Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
IP FILTER(4)           FreeBSD Kernel Interfaces Manual           IP FILTER(4)

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)

NAME | DESCRIPTION | FEATURES | Tools | PACKET PROCESSING FLOW | MORE INFORMATION | SEE ALSO

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=ipfilter&manpath=FreeBSD+10.3-RELEASE+and+Ports>

home | help