Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect

contrib/tools/server/Makefile

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+server:
+	$(CC) -g -o server server.c
+
+clean:
+	rm server
+

contrib/tools/server/README.md

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+# Sample TCP Server
+
+This is a simple TCP server that listens on the specified port (3490 by default) and replies to incoming connections by providing info about src/destination ip/port.
+
+It demonstrates the use of the getsockopt() system call with the ORIGINAL_DST option to retrieve the original destination ip/port after an iptables redirect.
+
+So, for example, if you have the server listening on port 3490 on the local machine and an iptables rule like:
+
+```
+iptables -t nat -I OUTPUT 1 -p tcp --dport 4000:5000 -j REDIRECT --to-port 3490
+```
+your will see:
+
+```
+$ telnet localhost 3490
+FROM 127.0.0.1:44978, TO 127.0.0.1:3490, ORIG DEST 127.0.0.1:3490
+
+$ telnet localhost 4100
+FROM 127.0.0.1:35476, TO 127.0.0.1:3490, ORIG DEST 127.0.0.1:4100
+
+$ telnet 1.1.1.1 5000
+FROM 100.100.100.100:60275, TO 127.0.0.1:3490, ORIG DEST 1.1.1.1:5000
+```
+
+

contrib/tools/server/server.c

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+/*
+** server.c
+ * Demo server to check that we can extract all parameters from an incoming
+*request
+ * and respond to it
+*/
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <linux/netfilter_ipv4.h>
+#include <netdb.h>
+#include <netinet/in.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#define DEFAULT_PORT \
+  "3490"  // the port users will be connecting to, if not specified on command
+          // line
+
+#define QUEUE_SIZE 10  // pending connections queue size
+
+void sigchld_handler(int s) {
+  int saved_errno = errno;
+
+  while (waitpid(-1, NULL, WNOHANG) > 0)
+    ;
+
+  errno = saved_errno;
+}
+
+int main(int argc, char *argv[]) {
+  struct sigaction sa;
+  const int yes = 1;
+  char *port = DEFAULT_PORT;
+  int rv;
+
+  if (argc > 1) {
+    port = argv[1];
+  }
+
+  struct addrinfo hints;
+  memset(&hints, 0, sizeof hints);
+  hints.ai_family = AF_INET;
+  hints.ai_socktype = SOCK_STREAM;
+  hints.ai_flags = AI_PASSIVE;  // use my IP
+
+  struct addrinfo *server_info = NULL;
+  if ((rv = getaddrinfo(NULL, port, &hints, &server_info)) != 0) {
+    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
+    return 1;
+  }
+
+  struct addrinfo *p;
+  int sockfd;  // listen on sock_fd
+  for (p = server_info; p != NULL; p = p->ai_next) {
+    if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) {
+      perror("server: socket");
+      continue;
+    }
+
+    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
+      perror("setsockopt");
+      exit(1);
+    }
+
+    if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
+      close(sockfd);
+      perror("server: bind");
+      continue;
+    }
+
+    break;
+  }
+
+  freeaddrinfo(server_info);
+
+  if (p == NULL) {
+    fprintf(stderr, "server: failed to bind\n");
+    exit(1);
+  }
+  if (listen(sockfd, QUEUE_SIZE) == -1) {
+    perror("listen");
+    exit(1);
+  }
+
+  sa.sa_handler = sigchld_handler;
+  sigemptyset(&sa.sa_mask);
+  sa.sa_flags = SA_RESTART;
+  if (sigaction(SIGCHLD, &sa, NULL) == -1) {
+    perror("sigaction");
+    exit(1);
+  }
+
+  char bind_addr_str[INET6_ADDRSTRLEN] = {0};
+  struct sockaddr_in *bind_sock_addr = (struct sockaddr_in *)p->ai_addr;
+  inet_ntop(p->ai_family, &bind_sock_addr->sin_addr, bind_addr_str,
+            p->ai_addrlen);
+  printf("server %s: waiting for connections on port %s:%u...\n", port,
+         bind_addr_str, ntohs(bind_sock_addr->sin_port));
+
+  while (1) {
+    socklen_t addr_len = sizeof(struct sockaddr_storage);
+    int addr_str_len = INET6_ADDRSTRLEN;
+
+    struct sockaddr_storage their_addr = {
+        0};  // connector's address information
+    int accepted_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_len);
+    if (accepted_fd == -1) {
+      perror("accept");
+      continue;
+    }
+
+    struct sockaddr_in *src_sock_addr = (struct sockaddr_in *)&their_addr;
+    char their_addr_str[INET6_ADDRSTRLEN] = {0};
+    inet_ntop(their_addr.ss_family, &src_sock_addr->sin_addr, their_addr_str,
+              addr_str_len);
+    printf("server %s: got connection FROM %s:%u\n", port, their_addr_str,
+           ntohs(src_sock_addr->sin_port));
+
+    struct sockaddr_storage my_addr = {0};  // my address information
+    struct sockaddr_in *dst_sock_addr = (struct sockaddr_in *)&my_addr;
+    char my_addr_str[INET6_ADDRSTRLEN] = {0};
+    getsockname(accepted_fd, (struct sockaddr *)dst_sock_addr, &addr_len);
+    inet_ntop(my_addr.ss_family, &dst_sock_addr->sin_addr, my_addr_str,
+              addr_str_len);
+    printf("server %s: got connection TO %s:%u\n", port, my_addr_str,
+           ntohs(dst_sock_addr->sin_port));
+
+    struct sockaddr_storage orig_addr = {0};  // orig address information
+    struct sockaddr_in *orig_sock_addr = (struct sockaddr_in *)&orig_addr;
+    char orig_addr_str[INET6_ADDRSTRLEN] = {0};
+    int status = getsockopt(accepted_fd, SOL_IP, SO_ORIGINAL_DST,
+                            orig_sock_addr, &addr_len);
+
+    if (status == 0) {
+      inet_ntop(orig_addr.ss_family, &orig_sock_addr->sin_addr, orig_addr_str,
+                addr_str_len);
+      printf("server %s: ORIG DEST %s:%u\n", port, orig_addr_str,
+             ntohs(orig_sock_addr->sin_port));
+    } else {
+      printf("Could not get orig destination from accepted socket.\n");
+    }
+
+    if (!fork()) {  // this is the child process
+
+      close(sockfd);
+      char msg[256] = {0};
+      snprintf(msg, 256, "FROM %s:%u, TO %s:%u, ORIG DEST %s:%u\n",
+               their_addr_str, ntohs(src_sock_addr->sin_port), my_addr_str,
+               ntohs(dst_sock_addr->sin_port), orig_addr_str,
+               ntohs(orig_sock_addr->sin_port));
+
+      if (send(accepted_fd, msg, strlen(msg), 0) == -1) {
+        perror("send");
+      }
+      close(accepted_fd);
+      exit(0);
+    }
+    close(accepted_fd);
+  }
+
+  return 0;
+}