net: don't do DNS for onion and local addresses

Fixes #13705

Change-Id: I86c60c78ce0394f830f904c9cba83ebbf3efc046
Reviewed-on: https://go-review.googlesource.com/21328
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -319,8 +319,25 @@ func lookup(name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
 	return
 }
 
+// avoidDNS reports whether this is a hostname for which we should not
+// use DNS. Currently this includes only .onion and .local names,
+// per RFC 7686 and RFC 6762, respectively. See golang.org/issue/13705.
+func avoidDNS(name string) bool {
+	if name == "" {
+		return true
+	}
+	if name[len(name)-1] == '.' {
+		name = name[:len(name)-1]
+	}
+	return stringsHasSuffixFold(name, ".onion") || stringsHasSuffixFold(name, ".local")
+}
+
 // nameList returns a list of names for sequential DNS queries.
 func (conf *dnsConfig) nameList(name string) []string {
+	if avoidDNS(name) {
+		return nil
+	}
+
 	// If name is rooted (trailing dot), try only that name.
 	rooted := len(name) > 0 && name[len(name)-1] == '.'
 	if rooted {

src/net/dnsclient_unix_test.go

@@ -94,6 +94,57 @@ func TestSpecialDomainName(t *testing.T) {
 	}
 }
 
+// Issue 13705: don't try to resolve onion addresses, etc
+func TestAvoidDNSName(t *testing.T) {
+	tests := []struct {
+		name  string
+		avoid bool
+	}{
+		{"foo.com", false},
+		{"foo.com.", false},
+
+		{"foo.onion.", true},
+		{"foo.onion", true},
+		{"foo.ONION", true},
+		{"foo.ONION.", true},
+
+		{"foo.local.", true},
+		{"foo.local", true},
+		{"foo.LOCAL", true},
+		{"foo.LOCAL.", true},
+
+		{"", true}, // will be rejected earlier too
+
+		// Without stuff before onion/local, they're fine to
+		// use DNS. With a search path,
+		// "onion.vegegtables.com" can use DNS. Without a
+		// search path (or with a trailing dot), the queries
+		// are just kinda useless, but don't reveal anything
+		// private.
+		{"local", false},
+		{"onion", false},
+		{"local.", false},
+		{"onion.", false},
+	}
+	for _, tt := range tests {
+		got := avoidDNS(tt.name)
+		if got != tt.avoid {
+			t.Errorf("avoidDNS(%q) = %v; want %v", tt.name, got, tt.avoid)
+		}
+	}
+}
+
+// Issue 13705: don't try to resolve onion addresses, etc
+func TestLookupTorOnion(t *testing.T) {
+	addrs, err := goLookupIP("foo.onion")
+	if len(addrs) > 0 {
+		t.Errorf("unexpected addresses: %v", addrs)
+	}
+	if err != nil {
+		t.Fatalf("lookup = %v; want nil", err)
+	}
+}
+
 type resolvConfTest struct {
 	dir  string
 	path string