add hw4

Author: chengscott

hw4/Makefile

@@ -0,0 +1,14 @@
+CC=gcc
+CFLAGS=-Os -Wall
+
+all: source.c
+	${CC} ${CFLAGS} source.c -o lanchat
+	strip lanchat
+
+.PHONY: format
+format:
+	clang-format -style=Google -i source.c
+
+.PHONY: clean
+clean:
+	rm -f lanchat

hw4/README

@@ -0,0 +1,10 @@
+# Chat in LAN
+
+## Usage
+
+`make` builds a binary `lanchat`
+
+## Note
+
+- the source interface is chosen by the last available interface (UP)
+- the registered protocol is 0x801 (ETH_P_IP + 1)

hw4/source.c

@@ -0,0 +1,114 @@
+#include <arpa/inet.h>
+#include <ifaddrs.h>
+#include <linux/if_packet.h>
+#include <net/ethernet.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <time.h>
+#include <unistd.h>
+
+int main(void) {
+  printf("Enumerated network interfaces:\n");
+  struct ifaddrs *ifap, *it;
+  getifaddrs(&ifap);
+  // MAC address
+  char MAC[12][18];
+  int index = 0;
+  for (it = ifap; it; it = it->ifa_next) {
+    unsigned char *mac = ((struct sockaddr_ll *)it->ifa_addr)->sll_addr;
+    if (it->ifa_addr->sa_family == AF_INET && (it->ifa_flags & IFF_UP) == 1)
+      index = if_nametoindex(it->ifa_name);
+    if (it->ifa_addr->sa_family == AF_PACKET)
+      sprintf(MAC[if_nametoindex(it->ifa_name)],
+              "%02x:%02x:%02x:%02x:%02x:%02x", mac[0], mac[1], mac[2], mac[3],
+              mac[4], mac[5]);
+  }
+  // Network address
+  for (it = ifap; it; it = it->ifa_next) {
+    if (it->ifa_addr->sa_family == AF_INET) {
+      char inet_addr[INET_ADDRSTRLEN], netmask_addr[INET_ADDRSTRLEN],
+          masked_addr[INET_ADDRSTRLEN];
+      struct in_addr *it_addr = &((struct sockaddr_in *)it->ifa_addr)->sin_addr,
+                     *it_netmask =
+                         &((struct sockaddr_in *)it->ifa_netmask)->sin_addr,
+                     masked_in;
+      inet_ntop(AF_INET, it_addr, inet_addr, INET_ADDRSTRLEN);
+      inet_ntop(AF_INET, it_netmask, netmask_addr, INET_ADDRSTRLEN);
+      masked_in.s_addr = it_addr->s_addr | (~it_netmask->s_addr);
+      inet_ntop(AF_INET, &masked_in, masked_addr, INET_ADDRSTRLEN);
+      unsigned int idx = if_nametoindex(it->ifa_name);
+      printf("%u - %-8s %16s %#x (%s) %s\n", idx, it->ifa_name, inet_addr,
+             ntohl(it_netmask->s_addr), masked_addr, MAC[idx]);
+    }
+  }
+  freeifaddrs(ifap);
+  // username
+  srand(time(NULL));
+  char username[48] = "[";
+  sprintf(username, "%05d%05d[", rand() % 100000, rand() % 100000);
+  printf("Enter your name: ");
+  scanf("%s", username + 11);
+  printf("Welcome, '%s'!\n", username + 11);
+  struct ether_header *hdr;
+  const size_t len_ether_header = sizeof(struct ether_header),
+               user_len = strlen(username);
+  const unsigned short protocol = htons(ETH_P_IP + 1);
+  // child receive message
+  unsigned int pid;
+  if ((pid = fork()) == 0) {
+    char recv_msg[4096];
+    int recv_sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
+    while (1) {
+      recv(recv_sock, recv_msg, 4096, 0);
+      hdr = (struct ether_header *)recv_msg;
+      if (hdr->ether_type == protocol &&
+          strncmp(username, recv_msg + len_ether_header, user_len) != 0) {
+        unsigned char *mac = hdr->ether_shost;
+        printf(">>> <%02x:%02x:%02x:%02x:%02x:%02x> %s", mac[0], mac[1], mac[2],
+               mac[3], mac[4], mac[5], recv_msg + len_ether_header + 10);
+      }
+    }
+    return 0;
+  }
+  // link layer address
+  struct sockaddr_ll addr;
+  addr.sll_family = AF_PACKET;
+  addr.sll_protocol = ETH_P_IP + 1;
+  addr.sll_halen = ETH_ALEN;
+  addr.sll_ifindex = index;
+  for (size_t i = 0; i < 6; ++i) addr.sll_addr[i] = 0xff;
+  // ethernet header
+  char send_msg[4096] = {};
+  hdr = (struct ether_header *)send_msg;
+  for (size_t i = 0; i < 6; ++i) {
+    char m1 = MAC[index][i * 3], m2 = MAC[index][i * 3 + 1];
+    int d1 = m1 >= '0' && m1 <= '9' ? m1 - '0' : m1 - 'a' + 10,
+        d2 = m2 >= '0' && m2 <= '9' ? m2 - '0' : m2 - 'a' + 10;
+    hdr->ether_shost[i] = d1 * 16 + d2;
+    hdr->ether_dhost[i] = 0xff;
+  }
+  hdr->ether_type = htons(ETH_P_IP + 1);
+  // send message
+  size_t len = len_ether_header, msg_len = 0;
+  for (size_t i = 0; i < user_len; ++i) send_msg[len++] = username[i];
+  send_msg[len++] = ']';
+  send_msg[len++] = ':';
+  send_msg[len++] = ' ';
+  int send_sock = socket(AF_PACKET, SOCK_RAW, protocol);
+  while (fgets(send_msg + len, 4000, stdin)) {
+    msg_len = strlen(send_msg + len) + 1;
+    if (msg_len == 2) continue;
+    send_msg[len + msg_len] = '\0';
+    sendto(send_sock, send_msg, len + msg_len, 0, (struct sockaddr *)&addr,
+           sizeof(struct sockaddr_ll));
+  }
+  kill(pid, SIGTERM);
+  return 0;
+}