Use /etc/hosts when resolving names.

http://code.google.com/p/go/issues/detail?id=313

This conflics with Chris' patch at:
https://golang.org/cl/181063

But I believe this is more complete since it has a simple caching and proper tests.

R=cw, rsc
CC=golang-dev
https://golang.org/cl/183066

src/pkg/net/Makefile

@@ -11,6 +11,7 @@ GOFILES=\
 	dnsmsg.go\
 	fd.go\
 	fd_$(GOOS).go\
+	hosts.go\
 	ip.go\
 	ipsock.go\
 	net.go\

src/pkg/net/dnsclient.go

@@ -224,7 +224,7 @@ func isDomainName(s string) bool {
 	return ok
 }
 
-// LookupHost looks up the host name using the local DNS resolver.
+// LookupHost looks for name using the local hosts file and DNS resolver.
 // It returns the canonical name for the host and an array of that
 // host's addresses.
 func LookupHost(name string) (cname string, addrs []string, err os.Error) {
@@ -236,7 +236,12 @@ func LookupHost(name string) (cname string, addrs []string, err os.Error) {
 		err = dnserr
 		return
 	}
-
+	// Use entries from /etc/hosts if they match.
+	addrs = lookupStaticHost(name)
+	if len(addrs) > 0 {
+		cname = name
+		return
+	}
 	// If name is rooted (trailing dot) or has enough dots,
 	// try it by itself first.
 	rooted := len(name) > 0 && name[len(name)-1] == '.'

src/pkg/net/hosts.go

+// 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.
+
+// Read static host/IP entries from /etc/hosts.
+
+package net
+
+import (
+	"os"
+	"sync"
+)
+
+const cacheMaxAge = int64(300) // 5 minutes.
+
+// hostsPath points to the file with static IP/address entries.
+var hostsPath = "/etc/hosts"
+
+// Simple cache.
+var hosts struct {
+	sync.Mutex
+	data map[string][]string
+	time int64
+	path string
+}
+
+func readHosts() {
+	now, _, _ := os.Time()
+	hp := hostsPath
+	if len(hosts.data) == 0 || hosts.time+cacheMaxAge <= now || hosts.path != hp {
+		hs := make(map[string][]string)
+		var file *file
+		file, _ = open(hp)
+		for line, ok := file.readLine(); ok; line, ok = file.readLine() {
+			if i := byteIndex(line, '#'); i >= 0 {
+				// Discard comments.
+				line = line[0:i]
+			}
+			f := getFields(line)
+			if len(f) < 2 || ParseIP(f[0]) == nil {
+				continue
+			}
+			h := f[1]
+			old, _ := hs[h]
+			hs[h] = appendHost(old, f[0])
+		}
+		// Update the data cache.
+		hosts.time, _, _ = os.Time()
+		hosts.path = hp
+		hosts.data = hs
+		file.close()
+	}
+}
+
+func appendHost(hosts []string, address string) []string {
+	n := len(hosts)
+	if n+1 > cap(hosts) { // reallocate
+		a := make([]string, n, 2*n+1)
+		copy(a, hosts)
+		hosts = a
+	}
+	hosts = hosts[0 : n+1]
+	hosts[n] = address
+	return hosts
+}
+
+// lookupStaticHosts looks up the addresses for the given host from /etc/hosts.
+func lookupStaticHost(host string) []string {
+	hosts.Lock()
+	defer hosts.Unlock()
+	readHosts()
+	if len(hosts.data) != 0 {
+		if ips, ok := hosts.data[host]; ok {
+			return ips
+		}
+	}
+	return nil
+}

src/pkg/net/hosts_test.go

+// 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 (
+	"testing"
+)
+
+type hostTest struct {
+	host string
+	ips  []IP
+}
+
+
+var hosttests = []hostTest{
+	hostTest{"odin", []IP{
+		IPv4(127, 0, 0, 2),
+		IPv4(127, 0, 0, 3),
+		ParseIP("::2"),
+	}},
+	hostTest{"thor", []IP{
+		IPv4(127, 1, 1, 1),
+	}},
+	hostTest{"loki", []IP{}},
+}
+
+func TestLookupStaticHost(t *testing.T) {
+	p := hostsPath
+	hostsPath = "hosts_testdata"
+	for i := 0; i < len(hosttests); i++ {
+		tt := hosttests[i]
+		ips := lookupStaticHost(tt.host)
+		if len(ips) != len(tt.ips) {
+			t.Errorf("# of hosts = %v; want %v",
+				len(ips), len(tt.ips))
+			return
+		}
+		for k, v := range ips {
+			if tt.ips[k].String() != v {
+				t.Errorf("lookupStaticHost(%q) = %v; want %v",
+					tt.host, v, tt.ips[k])
+			}
+		}
+	}
+	hostsPath = p
+}

src/pkg/net/hosts_testdata

+255.255.255.255	broadcasthost
+127.0.0.2	odin
+127.0.0.3	odin  # inline comment 
+::2             odin
+127.1.1.1	thor
+# Bogus entries that must be ignored.
+123.123.123	loki
+321.321.321.321
+# TODO(yvesj): Should we be able to parse this? From a Darwin system.
+fe80::1%lo0	localhost

src/pkg/net/ip.go

@@ -278,6 +278,10 @@ func parseIPv4(s string) IP {
 	var p [IPv4len]byte
 	i := 0
 	for j := 0; j < IPv4len; j++ {
+		if i >= len(s) {
+			// Missing octets.
+			return nil
+		}
 		if j > 0 {
 			if s[i] != '.' {
 				return nil