tun0_changeip.diff

--- tun0.c  2017-11-29 22:57:44.000000000 +0300
+++ tun0_changeip.c 2017-12-04 17:36:04.000000000 +0300
@@ -16,6 +16,47 @@
 #include <stdarg.h>
 #include <netinet/ip.h>
 #include <netinet/ip6.h>
+/* for IPPROTO_XXX definitions */
+#include <netinet/in.h>
+/* for TCP header */
+#include <netinet/tcp.h>
+/* for UDP header */
+#include <netinet/udp.h>
+
+/* the code snippet below is taken "as is" from the openvpn project, file src/openvpn/proto.h
+/* it's used to correct internet checksums for IP, TCP and UDP headers after IP header modifications
+/*
+ * The following macro is used to update an
+ * internet checksum.  "acc" is a 32-bit
+ * accumulation of all the changes to the
+ * checksum (adding in old 16-bit words and
+ * subtracting out new words), and "cksum"
+ * is the checksum value to be updated.
+ */
+#define ADJUST_CHECKSUM(acc, cksum) { \
+  int _acc = acc; \
+  _acc += (cksum); \
+  if (_acc < 0) { \
+    _acc = -_acc; \
+    _acc = (_acc >> 16) + (_acc & 0xffff); \
+    _acc += _acc >> 16; \
+    (cksum) = (uint16_t) ~_acc; \
+  } else { \
+    _acc = (_acc >> 16) + (_acc & 0xffff); \
+    _acc += _acc >> 16; \
+    (cksum) = (uint16_t) _acc; \
+  } \
+}
+
+#define ADD_CHECKSUM_32(acc, u32) { \
+  acc += (u32) & 0xffff; \
+  acc += (u32) >> 16;   \
+}
+
+#define SUB_CHECKSUM_32(acc, u32) { \
+  acc -= (u32) & 0xffff; \
+  acc -= (u32) >> 16;   \
+}

 /* buffer for reading from tun/tap interface, must be >= 1500 */
 #define BUFSIZE 2000
@@ -96,6 +137,13 @@

   set_if_flags(tun_name, IFF_UP);

+  /* For now just hardcode the source and replacement IP addresses */
+  struct in_addr tun_ip_input, tun_ip_output;
+  if (!inet_aton("10.0.0.1", &tun_ip_input) || !inet_aton("10.0.0.2", &tun_ip_output)) {
+    printf("inet_aton() failed!\n");
+    exit(1);
+  }
+
   struct layer3_frame
   {
       uint16_t flags; // FLAGS from TUN
@@ -135,14 +183,50 @@
     if (ipv == 4) {

       puts("PARSING V4!");
-      const struct ip* ippacket = (struct ip*)(l3p->payload);
+      struct ip* ippacket = (struct ip*)(l3p->payload);
       printf("VER:%d\n", ippacket->ip_v);
       struct in_addr ip_dst = ippacket->ip_dst;
       struct sockaddr_in sin;
       sin.sin_family = AF_INET;
       sin.sin_addr = ip_dst;

-      printf("The IP address is %s\n", inet_ntoa(ip_dst));
+      printf("The source IP address is %s\n", inet_ntoa(ippacket->ip_src));
+      printf("The destination IP address is %s\n", inet_ntoa(ippacket->ip_dst));
+
+      /* Switch IP addresses */
+      int accumulate = 0;
+      if (ippacket->ip_src.s_addr == tun_ip_input.s_addr) {
+        /* pre-adjust IP checksum */
+        ADD_CHECKSUM_32(accumulate, ippacket->ip_src.s_addr);
+        /* change source IP */
+        ippacket->ip_src = tun_ip_output;
+        /* post-adjust IP checksum */
+        SUB_CHECKSUM_32(accumulate, ippacket->ip_src.s_addr);
+      }
+      else if (ippacket->ip_dst.s_addr == tun_ip_output.s_addr) {
+        /* pre-adjust IP checksum */
+        ADD_CHECKSUM_32(accumulate, ippacket->ip_dst.s_addr);
+        /* change destination IP */
+        ippacket->ip_dst = tun_ip_input;
+        /* post-adjust IP checksum */
+        SUB_CHECKSUM_32(accumulate, ippacket->ip_dst.s_addr);
+      }
+
+      printf("New source IP address is %s\n", inet_ntoa(ippacket->ip_src));
+      printf("New destination IP address is %s\n", inet_ntoa(ippacket->ip_dst));
+
+      /* Correct packet checksums */
+      ADJUST_CHECKSUM(accumulate, ippacket->ip_sum);
+      if (ippacket->ip_p == IPPROTO_TCP &&
+          nread >= sizeof(struct layer3_frame) + sizeof(struct ip) + sizeof(struct tcphdr)) {
+        struct tcphdr* tcp_hdr = (struct tcphdr*)(l3p->payload + sizeof(struct ip));
+        ADJUST_CHECKSUM(accumulate, tcp_hdr->check);
+      }
+      else if (ippacket->ip_p == IPPROTO_UDP &&
+          nread >= sizeof(struct layer3_frame) + sizeof(struct ip) + sizeof(struct udphdr)) {
+        struct udphdr* udp_hdr = (struct udphdr*)(l3p->payload + sizeof(struct ip));
+        ADJUST_CHECKSUM(accumulate, udp_hdr->check);
+      }

       /*
       int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);