Provided by: iptables-dev_1.4.12-1ubuntu4_i386 bug

NAME

       libipq -- iptables userspace packet queuing library.

SYNOPSIS

       #include <linux/netfilter.h>
       #include <libipq.h>

DESCRIPTION

       libipq is a development library for iptables userspace packet queuing.

   Userspace Packet Queuing
       Netfilter provides a mechanism for passing packets out of the stack for
       queueing to userspace, then  receiving  these  packets  back  into  the
       kernel  with  a verdict specifying what to do with the packets (such as
       ACCEPT or DROP).  These packets may also be modified in userspace prior
       to reinjection back into the kernel.

       For each supported protocol, a kernel module called a queue handler may
       register with Netfilter to perform the mechanics of passing packets  to
       and from userspace.

       The  standard queue handler for IPv4 is ip_queue.  It is provided as an
       experimental module with 2.4 kernels, and uses  a  Netlink  socket  for
       kernel/userspace communication.

       Once  ip_queue  is loaded, IP packets may be selected with iptables and
       queued for userspace processing via the  QUEUE  target.   For  example,
       running the following commands:

            # modprobe iptable_filter
            # modprobe ip_queue
            # iptables -A OUTPUT -p icmp -j QUEUE

       will  cause any locally generated ICMP packets (e.g. ping output) to be
       sent to the ip_queue module, which will then  attempt  to  deliver  the
       packets  to  a  userspace  application.  If no userspace application is
       waiting, the packets will be dropped

       An application may receive and process these packets via libipq.

   Libipq Overview
       Libipq provides an API for communicating with ip_queue.  The  following
       is  an  overview  of  API usage, refer to individual man pages for more
       details on each function.

       Initialisation
       To  initialise  the  library,  call  ipq_create_handle(3).   This  will
       attempt  to  bind  to the Netlink socket used by ip_queue and return an
       opaque context handle for subsequent library calls.

       Setting the Queue Mode
       ipq_set_mode(3)  allows  the  application  to  specify  whether  packet
       metadata,  or  packet  payloads  as  well  as  metadata  are  copied to
       userspace.  It is also  used  to  initially  notify  ip_queue  that  an
       application is ready to receive queue messages.

       Receiving Packets from the Queue
       ipq_read(3) waits for queue messages to arrive from ip_queue and copies
       them into a supplied buffer.  Queue messages may be packet messages  or
       error messages.

       The type of packet may be determined with ipq_message_type(3).

       If  it's  a  packet  message,  the metadata and optional payload may be
       retrieved with ipq_get_packet(3).

       To retrieve the value of an error message, use ipq_get_msgerr(3).

       Issuing Verdicts on Packets
       To issue a verdict on  a  packet,  and  optionally  return  a  modified
       version of the packet to the kernel, call ipq_set_verdict(3).

       Error Handling
       An  error  string  corresponding  to  the current value of the internal
       error variable ipq_errno may be obtained with ipq_errstr(3).

       For simple applications, calling  ipq_perror(3)  will  print  the  same
       message  as  ipq_errstr(3),  as well as the string corresponding to the
       global errno value (if set) to stderr.

       Cleaning Up
       To free up the Netlink socket and destroy resources associated with the
       context handle, call ipq_destroy_handle(3).

SUMMARY

       ipq_create_handle(3)
           Initialise library, return context handle.

       ipq_set_mode(3)
           Set  the queue mode, to copy either packet metadata, or payloads as
           well as metadata to userspace.

       ipq_read(3)
           Wait for a queue message to arrive from ip_queue and read it into a
           buffer.

       ipq_message_type(3)
           Determine message type in the buffer.

       ipq_get_packet(3)
           Retrieve a packet message from the buffer.

       ipq_get_msgerr(3)
           Retrieve an error message from the buffer.

       ipq_set_verdict(3)
           Set a verdict on a packet, optionally replacing its contents.

       ipq_errstr(3)
           Return  an  error  message  corresponding to the internal ipq_errno
           variable.

       ipq_perror(3)
           Helper function to print error messages to stderr.

       ipq_destroy_handle(3)
           Destroy context handle and associated resources.

EXAMPLE

       The following is an example of  a  simple  application  which  receives
       packets and issues NF_ACCEPT verdicts on each packet.
              /*
               * This code is GPL.
               */
              #include <linux/netfilter.h>
              #include <libipq.h>
              #include <stdio.h>

              #define BUFSIZE 2048

              static void die(struct ipq_handle *h)
              {
                   ipq_perror("passer");
                   ipq_destroy_handle(h);
                   exit(1);
              }

              int main(int argc, char **argv)
              {
                   int status;
                   unsigned char buf[BUFSIZE];
                   struct ipq_handle *h;

                   h = ipq_create_handle(0, NFPROTO_IPV4);
                   if (!h)
                        die(h);

                   status = ipq_set_mode(h, IPQ_COPY_PACKET, BUFSIZE);
                   if (status < 0)
                        die(h);

                   do{
                        status = ipq_read(h, buf, BUFSIZE, 0);
                        if (status < 0)
                             die(h);

                        switch (ipq_message_type(buf)) {
                             case NLMSG_ERROR:
                                  fprintf(stderr, "Received error message %d\n",
                                          ipq_get_msgerr(buf));
                                  break;

                             case IPQM_PACKET: {
                                  ipq_packet_msg_t *m = ipq_get_packet(buf);

                                  status = ipq_set_verdict(h, m->packet_id,
                                                           NF_ACCEPT, 0, NULL);
                                  if (status < 0)
                                       die(h);
                                  break;
                             }

                             default:
                                  fprintf(stderr, "Unknown message type!\n");
                                  break;
                        }
                   } while (1);

                   ipq_destroy_handle(h);
                   return 0;
              }

       Pointers  to  more  libipq  application  examples  may  be found in The
       Netfilter FAQ.

DIAGNOSTICS

       For information about monitoring and  tuning  ip_queue,  refer  to  the
       Linux 2.4 Packet Filtering HOWTO.

       If  an  application  modifies  a  packet,  it  needs to also update any
       checksums for the packet.  Typically, the kernel will silently  discard
       modified packets with invalid checksums.

SECURITY

       Processes require CAP_NET_ADMIN capabilty to access the kernel ip_queue
       module.  Such processes  can  potentially  access  and  modify  any  IP
       packets received, generated or forwarded by the kernel.

TODO

       Per-handle ipq_errno values.

BUGS

       Probably.

AUTHOR

       James Morris <jmorris@intercode.com.au>

COPYRIGHT

       Copyright (c) 2000-2001 Netfilter Core Team.

       Distributed under the GNU General Public License.

CREDITS

       Joost  Remijn  implemented the ipq_read timeout feature, which appeared
       in the 1.2.4 release of iptables.

       Fernando Anton added support for IPv6.

SEE ALSO

       iptables(8),        ipq_create_handle(3),        ipq_destroy_handle(3),
       ipq_errstr(3),           ipq_get_msgerr(3),          ipq_get_packet(3),
       ipq_message_type(3),   ipq_perror(3),   ipq_read(3),   ipq_set_mode(3),
       ipq_set_verdict(3).

       The  Netfilter home page at http://netfilter.samba.org/ which has links
       to The Networking Concepts HOWTO, The Linux 2.4 Packet Filtering HOWTO,
       The Linux 2.4 NAT HOWTO, The Netfilter Hacking HOWTO, The Netfilter FAQ
       and many other useful resources.