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NAME

       ipfilter - Introduction to IP packet filtering

DESCRIPTION

       IP  Filter  is a TCP/IP packet filter, suitable for use in a firewall environment. To use,
       it can either be used as a loadable kernel module or incorporated into your  UNIX  kernel;
       use as a loadable kernel module where possible is highly recommended. Scripts are provided
       to install and patch system files, as required.

FEATURES

       The IP packet filter can:

              explicitly deny/permit any packet from passing through

              distinguish between various interfaces

              filter by IP networks or hosts

              selectively filter any IP protocol

              selectively filter fragmented IP packets

              selectively filter packets with IP options

              send back an ICMP error/TCP reset for blocked packets

              keep packet state information for TCP, UDP and ICMP packet flows

              keep fragment state information for any IP packet, applying the same  rule  to  all
              fragments.

              act as a Network Address Translator (NAT)

              use redirection to setup true transparent proxy connections

              provide packet header details to a user program for authentication

              in  addition,  supports  temporary  storage  of pre-authenticated rules for passing
              packets through

       Special provision is made for the three most common Internet protocols, TCP, UDP and ICMP.
       The IP Packet filter allows filtering of:

              Inverted host/net matchingTCP/UDP packets by port number or a port number range

              ICMP packets by type/code

              "established" TCP packets

              On any arbitrary combination of TCP flags

              "short" (fragmented) IP packets with incomplete headers can be filtered

              any  of the 19 IP options or 8 registered IP security classes TOS (Type of Service)
              field in packets

       To keep track of the performance of the IP packet filter, a logging device is  used  which
       supports logging of:

              the TCP/UDP/ICMP and IP packet headers

              the first 128 bytes of the packet (including headers)

       A packet can be logged when:

              it is successfully passed through

              it is blocked from passing through

              it matches a rule setup to look for suspicious packets

       IP Filter keeps its own set of statistics on:

              packets blocked

              packets (and bytes!) used for accounting

              packets passed

              packets logged

              attempts to log which failed (buffer full)

              and much more, for packets going both in and out.

Tools

       The  current  implementation  provides  a small set of tools, which can easily be used and
       integrated with regular unix shells and tools. A brief description of the tools provided:

       ipf(8) reads in a set of rules, from either stdin or a file, and adds them to the  kernels
       current  list  (appending  them).  It  can also be used to flush the current filter set or
       delete individual filter rules. The file format is described in ipf(5).

       ipfs(8) is a utility to temporarily lock the IP Filter kernel tables (state tables and NAT
       mappings)  and  write them to disk. After that the system can be rebooted, and ipfs can be
       used to read these tables from disk and restore them into the kernel. This way the  system
       can be rebooted without the connections being terminated.

       ipfstat(8)  interrogates  the  kernel  for  statistics  on  packet  filtering, so far, and
       retrieves the list of filters in operation for inbound and outbound packets.

       ipftest(1) reads in a filter rule file and then applies sample  IP  packets  to  the  rule
       file.  This  allows  for  testing of filter list and examination of how a packet is passed
       along through it.

       ipmon(8) reads buffered data from the logging device (default is /dev/ipl) for  output  to
       either:

              screen (standard output)

              file

              syslog

       ipsend(1) generates arbitary IP packets for ethernet connected machines.

       ipresend(1)  reads  in a data file of saved IP packets (ie snoop/tcpdump/etherfind output)
       and sends it back across the network.

       iptest(1) contains a set of test "programs" which send out a series of IP  packets,  aimed
       at  testing  the  strength  of the TCP/IP stack at which it is aimed at. WARNING: this may
       crash machine(s) targeted!

       ipnat(8) reads in a set of rules, from either stdin or a file and adds them to the kernels
       current list of active NAT rules. NAT rules can also be deleted using ipnat. The format of
       the configuration file to be used with ipnat is described in ipnat(5).

       For use in your own programs (e.g. for writing of transparent  application  proxies),  the
       programming interface and the associated ioctl's are documented in ipf(4).

       Documentation  on  ioctl's and the format of data saved to the logging character device is
       provided in ipl(4) so that you may develop your own applications to work with or in  place
       of any of the above.

       Similar, the interface to the NAT code is documented in ipnat(4).

PACKET PROCESSING FLOW

       The  following  diagram  illustrates the flow of TCP/IP packets through the various stages
       introduced by IP Filter.

                                          IN
                                           |
                                           V
                 +-------------------------+--------------------------+
                 |                         |                          |
                 |                         V                          |
                 |            Network Address Translation             |
                 |                         |                          |
                 |         authenticated   |                          |
                 |       +-------<---------+                          |
                 |       |                 |                          |
                 |       |                 V                          |
                 |       V           IP Accounting                    |
                 |       |                 |                          |
                 |       |                 V                          |
                 |       |        Fragment Cache Check--+             |
                 |       |                 |            |             |
                 |       V                 V            V             |
                 |       |         Packet State Check-->+             |
                 |       |                 |            |             |
                 |       |       +->--+    |            |             |
                 |       |       |    |    V            |             |
                 |       V   groups   IP Filtering      V             |
                 |       |       |    |    |            |             |
                 |       |       +--<-+    |            |             |
                 |       |                 |            |             |
                 |       +---------------->|<-----------+             |
                 |                         |                          |
                 |                         V                          |
                 |                +---<----+                          |
                 |                |        |                          |
                 |            function     |                          |
                 |                |        V                          |
                 |                +--->----+                          |
                 |                         |                          |
                 |                         V                          |
              +--|---<--- fast-route ---<--+                          |
              |  |                         |                          |
              |  |                         V                          |
              |  +-------------------------+--------------------------+
              |                            |
              |                        pass only
              |                            |
              |                            V
              V               [KERNEL TCP/IP Processing]
              |                            |
              |  +-------------------------+--------------------------+
              |  |                         |                          |
              |  |                         V                          |
              |  |                Fragment Cache Check--+             |
              |  |                         |            |             |
              |  |                         V            V             |
              |  |                 Packet State Check-->+             |
              |  |                         |            |             |
              |  |                         V            |             |
              V  |                    IP Filtering      |             |
              |  |                         |            V             |
              |  |                         |<-----------+             |
              |  |                         V                          |
              |  |                   IP Accounting                    |
              |  |                         |                          |
              |  |                         V                          |
              |  |            Network Address Translation             |
              |  |                         |                          |
              |  |                         V                          |
              |  +-------------------------+--------------------------+
              |                            |
              |                        pass only
              V                            |
              +--------------------------->|
                                           V
                                          OUT

MORE INFORMATION

       More information (including pointers to the FAQ and the mailing list) can be obtained from
       the sofware's official homepage: www.ipfilter.org

SEE ALSO

       ipf(4),  ipf(5),  ipf(8),  ipfilter(5), ipfs(8), ipfstat(8), ipftest(1), ipl(4), ipmon(8),
       ipnat(8), ipnat(4),

                                                                                     IP FILTER(4)