feat: authenticate via HTTP/1.1 before establishing SSH tunnel

Add --http-auth and --auth-token flags to perform HTTP authentication
over TLS (with http/1.1 ALPN) before establishing the SSH tunnel.

Add --http-upgrade flag to use HTTP Upgrade header mechanism, allowing
the proxy to hijack/switch the connection to SSH on the same TLS
connection (similar to WebSocket upgrade but without websockets).

Two modes:
- Without --http-upgrade: Auth connection is closed after auth, then a
  second TLS+SSH connection is established (two connections)
- With --http-upgrade <proto>: After auth, if proxy responds with 101
  Switching Protocols + Upgrade: <proto>, the same connection is used
  for SSH (single connection, protocol switch)

No changes to existing behavior when --http-auth is not used.

cmd/sclient/main.go

@@ -51,6 +51,9 @@ func main() {
 	var insecure bool
 	var servername string
 	var silent bool
+	var httpAuthURL string
+	var authToken string
+	var httpUpgradeProtocol string
 
 	flag.Usage = usage
 
@@ -60,6 +63,9 @@ func main() {
 	flag.StringVar(&servername, "servername", "", "specify a servername different from <remote> (to disable SNI use an IP as <remote> and do not use this option)")
 	flag.BoolVar(&insecure, "insecure", false, "ignore bad TLS/SSL/HTTPS certificates")
 	flag.BoolVar(&silent, "silent", false, "less verbose output")
+	flag.StringVar(&httpAuthURL, "http-auth", "", "authenticate via HTTP/1.1 before establishing tunnel (ex: 'proxy.example.com/auth')")
+	flag.StringVar(&authToken, "auth-token", "", "authentication token (sent as Bearer token in Authorization header)")
+	flag.StringVar(&httpUpgradeProtocol, "http-upgrade", "", "protocol to upgrade to after authentication (e.g., 'ssh') - uses HTTP Upgrade header to switch protocols on the same connection")
 
 	flag.Parse()
 
@@ -78,6 +84,18 @@ func main() {
 		}
 	}
 
+	authTokenFromURL := ""
+	if httpAuthURL != "" {
+		if idx := strings.Index(httpAuthURL, "token="); idx != -1 {
+			authTokenFromURL = httpAuthURL[idx+6:]
+			if idx2 := strings.Index(authTokenFromURL, "&"); idx2 != -1 {
+				authTokenFromURL = authTokenFromURL[:idx2]
+			} else if idx2 := strings.Index(authTokenFromURL, " "); idx2 != -1 {
+				authTokenFromURL = authTokenFromURL[:idx2]
+			}
+		}
+	}
+
 	sclient := &sclient.Tunnel{
 		RemotePort:          443,
 		LocalAddress:        "localhost",
@@ -85,6 +103,13 @@ func main() {
 		ServerName:          servername,
 		Silent:              silent,
 		NextProtos:          alpns,
+		HttpAuthURL:         httpAuthURL,
+		AuthToken:           authToken,
+		HttpUpgradeProtocol: httpUpgradeProtocol,
+	}
+
+	if sclient.AuthToken == "" {
+		sclient.AuthToken = authTokenFromURL
 	}
 
 	remote := strings.Split(remotestr, ":")

sclient.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/http"
 	"os"
 	"strconv"
 	"strings"
@@ -20,16 +21,76 @@ type Tunnel struct {
 	NextProtos          []string
 	ServerName          string
 	Silent              bool
+	HttpAuthURL         string
+	AuthToken           string
+	HttpUpgradeProtocol string
+}
+
+func (t *Tunnel) httpAuth(conn net.Conn) (bool, error) {
+	url := t.HttpAuthURL
+	if !strings.HasPrefix(url, "http://") && !strings.HasPrefix(url, "https://") {
+		url = "http://" + url
+	}
+	parts := strings.SplitN(url, "//", 2)
+	path := "/"
+	if idx := strings.Index(parts[1], "/"); idx != -1 {
+		path = parts[1][idx:]
+	}
+	host := parts[0]
+
+	req, _ := http.NewRequest("GET", path, nil)
+	req.Host = host
+	if t.AuthToken != "" {
+		req.Header.Set("Authorization", "Bearer "+t.AuthToken)
+	}
+	if t.HttpUpgradeProtocol != "" {
+		req.Header.Set("Upgrade", t.HttpUpgradeProtocol)
+	}
+	req.ContentLength = 0
+	req.Close = true
+
+	if err := req.Write(conn); nil != err {
+		return false, err
+	}
+
+	buf := make([]byte, 4096)
+	n, err := conn.Read(buf)
+	if nil != err {
+		return false, err
+	}
+
+	resp := string(buf[:n])
+
+	if t.HttpUpgradeProtocol != "" {
+		if strings.Contains(resp, "101") && strings.Contains(strings.ToLower(resp), "upgrade: "+strings.ToLower(t.HttpUpgradeProtocol)) {
+			return true, nil
+		}
+		if !strings.Contains(resp, "200") && !strings.Contains(resp, "201") && !strings.Contains(resp, "202") && !strings.Contains(resp, "204") {
+			return false, fmt.Errorf("authentication failed: %s", resp)
+		}
+	}
+
+	if !strings.Contains(resp, "200") && !strings.Contains(resp, "201") && !strings.Contains(resp, "202") && !strings.Contains(resp, "204") {
+		return false, fmt.Errorf("authentication failed: %s", resp)
+	}
+
+	return false, nil
 }
 
 // DialAndListen will create a test TLS connection to the remote address and then
 // begin listening locally. Each local connection will result in a separate remote connection.
 func (t *Tunnel) DialAndListen() error {
 	remote := t.RemoteAddress + ":" + strconv.Itoa(t.RemotePort)
+
+	var nextProtos []string = t.NextProtos
+	if t.HttpAuthURL != "" {
+		nextProtos = []string{"http/1.1"}
+	}
+
 	conn, err := tls.Dial("tcp", remote,
 		&tls.Config{
 			ServerName:         t.ServerName,
-			NextProtos:         t.NextProtos,
+			NextProtos:         nextProtos,
 			InsecureSkipVerify: t.InsecureSkipVerify,
 		})
 
@@ -142,10 +203,44 @@ func pipe(r netReadWriteCloser, w netReadWriteCloser, t string) {
 }
 
 func (t *Tunnel) handleConnection(remote string, conn netReadWriteCloser) {
-	sclient, err := tls.Dial("tcp", remote,
+	var sclient net.Conn
+
+	if t.HttpAuthURL != "" {
+		authConn, err := tls.Dial("tcp", remote,
 			&tls.Config{
 				ServerName:         t.ServerName,
-			NextProtos:         t.NextProtos,
+				NextProtos:         []string{"http/1.1"},
+				InsecureSkipVerify: t.InsecureSkipVerify,
+			})
+
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "[error] (auth connection) %s\n", err)
+			_ = conn.Close()
+			return
+		}
+
+		upgraded, err := t.httpAuth(authConn)
+		if nil != err {
+			fmt.Fprintf(os.Stderr, "[error] (authentication) %s\n", err)
+			_ = conn.Close()
+			_ = authConn.Close()
+			return
+		}
+
+		if t.HttpUpgradeProtocol != "" && upgraded {
+			sclient = authConn
+		} else {
+			_ = authConn.Close()
+
+			var nextProtos []string = t.NextProtos
+			if len(nextProtos) == 0 {
+				nextProtos = []string{"ssh"}
+			}
+
+			sclient, err = tls.Dial("tcp", remote,
+				&tls.Config{
+					ServerName:         t.ServerName,
+					NextProtos:         nextProtos,
 					InsecureSkipVerify: t.InsecureSkipVerify,
 				})
 
@@ -154,6 +249,22 @@ func (t *Tunnel) handleConnection(remote string, conn netReadWriteCloser) {
 				_ = conn.Close()
 				return
 			}
+		}
+	} else {
+		var dialErr error
+		sclient, dialErr = tls.Dial("tcp", remote,
+			&tls.Config{
+				ServerName:         t.ServerName,
+				NextProtos:         t.NextProtos,
+				InsecureSkipVerify: t.InsecureSkipVerify,
+			})
+
+		if dialErr != nil {
+			fmt.Fprintf(os.Stderr, "[error] (remote) %s\n", dialErr)
+			_ = conn.Close()
+			return
+		}
+	}
 
 	if !t.Silent {
 		if conn.RemoteAddr().Network() == "stdio" {