daemon.c for testing C socket ipv6 / ipv4 compatibility

Author: tonymet

internal/socket-testing/.gitignore

@@ -0,0 +1,10 @@
+# Ignore all files
+daemon
+daemon2
+*
+
+# But not directories (so Git can check inside them)
+# !*/
+
+# Except daemon.c (allow this specific file)
+!daemon.c

internal/socket-testing/daemon.c

@@ -0,0 +1,99 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+#define PORT 8080
+#define BUFFER_SIZE 1024
+
+int main() {
+    int server_fd, new_socket;
+    struct sockaddr_in6 address;
+    int addrlen = sizeof(address);
+    char buffer[BUFFER_SIZE] = {0};
+    const char *http_response = "HTTP/1.1 200 OK\nContent-Type: text/plain\nContent-Length: 14\n\nHello, world!\n";
+    
+    // Create an IPv6 socket
+    if ((server_fd = socket(AF_INET6, SOCK_STREAM, 0)) == 0) {
+        perror("socket failed");
+        exit(EXIT_FAILURE);
+    }
+    
+    // Set SO_REUSEADDR to reuse the address immediately
+    int opt = 1;
+    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt))) {
+        perror("setsockopt SO_REUSEADDR failed");
+        close(server_fd);
+        exit(EXIT_FAILURE);
+    }
+
+    // Set IPV6_V6ONLY to 0 to allow dual-stack operation.
+    // This is the crucial part for the test. If this is 0,
+    // the IPv6 socket can also accept IPv4 connections.
+    // If it were 1, it would only accept IPv6 connections.
+    int v6only = 0;
+    if (setsockopt(server_fd, IPPROTO_IPV6, IPV6_V6ONLY, &v6only, sizeof(v6only))) {
+        perror("setsockopt IPV6_V6ONLY failed");
+        close(server_fd);
+        exit(EXIT_FAILURE);
+    }
+    int returned_v6only;
+    socklen_t len = sizeof(returned_v6only);
+    if (getsockopt(server_fd, IPPROTO_IPV6, IPV6_V6ONLY, &returned_v6only, &len) == 0) {
+        printf("IPV6_V6ONLY is currently set to: %d\n", returned_v6only);
+    } else {
+        perror("getsockopt IPV6_V6ONLY failed");
+    }
+
+    // Configure the server address
+    // Use IN6ADDR_LOOPBACK_INIT to bind to [::1]
+    memset(&address, 0, sizeof(address));
+    address.sin6_family = AF_INET6;
+    address.sin6_addr = in6addr_loopback; // This is the [::1] address
+    address.sin6_port = htons(PORT);
+    
+    // Bind the socket to the configured address
+    if (bind(server_fd, (struct sockaddr *)&address, addrlen) < 0) {
+        perror("bind failed");
+        close(server_fd);
+        exit(EXIT_FAILURE);
+    }
+    
+    // Start listening for incoming connections
+    if (listen(server_fd, 3) < 0) {
+        perror("listen failed");
+        close(server_fd);
+        exit(EXIT_FAILURE);
+    }
+    
+    printf("Server listening on [::1] port %d\n", PORT);
+    printf("Test with curl: \n");
+    printf("  curl -6 http://[::1]:%d\n", PORT);
+    printf("  curl -4 http://127.0.0.1:%d\n\n", PORT);
+
+    while (1) {
+        // Accept a new connection
+        if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
+            perror("accept failed");
+            continue;
+        }
+
+        // Read the incoming request
+        read(new_socket, buffer, BUFFER_SIZE);
+        printf("Request received:\n---\n%s\n---\n", buffer);
+        
+        // Send the HTTP response
+        write(new_socket, http_response, strlen(http_response));
+        
+        // Close the client socket
+        close(new_socket);
+    }
+    
+    // Close the server socket (unreachable in this infinite loop)
+    close(server_fd);
+    
+    return 0;
+}

multilistener/multilistener.go

@@ -31,10 +31,12 @@ type MultiListener struct {
 
 type Addresses = []string
 
-// net.Addr.Network() implementation
+// Network() implementation for net.Addr
 func (dl *MultiListener) Network() string {
 	return "tcp+multi"
 }
+
+// String() joins all addresses, comma separated, for logs & debug
 func (dl *MultiListener) String() string {
 	if len(dl.listeners) < 1 {
 		return ""
@@ -153,6 +155,5 @@ func (dl *MultiListener) Close() error {
 
 // Addr returns the preferred (first) interface Addr
 func (dl *MultiListener) Addr() net.Addr {
-	// Return one of the addresses for display
 	return dl.listeners[0].Addr()
 }