--- 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);

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/time.h>
#include <errno.h>
#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
 
int tun_alloc(char *dev)
{
    struct ifreq ifr;
    int fd, err;
 
    if (!dev) {
      return -1;
    }
 
    memset(&ifr, 0, sizeof(ifr));
    /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
     *        IFF_TAP   - TAP device
     *
     *        IFF_NO_PI - Do not provide packet information
     *        IFF_MULTI_QUEUE - Create a queue of multiqueue device
     */
    ifr.ifr_flags = IFF_TUN;
    strcpy(ifr.ifr_name, dev);
 
    if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
       return fd;
 
    err = ioctl(fd, TUNSETIFF, (void *)&ifr);
    if (err) {
       close(fd);
       goto err;
    }
 
    strcpy(dev, ifr.ifr_name);
 
    return fd;
err:
    close(fd);
    return err;
}
 
static int set_if_flags(char *ifname, short flags)
{
  struct ifreq ifr;
  int res = 0;
  ifr.ifr_flags = flags;
 
  strncpy(ifr.ifr_name, ifname, IFNAMSIZ);
 
  int skfd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
 
  res = ioctl(skfd, SIOCSIFFLAGS, &ifr);
  if (res < 0) {
    puts("Interface '%s': Error: SIOCSIFFLAGS failed: %s\n");
  } else {
    puts("Interface '%s': flags set to %04X.\n");
  }
 
  return res;
}
 
int main() {
 
  char *tun_name;
  tun_name = malloc(IFNAMSIZ);
  tun_name[0] = '\0';
 
  int tun_fd = tun_alloc(tun_name);
  if (tun_fd < 0) {
    puts("Try as root");
    exit(1);
  }
 
  if (ioctl(tun_fd, TUNSETPERSIST, 0) < 0) {
    perror("disabling TUNSETPERSIST");
    exit(1);
  }
  printf("Set interface '%s' nonpersistent\n", tun_name);
 
  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
      uint16_t proto; // PRPTO from TUN
      unsigned char payload[]; // FRAME/PACKET
  } __attribute__((packed));
 
  int nread;
  char buffer[BUFSIZE];
 
  while(1) {
 
    nread = read(tun_fd, buffer, sizeof(buffer));
    if(nread < 0) {
      perror("Reading from interface");
      close(tun_fd);
      exit(1);
    }
 
    /* Do whatever with the data */
    printf("Read %d bytes from device %s\n", nread, tun_name);
 
    struct layer3_frame* l3p = (struct layer3_frame*)(buffer);
    printf("FLAGS %x, PROTO %x\n", l3p->flags, ntohs(l3p->proto));
 
    int ethertype = ntohs(l3p->proto);
    int ipv = 4;
    if (ethertype == 0x800) {
      ipv = 4;
    } else if (ethertype == 0x86DD) {
      ipv = 6;
    } else {
      printf("Unknown ethertype %x", ethertype);
      exit(1);
    }
 
    if (ipv == 4) {
 
      puts("PARSING V4!");
      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 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);
      int code = sendto (s, ippacket, nread, 0, (struct sockaddr*)&sin, sizeof (sin));
      if (code < 0) {
        perror("Failed to send");
      } else {
        puts("Sent IPV4");
      }
      */
 
    } else if (ipv == 6) {
 
      puts("PARSING V6!");
      const struct ip6_hdr* ippacket = (struct ip6_hdr*)(l3p->payload);
      struct in6_addr ip_dst = ippacket->ip6_dst;
 
      static struct sockaddr_in6 remote;
      remote.sin6_family= AF_INET;
      remote.sin6_addr = ip_dst;
 
      /*
      int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
      int code = sendto (s, ippacket, nread, 0, (struct sockaddr*)&remote, sizeof (remote));
      if (code < 0) {
        perror("Failed to send");
      } else {
        puts("Sent IPV6");
      }
      */
 
    }
    write(tun_fd, buffer, nread);
 
  }
 
  return 0;
 
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/time.h>
#include <errno.h>
#include <stdarg.h>
#include <netinet/ip.h>
#include <netinet/ip6.h>

/* buffer for reading from tun/tap interface, must be >= 1500 */
#define BUFSIZE 2000

int tun_alloc(char *dev)
{
    struct ifreq ifr;
    int fd, err;

    if (!dev) {
      return -1;
    }

    memset(&ifr, 0, sizeof(ifr));
    /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
     *        IFF_TAP   - TAP device
     *
     *        IFF_NO_PI - Do not provide packet information
     *        IFF_MULTI_QUEUE - Create a queue of multiqueue device
     */
    ifr.ifr_flags = IFF_TUN;
    strcpy(ifr.ifr_name, dev);

    if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
       return fd;

    err = ioctl(fd, TUNSETIFF, (void *)&ifr);
    if (err) {
       close(fd);
       goto err;
    }

    strcpy(dev, ifr.ifr_name);

    return fd;
err:
    close(fd);
    return err;
}

static int set_if_flags(char *ifname, short flags)
{
  struct ifreq ifr;
  int res = 0;
  ifr.ifr_flags = flags;

  strncpy(ifr.ifr_name, ifname, IFNAMSIZ);

  int skfd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);

  res = ioctl(skfd, SIOCSIFFLAGS, &ifr);
  if (res < 0) {
    puts("Interface '%s': Error: SIOCSIFFLAGS failed: %s\n");
  } else {
    puts("Interface '%s': flags set to %04X.\n");
  }

  return res;
}

int main() {

  char *tun_name;
  tun_name = malloc(IFNAMSIZ);
  tun_name[0] = '\0';

  int tun_fd = tun_alloc(tun_name);
  if (tun_fd < 0) {
    puts("Try as root");
    exit(1);
  }

  if (ioctl(tun_fd, TUNSETPERSIST, 0) < 0) {
    perror("disabling TUNSETPERSIST");
    exit(1);
  }
  printf("Set interface '%s' nonpersistent\n", tun_name);

  set_if_flags(tun_name, IFF_UP);

  struct layer3_frame
  {
      uint16_t flags; // FLAGS from TUN
      uint16_t proto; // PRPTO from TUN
      unsigned char payload[]; // FRAME/PACKET
  } __attribute__((packed));

  int nread;
  char buffer[BUFSIZE];

  while(1) {

    nread = read(tun_fd, buffer, sizeof(buffer));
    if(nread < 0) {
      perror("Reading from interface");
      close(tun_fd);
      exit(1);
    }

    /* Do whatever with the data */
    printf("Read %d bytes from device %s\n", nread, tun_name);

    struct layer3_frame* l3p = (struct layer3_frame*)(buffer);
    printf("FLAGS %x, PROTO %x\n", l3p->flags, ntohs(l3p->proto));

    int ethertype = ntohs(l3p->proto);
    int ipv = 4;
    if (ethertype == 0x800) {
      ipv = 4;
    } else if (ethertype == 0x86DD) {
      ipv = 6;
    } else {
      printf("Unknown ethertype %x", ethertype);
      exit(1);
    }

    if (ipv == 4) {

      puts("PARSING V4!");
      const 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));

      /*
      int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
      int code = sendto (s, ippacket, nread, 0, (struct sockaddr*)&sin, sizeof (sin));
      if (code < 0) {
        perror("Failed to send");
      } else {
        puts("Sent IPV4");
      }
      */

    } else if (ipv == 6) {

      puts("PARSING V6!");
      const struct ip6_hdr* ippacket = (struct ip6_hdr*)(l3p->payload);
      struct in6_addr ip_dst = ippacket->ip6_dst;

      static struct sockaddr_in6 remote;
      remote.sin6_family= AF_INET;
      remote.sin6_addr = ip_dst;

      /*
      int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
      int code = sendto (s, ippacket, nread, 0, (struct sockaddr*)&remote, sizeof (remote));
      if (code < 0) {
        perror("Failed to send");
      } else {
        puts("Sent IPV6");
      }
      */

    }
    write(tun_fd, buffer, nread);

  }

  return 0;

}