeout: true}
}

// Do a lookup for a single name, which must be rooted
// (otherwise answer will not find the answers).
func tryOneName(cfg *dnsConfig, name string, qtype uint16) (cname string, addrs []RR, err error) {
	if len(cfg.servers) == 0 {
		return "", nil, &Error{Err: "no DNS servers", Name: name}
	}
	for i := 0; i < len(cfg.servers); i++ {
		// Calling Dial here is scary -- we have to be sure
		// not to dial a name that will require a DNS lookup,
		// or Dial will call back here to translate it.
		// The DNS config parser has already checked that
		// all the cfg.servers[i] are IP addresses, which
		// Dial will use without a DNS lookup.
		server := cfg.servers[i] + ":53"
		c, cerr := net.Dial("udp", server)
		if cerr != nil {
			err = cerr
			continue
		}
		msg, merr := exchange(cfg, c, name, qtype)
		c.Close()
		if merr != nil {
			err = merr
			continue
		}
		cname, addrs, err = Answer(name, qtype, msg, server)
		if err == nil || err.(*Error).Err == noSuchHost {
			break
		}
	}
	return
}

func convertRR_A(records []RR) []net.IP {
	addrs := make([]net.IP, len(records))
	for i, rr := range records {
		a := rr.(*RR_A).A
		addrs[i] = net.IPv4(byte(a>>24), byte(a>>16), byte(a>>8), byte(a))
	}
	return addrs
}

func convertRR_AAAA(records []RR) []net.IP {
	addrs := make([]net.IP, len(records))
	for i, rr := range records {
		a := make(net.IP, net.IPv6len)
		copy(a, rr.(*RR_AAAA).AAAA[:])
		addrs[i] = a
	}
	return addrs
}

var cfg *dnsConfig
var dnserr error

func loadConfig() { cfg, dnserr = dnsReadConfig() }

var onceLoadConfig sync.Once

var dnsHelper func(name string, qtype uint16) (cname string, addrs []RR, err error, handled bool)

// RegisterHelper registers a helper that will be asked to perform each
// DNS lookup.  If a helper returns handled=true, its results are used.
// If a helper returns handled=false, the standard system DNS is checked
// as a fallback.  It must be safe to call helper from multiple goroutines
// simultaneously.  It is not allowed to call RegisterHelper multiple times.
func RegisterHelper(helper func(name string, qtype uint16) (cname string, addrs []RR, err error, handled bool)) {
	if dnsHelper != nil {
		panic("dns: helper already registered")
	}
	if helper == nil {
		panic("dns: RegisterHelper called with nil helper")
	}
	dnsHelper = helper
}

func lookup(name string, qtype uint16) (cname string, addrs []RR, err error) {
	if !isDomainName(name) {
		return name, nil, &Error{Err: "invalid domain name", Name: name}
	}
	if dnsHelper != nil {
		cname, addrs, err, handled := dnsHelper(name, qtype)
		if handled {
			return cname, addrs, err
		}
	}
	onceLoadConfig.Do(loadConfig)
	if dnserr != nil || cfg == nil {
		err = dnserr
		return
	}
	// If name is rooted (trailing dot) or has enough dots,
	// try it by itself first.
	rooted := len(name) > 0 && name[len(name)-1] == '.'
	if rooted || strings.Count(name, ".") >= cfg.ndots {
		rname := name
		if !rooted {
			rname += "."
		}
		// Can try as ordinary name.
		cname, addrs, err = tryOneName(cfg, rname, qtype)
		if err == nil {
			return
		}
	}
	if rooted {
		return
	}

	// Otherwise, try suffixes.
	for i := 0; i < len(cfg.search); i++ {
		rname := name + "." + cfg.search[i]
		if rname[len(rname)-1] != '.' {
			rname += "."
		}
		cname, addrs, err = tryOneName(cfg, rname, qtype)
		if err == nil {
			return
		}
	}

	// Last ditch effort: try unsuffixed.
	rname := name
	if !rooted {
		rname += "."
	}
	cname, addrs, err = tryOneName(cfg, rname, qtype)
	if err == nil {
		return
	}
	return
}

// LookupHost looks up the given host using the local resolver.
// It returns an array of that host's addresses.
func LookupHost(name string) (addrs []string, err error) {
	// Use entries from /etc/hosts if they match.
	addrs = lookupStaticHost(name)
	if len(addrs) > 0 {
		return
	}
	onceLoadConfig.Do(loadConfig)
	if dnserr != nil || cfg == nil {
		err = dnserr
		return
	}
	ips, err := LookupIP(name)
	if err != nil {
		return
	}
	addrs = make([]string, 0, len(ips))
	for _, ip := range ips {
		addrs = append(addrs, ip.String())
	}
	return
}

// LookupIP looks up host using the local resolver.
// It returns an array of that host's IPv4 and IPv6 addresses.
func LookupIP(name string) (addrs []net.IP, err error) {
	// Use entries from /etc/hosts if possible.
	haddrs := lookupStaticHost(name)
	if len(haddrs) > 0 {
		for _, haddr := range haddrs {
			if ip := net.ParseIP(haddr); ip != nil {
				addrs = append(addrs, ip)
			}
		}
		if len(addrs) > 0 {
			return
		}
	}
	onceLoadConfig.Do(loadConfig)
	if dnserr != nil || cfg == nil {
		err = dnserr
		return
	}
	var records []RR
	var cname string
	cname, records, err = lookup(name, TypeA)
	if err != nil {
		return
	}
	addrs = convertRR_A(records)
	if cname != "" {
		name = cname
	}
	_, records, err = lookup(name, TypeAAAA)
	if err != nil && len(addrs) > 0 {
		// Ignore error because A lookup succeeded.
		err = nil
	}
	if err != nil {
		return
	}
	addrs = append(addrs, convertRR_AAAA(records)...)
	return
}

// LookupCNAME returns the canonical DNS host for the given name.
// Callers that do not care about the canonical name can call
// LookupHost or LookupIP directly; both take care of resolving
// the canonical name as part of the lookup.
func LookupCNAME(name string) (cname string, err error) {
	onceLoadConfig.Do(loadConfig)
	if dnserr != nil || cfg == nil {
		err = dnserr
		return
	}
	_, rr, err := lookup(name, TypeCNAME)
	if err != nil {
		return
	}
	cname = rr[0].(*RR_CNAME).Cname
	return
}