src/net/dnsclient.go - go - Git at Google

 // 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.

 package net

 import (
 	"math/rand"
 	"sort"
 )

 // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
 // address addr suitable for rDNS (PTR) record lookup or an error if it fails
 // to parse the IP address.
 func reverseaddr(addr string) (arpa string, err error) {
 	ip := ParseIP(addr)
 	if ip == nil {
 		return "", &DNSError{Err: "unrecognized address", Name: addr}
 	}
 	if ip.To4() != nil {
 		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
 	}
 	// Must be IPv6
 	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
 	// Add it, in reverse, to the buffer
 	for i := len(ip) - 1; i >= 0; i-- {
 		v := ip[i]
 		buf = append(buf, hexDigit[v&0xF])
 		buf = append(buf, '.')
 		buf = append(buf, hexDigit[v>>4])
 		buf = append(buf, '.')
 	}
 	// Append "ip6.arpa." and return (buf already has the final .)
 	buf = append(buf, "ip6.arpa."...)
 	return string(buf), nil
 }

 // Find answer for name in dns message.
 // On return, if err == nil, addrs != nil.
 func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err error) {
 	addrs = make([]dnsRR, 0, len(dns.answer))

 	if dns.rcode == dnsRcodeNameError {
 		return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
 	}
 	if dns.rcode != dnsRcodeSuccess {
 		// None of the error codes make sense
 		// for the query we sent. If we didn't get
 		// a name error and we didn't get success,
 		// the server is behaving incorrectly or
 		// having temporary trouble.
 		err := &DNSError{Err: "server misbehaving", Name: name, Server: server}
 		if dns.rcode == dnsRcodeServerFailure {
 			err.IsTemporary = true
 		}
 		return "", nil, err
 	}

 	// Look for the name.
 	// Presotto says it's okay to assume that servers listed in
 	// /etc/resolv.conf are recursive resolvers.
 	// We asked for recursion, so it should have included
 	// all the answers we need in this one packet.
 Cname:
 	for cnameloop := 0; cnameloop < 10; cnameloop++ {
 		addrs = addrs[0:0]
 		for _, rr := range dns.answer {
 			if _, justHeader := rr.(*dnsRR_Header); justHeader {
 				// Corrupt record: we only have a
 				// header. That header might say it's
 				// of type qtype, but we don't
 				// actually have it. Skip.
 				continue
 			}
 			h := rr.Header()
 			if h.Class == dnsClassINET && equalASCIILabel(h.Name, name) {
 				switch h.Rrtype {
 				case qtype:
 					addrs = append(addrs, rr)
 				case dnsTypeCNAME:
 					// redirect to cname
 					name = rr.(*dnsRR_CNAME).Cname
 					continue Cname
 				}
 			}
 		}
 		if len(addrs) == 0 {
 			return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
 		}
 		return name, addrs, nil
 	}

 	return "", nil, &DNSError{Err: "too many redirects", Name: name, Server: server}
 }

 func equalASCIILabel(x, y string) bool {
 	if len(x) != len(y) {
 		return false
 	}
 	for i := 0; i < len(x); i++ {
 		a := x[i]
 		b := y[i]
 		if 'A' <= a && a <= 'Z' {
 			a += 0x20
 		}
 		if 'A' <= b && b <= 'Z' {
 			b += 0x20
 		}
 		if a != b {
 			return false
 		}
 	}
 	return true
 }

 // isDomainName checks if a string is a presentation-format domain name
 // (currently restricted to hostname-compatible "preferred name" LDH labels and
 // SRV-like "underscore labels"; see golang.org/issue/12421).
 func isDomainName(s string) bool {
 	// See RFC 1035, RFC 3696.
 	// Presentation format has dots before every label except the first, and the
 	// terminal empty label is optional here because we assume fully-qualified
 	// (absolute) input. We must therefore reserve space for the first and last
 	// labels' length octets in wire format, where they are necessary and the
 	// maximum total length is 255.
 	// So our _effective_ maximum is 253, but 254 is not rejected if the last
 	// character is a dot.
 	l := len(s)
 	if l == 0 || l > 254 || l == 254 && s[l-1] != '.' {
 		return false
 	}

 	last := byte('.')
 	ok := false // Ok once we've seen a letter.
 	partlen := 0
 	for i := 0; i < len(s); i++ {
 		c := s[i]
 		switch {
 		default:
 			return false
 		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
 			ok = true
 			partlen++
 		case '0' <= c && c <= '9':
 			// fine
 			partlen++
 		case c == '-':
 			// Byte before dash cannot be dot.
 			if last == '.' {
 				return false
 			}
 			partlen++
 		case c == '.':
 			// Byte before dot cannot be dot, dash.
 			if last == '.' || last == '-' {
 				return false
 			}
 			if partlen > 63 || partlen == 0 {
 				return false
 			}
 			partlen = 0
 		}
 		last = c
 	}
 	if last == '-' || partlen > 63 {
 		return false
 	}

 	return ok
 }

 // absDomainName returns an absolute domain name which ends with a
 // trailing dot to match pure Go reverse resolver and all other lookup
 // routines.
 // See golang.org/issue/12189.
 // But we don't want to add dots for local names from /etc/hosts.
 // It's hard to tell so we settle on the heuristic that names without dots
 // (like "localhost" or "myhost") do not get trailing dots, but any other
 // names do.
 func absDomainName(b []byte) string {
 	hasDots := false
 	for _, x := range b {
 		if x == '.' {
 			hasDots = true
 			break
 		}
 	}
 	if hasDots && b[len(b)-1] != '.' {
 		b = append(b, '.')
 	}
 	return string(b)
 }

 // An SRV represents a single DNS SRV record.
 type SRV struct {
 	Target   string
 	Port     uint16
 	Priority uint16
 	Weight   uint16
 }

 // byPriorityWeight sorts SRV records by ascending priority and weight.
 type byPriorityWeight []*SRV

 func (s byPriorityWeight) Len() int { return len(s) }
 func (s byPriorityWeight) Less(i, j int) bool {
 	return s[i].Priority < s[j].Priority || (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
 }
 func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

 // shuffleByWeight shuffles SRV records by weight using the algorithm
 // described in RFC 2782.
 func (addrs byPriorityWeight) shuffleByWeight() {
 	sum := 0
 	for _, addr := range addrs {
 		sum += int(addr.Weight)
 	}
 	for sum > 0 && len(addrs) > 1 {
 		s := 0
 		n := rand.Intn(sum)
 		for i := range addrs {
 			s += int(addrs[i].Weight)
 			if s > n {
 				if i > 0 {
 					addrs[0], addrs[i] = addrs[i], addrs[0]
 				}
 				break
 			}
 		}
 		sum -= int(addrs[0].Weight)
 		addrs = addrs[1:]
 	}
 }

 // sort reorders SRV records as specified in RFC 2782.
 func (addrs byPriorityWeight) sort() {
 	sort.Sort(addrs)
 	i := 0
 	for j := 1; j < len(addrs); j++ {
 		if addrs[i].Priority != addrs[j].Priority {
 			addrs[i:j].shuffleByWeight()
 			i = j
 		}
 	}
 	addrs[i:].shuffleByWeight()
 }

 // An MX represents a single DNS MX record.
 type MX struct {
 	Host string
 	Pref uint16
 }

 // byPref implements sort.Interface to sort MX records by preference
 type byPref []*MX

 func (s byPref) Len() int           { return len(s) }
 func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
 func (s byPref) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

 // sort reorders MX records as specified in RFC 5321.
 func (s byPref) sort() {
 	for i := range s {
 		j := rand.Intn(i + 1)
 		s[i], s[j] = s[j], s[i]
 	}
 	sort.Sort(s)
 }

 // An NS represents a single DNS NS record.
 type NS struct {
 	Host string
 }
	// 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.

	package net

	import (
	"math/rand"
	"sort"
	)

	// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
	// address addr suitable for rDNS (PTR) record lookup or an error if it fails
	// to parse the IP address.
	func reverseaddr(addr string) (arpa string, err error) {
	ip := ParseIP(addr)
	if ip == nil {
	return "", &DNSError{Err: "unrecognized address", Name: addr}
	}
	if ip.To4() != nil {
	return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
	}
	// Must be IPv6
	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
	// Add it, in reverse, to the buffer
	for i := len(ip) - 1; i >= 0; i-- {
	v := ip[i]
	buf = append(buf, hexDigit[v&0xF])
	buf = append(buf, '.')
	buf = append(buf, hexDigit[v>>4])
	buf = append(buf, '.')
	}
	// Append "ip6.arpa." and return (buf already has the final .)
	buf = append(buf, "ip6.arpa."...)
	return string(buf), nil
	}

	// Find answer for name in dns message.
	// On return, if err == nil, addrs != nil.
	func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err error) {
	addrs = make([]dnsRR, 0, len(dns.answer))

	if dns.rcode == dnsRcodeNameError {
	return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
	}
	if dns.rcode != dnsRcodeSuccess {
	// None of the error codes make sense
	// for the query we sent. If we didn't get
	// a name error and we didn't get success,
	// the server is behaving incorrectly or
	// having temporary trouble.
	err := &DNSError{Err: "server misbehaving", Name: name, Server: server}
	if dns.rcode == dnsRcodeServerFailure {
	err.IsTemporary = true
	}
	return "", nil, err
	}

	// Look for the name.
	// Presotto says it's okay to assume that servers listed in
	// /etc/resolv.conf are recursive resolvers.
	// We asked for recursion, so it should have included
	// all the answers we need in this one packet.
	Cname:
	for cnameloop := 0; cnameloop < 10; cnameloop++ {
	addrs = addrs[0:0]
	for _, rr := range dns.answer {
	if _, justHeader := rr.(*dnsRR_Header); justHeader {
	// Corrupt record: we only have a
	// header. That header might say it's
	// of type qtype, but we don't
	// actually have it. Skip.
	continue
	}
	h := rr.Header()
	if h.Class == dnsClassINET && equalASCIILabel(h.Name, name) {
	switch h.Rrtype {
	case qtype:
	addrs = append(addrs, rr)
	case dnsTypeCNAME:
	// redirect to cname
	name = rr.(*dnsRR_CNAME).Cname
	continue Cname
	}
	}
	}
	if len(addrs) == 0 {
	return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
	}
	return name, addrs, nil
	}

	return "", nil, &DNSError{Err: "too many redirects", Name: name, Server: server}
	}

	func equalASCIILabel(x, y string) bool {
	if len(x) != len(y) {
	return false
	}
	for i := 0; i < len(x); i++ {
	a := x[i]
	b := y[i]
	if 'A' <= a && a <= 'Z' {
	a += 0x20
	}
	if 'A' <= b && b <= 'Z' {
	b += 0x20
	}
	if a != b {
	return false
	}
	}
	return true
	}

	// isDomainName checks if a string is a presentation-format domain name
	// (currently restricted to hostname-compatible "preferred name" LDH labels and
	// SRV-like "underscore labels"; see golang.org/issue/12421).
	func isDomainName(s string) bool {
	// See RFC 1035, RFC 3696.
	// Presentation format has dots before every label except the first, and the
	// terminal empty label is optional here because we assume fully-qualified
	// (absolute) input. We must therefore reserve space for the first and last
	// labels' length octets in wire format, where they are necessary and the
	// maximum total length is 255.
	// So our _effective_ maximum is 253, but 254 is not rejected if the last
	// character is a dot.
	l := len(s)
	if l == 0 \|\| l > 254 \|\| l == 254 && s[l-1] != '.' {
	return false
	}

	last := byte('.')
	ok := false // Ok once we've seen a letter.
	partlen := 0
	for i := 0; i < len(s); i++ {
	c := s[i]
	switch {
	default:
	return false
	case 'a' <= c && c <= 'z' \|\| 'A' <= c && c <= 'Z' \|\| c == '_':
	ok = true
	partlen++
	case '0' <= c && c <= '9':
	// fine
	partlen++
	case c == '-':
	// Byte before dash cannot be dot.
	if last == '.' {
	return false
	}
	partlen++
	case c == '.':
	// Byte before dot cannot be dot, dash.
	if last == '.' \|\| last == '-' {
	return false
	}
	if partlen > 63 \|\| partlen == 0 {
	return false
	}
	partlen = 0
	}
	last = c
	}
	if last == '-' \|\| partlen > 63 {
	return false
	}

	return ok
	}

	// absDomainName returns an absolute domain name which ends with a
	// trailing dot to match pure Go reverse resolver and all other lookup
	// routines.
	// See golang.org/issue/12189.
	// But we don't want to add dots for local names from /etc/hosts.
	// It's hard to tell so we settle on the heuristic that names without dots
	// (like "localhost" or "myhost") do not get trailing dots, but any other
	// names do.
	func absDomainName(b []byte) string {
	hasDots := false
	for _, x := range b {
	if x == '.' {
	hasDots = true
	break
	}
	}
	if hasDots && b[len(b)-1] != '.' {
	b = append(b, '.')
	}
	return string(b)
	}

	// An SRV represents a single DNS SRV record.
	type SRV struct {
	Target string
	Port uint16
	Priority uint16
	Weight uint16
	}

	// byPriorityWeight sorts SRV records by ascending priority and weight.
	type byPriorityWeight []*SRV

	func (s byPriorityWeight) Len() int { return len(s) }
	func (s byPriorityWeight) Less(i, j int) bool {
	return s[i].Priority < s[j].Priority \|\| (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
	}
	func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

	// shuffleByWeight shuffles SRV records by weight using the algorithm
	// described in RFC 2782.
	func (addrs byPriorityWeight) shuffleByWeight() {
	sum := 0
	for _, addr := range addrs {
	sum += int(addr.Weight)
	}
	for sum > 0 && len(addrs) > 1 {
	s := 0
	n := rand.Intn(sum)
	for i := range addrs {
	s += int(addrs[i].Weight)
	if s > n {
	if i > 0 {
	addrs[0], addrs[i] = addrs[i], addrs[0]
	}
	break
	}
	}
	sum -= int(addrs[0].Weight)
	addrs = addrs[1:]
	}
	}

	// sort reorders SRV records as specified in RFC 2782.
	func (addrs byPriorityWeight) sort() {
	sort.Sort(addrs)
	i := 0
	for j := 1; j < len(addrs); j++ {
	if addrs[i].Priority != addrs[j].Priority {
	addrs[i:j].shuffleByWeight()
	i = j
	}
	}
	addrs[i:].shuffleByWeight()
	}

	// An MX represents a single DNS MX record.
	type MX struct {
	Host string
	Pref uint16
	}

	// byPref implements sort.Interface to sort MX records by preference
	type byPref []*MX

	func (s byPref) Len() int { return len(s) }
	func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
	func (s byPref) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

	// sort reorders MX records as specified in RFC 5321.
	func (s byPref) sort() {
	for i := range s {
	j := rand.Intn(i + 1)
	s[i], s[j] = s[j], s[i]
	}
	sort.Sort(s)
	}

	// An NS represents a single DNS NS record.
	type NS struct {
	Host string
	}