// 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.
// Extended and bugfixes by Miek Gieben. Copyright 2010-2012.

// DOMAIN NAME SYSTEM
//
// Package dns implements a full featured interface to the Domain Name System.
// The package allows complete control over what is send out to the DNS. The package
// API follows the less-is-more principle, but presenting a small, clean interface.
//
// Resource records are native types. They are not stored in wire format.
// Basic usage pattern for creating a new resource record:
//
//      r := new(RR_TXT)
//      r.Hdr = RR_Header{Name: "miek.nl.", Rrtype: TypeMX, Class: ClassINET, Ttl: 3600}
//      r.Pref = 10
//      r.Mx = "mx.miek.nl."
//
// Or directly from a string:
//
//      mx := NewRR("miek.nl. 3600 IN MX 10 mx.miek.nl.")
//
// Or when the default TTL (3600) and class (IN) suit you:
//
//      mx := NewRR("miek.nl. MX 10 mx.miek.nl.")
//
// Or even:
//
//      mx := NewRR("$ORIGIN nl.\nmiek 1H IN MX 10 mx.miek"
// 
// The package dns supports (async) querying/replying, incoming/outgoing Axfr/Ixfr, 
// TSIG, EDNS0, dynamic updates, notifies and DNSSEC validation/signing.
// Note that domain names MUST be full qualified, before sending them. The packages
// enforces this, by throwing a panic().
//
// In the DNS messages are exchanged. Use pattern for creating one:
//
//      m := new(Msg)
//      m.SetQuestion("miek.nl.", TypeMX)
//
// The message m is now a message with the question section set to ask
// the MX records for the miek.nl. zone.
//
// The following is slightly more verbose, but more flexible:
//
//      m1 := new(Msg)
//      m1.MsgHdr.Id = Id()
//      m1.MsgHdr.RecursionDesired = false
//      m1.Question = make([]Question, 1)
//      m1.Question[0] = Question{"miek.nl.", TypeMX, ClassINET}
//
// After creating a message it can be send.
// Basic use pattern for synchronous querying the DNS: 
//
//      // We are sending the message 'm' to the server 127.0.0.1 
//      // on port 53 and wait for the reply.
//      c := NewClient()
//      // c.Net = "tcp" // If you want to use TCP
//      in := c.Exchange(m, "127.0.0.1:53")
//
// An asynchronous query is also possible, setting up is more elaborate then
// a synchronous query. The Basic use pattern is:
// 
//      HandleQueryFunc(".", handler)
//      ListenAndQuery(nil, nil)
//      c.Do(m1, "127.0.0.1:53")
//      // Do something else
//      r := <- DefaultReplyChan
//      // r.Reply is the answer
//      // r.Request is the original request
package dns

import (
	"net"
	"strconv"
)

const (
	Year68         = 1 << 32 // For RFC1982 (Serial Arithmetic) calculations in 32 bits.
	DefaultMsgSize = 4096    // Standard default for larger than 512 packets.
	UDPMsgSize     = 512     // Default buffer size for servers receiving UDP packets.
	MaxMsgSize     = 65536   // Largest possible DNS packet.
	DefaultTtl     = 3600    // Default TTL.
)

// Error represents a DNS error
type Error struct {
	Err     string
	Name    string
	Server  net.Addr
	Timeout bool
}

func (e *Error) Error() string {
	if e == nil {
		return "<nil>"
	}
	if e.Name == "" {
		return e.Err
	}
	return e.Name + ": " + e.Err

}

type RR interface {
	Header() *RR_Header
	String() string
	Len() int
}

// Exchange is used in communicating with the resolver.
type Exchange struct {
	Request *Msg  // The question sent.
	Reply   *Msg  // The answer to the question that was sent.
	Error   error // If something went wrong, this contains the error.
}

// DNS resource records.
// There are many types of messages,
// but they all share the same header.
type RR_Header struct {
	Name     string "cdomain-name"
	Rrtype   uint16
	Class    uint16
	Ttl      uint32
	Rdlength uint16 // length of data after header
}

func (h *RR_Header) Header() *RR_Header {
	return h
}

func (h *RR_Header) String() string {
	var s string

	if h.Rrtype == TypeOPT {
		s = ";"
		// and maybe other things
	}

	if len(h.Name) == 0 {
		s += ".\t"
	} else {
		s += h.Name + "\t"
	}
	s = s + strconv.Itoa(int(h.Ttl)) + "\t"

	if _, ok := Class_str[h.Class]; ok {
		s += Class_str[h.Class] + "\t"
	} else {
		s += "CLASS" + strconv.Itoa(int(h.Class)) + "\t"
	}

	if _, ok := Rr_str[h.Rrtype]; ok {
		s += Rr_str[h.Rrtype] + "\t"
	} else {
		s += "TYPE" + strconv.Itoa(int(h.Rrtype)) + "\t"
	}
	return s
}

func (h *RR_Header) Len() int {
	l := len(h.Name) + 1
	l += 10 // rrtype(2) + class(2) + ttl(4) + rdlength(2)
	return l
}

func zoneMatch(pattern, zone string) (ok bool) {
	if len(pattern) == 0 {
		return
	}
	if len(zone) == 0 {
		zone = "."
	}
	pattern = Fqdn(pattern)
	zone = Fqdn(zone)
	i := 0
	for {
		ok = pattern[len(pattern)-1-i] == zone[len(zone)-1-i]
		i++

		if !ok {
			break
		}
		if len(pattern)-1-i < 0 || len(zone)-1-i < 0 {
			break
		}

	}
	return
}