Source: hosts.go in package net


package net

import (
	"internal/bytealg"
	"sync"
	"time"
)

const cacheMaxAge = 5 * time.Second

func parseLiteralIP(addr string) string {
	var ip IP
	var zone string
	ip = parseIPv4(addr)
	if ip == nil {
		ip, zone = parseIPv6Zone(addr)
	}
	if ip == nil {
		return ""
	}
	if zone == "" {
		return ip.String()
	}
	return ip.String() + "%" + zone
}

hosts contains known host entries.

var hosts struct {
	sync.Mutex

Key for the list of literal IP addresses must be a host name. It would be part of DNS labels, a FQDN or an absolute FQDN. For now the key is converted to lower case for convenience.

	byName map[string][]string

Key for the list of host names must be a literal IP address including IPv6 address with zone identifier. We don't support old-classful IP address notation.

	byAddr map[string][]string

	expire time.Time
	path   string
	mtime  time.Time
	size   int64
}

func readHosts() {
	now := time.Now()
	hp := testHookHostsPath

	if now.Before(hosts.expire) && hosts.path == hp && len(hosts.byName) > 0 {
		return
	}
	mtime, size, err := stat(hp)
	if err == nil && hosts.path == hp && hosts.mtime.Equal(mtime) && hosts.size == size {
		hosts.expire = now.Add(cacheMaxAge)
		return
	}

	hs := make(map[string][]string)
	is := make(map[string][]string)
	var file *file
	if file, _ = open(hp); file == nil {
		return
	}
	for line, ok := file.readLine(); ok; line, ok = file.readLine() {

Discard comments.

			line = line[0:i]
		}
		f := getFields(line)
		if len(f) < 2 {
			continue
		}
		addr := parseLiteralIP(f[0])
		if addr == "" {
			continue
		}
		for i := 1; i < len(f); i++ {
			name := absDomainName([]byte(f[i]))
			h := []byte(f[i])
			lowerASCIIBytes(h)
			key := absDomainName(h)
			hs[key] = append(hs[key], addr)
			is[addr] = append(is[addr], name)
		}

Update the data cache.

	hosts.expire = now.Add(cacheMaxAge)
	hosts.path = hp
	hosts.byName = hs
	hosts.byAddr = is
	hosts.mtime = mtime
	hosts.size = size
	file.close()
}

lookupStaticHost looks up the addresses for the given host from /etc/hosts.

func lookupStaticHost(host string) []string {
	hosts.Lock()
	defer hosts.Unlock()
	readHosts()

TODO(jbd,bradfitz): avoid this alloc if host is already all lowercase? or linear scan the byName map if it's small enough?

		lowerHost := []byte(host)
		lowerASCIIBytes(lowerHost)
		if ips, ok := hosts.byName[absDomainName(lowerHost)]; ok {
			ipsCp := make([]string, len(ips))
			copy(ipsCp, ips)
			return ipsCp
		}
	}
	return nil
}

lookupStaticAddr looks up the hosts for the given address from /etc/hosts.

func lookupStaticAddr(addr string) []string {
	hosts.Lock()
	defer hosts.Unlock()
	readHosts()
	addr = parseLiteralIP(addr)
	if addr == "" {
		return nil
	}
	if len(hosts.byAddr) != 0 {
		if hosts, ok := hosts.byAddr[addr]; ok {
			hostsCp := make([]string, len(hosts))
			copy(hostsCp, hosts)
			return hostsCp
		}
	}
	return nil