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.Dd December 8, 2000
.Dt IP\ FILTER 4
.Os
.Sh NAME
.Nm IP Filter
.Nd Introduction to IP packet filtering
.Sh 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.
.Sh FEATURES
The IP packet filter can:
.Bl -bullet -offset indent -compact
.It
explicitly deny/permit any packet from passing through
.It
distinguish between various interfaces
.It
filter by IP networks or hosts
.It
selectively filter any IP protocol
.It
selectively filter fragmented IP packets
.It
selectively filter packets with IP options
.It
send back an ICMP error/TCP reset for blocked packets
.It
keep packet state information for TCP, UDP and ICMP packet flows
.It
keep fragment state information for any IP packet, applying the same rule
to all fragments.
.It
act as a Network Address Translator (NAT)
.It
use redirection to setup true transparent proxy connections
.It
provide packet header details to a user program for authentication
.It
in addition, supports temporary storage of pre-authenticated rules for passing packets through
.El
.Pp
Special provision is made for the three most common Internet protocols, TCP,
UDP and ICMP. The IP Packet filter allows filtering of:
.Bl -bullet -offset indent -compact
.It
Inverted host/net matchingTCP/UDP packets by port number or a port number
range
.It
ICMP packets by type/code
.It
"established" TCP packets
.It
On any arbitrary combination of TCP flags
.It
"short" (fragmented) IP packets with incomplete headers can be filtered
.It
any of the 19 IP options or 8 registered IP security classes TOS (Type of
Service) field in packets
.El
.Pp
To keep track of the performance of the IP packet filter, a logging device
is used which supports logging of:
.Bl -bullet -offset indent -compact
.It
the TCP/UDP/ICMP and IP packet headers
.It
the first 128 bytes of the packet (including headers)
.El
.Pp
A packet can be logged when:
.Bl -bullet -offset indent -compact
.It
it is successfully passed through
.It
it is blocked from passing through
.It
it matches a rule setup to look for suspicious packets
.El
.Pp
IP Filter keeps its own set of statistics on:
.Bl -bullet -offset indent -compact
.It
packets blocked
.It
packets (and bytes!) used for accounting
.It
packets passed
.li
packets logged
.It
attempts to log which failed (buffer full)
.El
and much more, for packets going both in and out.
.Sh 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:
.Pp
.Xr 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
.Xr ipf 5 .
.Pp
.Xr 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.
.Pp
.Xr 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.
.Pp
.Xr 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.
.Pp
.Xr ipmon 8
reads buffered data from the logging device (default is /dev/ipl)
for output to either:
.Bl -bullet -offset indent -compact
.It
screen (standard output)
.It
file
.It
syslog
.El
.Pp
.Xr ipsend 1
generates arbitary IP packets for ethernet connected machines.
.Pp
.Xr ipresend 1
reads in a data file of saved IP packets (ie
snoop/tcpdump/etherfind output) and sends it back across the network.
.Pp
.Xr 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!
.Pp
.Xr 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
.Xr ipnat 5 .
.Pp
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
.Xr ipf 4 .
Documentation on ioctl's and the format of data saved
to the logging character device is provided in
.Xr 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
.Xr ipnat 4 .
.Sh PACKET PROCESSING FLOW
The following diagram illustrates the flow of TCP/IP packets through the
various stages introduced by IP Filter.
.Pp
.nf
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
.fi
.Sh 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
.Sh SEE ALSO
.Xr ipf 4 ,
.Xr ipf 5 ,
.Xr ipf 8 ,
.Xr ipfilter 5 ,
.Xr ipfs 8 ,
.Xr ipfstat 8 ,
.Xr ipftest 1 ,
.Xr ipl 4 ,
.Xr ipmon 8 ,
.Xr ipnat 4 ,
.Xr ipnat 8 ,