Source: dnsclient.go in package net


package net

import (
	"sort"

	"golang.org/x/net/dns/dnsmessage"
)

provided by runtime

func fastrand() uint32

func randInt() int {
	x, y := fastrand(), fastrand()    // 32-bit halves
	u := uint(x)<<31 ^ uint(int32(y)) // full uint, even on 64-bit systems; avoid 32-bit shift on 32-bit systems
	i := int(u >> 1)                  // clear sign bit, even on 32-bit systems
	return i
}

func randIntn(n int) int {
	return randInt() % n
}

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

Add it, in reverse, to the buffer

	for i := len(ip) - 1; i >= 0; i-- {
		v := ip[i]
		buf = append(buf, hexDigit[v&0xF],
			'.',
			hexDigit[v>>4],
			'.')

Append "ip6.arpa." and return (buf already has the final .)

	buf = append(buf, "ip6.arpa."...)
	return string(buf), nil
}

func equalASCIIName(x, y dnsmessage.Name) bool {
	if x.Length != y.Length {
		return false
	}
	for i := 0; i < int(x.Length); i++ {
		a := x.Data[i]
		b := y.Data[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).

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('.')
	nonNumeric := false // true once we've seen a letter or hyphen
	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 == '_':
			nonNumeric = true
			partlen++

fine

			partlen++

Byte before dash cannot be dot.

			if last == '.' {
				return false
			}
			partlen++
			nonNumeric = true

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 nonNumeric
}

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 := randIntn(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 := randIntn(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