Opcua Client Ext Reverse Connect

php-opcua/opcua-client-ext-reverse-connect — v4.4.0 skill

An optional extension of php-opcua/opcua-client implementing the client side of OPC UA Reverse Connect (Part 6 §7.1.2.3). In Reverse Connect the server initiates the TCP connection to the client and announces itself with a ReverseHello (RHE) frame; from there the normal UA-TCP handshake proceeds on the same socket. This package binds the listener, decodes and validates the RHE, and bridges the live socket into the standard ClientBuilder flow.

Everything lives under the PhpOpcua\Client\ExtReverseConnect\* namespace. Applications that do not need Reverse Connect take no extra dependency.

When to use this skill

Activate when any of these apply:

• The task mentions Reverse Connect, ReverseHello, RHE, server-initiated OPC UA connections, or "the server connects to the client"
• An OPC UA edge gateway / PLC behind NAT or a firewall needs to "call home" to a central client over opc.tcp://
• A ReverseConnectListener, ReverseHelloValidator, ReverseConnectClientFactory, or ReverseConnectSession appears in code
• The task references OPC UA Part 6 §7.1.2.3, or the StartReverseConnect / StopReverseConnect test-suite methods

Do NOT activate for: ordinary client-initiated OPC UA connections (use the core opcua-client skill), generic PHP/networking work, or opc.https:// transport (use opcua-client-ext-transport-https).

The 60-second mental model

            (server dials the client)
  OPC UA Server ───TCP connect──► ReverseConnectListener  (bind host:port)
        │                                  │ accept(timeout)  ── stream_select()
        │ sends RHE frame ────────────────►│ readFrame()
        │                                  │ ReverseHelloParser::parse()
        │                                  ▼
        │                          ReverseHelloValidator::ensureAccepted()
        │                          (ServerUri whitelist + opc.tcp:// scheme)
        │                                  ▼
        │                          ReverseConnectSession {serverUri, endpointUrl, socket}
        │                                  ▼
        └──── same socket ────────► ReverseConnectClientFactory::buildClient()
                                           │ TcpTransport::fromConnectedSocket()
                                           ▼
                                    Client (core opcua-client, fully connected)

Four things to know:

1. The listener is blocking and single-threaded. accept(float $timeoutSeconds) waits via stream_select() for one inbound connection, reads the RHE, validates it, and returns a ReverseConnectSession. Loop over accept() to service multiple servers. There is no event loop.
2. The validator is the security boundary. Anyone who can reach the listener port can send an RHE, so the ServerUri is whitelisted before the secure channel opens. The whitelist is fail-secure: empty whitelist ⇒ every message rejected.
3. The socket is reused, not re-dialed. The factory wraps the already-connected socket with TcpTransport::fromConnectedSocket(); the core's ManagesConnectionTrait::performConnect() detects isConnected() and skips the redundant outbound connect(), jumping straight to HEL/ACK on the inherited socket.
4. This package speaks no OPC UA services itself. Once buildClient() returns a Client, everything else (read/write/browse/subscribe) is the core opcua-client API.

Quick start (the canonical shape)

use PhpOpcua\Client\ClientBuilder;
use PhpOpcua\Client\ExtReverseConnect\ReverseConnectClientFactory;
use PhpOpcua\Client\ExtReverseConnect\ReverseConnectListener;
use PhpOpcua\Client\ExtReverseConnect\ReverseHelloValidator;
use PhpOpcua\Client\Security\SecurityMode;
use PhpOpcua\Client\Security\SecurityPolicy;

// 1. Whitelist the servers you trust (exact, case-sensitive ApplicationUri).
$validator = new ReverseHelloValidator(['urn:opcua:testserver:nodes']);

// 2. Bind a listener. Use 0.0.0.0 when the server is remote / containerised;
//    127.0.0.1 only when the server can reach loopback.
$listener = new ReverseConnectListener('0.0.0.0', 4840, $validator);
$listener->listen();

try {
    // 3. Block until a trusted server announces itself (or time out).
    $session = $listener->accept(timeoutSeconds: 30.0);

    // 4. Bridge the live socket into a standard, fully connected Client.
    $client = (new ReverseConnectClientFactory())->buildClient(
        $session,
        static function (ClientBuilder $b): void {
            $b->setSecurityPolicy(SecurityPolicy::None)
              ->setSecurityMode(SecurityMode::None);
        },
    );

    // 5. From here it's the ordinary opcua-client API.
    $value = $client->read('ns=2;s=Sensors/Temp')->getValue();

    $client->disconnect();
} finally {
    $listener->close();
}

When to load deeper references

Public API surface (must-know)

All classes are in PhpOpcua\Client\ExtReverseConnect.

Events (PSR-14) — dispatched only when a dispatcher is supplied to the listener

Exceptions — all extend Exception\ReverseConnectException (which extends RuntimeException)

Idiomatic patterns AI agents should follow

1. Bind 0.0.0.0 for remote/containerised servers, 127.0.0.1 only for loopback-reachable ones. A server in a Docker container reaches the host via host.docker.internal → the bridge gateway IP, which cannot reach a 127.0.0.1-bound listener. See references/PITFALLS.md.
2. Always whitelist real ServerUris. Never pass an empty whitelist expecting "accept all" — it does the opposite (fail-secure). Never disable validation.
3. Treat accept() as one-shot per call. Wrap it in a loop for multiple servers; each call returns one session. There is no built-in concurrency.
4. Reuse the socket via the factory — don't re-connect() to endpointUrl yourself. That would open a second outbound channel and defeat the whole point of Reverse Connect (the server may be behind NAT and unreachable outbound).
5. close() the listener in finally; disconnect() the client in finally. The session owns the socket until the factory's transport takes it over.
6. Logs via PSR-3 ($logger ctor arg), events via PSR-14 ($dispatcher ctor arg). Events are not even constructed when no dispatcher is supplied — zero overhead.
7. Match the security config on both ends. The $configure callback sets SecurityPolicy/SecurityMode/identity exactly as a normal client would; the announced endpointUrl and the server certificate drive validation, not the listener bind interface.

Common pitfalls (read before generating code)

Don't write code that:

• Binds 127.0.0.1 when the server lives in another host/container — the RHE never arrives and accept() throws ReverseConnectTimeoutException.
• Passes an empty ReverseHelloValidator([]) thinking it accepts everything — it rejects everything.
• Calls ClientBuilder::connect($session->endpointUrl) directly instead of factory->buildClient($session) — re-dials outbound and ignores the inherited socket.
• Fails the test/run on the first connect against a freshly booted server (BadServerHalted, 0x800E0000) instead of retrying through the boot window.
• Leaks the listener or session socket by not closing in finally.
• Assumes accept() returning means the OPC UA session is up — it only means the RHE was received and validated; the UA handshake happens in buildClient().

Full catalog in references/PITFALLS.md.

Related packages in the php-opcua ecosystem

• opcua-client — the core client this package extends. Once buildClient() returns, you are using its API. Load the opcua-client skill for read/write/browse/subscribe/history.
• opcua-client-ext-transport-https — opc.https:// wire transport (Part 6 §7.4). Sibling extension on the same ClientTransportInterface seam.
• opcua-session-manager — keeps sessions alive across PHP requests; pair with reverse connect when the accepted Client must outlive a single request.
• uanetstandard-test-suite — Docker test servers exposing TestServer/ReverseConnect/StartReverseConnect / StopReverseConnect methods to drive the integration suite. Requires v1.4.0+ (methods) / v1.5.1+ (readiness-gated healthcheck).