http: don't proxy loopback addresses

Fixes #1589

R=rsc
CC=golang-dev
https://golang.org/cl/4443053

src/pkg/http/proxy_test.go

@@ -16,9 +16,15 @@ var UseProxyTests = []struct {
 	host  string
 	match bool
 }{
-	{"localhost", false},      // match completely
+	// Never proxy localhost:
+	{"localhost:80", false},
+	{"127.0.0.1", false},
+	{"127.0.0.2", false},
+	{"[::1]", false},
+	{"[::2]", true}, // not a loopback address
+
 	{"barbaz.net", false},     // match as .barbaz.net
-	{"foobar.com:443", false}, // have a port but match 
+	{"foobar.com", false},     // have a port but match 
 	{"foofoobar.com", true},   // not match as a part of foobar.com
 	{"baz.com", true},         // not match as a part of barbaz.com
 	{"localhost.net", true},   // not match as suffix of address
@@ -29,19 +35,16 @@ var UseProxyTests = []struct {
 
 func TestUseProxy(t *testing.T) {
 	oldenv := os.Getenv("NO_PROXY")
-	no_proxy := "foobar.com, .barbaz.net   , localhost"
-	os.Setenv("NO_PROXY", no_proxy)
 	defer os.Setenv("NO_PROXY", oldenv)
 
+	no_proxy := "foobar.com, .barbaz.net"
+	os.Setenv("NO_PROXY", no_proxy)
+
 	tr := &Transport{}
 
 	for _, test := range UseProxyTests {
-		if tr.useProxy(test.host) != test.match {
-			if test.match {
-				t.Errorf("useProxy(%v) = %v, want %v", test.host, !test.match, test.match)
-			} else {
-				t.Errorf("not expected: '%s' shouldn't match as '%s'", test.host, no_proxy)
-			}
+		if tr.useProxy(test.host+":80") != test.match {
+			t.Errorf("useProxy(%v) = %v, want %v", test.host, !test.match, test.match)
 		}
 	}
 }

src/pkg/http/transport.go

@@ -6,6 +6,7 @@ package http
 
 import (
 	"bufio"
+	"bytes"
 	"compress/gzip"
 	"crypto/tls"
 	"encoding/base64"
@@ -291,10 +292,28 @@ func (t *Transport) getConn(cm *connectMethod) (*persistConn, os.Error) {
 
 // useProxy returns true if requests to addr should use a proxy,
 // according to the NO_PROXY or no_proxy environment variable.
+// addr is always a canonicalAddr with a host and port.
 func (t *Transport) useProxy(addr string) bool {
 	if len(addr) == 0 {
 		return true
 	}
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return false
+	}
+	if host == "localhost" {
+		return false
+	}
+	if ip := net.ParseIP(host); ip != nil {
+		if ip4 := ip.To4(); ip4 != nil && ip4[0] == 127 {
+			// 127.0.0.0/8 loopback isn't proxied.
+			return false
+		}
+		if bytes.Equal(ip, net.IPv6loopback) {
+			return false
+		}
+	}
+
 	no_proxy := t.getenvEitherCase("NO_PROXY")
 	if no_proxy == "*" {
 		return false

src/pkg/net/ip.go

@@ -75,7 +75,8 @@ var (
 
 // Well-known IPv6 addresses
 var (
-	IPzero = make(IP, IPv6len) // all zeros
+	IPzero       = make(IP, IPv6len) // all zeros
+	IPv6loopback = IP([]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1})
 )
 
 // Is p all zeros?