net: expand nss myhostname fallback detection

Expand myhostname fallback detection to properly detect the local
hostname in addition to other supported special names and suffixes.

Fixes #17967

Change-Id: I1fe141fd9838b25886c08b6f2fd325e58be60457
Reviewed-on: https://go-review.googlesource.com/33550Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/conf.go

@@ -179,8 +179,6 @@ func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
 		}
 	}
 
-	hasDot := byteIndex(hostname, '.') != -1
-
 	// Canonicalize the hostname by removing any trailing dot.
 	if stringsHasSuffix(hostname, ".") {
 		hostname = hostname[:len(hostname)-1]
@@ -220,10 +218,14 @@ func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
 	var first string
 	for _, src := range srcs {
 		if src.source == "myhostname" {
-			if hostname == "" || hasDot {
-				continue
+			if isLocalhost(hostname) || isGateway(hostname) {
+				return fallbackOrder
 			}
-			return fallbackOrder
+			hn, err := getHostname()
+			if err != nil || stringsEqualFold(hostname, hn) {
+				return fallbackOrder
+			}
+			continue
 		}
 		if src.source == "files" || src.source == "dns" {
 			if !src.standardCriteria() {
@@ -306,3 +308,15 @@ func goDebugNetDNS() (dnsMode string, debugLevel int) {
 	parsePart(goDebug)
 	return
 }
+
+// isLocalhost reports whether h should be considered a "localhost"
+// name for the myhostname NSS module.
+func isLocalhost(h string) bool {
+	return stringsEqualFold(h, "localhost") || stringsEqualFold(h, "localhost.localdomain") || stringsHasSuffixFold(h, ".localhost") || stringsHasSuffixFold(h, ".localhost.localdomain")
+}
+
+// isGateway reports whether h should be considered a "gateway"
+// name for the myhostname NSS module.
+func isGateway(h string) bool {
+	return stringsEqualFold(h, "gateway")
+}

src/net/parse.go

@@ -334,22 +334,28 @@ func stringsHasSuffix(s, suffix string) bool {
 // stringsHasSuffixFold reports whether s ends in suffix,
 // ASCII-case-insensitively.
 func stringsHasSuffixFold(s, suffix string) bool {
-	if len(suffix) > len(s) {
+	return len(s) >= len(suffix) && stringsEqualFold(s[len(s)-len(suffix):], suffix)
+}
+
+// stringsHasPrefix is strings.HasPrefix. It reports whether s begins with prefix.
+func stringsHasPrefix(s, prefix string) bool {
+	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
+}
+
+// stringsEqualFold is strings.EqualFold, ASCII only. It reports whether s and t
+// are equal, ASCII-case-insensitively.
+func stringsEqualFold(s, t string) bool {
+	if len(s) != len(t) {
 		return false
 	}
-	for i := 0; i < len(suffix); i++ {
-		if lowerASCII(suffix[i]) != lowerASCII(s[len(s)-len(suffix)+i]) {
+	for i := 0; i < len(s); i++ {
+		if lowerASCII(s[i]) != lowerASCII(t[i]) {
 			return false
 		}
 	}
 	return true
 }
 
-// stringsHasPrefix is strings.HasPrefix. It reports whether s begins with prefix.
-func stringsHasPrefix(s, prefix string) bool {
-	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
-}
-
 func readFull(r io.Reader) (all []byte, err error) {
 	buf := make([]byte, 1024)
 	for {