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6.4. Firewalls

Contributed by Gary Palmer and Alex Nash .

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.

Note: 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 cracker from penetrating your internal network. If you let internal security lapse because you believe your firewall to be impenetrable, you have just made the crackers job that bit easier.

6.4.1. What is a firewall?

There are currently two distinct types of firewalls in common use on the Internet today. The first type is more properly called a packet filtering router, 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 proxy servers, rely on daemons to provide authentication and to forward packets, possibly on a multi-homed machine which has kernel packet forwarding disabled.

Sometimes sites combine the two types of firewalls, so that only a certain machine (known as a bastion host) 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.

FreeBSD comes with a kernel packet filter (known as IPFW), 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.

6.4.1.1. Packet filtering routers

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.

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''.

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).

6.4.1.2. Proxy servers

Proxy servers are machines which have had the normal system daemons (telnetd, ftpd, etc) replaced with special servers. These servers are called proxy servers 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).

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.

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.

6.4.2. What does IPFW allow me to do?

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

There are two related parts to IPFW. 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.

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

6.4.3. Enabling IPFW on FreeBSD

As the main part of the IPFW 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 reconfiguring the kernel for more details on how to recompile your kernel.

There are currently three kernel configuration options relevant to IPFW:

options IPFIREWALL

Compiles into the kernel the code for packet filtering.

options IPFIREWALL_VERBOSE

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

options IPFIREWALL_VERBOSE_LIMIT=10

Limits the number of packets logged through syslogd(8) 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.

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 ipfw(8) utility:

    # ipfw zero 4500

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

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

6.4.4. Configuring IPFW

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

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.

6.4.4.1. Altering the IPFW rules

The syntax for this form of the command is:

ipfw [-N] command [index] action [log] protocol addresses [options]

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

-N

Resolve addresses and service names in output.

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

add

Add an entry to the firewall/accounting rule list

delete

Delete an entry from the firewall/accounting rule list

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

If an index 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).

The log option causes matching rules to be output to the system console if the kernel was compiled with IPFIREWALL_VERBOSE.

Valid actions are:

reject

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

allow

Pass the packet on as normal. (aliases: pass and accept)

deny

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

count

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

Each action will be recognized by the shortest unambiguous prefix.

The protocols which can be specified are:

all

Matches any IP packet

icmp

Matches ICMP packets

tcp

Matches TCP packets

udp

Matches UDP packets

The address specification is:

from address/mask [port] to address/mark [port] [via interface]

You can only specify port in conjunction with protocols which support ports (UDP and TCP).

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

The syntax used to specify an address/mask is:

    address
or
    address/mask-bits
or
    address:mask-pattern

A valid hostname may be specified in place of the IP address. mask-bits is a decimal number representing how many bits in the address mask should be set. e.g. specifying 192.216.222.1/24 will create a mask which will allow any address in a class C subnet (in this case, 192.216.222) to be matched. mask-pattern is an IP address which will be logically AND'ed with the address given. The keyword any may be used to specify ``any IP address''.

The port numbers to be blocked are specified as:

port [,port [,port [...]]]

to specify either a single port or a list of ports, or

port-port

to specify a range of ports. You may also combine a single range with a list, but the range must always be specified first.

The options available are:

frag

Matches if the packet is not the first fragment of the datagram.

in

Matches if the packet is on the way in.

out

Matches if the packet is on the way out.

ipoptions spec

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

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 established rules early in the chain.

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).

tcpflags flags

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

icmptypes types

Matches if the ICMP type is present in the list types. The list may be specified as any combination of ranges and/or individual types separated by commas. Commonly used ICMP types are: 0 echo reply (ping reply), 3 destination unreachable, 5 redirect, 8 echo request (ping request), and 11 time exceeded (used to indicate TTL expiration as with traceroute(8)).

6.4.4.2. Listing the IPFW rules

The syntax for this form of the command is:

ipfw [-a] [-t] [-N] l

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

-a

While listing, show counter values. This option is the only way to see accounting counters.

-t

Display the last match times for each chain entry. The time listing is incompatible with the input syntax used by the ipfw(8) utility.

-N

Attempt to resolve given addresses and service names.

6.4.4.3. Flushing the IPFW rules

The syntax for flushing the chain is:

ipfw flush

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.

6.4.4.4. Clearing the IPFW packet counters

The syntax for clearing one or more packet counters is:

ipfw zero [index]

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

6.4.5. Example commands for ipfw

This command will deny all packets from the host evil.crackers.org to the telnet port of the host nice.people.org by being forwarded by the router:

    # ipfw add deny tcp from evil.crackers.org to nice.people.org 23

The next example denies and logs any TCP traffic from the entire crackers.org network (a class C) to the nice.people.org machine (any port).

    # ipfw add deny log tcp from evil.crackers.org/24 to nice.people.org

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:

    # ipfw add deny tcp from any to my.org/28 6000 setup

To see the accounting records:

    # ipfw -a list
or in the short form
    # ipfw -a l

You can also see the last time a chain entry was matched with:

    # ipfw -at l

6.4.6. Building a packet filtering firewall

Note: 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.

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.

Note: If you use the logging versions of the accept command, it can generate large 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 /var/log is located on.

You should enable your firewall from /etc/rc.conf.local or /etc/rc.conf. The associated manpage explains which knobs to fiddle and lists some preset firewall configurations. If you do not use a preset configuration, ipfw list will output the current ruleset into a file that you can pass to rc.conf. If you do not use /etc/rc.conf.local or /etc/rc.conf to enable your firewall, it is important to make sure your firewall is enabled before any IP interfaces are configured.

The next problem is what your firewall should actually do! 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:

Another checklist for firewall configuration is available from CERT at ftp://ftp.cert.org/pub/tech_tips/packet_filtering

As I said above, these are only guidelines. 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.