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5d39ab9169
(but I'm actually just as happy to have in the attic, for reference).
528 lines
24 KiB
Groff
528 lines
24 KiB
Groff
.\"
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.\" ipfw - a utility for manipulating the configuration of an IP firewall.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" @(#)ipfw.1
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.\"
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.TH ipfw 1 "October 27, 1994" "" "FreeBSD"
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.SH NAME
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ipfw - a utility for manipulating the configuration of an IP firewall.
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.SH SYNOPSIS
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.na
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.B ipfw
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.RB [options]
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.SH DESCRIPTION
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The
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.B ipfw
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command is used to configure an active IP firewall, setting masks on just
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what sites are allowed to connect through it, which packets are rejected,
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etc.
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.SH OPTIONS
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The command-line syntax of this command is rather involved, and rather than
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spend a lot of time that I just don't have at the moment creating a
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.B real
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man page, with properly formatted sections and all, I'm just going to loosely
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format the README I got. This really needs an nroff expert to go through
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it with a chainsaw and do a
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.N REAL
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job of formatting it! This all looks rather horrible at present, and
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I would actually almost recommend that you simply read the man page text
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directly, rather than trying to format it. Sorry, but I do NOT speak
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nroff, nor do I ever wish to learn how! :-) [-jkh].
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.PP
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For a sample kernel configuration file that will enable the right kernel
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features necessary for firewalling, see
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.I /sys/i386/conf/IPFIREWALL
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.
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.PP
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.B WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!!
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.PP
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This utility can be used to put your machine into very dysfunctional state,
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so if you want to test it then you should first make sure to read this man page
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all the way through, and don't run it anywhere from the system console!
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Using
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.I ipfw
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incorrectly is a really good way to kick yourself off your own machine
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if you're logged in over a network! Also make sure to never set this
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utility to be setuid root! It's a blatant security hole that way.
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Instead, run it as root or from "/etc/rc.local" as part of the boot process.
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It's also a good idea to use the checkb or checkf command options (see below)
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to pass some test packets through the firewalls that you've defined before
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going "live".
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.PP
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You may find it useful to create a file in which the first line is
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.I firewall flush
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to flush any existing firewalls before defining the explicit firewalls
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that you wish to use. This will ensure that you're always working from a
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known state.
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.PP
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The syntax for the
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.BI ipfirewall
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command option is rather complex and yet simple at the same time (if you know
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what I mean). There are seven sub-commands, and probably the easiest way to
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get into this is to give you a roughly BNF style grammar for the command
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(curly brackets are used for precedence, alternatives are separated by |,
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optional things are enclosed in square brackets, white space is required if
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it appears below and must not appear if there isn't any between the tokens
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below (i.e. no white space around periods, colons or slashes, whitespace
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required between all other tokens)):
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.PP
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.nf
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command ::= ipfirewall <list> | <flush> | <check> | <add> | <del>
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<list> ::= list
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<flush> ::= flush
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<check> ::= { checkb[locking] | checkf[orwarding] } <chkparms>
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<add> ::= { addb[locking] | addf[orwarding] } <add-del-parms>
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<del> ::= { delb[locking] | delf[orwarding] } <add-del-parms>
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<chkparms> ::= <protocol> from <ipaddr> <port> to <ipaddr> <port>
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<protocol> ::= tcp | udp
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<ipaddr> ::= <int>.<int>.<int>.<int> | <hostname>
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<hostname> ::= a host name from /etc/hosts
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<port> ::= <int> | <service>
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<service> ::= a service from /etc/services
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<int> ::= a non-negative integer
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<add-del-parms> ::= { accept | deny } { <universal_firewall> | <protocol_firewall> }
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<universal_firewall> ::= all from <masked_ipaddr> to <masked_ipaddr>
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<masked_ipaddr> ::= { <ipaddr>/<bits> } | { <ipaddr>:<ipaddr> } | <ipaddr>
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<bits> ::= integer in the range 0 to 32 inclusive
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<protocol_firewall> ::= <protocol> from <end_firewall> to <end_firewall>
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<end_firewall> ::= <masked_ipaddr> <port_list>
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<port_list> ::= [ <port>:<port> ] <sub_port_list>
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<sub_port_list> ::= <port> [ <sub_port_list> ]
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.fi
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.PP
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Although I think that the above grammar is complete, it isn't exactly what
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one would call easy to comprehend! Here's the basic idea along with what
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each of the forms mean:
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.PP
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The
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.I ipfirewall list
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command prints a list of the firewalls on both the
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forwarding and blocking chain in some more or less comprehensible format.
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.PP
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The
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.I ipfirewall flush
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command empties the two firewall chains.
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.PP
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The
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.I ipfirewall addblocking
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and
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.I ipfirewall addforwarding
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commands take a firewall description and add the firewall to the appropriate
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firewall chain. Note that you'll probably need to add some descriptions more
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then once, which will naturally take more then one entry in memory. It does
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not lead to significant degradation of performance, so don't worry about it.
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.PP
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The
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.I ipfirewall delblocking
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and
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.I ipfirewall delforwarding
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commands take a firewall description and delete the firewall from the
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appropriate firewall chain. The description must exactly match that given
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to an earlier add command. One delete command removes ALL matching entries
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from firewall chains.
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.PP
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There are two basic kinds of firewall descriptions. Universal firewall
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descriptions match all IP packets between specified pairs of hosts.
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Universal firewalls only check IP addresses (e.g. they match any combination
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of protocol and port numbers). Protocol-specific firewalls match either
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TCP/IP or UDP/IP packets between specified pairs of hosts. In addition
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to host descriptions, protocol-specific firewalls optionally take a
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description of which port numbers to match.
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.PP
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A host description consists of an IP address and a mask. The IP address
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is specified as either a domain name or in the familiar
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nn.nn.nn.nn format. The mask indicates how much of the IP address
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should be looked at when vetting packets. There are two ways to
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specify the mask. The first way is to suffix the IP address in the
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firewall with a slash and an integer in the range 0 through 32 inclusive.
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This integer is taken to be the number of high order bits of the IP
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address which are to be checked (for example, 192.153.211.0/24 checks
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the top 24 bits of the IP address, 192.153.211.17/32 checks all the
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bits and 0.0.0.0/0 checks none of the bits (i.e. all IP addresses are
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matched by this example)). The second way to specify a mask is to
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suffix the IP address with a colon followed by another IP address.
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This second address is the mask. Specifications equivalent to the
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above three examples using this syntax would be
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.PP
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.nf
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192.153.211.0:255.255.255.0
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192.153.211.17:255.255.255.255
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0.0.0.0:0.0.0.0
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.fi
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.PP
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The first form is taken from the syntax accepted by a Telebit NetBlazer.
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The second form is more along the lines of how a netmask is specified
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in /etc/netmasks. Finally, if no mask is specified then a mask of all
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1's is supplied (i.e. no mask is equivalent to /32 or :255.255.255.255).
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.PP
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The optional description of port numbers to mask can take three forms.
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The simplest form is to omit the list in which case all port numbers
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match. The next form is to specify a list of port numbers (either as
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positive integers or service names from /etc/services). The final form
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is actually a special case of the second form in which the first pair
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of port numbers is separated by a colon instead of white space. This
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pair specifies a range of port numbers (i.e. x:y specifies that all
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ports between x and y inclusive should match). A port description
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matches a particular port number if any of the following is true:
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.nf
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- the port description is null
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- the first pair of port numbers is a range and the port number
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is in the range (inclusive)
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- the port number is equal to any of the port numbers in the list
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.fi
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.PP
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There is a limit of a total of 10 port numbers in the source and
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destination port lists. This limit is arbitrary and easy to increase.
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It is determined by the value of the IP_FIREWALL_MAX_PORTS #define
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variable in ip_firewall.h. Each increase of 1 for this value adds two
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bytes to the size of each firewall. Since the size of a firewall is only
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slightly over 30 bytes right now, this limit of 10 could probably
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be increased by quite a bit before it became a concern. I've been
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thinking of increasing it to 20 which would be longer than any
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reasonable firewall would need and would only consume 20 more bytes
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per firewall. The counter argument to any increase is that it is
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always possible to construct an equivalent set of two or more firewalls
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that behaves like a single firewall with a really long port list.
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.PP
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This probably all sounds hopelessly complicated. It is actually not
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all that tricky (I'm just not very good at explaining it yet). A few
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examples will probably help a lot now:
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.PP
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Block all IP packets originating from the host hackers-den:
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.PP
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.nf
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ipfirewall addb deny all from hackers-den to 0.0.0.0/0
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.fi
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.PP
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Block all telnet packets to our telnet server from anywhere:
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.PP
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.nf
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ipfirewall addb deny tcp from 0.0.0.0/0 to mymachine/32 telnet
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.fi
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.PP
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Don't forward telnet, rlogin and rsh packets onto our local
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class C network:
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.PP
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.nf
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ipfirewall addf deny tcp from 0.0.0.0/0 to ournetwork/24 telnet login shell
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.fi
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.PP
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Don't let anyone on the local machine or any machine inside
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our local network ftp access to games.com:
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.PP
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.nf
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ipfirewall addb deny tcp from games.com ftp to 0.0.0.0/0
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.fi
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.PP
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This last one might look a little strange. It doesn't prevent
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anyone from sending packets to the games.com ftp server. What it
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does do is block any packets that the games.com ftp server sends
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back!
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.PP
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The
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.I ipfirewall checkblocking
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and
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.I ipfirewall checkforwarding
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commands take a description of an IP packet and check to see if the blocking
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or forwarding chain of firewalls respectively accept or reject the packet.
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It is used to make sure that the firewalls that you've defined work as
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expected. The basic syntax is probably best understood by looking at
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a couple of examples:
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.PP
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.nf
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ipfirewall checkb from bsdi.com 3001 to mymachine telnet
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.fi
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.PP
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checks to see if the blocking firewall will block a telnet packet from
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a telnet session originating on bsdi.com to the host mymachine will be
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blocked or not. Note that someone connecting to our telnet server
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could be using practically any port number. To be really sure, the
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firewall used to prevent access should be as simple as possible and/or
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you should try a variety of port numbers in addition to the rather
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arbitrarily chosen port of 3001.
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.PP
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One final note on the check* ,add* and del* command syntax. The noise word
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"to" exists in the syntax so that I can detect the end of a list of
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port numbers in the from description. Since I needed a noise word to
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detect this case, I added the noise word "from" in front of the from
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case for consistency.
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.PP
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Finally, have a look at the file
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.I "/usr/share/misc/ipfw.samp.filters"
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. It is the set of filters that I run at home [Danny].
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.PP
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Also check
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.I "/usr/share/misc/ipfw.samp.scripts"
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For examples of individual access restrictions.
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We [NetVision] use those for our dial-in PPP/SLIP users to allow some of them
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to access our internal networks, while disallowing others.
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This way we open access for the user's IP when he enters the system and shut it
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down when he leaves. All such changes may be applyed at any time,
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and so entries added and deleted from firewall while the system is
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is working have no other side effects [Ugen].
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.SH "TECHNICAL DETAILS"
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A bit of a description of how the firewalls are applied (i.e. what happens in
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the kernel) may be instructive to the advanced firewall-builder:
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.PP
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When an IP packet is received, the ipintr() routine in ip_input.c is
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called. This routine does a bit of basic error checking. If it
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detects any errors in the packet it generally drops the packet on
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the floor. The idea behind the ipfirewall facility is to treat packets
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that we don't want to accept as bad packets (i.e. drop them on the
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floor). The ipfirewall facility intercedes in the normal processing
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at two points. Just after the basic sanity checks are done, we pass
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any packets not targeted at the loopback network (127.0.0.0/8) to the
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firewall checker along with the chain of blocking firewalls.If the firewall
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checker tells us to block the packet then we branch to the "bad:" label
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in ipintr() which is where all bad packets are dropped on the floor.
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Otherwise, we allow normal processing of the packet to continue. The
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exact point at which we intercede was chosen to be after the basic
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sanity checking and before the option processing is done. We want to
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be after the basic sanity checking so that we don't have to be able
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to handle complete garbage. We want to be before the option processing
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because option processing is done in separate rather complex routine.
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Why bother doing this special processing if we might be dropping the
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packet?
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.PP
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The second point at which we intercede is when a packet is about to be
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forwarded to another host. All such packets are passed to the ip_forward
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routine. The ipfirewall code is at the very top of this routine. If
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the packet isn't targetted at the loopback interface (is it possible
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that it could be when we reach this point? I doubt it but safety first)
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then pass the packet to the firewall checker along with the forwarding
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firewall chain. If the firewall checker indicates that the packet should
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not be forwarded then we drop in (using code copied from a few lines
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further into the routine which drops broadcast packets which are not
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to be forwarded).
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.PP
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There are a couple of consequences of this approach:
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.PP
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1) Packets which are blocked are never forwarded (something to keep
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in mind when designing firewalls).
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.PP
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2) Packets targeted at the loopback interface (127.0.0.0/8) are never
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blocked. Blocking packets to the loopback interface seems pointless
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and potentially quite confusing. It also makes a possibly common
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case very cheap.
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.PP
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3) The sender of a packet which is blocked receives no indication that
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the packet was dropped. The Telebit NetBlazer can be configured to
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silently drop a blocked packet or to send back a "you can't get there
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from here" packet to the sender. Implementing the later would have
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been more work (possibly quite a bit more, I don't really know). Also,
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I don't see any reason to give a potential hacker any more information
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than necessary. Dropping the packet into the bit bucket seems like
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the best way to keep a hacker guessing. [Danny]
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.PP
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(I am working on this feature, it would be made optional and
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configurable by some ICMP_UNREACH_ON_DROP option, or such [Ugen]).
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.PP
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The firewall checker takes two parameters. The first parameter is a pointer
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to the packet in question. The second parameter is a pointer to the
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appropriate firewall chain. At the present time, the firewall checker passes
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these parameters to a second routine which is the real firewall checker.
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If the real checker says NO then an appropriate message is printed
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onto the console. This is useful for debugging purposes. Whether or
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not it remains in the long term depends on whether it is considered useful
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for logging purposes (I'm a little reluctant to leave it in since it
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provides a hacker with a way to commit a "denial of service" offense
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against you by filling up your /var/log/messages file's file system
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with error messages. There are ways of preventing this but ... [Danny]).
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In default configuration now no information about dropped packets
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printed.You may, however, define it as i do by adding
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.I options IPFIREWALL_VERBOSE
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to your kernel configuration file. Very useful thingy! [Ugen]
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.PP
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A return value of 0 from this routine (or the real firewall checker)
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indicates that the packet is to be dropped. A value of 1 indicates
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that the packet is to be accepted. In the early testing stages you
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might want to make the top level firewall checker always return 1 even
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if the real checker returns 0 just in case the real firewall checker
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screws up (or your firewalls aren't as well designed as they should be).
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In fact, this might be a useful optional feature (providing a way to
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leave a door unlocked doesn't seem all that wise but it has to be
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balanced against the inconvenience to legitimate users who might get
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screwed up by poorly designed firewalls).
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.PP
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The real firewall returns 1 (accept the packet) if the chain is empty. If
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efficiency is a concern (which it is in this code), this check should
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be done in ip_input.c before calling the firewall checker.
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.PP
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Assuming that there is a firewall chain to scan through, the real firewall
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checker picks up the src and dst IP addresses from the IP packet. It
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then goes through the firewall chain looking for the first firewall that
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matches the packet. Once a matching firewall has been found, a value of
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1 is returned if the firewall is an accept firewall and a value of 0 is
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returned otherwise.
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.PP
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The following processing is done for each firewall on the chain:
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.PP
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1) check the src and dst IP addresses. If they don't match then
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there isn't any point in looking any further at this firewall.
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This check is done by anding the packet's IP addresses the
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with appropriate masks and comparing the results to the
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appropriate addresses in the firewall. Note that the mask is
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NOT applied to the address in the firewall. If it has any 1
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bits that are 0 bits in the mask then the firewall will never
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match (this will be checked in ipfirewall soon). If the addresses
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match then we continue with the next step.
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.PP
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2) If the firewall is a universal firewall then we've got a match.
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Return either 0 or 1 as appropriate. Otherwise, continue with
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the next step.
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.PP
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3) Examine the IP protocol from the packet. If we havn't had to
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look at it before then we get it and set a local variable to
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IP_FIREWALL_TCP for TCP/IP packets, IP_FIREWALL_UDP for UDP/IP
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packets, IP_FIREWALL_ICMP for ICMP packets, and IP_FIREWALL_UNIVERSAL
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for all other packet types. Also, if the packet is a TCP/IP or
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a UDP/IP packet, save the source and destination port numbers
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at this point (taking advantage of the fact that the port numbers
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are stored in the same place in either a TCP/IP or a UDP/IP
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packet header). If the packet is neither a TCP/IP or a UDP/IP
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packet then this firewall won't match it (on to the next firewall).
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If this packet's protocol doesn't match this firewall's protocol
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(which can't be universal or we wouldn't be here) then on to
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the next firewall. Otherwise, continue with the next step.
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.PP
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4) We're checking either a TCP/IP or a UDP/IP packet. If the
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firewall's source port list is empty or the packet's source
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port matches something in the source port list AND if the firewall's
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destination port list is empty or the packet's destination
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port matches something in the destination port list then
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we've got a match (return 0 or 1 as appropriate). Otherwise,
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on to the next firewall.
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.PP
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As indicated above, if no packet on the chain matches the packet then
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it is accepted if the first firewall was a deny firewall and it is rejected
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if the first firewall was an accept packet. This is equivalent to the
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default behaviour of a Telebit NetBlazer. They provide a way to override
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this behaviour. I'm not convinced that it is necessary (I'm open to
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suggestions).
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.PP
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That's about it for the firewall checker. The
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.I ipfw
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program communicates with the kernel part of the firewall facility by making
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setsockopt calls on RAW IP sockets. Only root is allowed to open a RAW IP
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socket. This ensures that only root uses
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.I ipfw to manipulate the firewall facility.
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|
Also, somewhere in the kernel source or on a man page, I read that the
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RAW IP setsockopt calls are intended for manipulating the IP protocol layer
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|
as opposed to manipulating any particular instance of a socket. This seems
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|
like a reasonable description of what the firewall setsockopt command
|
|
codes do.
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|
.PP
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|
There are seven setsockopt command codes defined by the firewall facility
|
|
(in netinet/in.h). They are:
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|
.PP
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|
.nf
|
|
IP_FLUSH_FIREWALLS flush (i.e. free) both firewall chains.
|
|
|
|
IP_ADD_FORWARDING_FIREWALL add firewall pointed at by optval parm to
|
|
the end of the forwarding firewall chain.
|
|
|
|
IP_ADD_BLOCKING_FIREWALL add firewall pointed at by optval parm to
|
|
the end of the blocking firewall chain.
|
|
|
|
IP_DEL_FORWARDING_FIREWALL delete firewall pointed at by optval parm
|
|
from the forwarding firewall chain.
|
|
|
|
IP_DEL_BLOCKING_FIREWALL delete firewall pointed at by optval parm
|
|
from the blocking firewall chain.
|
|
|
|
IP_CHECK_FORWARDING_FIREWALL pass the IP packet do the firewall checker
|
|
along with the forwarding firewall chain.
|
|
Return 0 if packet was accepted, -1 (with
|
|
errno set to EACCES) if it wasn't.
|
|
|
|
IP_CHECK_BLOCKING_FIREWALL pass the IP packet do the firewall checker
|
|
along with the blocking firewall chain.
|
|
Return 0 if packet was accepted, -1 (with
|
|
errno set to EACCES) if it wasn't.
|
|
|
|
The IP_ADD_* and IP_DEL_* command codes do a fair bit of validity checking.
|
|
It is quite unlikely that a garbage firewall could get past them that
|
|
would cause major problems in the firewall checker. It IS possible for
|
|
a garbage packet to get past the checks which causes major grief because
|
|
it either blocks or accepts packets according to unusual rules (the rules
|
|
will conform to the ones described above but will probably come as quite
|
|
a surprise).
|
|
|
|
The IP_CHECK_* command codes expect the optval parameter to point
|
|
to a struct ip immediately followed by a header appropriate to the protocol
|
|
value described in the ip_p field of the ip header. The exact requirements
|
|
are as follows:
|
|
|
|
- The length of the optval parameter must be at least
|
|
|
|
sizeof(struct ip) + 2 * sizeof(u_short)
|
|
|
|
since this is the amount of memory that might be referenced by
|
|
the firewall checker.
|
|
|
|
- The ip_hl field of the ip structure must be equal to
|
|
|
|
sizeof(struct ip) / sizeof(int)
|
|
|
|
since this value indicates that the tcp/udp/??? header immediately
|
|
follows the ip header (appropriate for the purposes that this
|
|
interface is intended for).
|
|
|
|
Failure to follow these rules (for either the IP_ADD_*,IP_DEL_* or the
|
|
IP_CHECK_*_FIREWALL commands) will result in a return value of -1 with
|
|
errno set to EINVAL (for now, it will also result in an appropriate
|
|
message on the console).
|
|
|
|
To read current configuration of firewalls,the kvm_read() function used.
|
|
Symbols,which you have to find are :
|
|
struct ip_firewall * ip_firewall_blocking_chain ;
|
|
struct ip_firewall * ip_firewall_forwarding_chain ;
|
|
Both are pointers to the linked list of firewall entries.
|
|
Of course, you must at least be a member of group kmem to read kernel
|
|
symbols.
|
|
.fi
|
|
.PP
|
|
There are a couple of additional details that are worth reading about in
|
|
the ip_firewall.h file. Other than that, let the authors know how you do!
|
|
If you have any problems, you may call Danny Boulet at home (403 449-1835)
|
|
or send e-mail to <danny@BouletFermat.ab.ca>. If you call, please keep in
|
|
mind that Danny lives in the Canadian Mountain timezone (GMT-0600).
|
|
.PP
|
|
You may also reach some commercial users of this package (and also those
|
|
responsible for porting it to FreeBSD and adding several additional
|
|
commands), at 972-4-550-330, or via email at <ugen@NetVision.net.il>.
|
|
If you call, remember that Ugen lives in the Israel timezone, which is GMT+02.
|
|
|
|
.SH FILES
|
|
/usr/share/misc/ipfw.samp.filters
|
|
/usr/share/misc/ipfw.samp.scripts
|
|
.SH "BUGS"
|
|
You can very easily hose your machine utterly if you don't know what you're
|
|
doing. Dieses Befehl ist nur fuer Experten!
|
|
.SH "SEE ALSO"
|
|
.BR reboot (1) ,
|
|
.PP
|
|
.BR /sys/i386/conf/IPFIREWALL
|
|
.SH AUTHORS
|
|
Daniel Boulet <danny@BouletFermat.ab.ca>
|
|
.PP
|
|
Ugen J.S.Antsilevich <ugen@NetVision.net.il>
|
|
.PP
|
|
Jordan K. Hubbard <jkh@FreeBSD.org> [Crimes committed in this manpage]
|