firewall(7)

NAME

firewall - simple firewalls under FreeBSD

FIREWALL BASICS

A Firewall is most commonly used to protect an internal net

work from an
outside network by preventing the outside network from mak

ing arbitrary
connections into the internal network. Firewalls are also

used to prevent outside entities from spoofing internal IP addresses

and to isolate
services such as NFS or SMBFS (Windows file sharing) within

LAN segments.

The FreeBSD firewalling system also has the capability to

limit bandwidth
using dummynet(4). This feature can be useful when you need

to guarantee
a certain amount of bandwidth for a critical purpose. For

example, if
you are doing video conferencing over the Internet via your

office T1
(1.5 MBits/s), you may wish to bandwidth-limit all other T1

traffic to 1
MBit/s in order to reserve at least 0.5 MBits for your video

conferencing
connections. Similarly if you are running a popular web or

ftp site from
a colocation facility you might want to limit bandwidth to

prevent excessive bandwidth charges from your provider.

Finally, FreeBSD firewalls may be used to divert packets or

change the
next-hop address for packets to help route them to the cor

rect destination. Packet diversion is most often used to support NAT

(network
address translation), which allows an internal network using

a private IP
space to make connections to the outside for browsing or

other purposes.

Constructing a firewall may appear to be trivial, but most

people get
them wrong. The most common mistake is to create an exclu

sive firewall
rather than an inclusive firewall. An exclusive firewall

allows all
packets through except for those matching a set of rules.

An inclusive
firewall allows only packets matching the ruleset through.

Inclusive
firewalls are much, much safer than exclusive firewalls but

a tad more
difficult to build properly. The second most common mistake

is to blackhole everything except the particular port you want to let

through.
TCP/IP needs to be able to get certain types of ICMP errors

to function
properly - for example, to implement MTU discovery. Also, a

number of
common system daemons make reverse connections to the auth

service in an
attempt to authenticate the user making a connection. Auth

is rather
dangerous but the proper implementation is to return a TCP

reset for the
connection attempt rather than simply blackholing the pack

et. We cover
these and other quirks involved with constructing a firewall

in the sample firewall section below.

IPFW KERNEL CONFIGURATION

You do not need to create a custom kernel to use the IP

firewalling features. If you enable firewalling in your /etc/rc.conf (see

below), the
ipfw kernel module will be loaded automatically when neces

sary. However,
if you are paranoid you can compile IPFW directly into the

FreeBSD kernel
by using the IPFIREWALL option set. If compiled in the ker

nel, ipfw
denies all packets by default, which means that, if you do

not load in a
permissive ruleset via /etc/rc.conf, rebooting into your new

kernel will
take the network offline. This can prevent you from being

able to access
your system if you are not sitting at the console. It is

also quite common to update a kernel to a new release and reboot before

updating the
binaries. This can result in an incompatibility between the

ipfw(8) program and the kernel which prevents it from running in the

boot sequence,
also resulting in an inaccessible machine. Because of these

problems the
IPFIREWALL_DEFAULT_TO_ACCEPT kernel option is also available

which
changes the default firewall to pass through all packets.

Note, however,
that using this option may open a small window of opportuni

ty during
booting where your firewall passes all packets. Still, it

is a good
option to use while getting up to speed with FreeBSD fire

walling. Get
rid of it once you understand how it all works to close the

loophole,
though. There is a third option called IPDIVERT which al

lows you to use
the firewall to divert packets to a user program and is nec

essary if you
wish to use natd(8) to give private internal networks access

to the outside world. If you want to be able to limit the bandwidth

used by certain types of traffic, the DUMMYNET option must be used to

enable ipfw
pipe rules.

SAMPLE IPFW-BASED FIREWALL

Here is an example ipfw-based firewall taken from a machine

with three
interface cards. fxp0 is connected to the 'exposed' LAN.

Machines on
this LAN are dual-homed with both internal 10. IP addresses

and Internetrouted IP addresses. In our example, 192.100.5.x represents

the Internet-routed IP block while 10.x.x.x represents the internal

networks.
While it is not relevant to the example, 10.0.1.x is as

signed as the
internal address block for the LAN on fxp0, 10.0.2.x for the

LAN on fxp1,
and 10.0.3.x for the LAN on fxp2.

In this example we want to isolate all three LANs from the

Internet as
well as isolate them from each other, and we want to give

all internal
addresses access to the Internet through a NAT gateway run

ning on this
machine. To make the NAT gateway work, the firewall machine

is given two
Internet-exposed addresses on fxp0 in addition to an inter

nal 10. address
on fxp0: one exposed address (not shown) represents the ma

chine's official address, and the second exposed address (192.100.5.5 in

our example)
represents the NAT gateway rendezvous IP. We make the exam

ple more complex by giving the machines on the exposed LAN internal

10.0.0.x
addresses as well as exposed addresses. The idea here is

that you can
bind internal services to internal addresses even on exposed

machines and
still protect those services from the Internet. The only

services you
run on exposed IP addresses would be the ones you wish to

expose to the
Internet.

It is important to note that the 10.0.0.x network in our ex

ample is not
protected by our firewall. You must make sure that your In

ternet router
protects this network from outside spoofing. Also, in our

example, we
pretty much give the exposed hosts free reign on our inter

nal network
when operating services through internal IP addresses

(10.0.0.x). This
is somewhat of security risk: what if an exposed host is

compromised? To
remove the risk and force everything coming in via LAN0 to

go through the
firewall, remove rules 01010 and 01011.

Finally, note that the use of internal addresses represents

a big piece
of our firewall protection mechanism. With proper spoofing

safeguards in
place, nothing outside can directly access an internal (LAN1

or LAN2)
host.

# /etc/rc.conf
#
firewall_enable="YES"
firewall_type="/etc/ipfw.conf"

# temporary port binding range let
# through the firewall.
#
# NOTE: heavily loaded services running through the firewall

may require
# a larger port range for local-size binding. 4000-10000 or

4000-30000
# might be a better choice.
ip_portrange_first=4000
ip_portrange_last=5000
...

# /etc/ipfw.conf
#
# FIREWALL: the firewall machine / nat gateway
# LAN0 10.0.0.X and 192.100.5.X (dual homed)
# LAN1 10.0.1.X
# LAN2 10.0.2.X
# sw: ethernet switch (unmanaged)
#
# 192.100.5.x represents IP addresses exposed to the Inter

net
# (i.e. Internet routeable). 10.x.x.x represent internal

IPs
# (not exposed)
#
# [LAN1]
# ^
#
# FIREWALL -->[LAN2]
#
# [LAN0]
#
# +--> exposed host A
# +--> exposed host B
# +--> exposed host C
#
# INTERNET (secondary firewall)
# ROUTER
#
# [Internet]
#
# NOT SHOWN: The INTERNET ROUTER must contain rules to dis

allow
# all packets with source IP addresses in the 10. block in

order
# to protect the dual-homed 10.0.0.x block. Exposed hosts

are
# not otherwise protected in this example - they should only

bind
# exposed services to exposed IPs but can safely bind inter

nal
# services to internal IPs.
#
# The NAT gateway works by taking packets sent from internal
# IP addresses to external IP addresses and routing them to

natd, which
# is listening on port 8668. This is handled by rule

00300. Data coming
# back to natd from the outside world must also be routed to

natd using
# rule 00301. To make the example interesting, we note that

we do
# NOT have to run internal requests to exposed hosts through

natd
# (rule 00290) because those exposed hosts know about our
# 10. network. This can reduce the load on natd. Also note

that we
# of course do not have to route internal<->internal traffic

through
# natd since those hosts know how to route our 10. internal

network.
# The natd command we run from /etc/rc.local is shown below.

See
# also the in-kernel version of natd, ipnat.
#
# natd -s -u -a 208.161.114.67
#
#
add 00290 skipto 1000 ip from 10.0.0.0/8 to 192.100.5.0/24
add 00300 divert 8668 ip from 10.0.0.0/8 to not 10.0.0.0/8
add 00301 divert 8668 ip from not 10.0.0.0/8 to 192.100.5.5

# Short cut the rules to avoid running high bandwidths

through
# the entire rule set. Allow established tcp connections

through,
# and shortcut all outgoing packets under the assumption

that
# we need only firewall incoming packets.
#
# Allowing established tcp connections through creates a

small
# hole but may be necessary to avoid overloading your fire

wall.
# If you are worried, you can move the rule to after the

spoof
# checks.
#
add 01000 allow tcp from any to any established
add 01001 allow all from any to any out via fxp0
add 01001 allow all from any to any out via fxp1
add 01001 allow all from any to any out via fxp2

# Spoof protection. This depends on how well you trust your
# internal networks. Packets received via fxp1 MUST come

from
# 10.0.1.x. Packets received via fxp2 MUST come from

10.0.2.x.
# Packets received via fxp0 cannot come from the LAN1 or

LAN2
# blocks. We cannot protect 10.0.0.x here, the Internet

router
# must do that for us.
#
add 01500 deny all from not 10.0.1.0/24 in via fxp1
add 01500 deny all from not 10.0.2.0/24 in via fxp2
add 01501 deny all from 10.0.1.0/24 in via fxp0
add 01501 deny all from 10.0.2.0/24 in via fxp0

# In this example rule set there are no restrictions between
# internal hosts, even those on the exposed LAN (as long as
# they use an internal IP address). This represents a
# potential security hole (what if an exposed host is
# compromised?). If you want full restrictions to apply
# between the three LANs, firewalling them off from each
# other for added security, remove these two rules.
#
# If you want to isolate LAN1 and LAN2, but still want
# to give exposed hosts free reign with each other, get
# rid of rule 01010 and keep rule 01011.
#
# (commented out, uncomment for less restrictive firewall)
#add 01010 allow all from 10.0.0.0/8 to 10.0.0.0/8
#add 01011 allow all from 192.100.5.0/24 to 192.100.5.0/24
#

# SPECIFIC SERVICES ALLOWED FROM SPECIFIC LANS
#
# If using a more restrictive firewall, allow specific LANs
# access to specific services running on the firewall it

self.
# In this case we assume LAN1 needs access to filesharing

running
# on the firewall. If using a less restrictive firewall
# (allowing rule 01010), you do not need these rules.
#
add 01012 allow tcp from 10.0.1.0/8 to 10.0.1.1 139
add 01012 allow udp from 10.0.1.0/8 to 10.0.1.1 137,138

# GENERAL SERVICES ALLOWED TO CROSS INTERNAL AND EXPOSED

LANS
#
# We allow specific UDP services through: DNS lookups,

ntalk, and ntp.
# Note that internal services are protected by virtue of

having
# spoof-proof internal IP addresses (10. net), so these

rules
# really only apply to services bound to exposed IPs. We

have
# to allow UDP fragments or larger fragmented UDP packets

will
# not survive the firewall.
#
# If we want to expose high-numbered temporary service ports
# for things like DNS lookup responses we can use a port

range,
# in this example 4000-65535, and we set to /etc/rc.conf

variables
# on all exposed machines to make sure they bind temporary

ports
# to the exposed port range (see rc.conf example above)
#
add 02000 allow udp from any to any 4000-65535,do

main,ntalk,ntp
add 02500 allow udp from any to any frag

# Allow similar services for TCP. Again, these only apply

to
# services bound to exposed addresses. NOTE: we allow
# through but do not actually run an identd server on any

exposed
# port. This allows the machine being authed to respond

with a
# TCP RESET. Throwing the packet away would result in de

lays
# when connecting to remote services that do reverse ident

lookups.
#
# Note that we do not allow tcp fragments through, and that

we do
# not allow fragments in general (except for UDP fragments).

We
# expect the TCP mtu discovery protocol to work properly so

there
# should be no TCP fragments.
#
add 03000 allow tcp from any to any http,https
add 03000 allow tcp from any to any 4000-65535,ssh,smtp,do

main,ntalk
add 03000 allow tcp from any to any auth,pop3,ftp,ftp-data

# It is important to allow certain ICMP types through, here

is a list
# of general ICMP types. Note that it is important to let

ICMP type 3
# through.
#
# 0 Echo Reply
# 3 Destination Unreachable (used by TCP MTU

discovery, aka
# packet-too-big)
# 4 Source Quench (typically not allowed)
# 5 Redirect (typically not allowed - can be

dangerous!)
# 8 Echo
# 11 Time Exceeded
# 12 Parameter Problem
# 13 Timestamp
# 14 Timestamp Reply
#
# Sometimes people need to allow ICMP REDIRECT packets,

which is
# type 5, but if you allow it make sure that your Internet

router
# disallows it.

add 04000 allow icmp from any to any icmptypes

0,3,8,11,12,13,14

# log any remaining fragments that get through. Might be

useful,
# otherwise do not bother. Have a final deny rule as a

safety to
# guarantee that your firewall is inclusive no matter how

the kernel
# is configured.
#
add 05000 deny log ip from any to any frag
add 06000 deny all from any to any

PORT BINDING INTERNAL AND EXTERNAL SERVICES

We have mentioned multi-homing hosts and binding services to

internal or
external addresses but we have not really explained it.

When you have a
host with multiple IP addresses assigned to it, you can bind

services run
on that host to specific IPs or interfaces rather than all

IPs. Take the
firewall machine for example: with three interfaces and two

exposed IP
addresses on one of those interfaces, the firewall machine

is known by 5
different IP addresses (10.0.0.1, 10.0.1.1, 10.0.2.1,

192.100.5.5, and
say 192.100.5.1). If the firewall is providing file sharing

services to
the windows LAN segment (say it is LAN1), you can use sam

ba's 'bind
interfaces' directive to specifically bind it to just the

LAN1 IP
address. That way the file sharing services will not be

made available
to other LAN segments. The same goes for NFS. If LAN2 has

your UNIX
engineering workstations, you can tell nfsd to bind specifi

cally to
10.0.2.1. You can specify how to bind virtually every ser

vice on the
machine and you can use a light jail(8) to indirectly bind

services that
do not otherwise give you the option.

SEE ALSO

dummynet(4), ipnat(5), rc.conf(5), smb.conf(5)

(ports/net/samba),
samba(7) (ports/net/samba), config(8), ipfw(8), ipnat(8),

jail(8),
natd(8), nfsd(8)

ADDITIONAL READING

Ipfilter

ipf(5), ipf(8), ipfstat(8)

Packet Filter

pf.conf(5), pfctl(8), pflogd(8)

HISTORY

The firewall manual page was originally written by Matthew

Dillon and
first appeared in FreeBSD 4.3, May 2001.

BSD May 26, 2001

BSD