Pull request: dnsforward: fix clientid check

Closes #3437.

Squashed commit of the following:

commit fc4207a6ee1a09ade9db5eb5c8b58f88011db2f9
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Aug 12 18:22:31 2021 +0300

    dnsforward: imp code, docs

commit 0c608e0b7ca0b68b7810fc1ca798fb7d80d6ac24
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Aug 12 18:01:22 2021 +0300

    dnsforward: fix clientid check

CHANGELOG.md

@@ -64,6 +64,7 @@ and this project adheres to
 
 ### Fixed
 
+- Client ID checking ([#3437]).
 - Discovering other DHCP servers on `darwin` and `freebsd` ([#3417]).
 - Switching listening address to unspecified one when bound to a single
   specified IPv4 address on Darwin (macOS) ([#2807]).
@@ -122,6 +123,7 @@ and this project adheres to
 [#3351]: https://github.com/AdguardTeam/AdGuardHome/issues/3351
 [#3372]: https://github.com/AdguardTeam/AdGuardHome/issues/3372
 [#3417]: https://github.com/AdguardTeam/AdGuardHome/issues/3417
+[#3437]: https://github.com/AdguardTeam/AdGuardHome/issues/3437
 
 
 

internal/dnsforward/clientid.go

@@ -24,21 +24,39 @@ func ValidateClientID(clientID string) (err error) {
 	return nil
 }
 
+// hasLabelSuffix returns true if s ends with suffix preceded by a dot.  It's
+// a helper function to prevent unnecessary allocations in code like:
+//
+// if strings.HasSuffix(s, "." + suffix) { /* … */ }
+//
+// s must be longer than suffix.
+func hasLabelSuffix(s, suffix string) (ok bool) {
+	return strings.HasSuffix(s, suffix) && s[len(s)-len(suffix)-1] == '.'
+}
+
 // clientIDFromClientServerName extracts and validates a client ID.  hostSrvName
 // is the server name of the host.  cliSrvName is the server name as sent by the
 // client.  When strict is true, and client and host server name don't match,
 // clientIDFromClientServerName will return an error.
-func clientIDFromClientServerName(hostSrvName, cliSrvName string, strict bool) (clientID string, err error) {
+func clientIDFromClientServerName(
+	hostSrvName string,
+	cliSrvName string,
+	strict bool,
+) (clientID string, err error) {
 	if hostSrvName == cliSrvName {
 		return "", nil
 	}
 
-	if !strings.HasSuffix(cliSrvName, hostSrvName) {
+	if !hasLabelSuffix(cliSrvName, hostSrvName) {
 		if !strict {
 			return "", nil
 		}
 
-		return "", fmt.Errorf("client server name %q doesn't match host server name %q", cliSrvName, hostSrvName)
+		return "", fmt.Errorf(
+			"client server name %q doesn't match host server name %q",
+			cliSrvName,
+			hostSrvName,
+		)
 	}
 
 	clientID = cliSrvName[:len(cliSrvName)-len(hostSrvName)-1]

internal/dnsforward/clientid_test.go

@@ -134,6 +134,15 @@ func TestServer_clientIDFromDNSContext(t *testing.T) {
 		wantClientID: "cli",
 		wantErrMsg:   "",
 		strictSNI:    true,
+	}, {
+		name:         "tls_client_id_issue3437",
+		proto:        proxy.ProtoTLS,
+		hostSrvName:  "example.com",
+		cliSrvName:   "cli.myexample.com",
+		wantClientID: "",
+		wantErrMsg: `client id check: client server name "cli.myexample.com" ` +
+			`doesn't match host server name "example.com"`,
+		strictSNI: true,
 	}}
 
 	for _, tc := range testCases {