freebsd-dev/share/doc/handbook/firewalls.sgml

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<sect><heading>Firewalls<label id="firewalls"></heading>

<p><em>Contributed by &a.gpalmer; and &a.alex;.</em>

Firewalls are an area of increasing interest for people who are
connected to the Internet, and are even finding applications on
private networks to provide enhanced security. This section will
hopefully explain what firewalls are, how to use them, and how to use
the facilities provided in the FreeBSD kernel to implement them.

<quote><bf>Note</bf>: People often think that having a firewall between
your companies internal network and the ``Big Bad Internet'' will
solve all your security problems. It may help, but a poorly setup
firewall system is more of a security risk than not having one at all.
A firewall can only add another layer of security to your systems, but
they will not be able to stop a really determined hacker from
penetrating your internal network. If you let internal security lapse
because you believe your firewall to be impenetrable, you have just
made the hackers job that bit easier.</quote>

<sect1><heading>What is a firewall?</heading>

<p>There are currently two distinct types of firewalls in common
use on the Internet today. The first type is more properly called
a <bf>packet filtering router</bf>, where the kernel on a
multi-homed machine chooses whether to forward or block packets
based on a set of rules. The second type, known as <bf>proxy
servers</bf>, rely on daemons to provide authentication and to
forward packets, possibly on a multi-homed machine which has
kernel packet forwarding disabled.

<p>Sometimes sites combine the two types of firewalls, so that only a
certain machine (known as a <bf>bastion host</bf>) is allowed to send
packets through a packet filtering router onto an internal
network. Proxy services are run on the bastion host, which are
generally more secure than normal authentication mechanisms.

<p>FreeBSD comes with a kernel packet filter (known as <tt>IPFW</tt>),
which is what the rest of this section will concentrate on. Proxy
servers can be built on FreeBSD from third party software, but there
is such a variety of proxy servers available that it would be
impossible to cover them in this document.

<sect2><heading>Packet filtering routers<label id="firewalls:packet_filters"></heading>

<p>A router is a machine which forwards packets between two or more
networks. A packet filtering router has an extra piece of code in its
kernel, which compares each packet to a list of rules before deciding
if it should be forwarded or not. Most modern IP routing software has
packet filtering code in it, which defaults to forwarding all
packets. To enable the filters, you need to define a set of rules for
the filtering code, so that it can decide if the packet should be
allowed to pass or not.

<p>To decide if a packet should be passed on or not, the code looks
through its set of rules for a rule which matches the contents of
this packets headers. Once a match is found, the rule action is
obeyed.  The rule action could be to drop the packet, to forward the
packet, or even to send an ICMP message back to the originator.  Only
the first match counts, as the rules are searched in order. Hence, the
list of rules can be referred to as a ``rule chain''.

<p>The packet matching criteria varies depending on the software used,
but typically you can specify rules which depend on the source IP
address of the packet, the destination IP address, the source port
number, the destination port number (for protocols which support
ports), or even the packet type (UDP, TCP, ICMP, etc).

<sect2><heading>Proxy servers<label id="firewalls:proxy_servers"></heading>

<p>Proxy servers are machines which have had the normal system daemons
(telnetd, ftpd, etc) replaced with special servers. These servers are
called <bf>proxy servers</bf> as they normally only allow onward
connections to be made. This enables you to run (for example) a proxy
telnet server on your firewall host, and people can telnet in to your
firewall from the outside, go through some authentication mechanism,
and then gain access to the internal network (alternatively, proxy
servers can be used for signals coming from the internal network and
heading out).

<p>Proxy servers are normally more secure than normal servers, and
often have a wider variety of authentication mechanisms available,
including ``one-shot'' password systems so that even if someone
manages to discover what password you used, they will not be able to use
it to gain access to your systems as the password instantly
expires. As they do not actually give users access to the host machine,
it becomes a lot more difficult for someone to install backdoors
around your security system.

<p>Proxy servers often have ways of restricting access further, so
that only certain hosts can gain access to the servers, and often they
can be set up so that you can limit which users can talk to which
destination machine. Again, what facilities are available depends
largely on what proxy software you choose.

<sect1><heading>What does IPFW allow me to do?</heading>

<p><tt>IPFW</tt>, the software supplied with FreeBSD, is a packet
filtering and accounting system which resides in the kernel, and has a
user-land control utility, <tt>ipfw(8)</tt>. Together, they
allow you to define and query the rules currently used by the kernel
in its routing decisions.

<p>There are two related parts to <tt>IPFW</tt>. The firewall section
allows you to perform packet filtering. There is also an IP accounting
section which allows you to track usage of your router, based on
similar rules to the firewall section. This allows you to see (for
example) how much traffic your router is getting from a certain
machine, or how much WWW (World Wide Web) traffic it is forwarding.

<p>As a result of the way that <tt>IPFW</tt> is designed, you can use
<tt>IPFW</tt> on non-router machines to perform packet filtering on
incoming and outgoing connections. This is a special case of the more
general use of <tt>IPFW</tt>, and the same commands and techniques
should be used in this situation.

<sect1><heading>Enabling IPFW on FreeBSD</heading>

<p>As the main part of the <tt>IPFW</tt> system lives in the kernel, you will
need to add one or more options to your kernel configuration
file, depending on what facilities you want, and recompile your kernel. See
<ref id="kernelconfig" name="reconfiguring the kernel"> for more
details on how to recompile your kernel.

<p>There are currently three kernel configuration options
relevant to IPFW:

<descrip>
<tag/options IPFIREWALL/ Compiles into the kernel the code for packet
filtering.

<tag/options IPFIREWALL_VERBOSE/ Enables code to allow logging of
packets through <tt>syslogd(8)</tt>. Without this option, even if you
specify that packets should be logged in the filter rules, nothing
will happen.

<tag/options IPFIREWALL_VERBOSE_LIMIT=10/ Limits the number of
packets logged through <tt>syslogd(8)</tt> on a per entry basis.
You may wish to use this option in hostile environments in which
you want to log firewall activity, but do not want to be open to
a denial of service attack via syslog flooding.

<p>When a chain entry reaches the packet limit specified, logging
is turned off for that particular entry.  To resume logging, you
will need to reset the associated counter using the <tt>ipfw(8)</tt>
utility:

<tscreen><verb>
ipfw zero 4500
</verb></tscreen>

Where 4500 is the chain entry you wish to continue logging.

</descrip>

Previous versions of FreeBSD contained an <tt>IPFIREWALL_ACCT</tt> 
option.  This is now obsolete as the firewall code automatically 
includes accounting facilities.

<sect1><heading>Configuring IPFW</heading>

<p>The configuration of the <tt>IPFW</tt> software is done through the
<tt>ipfw(8)</tt> utility. The syntax for this command looks
quite complicated, but it is relatively simple once you understand
its structure.

<p>There are currently four different command categories used by the
utility: addition/deletion, listing, flushing, and clearing.
Addition/deletion is used to build the rules that control how packets
are accepted, rejected, and logged.  Listing is used to examine the
contents of your rule set (otherwise known as the chain) and packet
counters (accounting).  Flushing is used to remove all entries from
the chain.  Clearing is used to zero out one or more accounting
entries.

<sect2><heading>Altering the IPFW rules</heading>

<p>The syntax for this form of the command is:
<tscreen>
ipfw &lsqb;-N&rsqb; <em>command</em> &lsqb;<em>index</em>&rsqb; 
<em>action</em> &lsqb;log&rsqb; <em>protocol</em> <em>addresses</em> 
&lsqb;<em>options</em>&rsqb;
</tscreen>

<p>There is one valid flag when using this form of the command:

<descrip>
<tag/-N/Resolve addresses and service names in output.
</descrip>

The <em>command</em> given can be shortened to the shortest unique
form. The valid <em>commands</em> are:

<descrip>

<tag/add/Add an entry to the firewall/accounting rule list

<tag/delete/Delete an entry from the firewall/accounting rule list

</descrip>

Previous versions of <tt>IPFW</tt> used separate firewall and
accounting entries.  The present version provides packet accounting
with each firewall entry.

<p>If an <tt>index</tt> value is supplied, it used to place the entry
at a specific point in the chain.  Otherwise, the entry is placed at
the end of the chain at an index 100 greater than the last chain
entry (this does not include the default policy, rule 65535, deny).

<p>The <bf>log</bf> option causes matching rules to be output to the
system console if the kernel was compiled with <bf>IPFIREWALL_VERBOSE</bf>.

<p>Valid <em>actions</em> are:

<descrip>

<tag/reject/Drop the packet, and send an ICMP host or port
unreachable (as appropriate) packet to the source.

<tag/allow/Pass the packet on as normal. (aliases: <bf>pass</bf> and
<bf>accept</bf>)

<tag/deny/Drop the packet.  The source is not notified via an ICMP
message (thus it appears that the packet never arrived at the
destination).

<tag/count/Update packet counters but do not allow/deny the packet
based on this rule.  The search continues with the next chain entry.

</descrip>

<p>Each <em>action</em> will be recognized by the shortest unambiguous
prefix.

The <em>protocols</em> which can be specified are:

<descrip>

<tag/all/Matches any IP packet

<tag/icmp/Matches ICMP packets

<tag/tcp/Matches TCP packets

<tag/udp/Matches UDP packets

</descrip>

<p>The <em>address</em> specification is:
<tscreen>
<bf>from</bf> &lt;<em>address/mask</em>&gt;&lsqb;<em>port</em>&rsqb; <bf>to</bf>
 &lt;<em>address/mask</em>&gt;&lsqb;<em>port</em>&rsqb &lsqb;<bf>via</bf> &lt;<em>interface</em>&gt;&rsqb;
</tscreen>

<p>You can only specify <em>port</em> in conjunction with
<em>protocols</em> which support ports (UDP and TCP).

<p>The <bf>via</bf> is optional and may specify the IP address or
domain name of a local IP interface, or an interface name (e.g.
<tt>ed0</tt>) to match only packets coming through this interface.
Interface unit numbers can be specified with an optional wildcard.
For example, <tt>ppp*</tt> would match all kernel PPP interfaces.

<p>The syntax used to specify an <tt>&lt;address/mask&gt;</tt> is:
<tscreen>
&lt;address&gt;
</tscreen>
or
<tscreen>
&lt;address&gt;/mask-bits
</tscreen>
or
<tscreen>
&lt;address&gt;:mask-pattern
</tscreen>

<p>A valid hostname may be specified in place of the IP
address. <tt>mask-bits</tt> is a decimal number representing how many
bits in the address mask should be set. e.g. specifying
<tscreen>
192.216.222.1/24
</tscreen>
will create a mask which will allow any address in a class C subnet
(in this case, 192.216.222) to be matched. <tt>mask-pattern</tt> is an IP
address which will be logically AND'ed with the address given.  The
keyword <tt>any</tt> may be used to specify ``any IP address''.
<p>The port numbers to be blocked are specified as:
<tscreen>
port&lsqb;,port&lsqb;,port&lsqb;...&rsqb;&rsqb;&rsqb;
</tscreen>
to specify either a single port or a list of ports, or
<tscreen><verb>
port-port
</verb></tscreen>
to specify a range of ports.  You may also combine a single range with a
list, but the range must always be specified first.

<p>The <em>options</em> available are:

<descrip>

<tag/frag/Matches if the packet is not the first fragment of the datagram.

<tag/in/Matches if the packet is on the way in.

<tag/out/Matches if the packet is on the way out.

<tag/ipoptions <em>spec</em>/Matches if the IP header contains the
comma separated list of options specified in <em>spec</em>.  The
supported list of IP options are: <bf>ssrr</bf> (strict source route),
<bf>lsrr</bf> (loose source route), <bf>rr</bf> (record packet route),
and <bf>ts</bf> (timestamp).  The absence of a particular option may
be denoted with a leading '!'.

<tag/established/Matches if the packet is part of an already established
TCP connection (i.e. it has the RST or ACK bits set).  You can optimize
the performance of the firewall by placing <em>established</em> rules
early in the chain.

<tag/setup/Matches if the packet is an attempt to establish a TCP connection
(the SYN bit set is set but the ACK bit is not).

<tag/tcpflags <em>flags</em>/Matches if the TCP header contains
the comma separated list of <em>flags</em>.  The supported flags
are <bf>fin</bf>, <bf>syn</bf>, <bf>rst</bf>, <bf>psh</bf>, <bf>ack</bf>,
and <bf>urg</bf>.  The absence of a particular flag may be indicated
by a leading '!'.

<tag/icmptypes <em>types</em>/Matches if the ICMP type is present in
the list <em>types</em>.  The list may be specified as any combination
of ranges and/or individual types separated by commas.  Commonly used
ICMP types are: <bf>0</bf> echo reply (ping reply), <bf>5</bf>
redirect, <bf>8</bf> echo request (ping request), and <bf>11</bf>
time exceeded (used to indicate TTL expiration as with
<tt>traceroute(8)</tt>).

</descrip>

<sect2><heading>Listing the IPFW rules</heading>

<p>The syntax for this form of the command is:
<tscreen>
ipfw &lsqb;-atN&rsqb; l
</tscreen>

<p>There are three valid flags when using this form of the command:

<descrip>

<tag/-a/While listing, show counter values. This option is the only
way to see accounting counters.

<tag/-t/Display the last match times for each chain entry.  The time
listing is incompatible with the input syntax used by the
<tt>ipfw(8)</tt> utility.

<tag/-N/Attempt to resolve given addresses and service names.

</descrip>

<sect2><heading>Flushing the IPFW rules</heading>

<p>The syntax for flushing the chain is:
<tscreen>
ipfw flush
</tscreen>

<p>This causes all entries in the firewall chain to be removed except
the fixed default policy enforced by the kernel (index 65535).  Use
caution when flushing rules, the default deny policy will leave your
system cut off from the network until allow entries are added to the
chain.

<sect2><heading>Clearing the IPFW packet counters</heading>

<p>The syntax for clearing one or more packet counters is:
<tscreen>
ipfw zero &lsqb;index&rsqb;
</tscreen>

<p>When used without an <em>index</em> argument, all packet counters
are cleared.  If an <em>index</em> is supplied, the clearing operation
only affects a specific chain entry.

<sect1><heading>Example commands for ipfw</heading>

<p>This command will deny all packets from the host
<bf>evil.hacker.org</bf> to the telnet port of the host
<bf>nice.people.org</bf> by being forwarded by the router:

<tscreen><verb>
ipfw add deny tcp from evil.hacker.org to nice.people.org 23
</verb></tscreen>

<p>The next example denies and logs any TCP traffic from the entire
<bf>hacker.org</bf> network (a class C) to the <bf>nice.people.org</bf>
machine (any port).

<tscreen><verb>
ipfw add deny log tcp from evil.hacker.org/24 to nice.people.org
</verb></tscreen>

If you do not want people sending X sessions to your internal network
(a subnet of a class C), the following command will do the necessary
filtering:

<tscreen><verb>
ipfw add deny from any to my.org/28 6000 setup
</verb></tscreen>

To allow access to the SUP server on <bf>sup.FreeBSD.ORG</bf>, use the
following command:

<tscreen><verb>
ipfw add accept from any to sup.FreeBSD.ORG 871
</verb></tscreen>

To see the accounting records:
<tscreen><verb>
ipfw -a list
</verb></tscreen>
or in the short form
<tscreen><verb>
ipfw -a l
</verb></tscreen>
You can also see the last time a chain entry was matched with
<tscreen><verb>
ipfw -at l
</verb></tscreen>

<sect1><heading>Building a packet filtering firewall</heading>

<p><quote><bf>Note:</bf> The following suggestions are just that:
suggestions. The requirements of each firewall are different and I
cannot tell you how to build a firewall to meet your particular
requirements.</quote>

<p>When initially setting up your firewall, unless you have a test
bench setup where you can configure your firewall host in a controlled
environment, I strongly recommend you use the logging version of the
commands and enable logging in the kernel. This will allow you to
quickly identify problem areas and cure them without too much
disruption. Even after the initial setup phase is complete, I
recommend using the logging for of `deny' as it allows tracing of
possible attacks and also modification of the firewall rules if your
requirements alter.

<quote><bf>Note:</BF> If you use the logging versions of the
<bf>accept</bf> command, it can generate <em>large</em> amounts
of log data as one log line will be generated for every packet
that passes through the firewall, so large ftp/http transfers,
etc, will really slow the system down. It also increases the
latencies on those packets as it requires more work to be done by
the kernel before the packet can be passed on.  syslogd with also
start using up a lot more processor time as it logs all the extra
data to disk, and it could quite easily fill the partition
<tt>/var/log</tt> is located on.</quote>

<p>As currently supplied, FreeBSD does not have the ability to
load firewall rules at boot time. My suggestion is to put a call
to a shell script in the <tt>/etc/netstart</tt> script. Put the
call early enough in the netstart file so that the firewall is
configured before any of the IP interfaces are configured. This
means that there is no window during which time your network is
open.

<p>The actual script used to load the rules is entirely up to
you. There is currently no support in the <tt>ipfw</tt> utility for
loading multiple rules in the one command. The system I use is to use
the command:

<tscreen><verb>
# ipfw list
</verb></tscreen>

to write a list of the current rules out to a file, and then use a
text editor to prepend ``<tt>ipfw </tt>'' before all the lines. This
will allow the script to be fed into /bin/sh and reload the rules into
the kernel. Perhaps not the most efficient way, but it works.

<p>The next problem is what your firewall should actually <bf>DO</bf>!
This is largely dependent on what access to your network you want to
allow from the outside, and how much access to the outside world you
want to allow from the inside. Some general rules are:

<itemize>

 <item>Block all incoming access to ports below 1024 for TCP. This is
where most of the security sensitive services are, like finger, SMTP
(mail) and telnet.

 <item>Block <bf>all</bf> incoming UDP traffic. There are very few
useful services that travel over UDP, and what useful traffic there is
is normally a security threat (e.g. Suns RPC and NFS protocols). This
has its disadvantages also, since UDP is a connectionless protocol,
denying incoming UDP traffic also blocks the replies to outgoing UDP
traffic. This can cause a problem for people (on the inside)
using external archie (prospero) servers.  If you want to allow access
to archie, you'll have to allow packets coming from ports 191 and 1525
to any internal UDP port through the firewall. ntp is another service
you may consider allowing through, which comes from port 123.

 <item>Block traffic to port 6000 from the outside. Port 6000 is the
port used for access to X11 servers, and can be a security threat
(especially if people are in the habit of doing <tt>xhost +</tt> on
their workstations). X11 can actually use a range of ports starting at
6000, the upper limit being how many X displays you can run on the
machine. The upper limit as defined by RFC 1700 (Assigned Numbers) is
6063.

 <item>Check what ports any internal servers use (e.g. SQL servers,
etc). It's probably a good idea to block those as well, as they
normally fall outside the 1-1024 range specified above.

</itemize>

<p>Another checklist for firewall configuration is available from CERT
at <htmlurl url="ftp://ftp.cert.org/pub/tech&lowbar;tips/packet&lowbar;filtering"
name="ftp://ftp.cert.org/pub/tech&lowbar;tips/packet&lowbar;filtering">

<p>As I said above, these are only <em>guidelines</em>. You will have
to decide what filter rules you want to use on your firewall
yourself. I cannot accept ANY responsibility if someone breaks into
your network, even if you follow the advice given above.