feat(tcp): ListenerTCP binds the address it is given, at port 0 if asked

A new constructor takes a numeric bind address (127.0.0.1, ::1, 0.0.0.0,
::) and accepts port 0, and Port() reports the port the listener got.
Until now every listener bound 0.0.0.0 at a port the caller chose, so a
helper meant for this machine's own processes was reachable from the
network, and the caller had to guess a free port. A name such as
`localhost` is refused instead of bound to whichever address the
resolver ranks first. The port-only constructor still listens on every
IPv4 interface.

`running` is atomic now: Stop() runs on another thread than the accept
loop that reads it. The accept loops take a sockaddr_storage, so an IPv6
peer's address is no longer cut short.

New: ShouldBindLoopbackEphemeral.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -75,6 +75,15 @@ Crafter::ListenerTCP listener(8080, callback);
 listener.ListenSyncSync(); // Synchronous listening
 ```
 
+The port-only constructor listens on every IPv4 interface. To keep a listener to this machine, or to let the kernel pick the port, name the address and pass port 0; `Port()` reports the port it got. The address must be numeric (`127.0.0.1`, `::1`, `0.0.0.0`, `::`): a name such as `localhost` is refused rather than bound to whichever address the resolver ranks first.
+
+```cpp
+Crafter::ListenerTCP broker("127.0.0.1", 0, callback);
+const std::uint16_t port = broker.Port();
+std::thread([&] { broker.ListenSyncSync(); }).detach();
+// ... broker.Stop() ends the loop from another thread.
+```
+
 ### HTTP/3 Components
 
 HTTP/3 runs over QUIC, which always requires TLS. Pass server credentials when constructing the listener (or set `selfSigned = true` for a development-only ephemeral cert) and matching client credentials when constructing the client (`insecureNoServerValidation = true` for self-signed servers).
@@ -301,6 +310,7 @@ The library includes tests covering:
 - Mutual TLS and the raw stream (`ShouldRequireClientCertificateHTTPS1`) — a client certificate the listener's CA vouches for is served and one absent is refused; `TLSStream` is also driven directly with hand-written HTTP/1.1 to keep `:TLS` usable without `:ClientHTTP1` on top
 - Dual-stack connect (`ShouldConnectDualStack`) — `localhost` reaches a server on `::1` only and one on `127.0.0.1` only, an IPv6 literal connects over IPv6, and the last failure is what is thrown when no address answers; skips on a host without IPv6 loopback
 - Connect errors (`ShouldReportConnectErrors`) — a refused connect and an unresolvable name throw `SocketErrorException` with their errno or getaddrinfo code and the message ClientTCP always threw, and still reach a `std::runtime_error` handler
+- Loopback listeners (`ShouldBindLoopbackEphemeral`) — a listener on `127.0.0.1` at port 0 reports its port, takes loopback clients and refuses them through the host's other address; `::1` works too, a name is refused, and the port-only constructor still listens on every interface
 
 The external-interop test requires outbound UDP/443; if your network blocks it the test will fail. `ShouldInteropCurlHTTP1` and `ShouldInteropCurlHTTPS1` skip whichever half is unavailable when `curl` or `python3` is not installed, and the mutual-TLS half of `ShouldRequireClientCertificateHTTPS1` skips without the `openssl` CLI (a *client* certificate needs the `clientAuth` extended key usage, which the built-in development certificate does not carry) — so all three pass on a bare machine. Install `curl`, `python3` and `openssl` to actually exercise them.
 

implementations/Crafter.Network-ListenerTCP.cpp

@@ -12,6 +12,7 @@ module;
 #include <sys/time.h>
 #include <sys/wait.h>
 #include <strings.h>
+#include <netdb.h>
 #include <cerrno>
 
 module Crafter.Network:ListenerTCP_impl;
@@ -21,14 +22,30 @@ import Crafter.Thread;
 
 using namespace Crafter;
 
-ListenerTCP::ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit, std::uint32_t totalClientLimit) : connectCallback(connectCallback), concurrentClientLimit(concurrentClientLimit), totalClientLimit(totalClientLimit) {
-	sockaddr_in servAddr;
-    bzero((char*)&servAddr, sizeof(servAddr));
-    servAddr.sin_family = AF_INET;
-    servAddr.sin_addr.s_addr = htonl(INADDR_ANY);
-    servAddr.sin_port = htons(port);
+ListenerTCP::ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit, std::uint32_t totalClientLimit)
+    : ListenerTCP("0.0.0.0", port, std::move(connectCallback), concurrentClientLimit, totalClientLimit) {}
 
-    s = socket(AF_INET, SOCK_STREAM, 0);
+ListenerTCP::ListenerTCP(std::string_view bindAddress, std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit, std::uint32_t totalClientLimit) : connectCallback(connectCallback), concurrentClientLimit(concurrentClientLimit), totalClientLimit(totalClientLimit) {
+    // AI_NUMERICHOST: no lookup, so `localhost` is refused here rather than
+    // bound to whichever of ::1 and 127.0.0.1 the resolver ranks first.
+    addrinfo hints{};
+    hints.ai_family = AF_UNSPEC;
+    hints.ai_socktype = SOCK_STREAM;
+    hints.ai_flags = AI_PASSIVE | AI_NUMERICHOST | AI_NUMERICSERV;
+    const std::string host(bindAddress);
+    const std::string service = std::to_string(port);
+    addrinfo* resolved = nullptr;
+    const int status = getaddrinfo(host.c_str(), service.c_str(), &hints, &resolved);
+    if (status != 0 || resolved == nullptr) {
+        throw std::runtime_error("Could not bind " + host + ": not a numeric IPv4 or IPv6 address (" + gai_strerror(status) + ")");
+    }
+    sockaddr_storage servAddr {};
+    std::memcpy(&servAddr, resolved->ai_addr, resolved->ai_addrlen);
+    const socklen_t servAddrLength = resolved->ai_addrlen;
+    const int family = resolved->ai_family;
+    freeaddrinfo(resolved);
+
+    s = socket(family, SOCK_STREAM, 0);
     if(s < 0)
     {
         throw std::runtime_error("Error establishing the server socket");
@@ -39,7 +56,7 @@ ListenerTCP::ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> con
     int reuse = 1;
     setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse));
 
-	if(bind(s, (struct sockaddr*) &servAddr, sizeof(servAddr)) < 0) {
+	if(bind(s, reinterpret_cast<const sockaddr*>(&servAddr), servAddrLength) < 0) {
         // Ignoring this left the listener silently accepting nothing at
         // all, with no hint as to why.
         const std::string reason = std::strerror(errno);
@@ -55,6 +72,18 @@ ListenerTCP::ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> con
     }
 }
 
+std::uint16_t ListenerTCP::Port() const {
+    sockaddr_storage address {};
+    socklen_t length = sizeof(address);
+    if (getsockname(s, reinterpret_cast<sockaddr*>(&address), &length) == -1) {
+        throw std::runtime_error(std::string("Could not read the listening port: ") + std::strerror(errno));
+    }
+    if (address.ss_family == AF_INET6) {
+        return ntohs(reinterpret_cast<const sockaddr_in6*>(&address)->sin6_port);
+    }
+    return ntohs(reinterpret_cast<const sockaddr_in*>(&address)->sin_port);
+}
+
 void ListenerTCP::Stop() {
 	running = false;
 	shutdown(s, SHUT_RDWR);
@@ -64,7 +93,7 @@ void ListenerTCP::Stop() {
 
 void ListenerTCP::ListenSyncSync() {
 	while (running && totalClientCounter < totalClientLimit) {
-        sockaddr_in newSockAddr;
+        sockaddr_storage newSockAddr;
         socklen_t newSockAddrSize = sizeof(newSockAddr);
 		int client = accept(s, (sockaddr*)&newSockAddr, &newSockAddrSize);
 		if (client > 0) {
@@ -81,7 +110,7 @@ void ListenerTCP::ListenSyncSync() {
 
 void ListenerTCP::ListenSyncAsync() {
 	while (running && totalClientCounter < totalClientLimit) {
-        sockaddr_in newSockAddr;
+        sockaddr_storage newSockAddr;
         socklen_t newSockAddrSize = sizeof(newSockAddr);
 		int client = accept(s, (sockaddr*)&newSockAddr, &newSockAddrSize);
 		if (client > 0) {
@@ -97,7 +126,7 @@ void ListenerTCP::ListenSyncAsync() {
 void ListenerTCP::ListenAsyncSync() {
     ThreadPool::Enqueue([this]() {
 	    while (running && totalClientCounter < totalClientLimit) {
-            sockaddr_in newSockAddr;
+            sockaddr_storage newSockAddr;
             socklen_t newSockAddrSize = sizeof(newSockAddr);
 		    int client = accept(s, (sockaddr*)&newSockAddr, &newSockAddrSize);
 		    if (client > 0) {
@@ -114,7 +143,7 @@ void ListenerTCP::ListenAsyncSync() {
 void ListenerTCP::ListenAsyncAsync() {
     ThreadPool::Enqueue([this]() {
 	    while (running && totalClientCounter < totalClientLimit) {
-            sockaddr_in newSockAddr;
+            sockaddr_storage newSockAddr;
             socklen_t newSockAddrSize = sizeof(newSockAddr);
 		    int client = accept(s, (sockaddr*)&newSockAddr, &newSockAddrSize);
 		    if (client > 0) {

interfaces/Crafter.Network-ListenerTCP.cppm

@@ -1,28 +1,40 @@
-//SPDX-License-Identifier: LGPL-3.0-only
-//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
-
-export module Crafter.Network:ListenerTCP;
-import std;
-import :ClientTCP;
-
-#ifndef CRAFTER_NETWORK_BROWSER
-namespace Crafter {
-	export class ListenerTCP {
-	public:
-		bool running = true;
-		std::function<void(ClientTCP*)> connectCallback;
-		std::uint32_t concurrentClientLimit;
-		std::uint32_t totalClientLimit;
-		ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit = std::numeric_limits<std::uint32_t>::max(), std::uint32_t totalClientLimit = std::numeric_limits<std::uint32_t>::max());
-		~ListenerTCP();
-		void Stop();
-		void ListenSyncSync();
-		void ListenSyncAsync();
-		void ListenAsyncSync();
-		void ListenAsyncAsync();
-	private:
-		std::uint32_t totalClientCounter = 0;
-		int s;
-	};
-}
-#endif
+//SPDX-License-Identifier: LGPL-3.0-only
+//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+export module Crafter.Network:ListenerTCP;
+import std;
+import :ClientTCP;
+
+#ifndef CRAFTER_NETWORK_BROWSER
+namespace Crafter {
+	export class ListenerTCP {
+	public:
+		// Atomic: Stop() is called from another thread than the one running
+		// an accept loop, which reads this on every turn.
+		std::atomic<bool> running = true;
+		std::function<void(ClientTCP*)> connectCallback;
+		std::uint32_t concurrentClientLimit;
+		std::uint32_t totalClientLimit;
+		// Every IPv4 interface (0.0.0.0) at `port`.
+		ListenerTCP(std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit = std::numeric_limits<std::uint32_t>::max(), std::uint32_t totalClientLimit = std::numeric_limits<std::uint32_t>::max());
+		// `bindAddress` is a numeric IPv4 or IPv6 address — 127.0.0.1 or ::1
+		// for this machine alone, 0.0.0.0 or :: for every interface — and a
+		// name such as `localhost` is refused: the address decides who can
+		// reach the listener, so it is spelled out rather than looked up. A
+		// `port` of 0 lets the kernel pick a free one; Port() says which.
+		ListenerTCP(std::string_view bindAddress, std::uint16_t port, std::function<void(ClientTCP*)> connectCallback, std::uint32_t concurrentClientLimit = std::numeric_limits<std::uint32_t>::max(), std::uint32_t totalClientLimit = std::numeric_limits<std::uint32_t>::max());
+		~ListenerTCP();
+		// The port the listener is bound to: the one asked for, or the
+		// kernel's pick when that was 0.
+		std::uint16_t Port() const;
+		void Stop();
+		void ListenSyncSync();
+		void ListenSyncAsync();
+		void ListenAsyncSync();
+		void ListenAsyncAsync();
+	private:
+		std::uint32_t totalClientCounter = 0;
+		int s;
+	};
+}
+#endif

project.cpp

@@ -176,6 +176,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("ShouldRecieveTCP").Dependencies({ &cfg });
         cfg.AddTest("ShouldConnectDualStack").Dependencies({ &cfg });
         cfg.AddTest("ShouldReportConnectErrors").Dependencies({ &cfg });
+        cfg.AddTest("ShouldBindLoopbackEphemeral").Dependencies({ &cfg });
         cfg.AddTest("ShouldInteropCurlHTTP1").Dependencies({ &cfg });
         cfg.AddTest("ShouldInteropCurlHTTPS1").Dependencies({ &cfg });
         cfg.AddTest("ShouldParseHTTP1").Dependencies({ &cfg });

tests/ShouldBindLoopbackEphemeral/main.cpp

@@ -0,0 +1,143 @@
+//SPDX-License-Identifier: LGPL-3.0-only
+//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// ListenerTCP can be told the address it binds and take port 0, with Port()
+// reporting what the kernel picked. Before, it always bound 0.0.0.0 at a port
+// the caller chose: a helper meant for this machine's own processes — a broker
+// that hands out database access, say — was reachable from the network, and
+// the caller had to guess a free port and hope.
+//
+// Also: a host name is refused rather than bound to whichever address the
+// resolver ranks first, and the port-only constructor keeps listening on every
+// IPv4 interface.
+
+#include <arpa/inet.h>
+#include <ifaddrs.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <cerrno>
+#include <cstdlib>
+
+import Crafter.Network;
+import std;
+using namespace Crafter;
+using namespace std::chrono_literals;
+
+namespace {
+
+int failures = 0;
+
+void Check(bool ok, std::string_view what) {
+    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
+    if (!ok) ++failures;
+}
+
+// An IPv4 address of this host that is not loopback, if it has one.
+std::optional<std::string> NonLoopbackIPv4() {
+    ifaddrs* interfaces = nullptr;
+    if (getifaddrs(&interfaces) != 0) return std::nullopt;
+    std::optional<std::string> found;
+    for (const ifaddrs* entry = interfaces; entry != nullptr && !found; entry = entry->ifa_next) {
+        if (entry->ifa_addr == nullptr || entry->ifa_addr->sa_family != AF_INET) continue;
+        const auto* v4 = reinterpret_cast<const sockaddr_in*>(entry->ifa_addr);
+        if (ntohl(v4->sin_addr.s_addr) >> 24 == 127) continue;
+        char text[INET_ADDRSTRLEN] = {};
+        inet_ntop(AF_INET, &v4->sin_addr, text, sizeof(text));
+        found = std::string(text);
+    }
+    freeifaddrs(interfaces);
+    return found;
+}
+
+// Runs a listener's accept loop on a thread of its own and counts the
+// connections it was handed.
+struct Serving {
+    ListenerTCP& listener;
+    std::atomic<int> accepted = 0;
+    std::thread loop;
+
+    explicit Serving(ListenerTCP& listener) : listener(listener) {
+        listener.connectCallback = [this](ClientTCP* client) {
+            ++accepted;
+            delete client;
+        };
+        loop = std::thread([this] { this->listener.ListenSyncSync(); });
+    }
+    ~Serving() {
+        listener.Stop();
+        loop.join();
+    }
+    bool WaitFor(int count) {
+        for (int i = 0; i < 200 && accepted.load() < count; ++i) std::this_thread::sleep_for(10ms);
+        return accepted.load() >= count;
+    }
+};
+
+} // namespace
+
+int main() {
+    {
+        ListenerTCP listener("127.0.0.1", 0, nullptr);
+        const std::uint16_t port = listener.Port();
+        Check(port != 0, std::format("port 0 gets a port from the kernel, which Port() reports ({})", port));
+        Serving serving(listener);
+        {
+            ClientTCP client("127.0.0.1", port, 2s);
+        }
+        Check(serving.WaitFor(1), "a client on loopback reaches it");
+
+        if (const std::optional<std::string> outside = NonLoopbackIPv4()) {
+            bool refused = false;
+            try {
+                ClientTCP client(*outside, port, 2s);
+            } catch (const SocketErrorException& e) {
+                refused = e.code == ECONNREFUSED;
+            } catch (const std::exception&) {
+            }
+            Check(refused, std::format("through {} it is refused: it listens on 127.0.0.1 alone", *outside));
+        } else {
+            std::println("SKIP  no non-loopback IPv4 address to try from");
+        }
+    }
+
+    {
+        std::optional<ListenerTCP> v6;
+        try {
+            v6.emplace("::1", 0, nullptr);
+        } catch (const std::exception&) {
+        }
+        if (v6) {
+            Serving serving(*v6);
+            {
+                ClientTCP client("::1", v6->Port(), 2s);
+            }
+            Check(serving.WaitFor(1), "an IPv6 loopback listener takes a client on ::1");
+        } else {
+            std::println("SKIP  no IPv6 loopback on this host");
+        }
+    }
+
+    {
+        bool refused = false;
+        try {
+            ListenerTCP named("localhost", 0, nullptr);
+        } catch (const std::runtime_error&) {
+            refused = true;
+        }
+        Check(refused, "a host name is refused: the address to bind is spelled out");
+    }
+
+    {
+        ListenerTCP any(0, nullptr);
+        Check(any.Port() != 0, "the port-only constructor takes port 0 as well");
+        Serving serving(any);
+        {
+            ClientTCP client("127.0.0.1", any.Port(), 2s);
+        }
+        Check(serving.WaitFor(1), "and still listens on every IPv4 interface, loopback included");
+    }
+
+    std::println("{}", failures == 0 ? "all checks passed" : "check(s) failed");
+    return failures == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+}