ipfw(8)

NAME

ipfw - IP firewall and traffic shaper control program

SYNOPSIS

ipfw [-cq] add rule
ipfw [-acdefnNStT] {list | show} [rule | first-last ...]
ipfw [-f | -q] flush
ipfw [-q] {delete | zero | resetlog} [set] [number ...]
ipfw enable
     {firewall |  altq  |  one_pass  |   debug   |   verbose
dyn_keepalive}
ipfw disable
     {firewall   |   altq  |  one_pass  |  debug  |  verbose
dyn_keepalive}
ipfw set [disable number ...] [enable number ...]
ipfw set move [rule] number to number
ipfw set swap number number
ipfw set show
ipfw table number add addr[/masklen] [value]
ipfw table number delete addr[/masklen]
ipfw table number flush
ipfw table number list
ipfw {pipe | queue} number config config-options
ipfw [-s [field]] {pipe | queue}  {delete  |  list  |  show}
[number ...]
ipfw [-cfnNqS] [-p preproc [preproc-flags]] pathname

DESCRIPTION

The ipfw utility is the user interface for controlling the
ipfw(4) firewall and the dummynet(4) traffic shaper in FreeBSD.
An ipfw configuration, or ruleset, is made of a list of
rules numbered
from 1 to 65535. Packets are passed to ipfw from a number
of different
places in the protocol stack (depending on the source and
destination of
the packet, it is possible that ipfw is invoked multiple
times on the
same packet). The packet passed to the firewall is compared
against each
of the rules in the firewall ruleset. When a match is
found, the action
corresponding to the matching rule is performed.
Depending on the action and certain system settings, packets
can be reinjected into the firewall at some rule after the matching one
for further
processing.
An ipfw ruleset always includes a default rule (numbered
65535) which
cannot be modified or deleted, and matches all packets. The
action associated with the default rule can be either deny or allow de
pending on how
the kernel is configured.
If the ruleset includes one or more rules with the
keep-state or limit
option, then ipfw assumes a stateful behaviour, i.e., upon a
match it
will create dynamic rules matching the exact parameters (ad
dresses and
ports) of the matching packet.
These dynamic rules, which have a limited lifetime, are
checked at the
first occurrence of a check-state, keep-state or limit rule,
and are typically used to open the firewall on-demand to legitimate
traffic only.
See the STATEFUL FIREWALL and EXAMPLES Sections below for
more information on the stateful behaviour of ipfw.
All rules (including dynamic ones) have a few associated
counters: a
packet count, a byte count, a log count and a timestamp in
dicating the
time of the last match. Counters can be displayed or reset
with ipfw
commands.
Rules can be added with the add command; deleted individual
ly or in
groups with the delete command, and globally (except those
in set 31)
with the flush command; displayed, optionally with the con
tent of the
counters, using the show and list commands. Finally, coun
ters can be
reset with the zero and resetlog commands.
Also, each rule belongs to one of 32 different sets , and
there are ipfw
commands to atomically manipulate sets, such as enable, dis
able, swap
sets, move all rules in a set to another one, delete all
rules in a set.
These can be useful to install temporary configurations, or
to test them.
See Section SETS OF RULES for more information on sets.
The following options are available:
-a While listing, show counter values. The show com
mand just
implies this option.
-b Only show the action and the comment, not the body
of a rule.
Implies -c.
-c When entering or showing rules, print them in com
pact form, i.e.,
without the optional "ip from any to any" string
when this does
not carry any additional information.
-d While listing, show dynamic rules in addition to
static ones.
-e While listing, if the -d option was specified, also
show expired
dynamic rules.
-f Do not ask for confirmation for commands that can
cause problems
if misused, i.e. flush. If there is no tty associ
ated with the
process, this is implied.
-n Only check syntax of the command strings, without
actually pass
ing them to the kernel.
-N Try to resolve addresses and service names in out
put.
-q While adding, zeroing, resetlogging or flushing, be
quiet about
actions (implies -f). This is useful for adjusting
rules by executing multiple ipfw commands in a script (e.g.,
`sh /etc/rc.firewall'), or by processing a file of
many ipfw
rules across a remote login session. If a flush is
performed in
normal (verbose) mode (with the default kernel con
figuration), it
prints a message. Because all rules are flushed,
the message
might not be delivered to the login session, causing
the remote
login session to be closed and the remainder of the
ruleset to
not be processed. Access to the console would then
be required
to recover.
-S While listing rules, show the set each rule belongs
to. If this
flag is not specified, disabled rules will not be
listed.
-s [field]
While listing pipes, sort according to one of the
four counters
(total or current packets or bytes).
-t While listing, show last match timestamp (converted
with
ctime()).
-T While listing, show last match timestamp (as seconds
from the
epoch). This form can be more convenient for post
processing by
scripts.
To ease configuration, rules can be put into a file which is
processed
using ipfw as shown in the last synopsis line. An absolute
pathname must
be used. The file will be read line by line and applied as
arguments to
the ipfw utility.
Optionally, a preprocessor can be specified using -p preproc
where
pathname is to be piped through. Useful preprocessors in
clude cpp(1) and
m4(1). If preproc does not start with a slash (`/') as its
first character, the usual PATH name search is performed. Care should
be taken with
this in environments where not all file systems are mounted
(yet) by the
time ipfw is being run (e.g. when they are mounted over
NFS). Once -p
has been specified, any additional arguments as passed on to
the preprocessor for interpretation. This allows for flexible config
uration files
(like conditionalizing them on the local hostname) and the
use of macros
to centralize frequently required arguments like IP address
es.
The ipfw pipe and queue commands are used to configure the
traffic
shaper, as shown in the TRAFFIC SHAPER (DUMMYNET)
CONFIGURATION Section
below.
If the world and the kernel get out of sync the ipfw ABI may
break, preventing you from being able to add any rules. This can ad
versely effect
the booting process. You can use ipfw disable firewall to
temporarily
disable the firewall to regain access to the network, allow
ing you to fix
the problem.

PACKET FLOW

A packet is checked against the active ruleset in multiple
places in the
protocol stack, under control of several sysctl variables.
These places
and variables are shown below, and it is important to have
this picture
in mind in order to design a correct ruleset.

^ to upper layers V
+----------->-----------+
^ V
[ip(6)_input] [ip(6)_output] net.in
et.ip.fw.enable=1

^ V
[ether_demux] [ether_output_frame]
net.link.ether.ipfw=1

+-->--[bdg_forward]-->--+
net.link.ether.bridge_ipfw=1
^ V
| to devices
As can be noted from the above picture, the number of times
the same
packet goes through the firewall can vary between 0 and 4
depending on
packet source and destination, and system configuration.
Note that as packets flow through the stack, headers can be
stripped or
added to it, and so they may or may not be available for in
spection.
E.g., incoming packets will include the MAC header when ipfw
is invoked
from ether_demux(), but the same packets will have the MAC
header
stripped off when ipfw is invoked from ip_input() or
ip6_input().
Also note that each packet is always checked against the
complete ruleset, irrespective of the place where the check occurs, or
the source of
the packet. If a rule contains some match patterns or ac
tions which are
not valid for the place of invocation (e.g. trying to match
a MAC header
within ip_input or ip6_input ), the match pattern will not
match, but a
not operator in front of such patterns will cause the pat
tern to always
match on those packets. It is thus the responsibility of
the programmer,
if necessary, to write a suitable ruleset to differentiate
among the possible places. skipto rules can be useful here, as an exam
ple:

# packets from ether_demux or bdg_forward
ipfw add 10 skipto 1000 all from any to any layer2 in
# packets from ip_input
ipfw add 10 skipto 2000 all from any to any not layer2
in
# packets from ip_output
ipfw add 10 skipto 3000 all from any to any not layer2
out
# packets from ether_output_frame
ipfw add 10 skipto 4000 all from any to any layer2 out
(yes, at the moment there is no way to differentiate between
ether_demux
and bdg_forward).

SYNTAX

In general, each keyword or argument must be provided as a
separate command line argument, with no leading or trailing spaces.
Keywords are
case-sensitive, whereas arguments may or may not be case
sensitive
depending on their nature (e.g. uid's are, hostnames are
not).
In ipfw2 you can introduce spaces after commas ',' to make
the line more
readable. You can also put the entire command (including
flags) into a
single argument. E.g., the following forms are equivalent:

ipfw -q add deny src-ip 10.0.0.0/24,127.0.0.1/8
ipfw -q add deny src-ip 10.0.0.0/24, 127.0.0.1/8
ipfw "-q add deny src-ip 10.0.0.0/24, 127.0.0.1/8"

RULE FORMAT

The format of ipfw rules is the following:
[rule_number] [set set_number] [prob
match_probability]
action [log [logamount number]] [altq queue] body
where the body of the rule specifies which information is
used for filtering packets, among the following:

Layer-2 header fields When available
IPv4 and IPv6 Protocol TCP, UDP, ICMP,
etc.
Source and dest. addresses and ports
Direction See Section PACKET
FLOW
Transmit and receive interface By name or address
Misc. IP header fields Version, type of
service, data
gram length, iden
tification,
fragment flag (non
zero IP offset), Time To Live
IP options
IPv6 Extension headers Fragmentation, Hop
by-Hop
options, source
routing, IPSec
options.
IPv6 Flow-ID
Misc. TCP header fields TCP flags (SYN,
FIN, ACK, RST,
etc.), sequence
number, acknowledgment number,
window
TCP options
ICMP types for ICMP packets
ICMP6 types for ICMP6 packets
User/group ID When the packet can
be associ
ated with a local
socket.
Divert status Whether a packet
came from a
divert socket
(e.g., natd(8)).
Note that some of the above information, e.g. source MAC or
IP addresses
and TCP/UDP ports, could easily be spoofed, so filtering on
those fields
alone might not guarantee the desired results.
rule_number
Each rule is associated with a rule_number in the
range 1..65535,
with the latter reserved for the default rule.
Rules are checked
sequentially by rule number. Multiple rules can
have the same
number, in which case they are checked (and listed)
according to
the order in which they have been added. If a rule
is entered
without specifying a number, the kernel will assign
one in such a
way that the rule becomes the last one before the
default rule.
Automatic rule numbers are assigned by incrementing
the last nondefault rule number by the value of the sysctl vari
able
net.inet.ip.fw.autoinc_step which defaults to 100.
If this is
not possible (e.g. because we would go beyond the
maximum allowed
rule number), the number of the last non-default
value is used
instead.
set set_number
Each rule is associated with a set_number in the
range 0..31.
Sets can be individually disabled and enabled, so
this parameter
is of fundamental importance for atomic ruleset ma
nipulation. It
can be also used to simplify deletion of groups of
rules. If a
rule is entered without specifying a set number, set
0 will be
used.
Set 31 is special in that it cannot be disabled, and
rules in set
31 are not deleted by the ipfw flush command (but
you can delete
them with the ipfw delete set 31 command). Set 31
is also used
for the default rule.
prob match_probability
A match is only declared with the specified proba
bility (floating
point number between 0 and 1). This can be useful
for a number
of applications such as random packet drop or (in
conjunction
with dummynet(4)) to simulate the effect of multiple
paths leading to out-of-order packet delivery.
Note: this condition is checked before any other
condition,
including ones such as keep-state or check-state
which might have
side effects.
log [logamount number]
When a packet matches a rule with the log keyword, a
message will
be logged to syslogd(8) with a LOG_SECURITY facili
ty. The logging only occurs if the sysctl variable
net.inet.ip.fw.verbose is
set to 1 (which is the default when the kernel is
compiled with
IPFIREWALL_VERBOSE) and the number of packets logged
so far for
that particular rule does not exceed the logamount
parameter. If
no logamount is specified, the limit is taken from
the sysctl
variable net.inet.ip.fw.verbose_limit. In both cas
es, a value of
0 removes the logging limit.
Once the limit is reached, logging can be re-enabled
by clearing
the logging counter or the packet counter for that
entry, see the
resetlog command.
Note: logging is done after all other packet match
ing conditions
have been successfully verified, and before perform
ing the final
action (accept, deny, etc.) on the packet.
altq queue
When a packet matches a rule with the altq keyword,
the ALTQ
identifier for the given queue (see altq(4)) will be
attached.
Note that this ALTQ tag is only meaningful for pack
ets going
"out" of IPFW, and not being rejected or going to
divert sockets.
Note that if there is insufficient memory at the
time the packet
is processed, it will not be tagged, so it is wise
to make your
ALTQ "default" queue policy account for this. If
multiple altq
rules match a single packet, only the first one adds
the ALTQ
classification tag. In doing so, traffic may be
shaped by using
count altq queue rules for classification early in
the ruleset,
then later applying the filtering decision. For ex
ample,
check-state and keep-state rules may come later and
provide the
actual filtering decisions in addition to the fall
back ALTQ tag.
You must run pfctl(8) to set up the queues before
IPFW will be
able to look them up by name, and if the ALTQ disci
plines are
rearranged, the rules in containing the queue iden
tifiers in the
kernel will likely have gone stale and need to be
reloaded.
Stale queue identifiers will probably result in mis
classification.
All system ALTQ processing can be turned on or off
via ipfw
enable altq and ipfw disable altq. The usage of net.inet.ip.fw.one_pass is irrelevant to ALTQ traf
fic shaping, as
the actual rule action is followed always after
adding an ALTQ
tag.
RULE ACTIONS
A rule can be associated with one of the following actions,
which will be
executed when the packet matches the body of the rule.
allow | accept | pass | permit
Allow packets that match rule. The search termi
nates.
check-state
Checks the packet against the dynamic ruleset. If a
match is
found, execute the action associated with the rule
which generated this dynamic rule, otherwise move to the next
rule.
Check-state rules do not have a body. If no
check-state rule is
found, the dynamic ruleset is checked at the first
keep-state or
limit rule.
count Update counters for all packets that match rule.
The search con
tinues with the next rule.
deny | drop
Discard packets that match this rule. The search
terminates.
divert port
Divert packets that match this rule to the divert(4)
socket bound
to port port. The search terminates.
fwd | forward ipaddr[,port]
Change the next-hop on matching packets to ipaddr,
which can be
an IP address or a host name. The search terminates
if this rule
matches.
If ipaddr is a local address, then matching packets
will be forwarded to port (or the port number in the packet if
one is not
specified in the rule) on the local machine.
If ipaddr is not a local address, then the port num
ber (if specified) is ignored, and the packet will be forwarded
to the remote
address, using the route as found in the local rout
ing table for
that IP.
A fwd rule will not match layer-2 packets (those re
ceived on
ether_input, ether_output, or bridged).
The fwd action does not change the contents of the
packet at all.
In particular, the destination address remains un
modified, so
packets forwarded to another system will usually be
rejected by
that system unless there is a matching rule on that
system to
capture them. For packets forwarded locally, the
local address
of the socket will be set to the original destina
tion address of
the packet. This makes the netstat(1) entry look
rather weird
but is intended for use with transparent proxy
servers.
To enable fwd a custom kernel needs to be compiled
with the
option options IPFIREWALL_FORWARD. With the addi
tional option
options IPFIREWALL_FORWARD_EXTENDED all safeguards
are removed
and it also makes it possible to redirect packets
destined to
locally configured IP addresses. Please note that
such rules
apply to locally generated packets as well and great
care is
required to ensure proper behaviour for automatical
ly generated
packets like ICMP message size exceeded and others.
pipe pipe_nr
Pass packet to a dummynet(4) ``pipe'' (for bandwidth
limitation,
delay, etc.). See the TRAFFIC SHAPER (DUMMYNET)
CONFIGURATION
Section for further information. The search termi
nates; however,
on exit from the pipe and if the sysctl(8) variable
net.inet.ip.fw.one_pass is not set, the packet is
passed again to
the firewall code starting from the next rule.
queue queue_nr
Pass packet to a dummynet(4) ``queue'' (for band
width limitation
using WF2Q+).
reject (Deprecated). Synonym for unreach host.
reset Discard packets that match this rule, and if the
packet is a TCP
packet, try to send a TCP reset (RST) notice. The
search terminates.
reset6 Discard packets that match this rule, and if the
packet is a TCP
packet, try to send a TCP reset (RST) notice. The
search terminates.
skipto number
Skip all subsequent rules numbered less than number.
The search
continues with the first rule numbered number or
higher.
tee port
Send a copy of packets matching this rule to the di
vert(4) socket
bound to port port. The search continues with the
next rule.
unreach code
Discard packets that match this rule, and try to
send an ICMP
unreachable notice with code code, where code is a
number from 0
to 255, or one of these aliases: net, host,
protocol, port,
needfrag, srcfail, net-unknown, host-unknown,
isolated,
net-prohib, host-prohib, tosnet, toshost,
filter-prohib,
host-precedence or precedence-cutoff. The search
terminates.
unreach6 code
Discard packets that match this rule, and try to
send an ICMPv6
unreachable notice with code code, where code is a
number from 0,
1, 3 or 4, or one of these aliases: no-route,
admin-prohib,
address or port. The search terminates.
netgraph cookie
Divert packet into netgraph with given cookie. The
search terminates. If packet is later returned from netgraph it
is either
accepted or continues with the next rule, depending
on
net.inet.ip.fw.one_pass sysctl variable.
ngtee cookie
A copy of packet is diverted into netgraph, original
packet is
either accepted or continues with the next rule, de
pending on
net.inet.ip.fw.one_pass sysctl variable. See
ng_ipfw(4) for more
information on netgraph and ngtee actions.
RULE BODY
The body of a rule contains zero or more patterns (such as
specific
source and destination addresses or ports, protocol options,
incoming or
outgoing interfaces, etc.) that the packet must match in
order to be
recognised. In general, the patterns are connected by (im
plicit) and
operators -- i.e., all must match in order for the rule to
match. Individual patterns can be prefixed by the not operator to re
verse the result
of the match, as in

ipfw add 100 allow ip from not 1.2.3.4 to any
Additionally, sets of alternative match patterns (or-blocks)
can be constructed by putting the patterns in lists enclosed between
parentheses (
) or braces { }, and using the or operator as follows:

ipfw add 100 allow ip from { x or not y or z } to any
Only one level of parentheses is allowed. Beware that most
shells have
special meanings for parentheses or braces, so it is advis
able to put a
backslash in front of them to prevent such interpretations.
The body of a rule must in general include a source and des
tination
address specifier. The keyword any can be used in various
places to
specify that the content of a required field is irrelevant.
The rule body has the following format:

[proto from src to dst] [options]
The first part (proto from src to dst) is for backward com
patibility with
earlier versions of FreeBSD. In modern FreeBSD any match
pattern
(including MAC headers, IP protocols, addresses and ports)
can be specified in the options section.
Rule fields have the following meaning:
proto: protocol | { protocol or ... }
protocol: [not] protocol-name | protocol-number
An IP protocol specified by number or name (for a
complete list
see /etc/protocols), or one of the following key
words:
ip4 | ipv4
Matches IPv4 packets.
ip6 | ipv6
Matches IPv6 packets.
ip | all
Matches any packet.
The ipv6 in proto option will be treated as inner
protocol. And,
the ipv4 is not available in proto option.
The { protocol or ... } format (an or-block) is pro
vided for convenience only but its use is deprecated.
src and dst: {addr | { addr or ... }} [[not] ports]
An address (or a list, see below) optionally fol
lowed by ports
specifiers.
The second format (or-block with multiple addresses)
is provided
for convenience only and its use is discouraged.
addr: [not] {any | me | me6 table(number[,value])
addr-list | addr-set}
any matches any IP address.
me matches any IP address configured on an interface in
the system.
me6 matches any IPv6 address configured on an interface
in the sys
tem. The address list is evaluated at the time the
packet is
analysed.
table(number[,value])
Matches any IPv4 address for which an entry exists
in the lookup
table number. If an optional 32-bit unsigned value
is also specified, an entry will match only if it has this val
ue. See the
LOOKUP TABLES section below for more information on
lookup
tables.
addr-list: ip-addr[,addr-list]
ip-addr:
A host or subnet address specified in one of the
following ways:
numeric-ip | hostname
Matches a single IPv4 address, specified as
dotted-quad
or a hostname. Hostnames are resolved at
the time the
rule is added to the firewall list.
addr/masklen
Matches all addresses with base addr (speci
fied as an IP
address or a hostname) and mask width of
masklen bits.
As an example, 1.2.3.4/25 will match all IP
numbers from
1.2.3.0 to 1.2.3.127 .
addr:mask
Matches all addresses with base addr (speci
fied as an IP
address or a hostname) and the mask of mask,
specified as
a dotted quad. As an example,
1.2.3.4:255.0.255.0 will
match 1.*.3.*. This form is advised only
for non-contiguous masks. It is better to resort to
the
addr/masklen format for contiguous masks,
which is more
compact and less error-prone.
addr-set: addr[/masklen]{list}
list: {num | num-num}[,list]
Matches all addresses with base address addr (speci
fied as an IP
address or a hostname) and whose last byte is in the
list between
braces { } . Note that there must be no spaces be
tween braces
and numbers (spaces after commas are allowed). Ele
ments of the
list can be specified as single entries or ranges.
The masklen
field is used to limit the size of the set of ad
dresses, and can
have any value between 24 and 32. If not specified,
it will be
assumed as 24.
This format is particularly useful to handle sparse
address sets
within a single rule. Because the matching occurs
using a bitmask, it takes constant time and dramatically re
duces the complexity of rulesets.
As an example, an address specified as
1.2.3.4/24{128,35-55,89}
will match the following IP addresses:
1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .
addr6-list: ip6-addr[,addr6-list]
ip6-addr:
A host or subnet specified one of the following
ways:
numeric-ip | hostname
Matches a single IPv6 address as allowed by
inet_pton(3)
or a hostname. Hostnames are resolved at
the time the
rule is added to the firewall list.
addr/masklen
Matches all IPv6 addresses with base addr
(specified as
allowed by inet_pton or a hostname) and mask
width of
masklen bits.
No support for sets of IPv6 addresses is provided
because IPv6
addresses are typically random past the initial pre
fix.
ports: {port | port-port}[,ports]
For protocols which support port numbers (such as
TCP and UDP),
optional ports may be specified as one or more ports
or port
ranges, separated by commas but no spaces, and an
optional not
operator. The `-' notation specifies a range of
ports (including
boundaries).
Service names (from /etc/services) may be used in
stead of numeric
port values. The length of the port list is limited
to 30 ports
or ranges, though one can specify larger ranges by
using an
or-block in the options section of the rule.
A backslash (`´) can be used to escape the dash
(`-') character
in a service name (from a shell, the backslash must
be typed
twice to avoid the shell itself interpreting it as
an escape
character).

ipfw add count tcp from any ftp-data-ftp to
any
Fragmented packets which have a non-zero offset
(i.e., not the
first fragment) will never match a rule which has
one or more
port specifications. See the frag option for de
tails on matching
fragmented packets.
RULE OPTIONS (MATCH PATTERNS)
Additional match patterns can be used within rules. Zero or
more of
these so-called options can be present in a rule, optionally
prefixed by
the not operand, and possibly grouped into or-blocks.
The following match patterns can be used (listed in alpha
betical order):
// this is a comment.
Inserts the specified text as a comment in the rule.
Everything
following // is considered as a comment and stored
in the rule.
You can have comment-only rules, which are listed as
having a
count action followed by the comment.
bridged
Alias for layer2.
diverted
Matches only packets generated by a divert socket.
diverted-loopback
Matches only packets coming from a divert socket
back into the IP
stack input for delivery.
diverted-output
Matches only packets going from a divert socket back
outward to
the IP stack output for delivery.
dst-ip ip-address
Matches IPv4 packets whose destination IP is one of
the
address(es) specified as argument.
{dst-ip6 | dst-ipv6} ip6-address
Matches IPv6 packets whose destination IP is one of
the
address(es) specified as argument.
dst-port ports
Matches IP packets whose destination port is one of
the port(s)
specified as argument.
established
Matches TCP packets that have the RST or ACK bits
set.
ext6hdr header
Matches IPv6 packets containing the extended header
given by
header. Supported headers are:
Fragment, (frag), Hop-to-hop options (hopopt),
Source routing
(route), Destination options (dstopt), IPSec authen
tication headers (ah), and IPSec encapsulated security payload
headers (esp).
flow-id labels
Matches IPv6 packets containing any of the flow la
bels given in
labels. labels is a comma seperate list of numeric
flow labels.
frag Matches packets that are fragments and not the first
fragment of
an IP datagram. Note that these packets will not
have the next
protocol header (e.g. TCP, UDP) so options that look
into these
headers cannot match.
gid group
Matches all TCP or UDP packets sent by or received
for a group.
A group may be specified by name or number. This
option should
be used only if debug.mpsafenet=0 to avoid possible
deadlocks due
to layering violations in its implementation.
jail prisonID
Matches all TCP or UDP packets sent by or received
for the jail
whos prison ID is prisonID. This option should be
used only if
debug.mpsafenet=0 to avoid possible deadlocks due to
layering
violations in its implementation.
icmptypes types
Matches ICMP packets whose ICMP type is in the list
types. The
list may be specified as any combination of individ
ual types
(numeric) separated by commas. Ranges are not
allowed. The supported ICMP types are:
echo reply (0), destination unreachable (3), source
quench (4),
redirect (5), echo request (8), router advertisement
(9), router
solicitation (10), time-to-live exceeded (11), IP
header bad
(12), timestamp request (13), timestamp reply (14),
information
request (15), information reply (16), address mask
request (17)
and address mask reply (18).
icmp6types types
Matches ICMP6 packets whose ICMP6 type is in the
list of types.
The list may be specified as any combination of in
dividual types
(numeric) separated by commas. Ranges are not
allowed.
in | out
Matches incoming or outgoing packets, respectively.
in and out
are mutually exclusive (in fact, out is implemented
as not in).
ipid id-list
Matches IPv4 packets whose ip_id field has value in
cluded in
id-list, which is either a single value or a list of
values or
ranges specified in the same way as ports.
iplen len-list
Matches IP packets whose total length, including
header and data,
is in the set len-list, which is either a single
value or a list
of values or ranges specified in the same way as
ports.
ipoptions spec
Matches packets whose IPv4 header contains the comma
separated
list of options specified in spec. The supported 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 `!'.
ipprecedence precedence
Matches IPv4 packets whose precedence field is equal
to
precedence.
ipsec Matches packets that have IPSEC history associated
with them
(i.e., the packet comes encapsulated in IPSEC, the
kernel has
IPSEC support and IPSEC_FILTERGIF option, and can
correctly
decapsulate it).
Note that specifying ipsec is different from speci
fying proto
ipsec as the latter will only look at the specific
IP protocol
field, irrespective of IPSEC kernel support and the
validity of
the IPSEC data.
Further note that this flag is silently ignored in
kernels without IPSEC support. It does not affect rule process
ing when given
and the rules are handled as if with no ipsec flag.
iptos spec
Matches IPv4 packets whose tos field contains the
comma separated
list of service types specified in spec. The sup
ported IP types
of service are:
lowdelay (IPTOS_LOWDELAY), throughput (IP
TOS_THROUGHPUT),
reliability (IPTOS_RELIABILITY), mincost (IPTOS_MIN
COST),
congestion (IPTOS_CE). The absence of a particular
type may be
denoted with a `!'.
ipttl ttl-list
Matches IPv4 packets whose time to live is included
in ttl-list,
which is either a single value or a list of values
or ranges
specified in the same way as ports.
ipversion ver
Matches IP packets whose IP version field is ver.
keep-state
Upon a match, the firewall will create a dynamic
rule, whose
default behaviour is to match bidirectional traffic
between
source and destination IP/port using the same proto
col. The rule
has a limited lifetime (controlled by a set of
sysctl(8) variables), and the lifetime is refreshed every time a
matching
packet is found.
layer2 Matches only layer2 packets, i.e., those passed to
ipfw from
ether_demux() and ether_output_frame().
limit {src-addr | src-port | dst-addr | dst-port} N
The firewall will only allow N connections with the
same set of
parameters as specified in the rule. One or more of
source and
destination addresses and ports can be specified.
Currently,
only IPv4 flows are supported.
{ MAC | mac } dst-mac src-mac
Match packets with a given dst-mac and src-mac ad
dresses, specified as the any keyword (matching any MAC address),
or six groups
of hex digits separated by colons, and optionally
followed by a
mask indicating the significant bits. The mask may
be specified
using either of the following methods:
1. A slash (/) followed by the number of sig
nificant bits.
For example, an address with 33 significant
bits could be
specified as:

MAC 10:20:30:40:50:60/33 any
2. An ampersand (&) followed by a bitmask spec
ified as six
groups of hex digits separated by colons.
For example,
an address in which the last 16 bits are
significant
could be specified as:

MAC
10:20:30:40:50:60&00:00:00:00:ff:ff any
Note that the ampersand character has a spe
cial meaning
in many shells and should generally be es
caped.
Note that the order of MAC addresses (destination
first, source
second) is the same as on the wire, but the opposite
of the one
used for IP addresses.
mac-type mac-type
Matches packets whose Ethernet Type field corre
sponds to one of
those specified as argument. mac-type is specified
in the same
way as port numbers (i.e., one or more comma-sepa
rated single
values or ranges). You can use symbolic names for
known values
such as vlan, ipv4, ipv6. Values can be entered as
decimal or
hexadecimal (if prefixed by 0x), and they are always
printed as
hexadecimal (unless the -N option is used, in which
case symbolic
resolution will be attempted).
proto protocol
Matches packets with the corresponding IP protocol.
recv | xmit | via {ifX | if* | ipno | any} Matches packets received, transmitted or going
through, respectively, the interface specified by exact name (ifX),
by device
name (if*), by IP address, or through some inter
face.
The via keyword causes the interface to always be
checked. If
recv or xmit is used instead of via, then only the
receive or
transmit interface (respectively) is checked. By
specifying
both, it is possible to match packets based on both
receive and
transmit interface, e.g.:

ipfw add deny ip from any to any out recv ed0
xmit ed1
The recv interface can be tested on either incoming
or outgoing
packets, while the xmit interface can only be tested
on outgoing
packets. So out is required (and in is invalid)
whenever xmit is
used.
A packet may not have a receive or transmit inter
face: packets
originating from the local host have no receive in
terface, while
packets destined for the local host have no transmit
interface.
setup Matches TCP packets that have the SYN bit set but no
ACK bit.
This is the short form of ``tcpflags syn,!ack''.
src-ip ip-address
Matches IPv4 packets whose source IP is one of the
address(es)
specified as an argument.
src-ip6 ip6-address
Matches IPv6 packets whose source IP is one of the
address(es)
specified as an argument.
src-port ports
Matches IP packets whose source port is one of the
port(s) specified as argument.
tcpack ack
TCP packets only. Match if the TCP header acknowl
edgment number
field is set to ack.
tcpdatalen tcpdatalen-list Matches TCP packets whose length of TCP data is
tcpdatalen-list,
which is either a single value or a list of values
or ranges
specified in the same way as ports.
tcpflags spec
TCP packets only. Match if the TCP header contains
the comma
separated list of flags specified in spec. The sup
ported TCP
flags are:
fin, syn, rst, psh, ack and urg. The absence of a
particular
flag may be denoted with a `!'. A rule which con
tains a tcpflags
specification can never match a fragmented packet
which has a
non-zero offset. See the frag option for details on
matching
fragmented packets.
tcpseq seq
TCP packets only. Match if the TCP header sequence
number field
is set to seq.
tcpwin win
TCP packets only. Match if the TCP header window
field is set to
win.
tcpoptions spec
TCP packets only. Match if the TCP header contains
the comma
separated list of options specified in spec. The
supported TCP
options are:
mss (maximum segment size), window (tcp window ad
vertisement),
sack (selective ack), ts (rfc1323 timestamp) and cc
(rfc1644
t/tcp connection count). The absence of a particu
lar option may
be denoted with a `!'.
uid user
Match all TCP or UDP packets sent by or received for
a user. A
user may be matched by name or identification num
ber. This
option should be used only if debug.mpsafenet=0 to
avoid possible
deadlocks due to layering violations in its imple
mentation.
verrevpath
For incoming packets, a routing table lookup is done
on the
packet's source address. If the interface on which
the packet
entered the system matches the outgoing interface
for the route,
the packet matches. If the interfaces do not match
up, the
packet does not match. All outgoing packets or
packets with no
incoming interface match.
The name and functionality of the option is inten
tionally similar
to the Cisco IOS command:

ip verify unicast reverse-path
This option can be used to make anti-spoofing rules
to reject all
packets with source addresses not from this inter
face. See also
the option antispoof.
versrcreach
For incoming packets, a routing table lookup is done
on the
packet's source address. If a route to the source
address
exists, but not the default route or a blackhole/re
ject route,
the packet matches. Otherwise, the packet does not
match. All
outgoing packets match.
The name and functionality of the option is inten
tionally similar
to the Cisco IOS command:

ip verify unicast source reachable-via any
This option can be used to make anti-spoofing rules
to reject all
packets whose source address is unreachable.
antispoof
For incoming packets, the packet's source address is
checked if
it belongs to a directly connected network. If the
network is
directly connected, then the interface the packet
came on in is
compared to the interface the network is connected
to. When
incoming interface and directly connected interface
are not the
same, the packet does not match. Otherwise, the
packet does
match. All outgoing packets match.
This option can be used to make anti-spoofing rules
to reject all
packets that pretend to be from a directly connected
network but
do not come in through that interface. This option
is similar to
but more restricted than verrevpath because it en
gages only on
packets with source addresses of directly connected
networks
instead of all source addresses.

LOOKUP TABLES

Lookup tables are useful to handle large sparse address
sets, typically
from a hundred to several thousands of entries. There may
be up to 128
different lookup tables, numbered 0 to 127.
Each entry is represented by an addr[/masklen] and will
match all
addresses with base addr (specified as an IP address or a
hostname) and
mask width of masklen bits. If masklen is not specified, it
defaults to
32. When looking up an IP address in a table, the most spe
cific entry
will match. Associated with each entry is a 32-bit unsigned
value, which
can optionally be checked by a rule matching code. When
adding an entry,
if value is not specified, it defaults to 0.
An entry can be added to a table (add), removed from a table
(delete), a
table can be examined (list) or flushed (flush).
Internally, each table is stored in a Radix tree, the same
way as the
routing table (see route(4)).
Lookup tables currently support IPv4 addresses only.
The tablearg feature provides the ability to use a value,
looked up in
the table, as the argument for a rule action. This can sig
nificantly
reduce number of rules in some configurations. The tablearg
argument can
be used with the following actions: pipe, queue, divert,
tee, netgraph,
ngtee. See the EXAMPLES Section for example usage of tables
and the
tablearg keyword.

SETS OF RULES

Each rule belongs to one of 32 different sets , numbered 0
to 31. Set 31
is reserved for the default rule.
By default, rules are put in set 0, unless you use the set N
attribute
when entering a new rule. Sets can be individually and
atomically
enabled or disabled, so this mechanism permits an easy way
to store multiple configurations of the firewall and quickly (and atomi
cally) switch
between them. The command to enable/disable sets is

ipfw set [disable number ...] [enable number ...]
where multiple enable or disable sections can be specified.
Command execution is atomic on all the sets specified in the command.
By default,
all sets are enabled.
When you disable a set, its rules behave as if they do not
exist in the
firewall configuration, with only one exception:

dynamic rules created from a rule before it had been
disabled will
still be active until they expire. In order to delete
dynamic
rules you have to explicitly delete the parent rule
which generated
them.
The set number of rules can be changed with the command

ipfw set move {rule rule-number | old-set} to new-set
Also, you can atomically swap two rulesets with the command

ipfw set swap first-set second-set
See the EXAMPLES Section on some possible uses of sets of
rules.

STATEFUL FIREWALL

Stateful operation is a way for the firewall to dynamically
create rules
for specific flows when packets that match a given pattern
are detected.
Support for stateful operation comes through the
check-state, keep-state
and limit options of rules.
Dynamic rules are created when a packet matches a keep-state
or limit
rule, causing the creation of a dynamic rule which will
match all and
only packets with a given protocol between a src-ip/src-port dst-ip/dst-port pair of addresses (src and dst are used here
only to
denote the initial match addresses, but they are completely
equivalent
afterwards). Dynamic rules will be checked at the first
check-state,
keep-state or limit occurrence, and the action performed up
on a match
will be the same as in the parent rule.
Note that no additional attributes other than protocol and
IP addresses
and ports are checked on dynamic rules.
The typical use of dynamic rules is to keep a closed fire
wall configuration, but let the first TCP SYN packet from the inside net
work install a
dynamic rule for the flow so that packets belonging to that
session will
be allowed through the firewall:

ipfw add check-state
ipfw add allow tcp from my-subnet to any setup keep
state
ipfw add deny tcp from any to any
A similar approach can be used for UDP, where an UDP packet
coming from
the inside will install a dynamic rule to let the response
through the
firewall:

ipfw add check-state
ipfw add allow udp from my-subnet to any keep-state
ipfw add deny udp from any to any
Dynamic rules expire after some time, which depends on the
status of the
flow and the setting of some sysctl variables. See Section
SYSCTL
VARIABLES for more details. For TCP sessions, dynamic rules
can be
instructed to periodically send keepalive packets to refresh
the state of
the rule when it is about to expire.
See Section EXAMPLES for more examples on how to use dynamic
rules.

TRAFFIC SHAPER (DUMMYNET) CONFIGURATION

ipfw is also the user interface for the dummynet(4) traffic
shaper.
dummynet operates by first using the firewall to classify
packets and
divide them into flows, using any match pattern that can be
used in ipfw
rules. Depending on local policies, a flow can contain
packets for a
single TCP connection, or from/to a given host, or entire
subnet, or a
protocol type, etc.
Packets belonging to the same flow are then passed to either
of two different objects, which implement the traffic regulation:

pipe A pipe emulates a link with given bandwidth,
propagation
delay, queue size and packet loss rate. Packets
are queued
in front of the pipe as they come out from the
classifier,
and then transferred to the pipe according to
the pipe's
parameters.
queue A queue is an abstraction used to implement the
WF2Q+ (Worst
case Fair Weighted Fair Queueing) policy, which
is an efficient variant of the WFQ policy.
The queue associates a weight and a reference
pipe to each
flow, and then all backlogged (i.e., with pack
ets queued)
flows linked to the same pipe share the pipe's
bandwidth proportionally to their weights. Note that weights
are not priorities; a flow with a lower weight is still
guaranteed to
get its fraction of the bandwidth even if a flow
with a
higher weight is permanently backlogged.
In practice, pipes can be used to set hard limits to the
bandwidth that a
flow can use, whereas queues can be used to determine how
different flow
share the available bandwidth.
The pipe and queue configuration commands are the following:

pipe number config pipe-configuration
queue number config queue-configuration
The following parameters can be configured for a pipe:
bw bandwidth | device
Bandwidth, measured in [K|M]{bit/s|Byte/s}.
A value of 0 (default) means unlimited bandwidth.
The unit must
immediately follow the number, as in

ipfw pipe 1 config bw 300Kbit/s
If a device name is specified instead of a numeric
value, as in

ipfw pipe 1 config bw tun0
then the transmit clock is supplied by the specified
device. At
the moment only the tun(4) device supports this
functionality,
for use in conjunction with ppp(8).
delay ms-delay
Propagation delay, measured in milliseconds. The
value is
rounded to the next multiple of the clock tick (typ
ically 10ms,
but it is a good practice to run kernels with ``op
tions HZ=1000''
to reduce the granularity to 1ms or less). Default
value is 0,
meaning no delay.
The following parameters can be configured for a queue:
pipe pipe_nr
Connects a queue to the specified pipe. Multiple
queues (with
the same or different weights) can be connected to
the same pipe,
which specifies the aggregate rate for the set of
queues.
weight weight
Specifies the weight to be used for flows matching
this queue.
The weight must be in the range 1..100, and defaults
to 1.
Finally, the following parameters can be configured for both
pipes and
queues:
buckets hash-table-size
Specifies the size of the hash table used for storing
the various
queues. Default value is 64 controlled by the
sysctl(8) variable
net.inet.ip.dummynet.hash_size, allowed range is 16 to
65536.
mask mask-specifier
Packets sent to a given pipe or queue by an ipfw rule
can be further classified into multiple flows, each of which is
then sent to
a different dynamic pipe or queue. A flow identifier
is constructed by masking the IP addresses, ports and proto
col types as
specified with the mask options in the configuration
of the pipe or
queue. For each different flow identifier, a new pipe
or queue is
created with the same parameters as the original ob
ject, and matching packets are sent to it.
Thus, when dynamic pipes are used, each flow will get
the same
bandwidth as defined by the pipe, whereas when dynamic
queues are
used, each flow will share the parent's pipe bandwidth
evenly with
other flows generated by the same queue (note that
other queues
with different weights might be connected to the same
pipe).
Available mask specifiers are a combination of one or
more of the
following:
dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask,
dst-port
mask, src-port mask, flow-id mask, proto mask or all,
where the latter means all bits in all fields are sig
nificant.
noerror
When a packet is dropped by a dummynet queue or pipe,
the error is
normally reported to the caller routine in the kernel,
in the same
way as it happens when a device queue fills up. Set
ting this
option reports the packet as successfully delivered,
which can be
needed for some experimental setups where you want to
simulate loss
or congestion at a remote router.
plr packet-loss-rate
Packet loss rate. Argument packet-loss-rate is a
floating-point
number between 0 and 1, with 0 meaning no loss, 1
meaning 100%
loss. The loss rate is internally represented on 31
bits.
queue {slots | sizeKbytes}
Queue size, in slots or KBytes. Default value is 50
slots, which
is the typical queue size for Ethernet devices. Note
that for slow
speed links you should keep the queue size short or
your traffic
might be affected by a significant queueing delay.
E.g., 50 maxsized ethernet packets (1500 bytes) mean 600Kbit or
20s of queue on
a 30Kbit/s pipe. Even worse effects can result if you
get packets
from an interface with a much larger MTU, e.g. the
loopback interface with its 16KB packets.
red | gred w_q/min_th/max_th/max_p Make use of the RED (Random Early Detection) queue
management algorithm. w_q and max_p are floating point numbers be
tween 0 and 1 (0
not included), while min_th and max_th are integer
numbers specifying thresholds for queue management (thresholds are
computed in
bytes if the queue has been defined in bytes, in slots
otherwise).
The dummynet(4) also supports the gentle RED variant
(gred). Three
sysctl(8) variables can be used to control the RED be
haviour:
net.inet.ip.dummynet.red_lookup_depth specifies the accuracy in computing the aver
age queue when
the link is idle (defaults to 256, must be
greater than
zero)
net.inet.ip.dummynet.red_avg_pkt_size specifies the expected average packet size
(defaults to
512, must be greater than zero)
net.inet.ip.dummynet.red_max_pkt_size specifies the expected maximum packet size,
only used when
queue thresholds are in bytes (defaults to
1500, must be
greater than zero).
When used with IPv6 data, dummynet currently has several
limitations.
First, debug.mpsafenet=0 must be set. Second, the informa
tion necessicary to route link-local packets to an interface is not
avalable after
processing by dummynet so those packets are dropped in the
output path.
Care should be taken to insure that link-local packets are
not passed to
dummynet.

CHECKLIST

Here are some important points to consider when designing
your rules:
+o Remember that you filter both packets going in and out.
Most connec
tions need packets going in both directions.
+o Remember to test very carefully. It is a good idea to
be near the
console when doing this. If you cannot be near the con
sole, use an
auto-recovery script such as the one in
/usr/share/examples/ipfw/change_rules.sh.
+o Do not forget the loopback interface.

FINE POINTS

+o There are circumstances where fragmented datagrams are
uncondition
ally dropped. TCP packets are dropped if they do not
contain at
least 20 bytes of TCP header, UDP packets are dropped if
they do not
contain a full 8 byte UDP header, and ICMP packets are
dropped if
they do not contain 4 bytes of ICMP header, enough to
specify the
ICMP type, code, and checksum. These packets are simply
logged as
``pullup failed'' since there may not be enough good da
ta in the
packet to produce a meaningful log entry.
+o Another type of packet is unconditionally dropped, a TCP
packet with
a fragment offset of one. This is a valid packet, but
it only has
one use, to try to circumvent firewalls. When logging
is enabled,
these packets are reported as being dropped by rule -1.
+o If you are logged in over a network, loading the kld(4)
version of
ipfw is probably not as straightforward as you would
think. I recommend the following command line:

kldload ipfw && ipfw add 32000 al
low ip from any to any
Along the same lines, doing an

ipfw flush
in similar surroundings is also a bad idea.
+o The ipfw filter list may not be modified if the system
security level
is set to 3 or higher (see init(8) for information on
system security
levels).

PACKET DIVERSION

A divert(4) socket bound to the specified port will receive
all packets
diverted to that port. If no socket is bound to the desti
nation port, or
if the divert module is not loaded, or if the kernel was not
compiled
with divert socket support, the packets are dropped.

SYSCTL VARIABLES

A set of sysctl(8) variables controls the behaviour of the
firewall and
associated modules (dummynet, bridge). These are shown be
low together
with their default value (but always check with the
sysctl(8) command
what value is actually in use) and meaning:
net.inet.ip.dummynet.expire: 1 Lazily delete dynamic pipes/queue once they have no
pending traffic. You can disable this by setting the variable
to 0, in which
case the pipes/queues will only be deleted when the
threshold is
reached.
net.inet.ip.dummynet.hash_size: 64 Default size of the hash table used for dynamic
pipes/queues.
This value is used when no buckets option is speci
fied when configuring a pipe/queue.
net.inet.ip.dummynet.max_chain_len: 16 Target value for the maximum number of pipes/queues
in a hash
bucket. The product max_chain_len*hash_size is used
to determine
the threshold over which empty pipes/queues will be
expired even
when net.inet.ip.dummynet.expire=0.
net.inet.ip.dummynet.red_lookup_depth: 256
net.inet.ip.dummynet.red_avg_pkt_size: 512
net.inet.ip.dummynet.red_max_pkt_size: 1500 Parameters used in the computations of the drop
probability for
the RED algorithm.
net.inet.ip.fw.autoinc_step: 100 Delta between rule numbers when auto-generating
them. The value
must be in the range 1..1000.
net.inet.ip.fw.curr_dyn_buckets: net.inet.ip.fw.dyn_buckets The current number of buckets in the hash table for
dynamic rules
(readonly).
net.inet.ip.fw.debug: 1
Controls debugging messages produced by ipfw.
net.inet.ip.fw.dyn_buckets: 256 The number of buckets in the hash table for dynamic
rules. Must
be a power of 2, up to 65536. It only takes effect
when all
dynamic rules have expired, so you are advised to
use a flush
command to make sure that the hash table is resized.
net.inet.ip.fw.dyn_count: 3 Current number of dynamic rules (read-only).
net.inet.ip.fw.dyn_keepalive: 1 Enables generation of keepalive packets for
keep-state rules on
TCP sessions. A keepalive is generated to both
sides of the connection every 5 seconds for the last 20 seconds of
the lifetime
of the rule.
net.inet.ip.fw.dyn_max: 8192 Maximum number of dynamic rules. When you hit this
limit, no
more dynamic rules can be installed until old ones
expire.
net.inet.ip.fw.dyn_ack_lifetime: 300
net.inet.ip.fw.dyn_syn_lifetime: 20
net.inet.ip.fw.dyn_fin_lifetime: 1
net.inet.ip.fw.dyn_rst_lifetime: 1
net.inet.ip.fw.dyn_udp_lifetime: 5
net.inet.ip.fw.dyn_short_lifetime: 30 These variables control the lifetime, in seconds, of
dynamic
rules. Upon the initial SYN exchange the lifetime
is kept short,
then increased after both SYN have been seen, then
decreased
again during the final FIN exchange or when a RST is
received.
Both dyn_fin_lifetime and dyn_rst_lifetime must be
strictly lower
than 5 seconds, the period of repetition of
keepalives. The
firewall enforces that.
net.inet.ip.fw.enable: 1
Enables the firewall. Setting this variable to 0
lets you run
your machine without firewall even if compiled in.
net.inet.ip.fw.one_pass: 1 When set, the packet exiting from the dummynet(4)
pipe or from
ng_ipfw(4) node is not passed though the firewall
again. Otherwise, after an action, the packet is reinjected into
the firewall
at the next rule.
net.inet.ip.fw.verbose: 1
Enables verbose messages.
net.inet.ip.fw.verbose_limit: 0 Limits the number of messages produced by a verbose
firewall.
net.inet6.ip6.fw.deny_unknown_exthdrs: 1 If enabled packets with unknown IPv6 Extension Head
ers will be
denied.
net.link.ether.ipfw: 0
Controls whether layer-2 packets are passed to ipfw.
Default is
no.
net.link.ether.bridge_ipfw: 0 Controls whether bridged packets are passed to ipfw.
Default is
no.

EXAMPLES

There are far too many possible uses of ipfw so this Section
will only
give a small set of examples.
BASIC PACKET FILTERING
This command adds an entry which denies all tcp packets from
cracker.evil.org to the telnet port of wolf.tambov.su from
being forwarded by the host:

ipfw add deny tcp from cracker.evil.org to wolf.tam
bov.su telnet
This one disallows any connection from the entire cracker's
network to my
host:

ipfw add deny ip from 123.45.67.0/24 to my.host.org
A first and efficient way to limit access (not using dynamic
rules) is
the use of the following rules:

ipfw add allow tcp from any to any established
ipfw add allow tcp from net1 portlist1 to net2
portlist2 setup
ipfw add allow tcp from net3 portlist3 to net3
portlist3 setup
...
ipfw add deny tcp from any to any
The first rule will be a quick match for normal TCP packets,
but it will
not match the initial SYN packet, which will be matched by
the setup
rules only for selected source/destination pairs. All other
SYN packets
will be rejected by the final deny rule.
If you administer one or more subnets, you can take advan
tage of the
address sets and or-blocks and write extremely compact rule
sets which
selectively enable services to blocks of clients, as below:

goodguys="{ 10.1.2.0/24{20,35,66,18} or
10.2.3.0/28{6,3,11} }"
badguys="10.1.2.0/24{8,38,60}"
ipfw add allow ip from ${goodguys} to any
ipfw add deny ip from ${badguys} to any
... normal policies ...
The verrevpath option could be used to do automated anti
spoofing by
adding the following to the top of a ruleset:

ipfw add deny ip from any to any not verrevpath in
This rule drops all incoming packets that appear to be com
ing to the system on the wrong interface. For example, a packet with a
source address
belonging to a host on a protected internal network would be
dropped if
it tried to enter the system from an external interface.
The antispoof option could be used to do similar but more
restricted
anti-spoofing by adding the following to the top of a rule
set:

ipfw add deny ip from any to any not antispoof in
This rule drops all incoming packets that appear to be com
ing from
another directly connected system but on the wrong inter
face. For example, a packet with a source address of 192.168.0.0/24 , con
figured on
fxp0 , but coming in on fxp1 would be dropped.
DYNAMIC RULES
In order to protect a site from flood attacks involving fake
TCP packets,
it is safer to use dynamic rules:

ipfw add check-state
ipfw add deny tcp from any to any established
ipfw add allow tcp from my-net to any setup keep-state
This will let the firewall install dynamic rules only for
those connection which start with a regular SYN packet coming from the
inside of our
network. Dynamic rules are checked when encountering the
first
check-state or keep-state rule. A check-state rule should
usually be
placed near the beginning of the ruleset to minimize the
amount of work
scanning the ruleset. Your mileage may vary.
To limit the number of connections a user can open you can
use the following type of rules:

ipfw add allow tcp from my-net/24 to any setup limit
src-addr 10
ipfw add allow tcp from any to me setup limit src-addr
4
The former (assuming it runs on a gateway) will allow each
host on a /24
network to open at most 10 TCP connections. The latter can
be placed on
a server to make sure that a single client does not use more
than 4
simultaneous connections.
BEWARE: stateful rules can be subject to denial-of-service
attacks by a
SYN-flood which opens a huge number of dynamic rules. The
effects of
such attacks can be partially limited by acting on a set of
sysctl(8)
variables which control the operation of the firewall.
Here is a good usage of the list command to see accounting
records and
timestamp information:

ipfw -at list
or in short form without timestamps:

ipfw -a list
which is equivalent to:

ipfw show
Next rule diverts all incoming packets from 192.168.2.0/24
to divert port
5000:

ipfw divert 5000 ip from 192.168.2.0/24 to any in
TRAFFIC SHAPING
The following rules show some of the applications of ipfw
and dummynet(4)
for simulations and the like.
This rule drops random incoming packets with a probability
of 5%:

ipfw add prob 0.05 deny ip from any to any in
A similar effect can be achieved making use of dummynet
pipes:

ipfw add pipe 10 ip from any to any
ipfw pipe 10 config plr 0.05
We can use pipes to artificially limit bandwidth, e.g. on a
machine acting as a router, if we want to limit traffic from local
clients on
192.168.2.0/24 we do:

ipfw add pipe 1 ip from 192.168.2.0/24 to any out
ipfw pipe 1 config bw 300Kbit/s queue 50KBytes
note that we use the out modifier so that the rule is not
used twice.
Remember in fact that ipfw rules are checked both on incom
ing and outgoing packets.
Should we want to simulate a bidirectional link with band
width limitations, the correct way is the following:

ipfw add pipe 1 ip from any to any out
ipfw add pipe 2 ip from any to any in
ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes
The above can be very useful, e.g. if you want to see how
your fancy Web
page will look for a residential user who is connected only
through a
slow link. You should not use only one pipe for both direc
tions, unless
you want to simulate a half-duplex medium (e.g. AppleTalk,
Ethernet,
IRDA). It is not necessary that both pipes have the same
configuration,
so we can also simulate asymmetric links.
Should we want to verify network performance with the RED
queue management algorithm:

ipfw add pipe 1 ip from any to any
ipfw pipe 1 config bw 500Kbit/s queue 100 red
0.002/30/80/0.1
Another typical application of the traffic shaper is to in
troduce some
delay in the communication. This can significantly affect
applications
which do a lot of Remote Procedure Calls, and where the
round-trip-time
of the connection often becomes a limiting factor much more
than bandwidth:

ipfw add pipe 1 ip from any to any out
ipfw add pipe 2 ip from any to any in
ipfw pipe 1 config delay 250ms bw 1Mbit/s
ipfw pipe 2 config delay 250ms bw 1Mbit/s
Per-flow queueing can be useful for a variety of purposes.
A very simple
one is counting traffic:

ipfw add pipe 1 tcp from any to any
ipfw add pipe 1 udp from any to any
ipfw add pipe 1 ip from any to any
ipfw pipe 1 config mask all
The above set of rules will create queues (and collect
statistics) for
all traffic. Because the pipes have no limitations, the on
ly effect is
collecting statistics. Note that we need 3 rules, not just
the last one,
because when ipfw tries to match IP packets it will not con
sider ports,
so we would not see connections on separate ports as differ
ent ones.
A more sophisticated example is limiting the outbound traf
fic on a net
with per-host limits, rather than per-network limits:

ipfw add pipe 1 ip from 192.168.2.0/24 to any out
ipfw add pipe 2 ip from any to 192.168.2.0/24 in
ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s
queue
20Kbytes
ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s
queue
20Kbytes
LOOKUP TABLES
In the following example, we need to create several traffic
bandwidth
classes and we need different hosts/networks to fall into
different
classes. We create one pipe for each class and configure
them accordingly. Then we create a single table and fill it with IP
subnets and
addresses. For each subnet/host we set the argument equal
to the number
of the pipe that it should use. Then we classify traffic
using a single
rule:

ipfw pipe 1 config bw 1000Kbyte/s
ipfw pipe 4 config bw 4000Kbyte/s
...
ipfw table 1 add 192.168.2.0/24 1
ipfw table 1 add 192.168.0.0/27 4
ipfw table 1 add 192.168.0.2 1
...
ipfw pipe tablearg ip from table(1) to any
SETS OF RULES
To add a set of rules atomically, e.g. set 18:

ipfw set disable 18
ipfw add NN set 18 ... # repeat as needed
ipfw set enable 18
To delete a set of rules atomically the command is simply:

ipfw delete set 18
To test a ruleset and disable it and regain control if some
thing goes
wrong:

ipfw set disable 18
ipfw add NN set 18 ... # repeat as needed
ipfw set enable 18; echo done; sleep 30 && ipfw set
disable 18
Here if everything goes well, you press control-C before the
"sleep" terminates, and your ruleset will be left active. Otherwise,
e.g. if you
cannot access your box, the ruleset will be disabled after
the sleep terminates thus restoring the previous situation.

SEE ALSO

cpp(1), m4(1), altq(4), bridge(4), divert(4), dummynet(4),
ip(4),
ipfirewall(4), ng_ipfw(4), protocols(5), services(5),
init(8),
kldload(8), reboot(8), sysctl(8), syslogd(8)

HISTORY

The ipfw utility first appeared in FreeBSD 2.0. dummynet(4)
was introduced in FreeBSD 2.2.8. Stateful extensions were introduced
in
FreeBSD 4.0. ipfw2 was introduced in Summer 2002.

AUTHORS

Ugen J. S. Antsilevich,
Poul-Henning Kamp,
Alex Nash,
Archie Cobbs,
Luigi Rizzo.

API based upon code written by Daniel Boulet for BSDI.

Work on dummynet(4) traffic shaper supported by Akamba Corp.

BUGS

Use of dummynet with IPv6 requires that debug.mpsafenet be
set to 0.
The syntax has grown over the years and sometimes it might
be confusing.
Unfortunately, backward compatibility prevents cleaning up
mistakes made
in the definition of the syntax.
!!! WARNING !!!
Misconfiguring the firewall can put your computer in an un
usable state,
possibly shutting down network services and requiring con
sole access to
regain control of it.
Incoming packet fragments diverted by divert are reassembled
before
delivery to the socket. The action used on those packet is
the one from
the rule which matches the first fragment of the packet.
Packets diverted to userland, and then reinserted by a user
land process
may lose various packet attributes. The packet source in
terface name
will be preserved if it is shorter than 8 bytes and the
userland process
saves and reuses the sockaddr_in (as does natd(8)); other
wise, it may be
lost. If a packet is reinserted in this manner, later rules
may be
incorrectly applied, making the order of divert rules in the
rule
sequence very important.
Dummynet drops all packets with IPv6 link-local addresses.
Rules using uid or gid may not behave as expected. In par
ticular, incoming SYN packets may have no uid or gid associated with them
since they do
not yet belong to a TCP connection, and the uid/gid associ
ated with a
packet may not be as expected if the associated process
calls setuid(2)
or similar system calls.
Rules which use uid, gid or jail based matching should be
used only if
debug.mpsafenet=0 to avoid possible deadlocks due to layer
ing violations
in its implementation.
BSD January 16, 2006

BSD

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