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6. Using Divert Sockets

This section will give you examples of how divert sockets can be used and how they are different of other packet interception mechanisms out there.

6.1 Divert sockets vs. other stuff

There are other mechanisms out there that have similar functionality. Here is why they are different:

Netlink sockets

Netlink sockets can intercept packets just like divert sockets by using firewall filter. They have a special type (AF_NETLINK) and on the surface seem to do the same thing. Two major differences are:

To be fair, the scope of netlink sockets is wider than this. In general, netlink mechanism is intended to allow communication between kernel and user space. There are, for instance, netlink routing sockets that allow you to communicate with the routing subsystem. However, as a packet interception mechanism, they are not as robust as divert sockets.

Raw sockets

RAW sockets can be a good way to listen in on traffic (especially under Linux, where RAW sockets can listen in on TCP and UDP traffic, although most other UNI*s do not allow that) but a RAW socket can't stop a packet from propagating through the IP stack - it simply gives you a copy of the packet and there is no way to inject it inbound (on the way up the stack) - only outbound. Also, you can only filter pockets out by the protocol number, which you specify when you open a RAW socket. There is no link between the firewall and RAW sockets.


More commonly known for the tool it facilitates - tcpdump, libpcap lets you listen in on traffic that hits your interface (whether it be ppp or eth or whatever). For ethernet it can also put your NIC into a promiscuous mode, so that it will forward to IP the traffic that not only is link-layer addressed to it, but to others on the same segment. Of course, libpcap allows for no way of actually stopping packets from propagating and no way to inject. In fact, libpcap is in many ways orthogonal to divert sockets.

6.2 Discussion on firewall chains

Linux provides you with three default chains: input, output and forward. There are also accounting chains, but they are of no consequence here. Depending on the packet origin it traverses one or more of these chains:

Input chain

is traversed by all packets that come into the host - packets that are addressed to it and packets that will be forwarded by it.

Output chain

is traversed by all packets originating in the host and by all forwarded packets

Forward chain

is traversed only by the forwarded packets.

The order in which a forwarded packet traverses the chains is:

  1. Input
  2. Forward
  3. Output
This may sometimes create problems for the interception if you are interested in a certain type of packets that may or may not originate on your host. A lot of times it is not clear which chain to use.

As a rule of thumb, forward chain should only be used to filter packets that are forwarded and are not originating and are not addressed to your host. If you are interested in a combination of both forwarded packets and packets that are originating or addressed to your host, then use input or output chain instead. Intercepting on forward and input or output chain for the same type of packet at the same time will create problems in reinjection and, more importantly, is unnecessary.

6.3 Using ipchains

The patched version of ipchains that you will need to retrieve from the website, is the tool that allows you to modify firewall rules from a shell (most people want that). It is also possible to set up firewall rules programmatically. See the example code for this - setting up a DIVERT rule would be similar to setting up a REDIRECT rule - specify DIVERT as a target and the divert port and you are set to go.

The ipchains syntax for setting up firewall rules remains the same. To specify a DIVERT rule you must specify -j DIVERT <port num> as a target, everything else remains the same. For instance

ipchains -A input -p ICMP -j DIVERT 1234
would set up a divert rule for ICMP packets to be diverted from input chain to a port 1234.

The following section explains how to use ipchains in conjunction with an interceptor user-space program.

6.4 Plain vanilla example

Example program

Here is an example program that reads packets from a divert socket, displays them and then reinjects them back. It requires that the divert port is specified on the command line.

#include <stdio.h>
#include <errno.h>
#include <limits.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <signal.h>

#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <netinet/udp.h>
#include <net/if.h>
#include <sys/param.h>

#include <linux/types.h>
#include <linux/icmp.h>
#include <linux/ip_fw.h>

#define IPPROTO_DIVERT 254
#define BUFSIZE 65535

char *progname;


char *fw_policy="DIVERT";
char *fw_chain="output";
struct ip_fw fw;
struct ip_fwuser ipfu;
struct ip_fwchange ipfc;
int fw_sock;

/* remove the firewall rule when exit */
void intHandler (int signo) {

  if (setsockopt(fw_sock, IPPROTO_IP, IP_FW_DELETE, &ipfc, sizeof(ipfc))==-1) {
    fprintf(stderr, "%s: could not remove rule: %s\n", progname, strerror(errno));



int main(int argc, char** argv) {
  int fd, rawfd, fdfw, ret, n;
  int on=1;
  struct sockaddr_in bindPort, sin;
  int sinlen;
  struct iphdr *hdr;
  unsigned char packet[BUFSIZE];
  struct in_addr addr;
  int i, direction;
  struct ip_mreq mreq;

  if (argc!=2) {
    fprintf(stderr, "Usage: %s <port number>\n", argv[0]);

  fprintf(stderr,"%s:Creating a socket\n",argv[0]);
  /* open a divert socket */

  if (fd==-1) {
    fprintf(stderr,"%s:We could not open a divert socket\n",argv[0]);


  fprintf(stderr,"%s:Binding a socket\n",argv[0]);
  ret=bind(fd, &bindPort, sizeof(struct sockaddr_in));

  if (ret!=0) {
    fprintf(stderr, "%s: Error bind(): %s",argv[0],strerror(ret));
  /* fill in the rule first */
  bzero(&fw, sizeof (struct ip_fw));
  fw.fw_proto=1; /* ICMP */

  /* fill in the fwuser structure */
  memcpy(ipfu.label, fw_policy, strlen(fw_policy));

  /* fill in the fwchange structure */
  memcpy(ipfc.fwc_label, fw_chain, strlen(fw_chain));

  /* open a socket */
  if ((fw_sock=socket(AF_INET, SOCK_RAW, IPPROTO_RAW))==-1) {
    fprintf(stderr, "%s: could not create a raw socket: %s\n", argv[0], strerror(errno));

  /* write a rule into it */
  if (setsockopt(fw_sock, IPPROTO_IP, IP_FW_APPEND, &ipfc, sizeof(ipfc))==-1) {
    fprintf(stderr, "%s could not set rule: %s\n", argv[0], strerror(errno));
  /* install signal handler to delete the rule */
  signal(SIGINT, intHandler);
#endif /* FIREWALL */
  printf("%s: Waiting for data...\n",argv[0]);
  /* read data in */
  sinlen=sizeof(struct sockaddr_in);
  while(1) {
    n=recvfrom(fd, packet, BUFSIZE, 0, &sin, &sinlen);
    hdr=(struct iphdr*)packet;
    printf("%s: The packet looks like this:\n",argv[0]);
        for( i=0; i<40; i++) {
                printf("%02x ", (int)*(packet+i));
                if (!((i+1)%16)) printf("\n");

    printf("%s: Source address: %s\n",argv[0], inet_ntoa(addr));
    printf("%s: Destination address: %s\n", argv[0], inet_ntoa(addr));
    printf("%s: Receiving IF address: %s\n", argv[0], inet_ntoa(sin.sin_addr));
    printf("%s: Protocol number: %i\n", argv[0], hdr->protocol);

    /* reinjection */

   if (IN_MULTICAST((ntohl(hdr->daddr)))) {
        printf("%s: Multicast address!\n", argv[0]);
        addr.s_addr = hdr->saddr;
        errno = 0;
        if (sin.sin_addr.s_addr == 0)
            printf("%s: set_interface returns %i with errno =%i\n", argv[0], setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)), errno);

   printf("%s Reinjecting DIVERT %i bytes\n", argv[0], n);
   n=sendto(fd, packet, n ,0, &sin, sinlen);
   printf("%s: %i bytes reinjected.\n", argv[0], n); 

   if (n<=0) 
     printf("%s: Oops: errno = %i\n", argv[0], errno);
   if (errno == EBADRQC)
     printf("errno == EBADRQC\n");
   if (errno == ENETUNREACH)
     printf("errno == ENETUNREACH\n");

You can simply cut-n-paste the code and compile it with your favorite compiler. If you want to enable reinjection - compile it with the -DREINJECT flag, otherwise it will only do the interception.

In order to get it to work, compile the kernel and ipchains-1.3.8 as described above. Insert a rule into any of the firewall chains: input, output or forward, then send the packets that would match the rule and watch them as they fly through the screen - your interceptor program will display them and then reinject them back, if appropriately compiled.

For example:

ipchains -A output -p TCP -s -j DIVERT 4321
interceptor 4321
will divert and display all TCP packets originating on host (for instance if your host is a gateway). It will intercept them on the output just before they go on the wire.

If you did not compile the pass through option into the kernel, then inserting the rule effectively will create a DENY rule in the firewall for the packets you specified until you start the interceptor program. See more on that above

If you want to set a firewall rule through your program, compile it with -DFIREWALL option and it will divert all ICMP packets from the output chain. It will also remove the DIVERT rule from the firewall when you use Ctrl-C to exit the program. In this case using pass-through vs. non-pass-through divert sockets makes virtually no difference.

6.5 The sky's the limit

As far as what you can use divert sockets for - your imagination would be the limiting factor. I would be interested to hear about applications that utilize divert sockets.

So, have fun!

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