net: make /etc/hosts lookups case-insensitive

The native Go host resolver was behaving differently than libc
and the entries in the /etc/hosts were handled in a case sensitive
way. In order to be compatible with libc's resolver, /etc/hosts
lookups must be case-insensitive.

Fixes #12806.

Change-Id: I3c14001abffadf7458fd1a027c91e6438a87f285
Reviewed-on: https://go-review.googlesource.com/15321
Run-TryBot: Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/hosts.go

@@ -60,9 +60,11 @@ func readHosts() {
 				continue
 			}
 			for i := 1; i < len(f); i++ {
-				h := f[i]
-				hs[h] = append(hs[h], addr)
-				is[addr] = append(is[addr], h)
+				h := []byte(f[i])
+				lowerASCIIBytes(h)
+				lh := string(h)
+				hs[lh] = append(hs[lh], addr)
+				is[addr] = append(is[addr], lh)
 			}
 		}
 		// Update the data cache.
@@ -80,7 +82,11 @@ func lookupStaticHost(host string) []string {
 	defer hosts.Unlock()
 	readHosts()
 	if len(hosts.byName) != 0 {
-		if ips, ok := hosts.byName[host]; ok {
+		// 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[string(lowerHost)]; ok {
 			return ips
 		}
 	}

src/net/hosts_test.go

@@ -74,6 +74,7 @@ var lookupStaticAddrTests = []struct {
 			{"255.255.255.255", []string{"broadcasthost"}},
 			{"127.0.0.2", []string{"odin"}},
 			{"127.0.0.3", []string{"odin"}},
+			{"127.0.0.4", []string{"bor"}},
 			{"::2", []string{"odin"}},
 			{"127.1.1.1", []string{"thor"}},
 			{"127.1.1.2", []string{"ullr", "ullrhost"}},

src/net/testdata/hosts

 255.255.255.255	broadcasthost
 127.0.0.2	odin
 127.0.0.3	odin  # inline comment 
+# case insensitivity
+127.0.0.4	Bor
 ::2             odin
 127.1.1.1	thor
 # aliases