Repository: kubernetes-sigs/apiserver-network-proxy

Issue:
http-connect: incorrect behaviour when the outbound connection not established
In its current state in http-connect mode, proxy-server sends OK 200 after sucessfully hijacking the connection from the client
[link to code](https://github.com/kubernetes-sigs/apiserver-network-proxy/blob/3737fb2ee2a985e8efd5829891cf838b474a1c85/pkg/server/tunnel.go#L57-#L69)

This forces the connection to be in tunnel mode as per the HTTP CONNECT [protocol](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/CONNECT)

```
The request target is unique to this method in that it consists of only the host and port number of the tunnel destination,
separated by a colon (see [Syntax](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/CONNECT#syntax) for details). 
Any [2XX successful response status code](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status#successful_responses)
means that the proxy will switch to 'tunnel mode' and any data in the success response body is from the server identified by the request target.
```

This behaviour is incorrect and it can be reproduced by dialing to a blackhole ip address of any other address that we know the agent will fail in dialing to.

Reproducing setup


Start server
```
./bin/proxy-server --mode=http-connect --server-ca-cert=certs/frontend/issued/ca.crt --server-cert=certs/frontend/issued/proxy-frontend.crt --server-key=certs/frontend/private/proxy-frontend.key --cluster-ca-cert=certs/agent/issued/ca.crt --cluster-cert=certs/agent/issued/proxy-frontend.crt --cluster-key=certs/agent/private/proxy-frontend.key
```

Start agent
```
./bin/proxy-agent --ca-cert=certs/agent/issued/ca.crt --agent-cert=certs/agent/issued/proxy-agent.crt --agent-key=certs/agent/private/proxy-agent.key
```

Test-client
```
./bin/proxy-test-client --mode=http-connect  --proxy-host=127.0.0.1 --ca-cert=certs/frontend/issued/ca.crt --client-cert=certs/frontend/issued/proxy-client.crt --client-key=certs/frontend/private/proxy-client.key --request-host=192.18.0.254 --request-path="" --request-port=1234 --request-proto=https
```

Response sent to the client:

```
❯ ./bin/proxy-test-client --mode=http-connect  --proxy-host=127.0.0.1 --ca-cert=certs/frontend/issued/ca.crt --client-cert=certs/frontend/issued/proxy-client.crt --client-key=certs/frontend/private/proxy-client.key --request-host=192.18.0.254 --request-path="" --request-port=1234 --request-proto=https


I0809 01:58:00.335502 1375650 main.go:115] ClientCert set to "certs/frontend/issued/proxy-client.crt".
I0809 01:58:00.335540 1375650 main.go:116] ClientKey set to "certs/frontend/private/proxy-client.key".
I0809 01:58:00.335549 1375650 main.go:117] CACert set to "certs/frontend/issued/ca.crt".
I0809 01:58:00.335556 1375650 main.go:118] RequestProto set to "https".
I0809 01:58:00.335563 1375650 main.go:119] RequestPath set to "".
I0809 01:58:00.335570 1375650 main.go:120] RequestHost set to "192.18.0.254".
I0809 01:58:00.335578 1375650 main.go:121] RequestPort set to 1234.
I0809 01:58:00.335585 1375650 main.go:122] ProxyHost set to "127.0.0.1".
I0809 01:58:00.335590 1375650 main.go:123] ProxyPort set to 8090.
I0809 01:58:00.335597 1375650 main.go:124] ProxyUdsName set to "".
I0809 01:58:00.335603 1375650 main.go:125] TestRequests set to '\x01'.
I0809 01:58:00.335611 1375650 main.go:126] TestDelaySec set to 0.
I0809 01:58:00.335618 1375650 main.go:127] AfterDelaySec set to 0.
I0809 01:58:00.335626 1375650 main.go:128] CloseIdleConn set to true.
E0809 01:58:05.342972 1375650 main.go:317] Failed request failed to send request to client, got Get "https://192.18.0.254:1234": http: server gave HTTP response to HTTPS client
Error: failed to send request to client, got Get "https://192.18.0.254:1234": http: server gave HTTP response to HTTPS client
E0809 01:58:05.342995 1375650 main.go:64] error: failed to send request to client, got Get "https://192.18.0.254:1234": http: server gave HTTP response to HTTPS client
```


Which is incorrect, we should first of all not be sending 200/OK before establishing the outbound connection. Even after delaying the response, the client should not be getting that error response.

The blanket error response of 503 thats sent [here](https://github.com/kubernetes-sigs/apiserver-network-proxy/blob/3737fb2ee2a985e8efd5829891cf838b474a1c85/pkg/server/server.go#L125-#L134) is incorrect.

Appropriate error mapping is required.

Hints from issue comments:

Solution patch:
diff --git a/pkg/server/server.go b/pkg/server/server.go
index 8ca3f0ce..ffec433c 100644
--- a/pkg/server/server.go
+++ b/pkg/server/server.go
@@ -108,6 +108,53 @@ const (
 	destHostKey key = iota
 )
 
+// mapDialErrorToHTTPStatus maps common TCP/network error strings to appropriate HTTP status codes
+func mapDialErrorToHTTPStatus(errStr string) int {
+	// Convert to lowercase for case-insensitive matching
+	errLower := strings.ToLower(errStr)
+
+	// Check each error pattern and return appropriate status code
+	switch {
+	// Timeouts - backend didn't respond in time -> 504 Gateway Timeout
+	case strings.Contains(errLower, "i/o timeout"),
+		strings.Contains(errLower, "deadline exceeded"),
+		strings.Contains(errLower, "context deadline exceeded"),
+		strings.Contains(errLower, "timeout"):
+		return 504
+
+	// Resource exhaustion errors -> 503 Service Unavailable
+	case strings.Contains(errLower, "too many open files"),
+		strings.Contains(errLower, "socket: too many open files"):
+		return 503
+
+	// Connection errors -> 502 Bad Gateway
+	case strings.Contains(errLower, "connection refused"),
+		strings.Contains(errLower, "connection reset by peer"),
+		strings.Contains(errLower, "broken pipe"),
+		strings.Contains(errLower, "network is unreachable"),
+		strings.Contains(errLower, "no route to host"),
+		strings.Contains(errLower, "host is unreachable"),
+		strings.Contains(errLower, "network is down"):
+		return 502
+
+	// DNS resolution failures -> 502 Bad Gateway
+	case strings.Contains(errLower, "no such host"),
+		strings.Contains(errLower, "name resolution"),
+		strings.Contains(errLower, "lookup") && strings.Contains(errLower, "no such host"):
+		return 502
+
+	// TLS/SSL errors -> 502 Bad Gateway
+	case strings.Contains(errLower, "tls"),
+		strings.Contains(errLower, "ssl"),
+		strings.Contains(errLower, "certificate"):
+		return 502
+
+	// Default to 502 Bad Gateway for unknown proxy errors
+	default:
+		return 502
+	}
+}
+
 func (c *ProxyClientConnection) send(pkt *client.Packet) error {
 	defer func(start time.Time) { metrics.Metrics.ObserveFrontendWriteLatency(time.Since(start)) }(time.Now())
 	if c.Mode == ModeGRPC {
@@ -122,11 +169,21 @@ func (c *ProxyClientConnection) send(pkt *client.Packet) error {
 			_, err := c.HTTP.Write(pkt.GetData().Data)
 			return err
 		} else if pkt.Type == client.PacketType_DIAL_RSP {
-			if pkt.GetDialResponse().Error != "" {
-				body := bytes.NewBufferString(pkt.GetDialResponse().Error)
+			dialErr := pkt.GetDialResponse().Error
+			if dialErr != "" {
+				// // Map the error to appropriate HTTP status code
+				statusCode := mapDialErrorToHTTPStatus(dialErr)
+				statusText := http.StatusText(statusCode)
+				body := bytes.NewBufferString(dialErr)
 				t := http.Response{
-					StatusCode: 503,
+					StatusCode: statusCode,
+					Status:     fmt.Sprintf("%d %s", statusCode, statusText),
 					Body:       io.NopCloser(body),
+					Header: http.Header{
+						"Content-Type": []string{"text/plain; charset=utf-8"},
+					},
+					Proto:      "HTTP/1.1",
+					ProtoMinor: 1,
 				}
 
 				t.Write(c.HTTP)
diff --git a/pkg/server/tunnel.go b/pkg/server/tunnel.go
index 5f04c02f..ad81179c 100644
--- a/pkg/server/tunnel.go
+++ b/pkg/server/tunnel.go
@@ -60,14 +60,6 @@ func (t *Tunnel) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Send the HTTP 200 OK status after a successful hijack
-	_, err = conn.Write([]byte("HTTP/1.1 200 Connection Established\r\n\r\n"))
-	if err != nil {
-		klog.ErrorS(err, "failed to send 200 connection established")
-		conn.Close()
-		return
-	}
-
 	var closeOnce sync.Once
 	defer closeOnce.Do(func() { conn.Close() })
 
@@ -110,11 +102,17 @@ func (t *Tunnel) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	t.Server.PendingDial.Add(random, connection)
 	if err := backend.Send(dialRequest); err != nil {
 		klog.ErrorS(err, "failed to tunnel dial request", "host", r.Host, "dialID", connection.dialID, "agentID", connection.agentID)
+		// Send proper HTTP error response
+		conn.Write([]byte(fmt.Sprintf("HTTP/1.1 502 Bad Gateway\r\nContent-Type: text/plain; charset=utf-8\r\n\r\nFailed to tunnel dial request: %v\r\n", err)))
+		conn.Close()
 		return
 	}
 	ctxt := backend.Context()
 	if ctxt.Err() != nil {
-		klog.ErrorS(err, "context reports failure")
+		klog.ErrorS(ctxt.Err(), "context reports failure")
+		conn.Write([]byte(fmt.Sprintf("HTTP/1.1 502 Bad Gateway\r\nContent-Type: text/plain; charset=utf-8\r\n\r\nBackend context error: %v\r\n", ctxt.Err())))
+		conn.Close()
+		return
 	}
 
 	select {
@@ -125,6 +123,15 @@ func (t *Tunnel) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	select {
 	case <-connection.connected: // Waiting for response before we begin full communication.
+		// Now that connection is established, send 200 OK to switch to tunnel mode
+		_, err = conn.Write([]byte("HTTP/1.1 200 Connection Established\r\n\r\n"))
+		if err != nil {
+			klog.ErrorS(err, "failed to send 200 connection established", "host", r.Host, "agentID", connection.agentID)
+			conn.Close()
+			return
+		}
+		klog.V(3).InfoS("Connection established, sent 200 OK", "host", r.Host, "agentID", connection.agentID, "connectionID", connection.connectID)
+
 	case <-closed: // Connection was closed before being established
 	}
 



Test patch:
diff --git a/tests/proxy_test.go b/tests/proxy_test.go
index 66fe7680..2d019d80 100644
--- a/tests/proxy_test.go
+++ b/tests/proxy_test.go
@@ -689,7 +689,7 @@ func TestFailedDNSLookupProxy_HTTPCONN(t *testing.T) {
 		t.Error(err)
 	}
 
-	urlString := "http://thissssssxxxxx.com:80"
+	urlString := "http://thisdefinitelydoesnotexist.com:80"
 	serverURL, _ := url.Parse(urlString)
 
 	// Send HTTP-Connect request
@@ -705,36 +705,12 @@ func TestFailedDNSLookupProxy_HTTPCONN(t *testing.T) {
 		t.Errorf("reading HTTP response from CONNECT: %v", err)
 	}
 
-	if res.StatusCode != 200 {
-		t.Errorf("expect 200; got %d", res.StatusCode)
-	}
-	if br.Buffered() > 0 {
-		t.Error("unexpected extra buffer")
-	}
-	dialer := func(_, _ string) (net.Conn, error) {
-		return conn, nil
-	}
-
-	c := &http.Client{
-		Transport: &http.Transport{
-			Dial: dialer,
-		},
-	}
-
-	resp, err := c.Get(urlString)
-	if err != nil {
-		t.Error(err)
+	if res.StatusCode != 502 {
+		t.Errorf("expect 502; got %d", res.StatusCode)
 	}
 
-	if resp.StatusCode != 503 {
-		t.Errorf("expect 503; got %d", res.StatusCode)
-	}
-
-	body, err := io.ReadAll(resp.Body)
-	resp.Body.Close()
-	if !strings.Contains(err.Error(), "connection reset by peer") {
-		t.Error(err)
-	}
+	body, err := io.ReadAll(res.Body)
+	res.Body.Close()
 
 	if !strings.Contains(string(body), "no such host") {
 		t.Errorf("Unexpected error: %v", err)
@@ -779,37 +755,21 @@ func TestFailedDial_HTTPCONN(t *testing.T) {
 	br := bufio.NewReader(conn)
 	res, err := http.ReadResponse(br, nil)
 	if err != nil {
-		t.Fatalf("reading HTTP response from CONNECT: %v", err)
-	}
-	if res.StatusCode != 200 {
-		t.Fatalf("expect 200; got %d", res.StatusCode)
+		t.Errorf("reading HTTP response from CONNECT: %v", err)
 	}
 
-	dialer := func(_, _ string) (net.Conn, error) {
-		return conn, nil
+	if res.StatusCode != 502 {
+		t.Errorf("expect 502; got %d", res.StatusCode)
 	}
 
-	c := &http.Client{
-		Transport: &http.Transport{
-			Dial: dialer,
-		},
-	}
-
-	resp, err := c.Get(server.URL)
+	body, err := io.ReadAll(res.Body)
+	res.Body.Close()
 	if err != nil {
-		t.Fatal(err)
-	}
-
-	body, err := io.ReadAll(resp.Body)
-	resp.Body.Close()
-	if err == nil {
 		t.Fatalf("Expected error reading response body; response=%q", body)
-	} else if !strings.Contains(err.Error(), "connection reset by peer") {
-		t.Error(err)
 	}
 
 	if !strings.Contains(string(body), "connection refused") {
-		t.Errorf("Unexpected error: %v", err)
+		t.Errorf("Expected 'connection refused' in error body, got: %s", string(body))
 	}
 
 	if err := ps.Metrics().ExpectServerDialFailure(metricsserver.DialFailureErrorResponse, 1); err != nil {