1// Copyright 2009 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// This package is copied from Go library net.
6// https://golang.org/src/net/dnsclient.go
7// The original private function reverseaddr
8// is exported as public function.
9
10package net
11
12import "net"
13
14// Reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
15// address addr suitable for rDNS (PTR) record lookup or an error if it fails
16// to parse the IP address.
17func Reverseaddr(addr string) (arpa string, err error) {
18	ip := net.ParseIP(addr)
19	if ip == nil {
20		return "", &net.DNSError{Err: "unrecognized address", Name: addr}
21	}
22	if ip.To4() != nil {
23		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
24	}
25	// Must be IPv6
26	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
27	// Add it, in reverse, to the buffer
28	for i := len(ip) - 1; i >= 0; i-- {
29		v := ip[i]
30		buf = append(buf, hexDigit[v&0xF],
31			'.',
32			hexDigit[v>>4],
33			'.')
34	}
35	// Append "ip6.arpa." and return (buf already has the final .)
36	buf = append(buf, "ip6.arpa."...)
37	return string(buf), nil
38}
39
40// Convert unsigned integer to decimal string.
41func uitoa(val uint) string {
42	if val == 0 { // avoid string allocation
43		return "0"
44	}
45	var buf [20]byte // big enough for 64bit value base 10
46	i := len(buf) - 1
47	for val >= 10 {
48		q := val / 10
49		buf[i] = byte('0' + val - q*10)
50		i--
51		val = q
52	}
53	// val < 10
54	buf[i] = byte('0' + val)
55	return string(buf[i:])
56}
57
58const hexDigit = "0123456789abcdef"
59