ipfw.8   [plain text]

.\" $FreeBSD: /repoman/r/ncvs/src/sbin/ipfw/ipfw.8,v 2003/07/28 07:15:13 luigi Exp $
.Dd August 13, 2002
.Dt IPFW 8
.Os Darwin
.Nm ipfw
.Nd IP firewall and traffic shaper control program
.Op Fl cq
.Cm add
.Ar rule
.Op Fl acdefnNStT
.Brq Cm list | show
.Op Ar rule | first-last ...
.Op Fl f | q
.Cm flush
.Op Fl q
.Brq Cm delete | zero | resetlog
.Op Cm set
.Op Ar number ...
.Cm enable
.Brq Cm firewall | one_pass | debug | verbose | dyn_keepalive
.Cm disable
.Brq Cm firewall | one_pass | debug | verbose | dyn_keepalive
.Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
.Cm set move
.Op Cm rule
.Ar number Cm to Ar number
.Cm set swap Ar number number
.Cm set show
.Brq Cm pipe | queue
.Ar number
.Cm config
.Ar config-options
.Op Fl s Op Ar field
.Brq Cm pipe | queue
.Brq Cm delete | list | show
.Op Ar number ...
.Op Fl cnNqS
.Fl p Ar preproc
.Ar preproc-flags
.Ar pathname
utility is the user interface for controlling the
.Xr ipfw 4
firewall and the
.Xr dummynet 4
traffic shaper in
.Fx .
configuration, or
.Em ruleset ,
is made of a list of
.Em rules
numbered from 1 to 65535.
Packets are passed to
from a number of different places in the protocol stack
(depending on the source and destination of the packet,
it is possible that
is invoked multiple times on the same packet).
The packet passed to the firewall is compared
against each of the rules in the firewall
.Em 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.
ruleset always includes a
.Em default
rule (numbered 65535) which cannot be modified or deleted,
and matches all packets.
The action associated with the
.Em default
rule can be either
.Cm deny
.Cm allow
depending on how the kernel is configured.
If the ruleset includes one or more rules with the
.Cm keep-state
.Cm limit
option, then
assumes a
.Em stateful
behaviour, i.e. upon a match it will create dynamic rules matching
the exact parameters (addresses and ports) of the matching packet.
These dynamic rules, which have a limited lifetime, are checked
at the first occurrence of a
.Cm check-state ,
.Cm keep-state
.Cm limit
rule, and are typically used to open the firewall on-demand to
legitimate traffic only.
See the
Sections below for more information on the stateful behaviour of
.Nm .
All rules (including dynamic ones) have a few associated counters:
a packet count, a byte count, a log count and a timestamp
indicating the time of the last match.
Counters can be displayed or reset with
Rules can be added with the
.Cm add
command; deleted individually or in groups with the
.Cm delete
command, and globally (except those in set 31) with the
.Cm flush
command; displayed, optionally with the content of the
counters, using the
.Cm show
.Cm list
Finally, counters can be reset with the
.Cm zero
.Cm resetlog
Also, each rule belongs to one of 32 different
.Em sets
, and there are
commands to atomically manipulate sets, such as enable,
disable, 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
for more information on
.Em sets .
The following options are available:
.Bl -tag -width indent
.It Fl a
While listing, show counter values.
.Cm show
command just implies this option.
.It Fl c
When entering or showing rules, print them in compact form,
i.e. without the optional "ip from any to any" string
when this does not carry any additional information.
.It Fl d
While listing, show dynamic rules in addition to static ones.
.It Fl e
While listing, if the
.Fl d
option was specified, also show expired dynamic rules.
.It Fl f
Don't ask for confirmation for commands that can cause problems
if misused,
.No i.e. Cm flush .
If there is no tty associated with the process, this is implied.
.It Fl n
Only check syntax of the command strings, without actually passing
them to the kernel.
.It Fl N
Try to resolve addresses and service names in output.
.It Fl q
.Cm add Ns ing ,
.Cm zero Ns ing ,
.Cm resetlog Ns ging
.Cm flush Ns ing ,
be quiet about actions
.Fl f ) .
This is useful for adjusting rules by executing multiple
commands in a script
.Ql sh\ /etc/rc.firewall ) ,
or by processing a file of many
rules across a remote login session.
If a
.Cm flush
is performed in normal (verbose) mode (with the default kernel
configuration), 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.
.It Fl S
While listing rules, show the
.Em set
each rule belongs to.
If this flag is not specified, disabled rules will not be
.It Fl s Op Ar field
While listing pipes, sort according to one of the four
counters (total or current packets or bytes).
.It Fl t
While listing, show last match timestamp (converted with ctime()).
.It Fl T
While listing, show last match timestamp (as seconds from the epoch).
This form can be more convenient for postprocessing by scripts.
To ease configuration, rules can be put into a file which is
processed using
as shown in the last synopsis line.
An absolute
.Ar pathname
must be used.
The file will be read line by line and applied as arguments to the
Optionally, a preprocessor can be specified using
.Fl p Ar preproc
.Ar pathname
is to be piped through.
Useful preprocessors include
.Xr cpp 1
.Xr m4 1 .
.Ar preproc
doesn't start with a slash
.Pq Ql /
as its first character, the usual
name search is performed.
Care should be taken with this in environments where not all
file systems are mounted (yet) by the time
is being run (e.g. when they are mounted over NFS).
.Fl p
has been specified, any additional arguments as passed on to the preprocessor
for interpretation.
This allows for flexible configuration files (like conditionalizing
them on the local hostname) and the use of macros to centralize
frequently required arguments like IP addresses.
.Cm pipe
.Cm queue
commands are used to configure the traffic shaper, as shown in the
Section below.
If the world and the kernel get out of sync the
ABI may break, preventing you from being able to add any rules.  This can
adversely effect the booting process.  You can use
.Cm disable
.Cm firewall
to temporarily disable the firewall to regain access to the network,
allowing you to fix the problem.
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.
.Bd -literal -offset indent
      ^	    to upper layers   V
      |                       |
      ^                       V
 [ip_input]              [ip_output]   net.inet.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 inspection.
E.g., incoming packets will include the MAC header when
is invoked from
.Cm ether_demux() ,
but the same packets will have the MAC header stripped off when
is invoked from
.Cm ip_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 actions which are not valid
for the place of invocation (e.g. trying to match a MAC header within
.Cm ip_input()
), the match pattern will not match, but a
.Cm not
operator in front of such patterns
.Em will
cause the pattern to
.Em 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.
.Cm skipto
rules can be useful here, as an example:
.Bd -literal -offset indent
# 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).
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).
.Nm 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:
.Bd -literal -offset indent
ipfw -q add deny src-ip,
ipfw -q add deny src-ip,
ipfw "-q add deny src-ip,"
The format of
rules is the following:
.Bd -ragged -offset indent
.Op Ar rule_number
.Op Cm set Ar set_number
.Op Cm prob Ar match_probability
.Ar "   " action
.Op Cm log Op Cm logamount Ar number
.Ar body
where the body of the rule specifies which information is used
for filtering packets, among the following:
.Bl -tag -width "Source and dest. addresses and ports" -offset XXX -compact
.It Layer-2 header fields
When available
.It IPv4 Protocol
TCP, UDP, ICMP, etc.
.It Source and dest. addresses and ports
.It Direction
See Section
.It Transmit and receive interface
By name or address
.It Misc. IP header fields
Version, type of service, datagram length, identification,
fragment flag (non-zero IP offset),
Time To Live
.It IP options
.It Misc. TCP header fields
TCP flags (SYN, FIN, ACK, RST, etc.),
sequence number, acknowledgment number,
.It TCP options
.It ICMP types
for ICMP packets
.It User/group ID
When the packet can be associated with a local socket.
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.
.Bl -tag -width indent
.It Ar rule_number
Each rule is associated with a
.Ar rule_number
in the range 1..65535, with the latter reserved for the
.Em default
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
.Em default
Automatic rule numbers are assigned by incrementing the last
non-default rule number by the value of the sysctl variable
.Ar 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.
.It Cm set Ar set_number
Each rule is associated with a
.Ar set_number
in the range 0..31.
Sets can be individually disabled and enabled, so this parameter
is of fundamental importance for atomic ruleset manipulation.
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
.Nm ipfw flush
command (but you can delete them with the
.Nm ipfw delete set 31
Set 31 is also used for the
.Em default
.It Cm prob Ar match_probability
A match is only declared with the specified probability
(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
.Xr 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.
.It Cm log Op Cm logamount Ar number
When a packet matches a rule with the
.Cm log
keyword, a message will be
logged to
.Xr syslogd 8
with a
The logging only occurs if the sysctl variable
.Em net.inet.ip.fw.verbose
is set to 1
(which is the default when the kernel is compiled with
) and the number of packets logged so far for that
particular rule does not exceed the
.Cm logamount
If no
.Cm logamount
is specified, the limit is taken from the sysctl variable
.Em net.inet.ip.fw.verbose_limit .
In both cases, 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
.Cm resetlog
Note: logging is done after all other packet matching conditions
have been successfully verified, and before performing the final
action (accept, deny, etc.) on the packet.
A rule can be associated with one of the following actions, which
will be executed when the packet matches the body of the rule.
.Bl -tag -width indent
.It Cm allow | accept | pass | permit
Allow packets that match rule.
The search terminates.
.It Cm 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.
.Cm Check-state
rules do not have a body.
If no
.Cm check-state
rule is found, the dynamic ruleset is checked at the first
.Cm keep-state
.Cm limit
.It Cm count
Update counters for all packets that match rule.
The search continues with the next rule.
.It Cm deny | drop
Discard packets that match this rule.
The search terminates.
.It Cm divert Ar port
Divert packets that match this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search terminates.
.It Cm fwd | forward Ar ipaddr Ns Op , Ns Ar port
Change the next-hop on matching packets to
.Ar ipaddr ,
which can be an IP address in dotted quad format or a host name.
The search terminates if this rule matches.
.Ar ipaddr
is a local address, then matching packets will be forwarded to
.Ar port
(or the port number in the packet if one is not specified in the rule)
on the local machine.
.Ar ipaddr
is not a local address, then the port number
(if specified) is ignored, and the packet will be
forwarded to the remote address, using the route as found in
the local routing table for that IP.
.Ar fwd
rule will not match layer-2 packets (those received
on ether_input, ether_output, or bridged).
.Cm fwd
action does not change the contents of the packet at all.
In particular, the destination address remains unmodified, 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 destination address of the packet.
This makes the
.Xr netstat 1
entry look rather weird but is intended for
use with transparent proxy servers.
.It Cm pipe Ar pipe_nr
Pass packet to a
.Xr dummynet 4
.Dq pipe
(for bandwidth limitation, delay, etc.).
See the
Section for further information.
The search terminates; however, on exit from the pipe and if
.Xr sysctl 8
.Em net.inet.ip.fw.one_pass
is not set, the packet is passed again to the firewall code
starting from the next rule.
.It Cm queue Ar queue_nr
Pass packet to a
.Xr dummynet 4
.Dq queue
(for bandwidth limitation using WF2Q+).
.It Cm reject
Synonym for
.Cm unreach host .
.It Cm 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.
.It Cm skipto Ar number
Skip all subsequent rules numbered less than
.Ar number .
The search continues with the first rule numbered
.Ar number
or higher.
.It Cm tee Ar port
Send a copy of packets matching this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search terminates and the original packet is accepted
(but see Section
.It Cm unreach Ar code
Discard packets that match this rule, and try to send an ICMP
unreachable notice with code
.Ar code ,
.Ar code
is a number from 0 to 255, or one of these aliases:
.Cm net , host , protocol , port ,
.Cm needfrag , srcfail , net-unknown , host-unknown ,
.Cm isolated , net-prohib , host-prohib , tosnet ,
.Cm toshost , filter-prohib , host-precedence
.Cm precedence-cutoff .
The search terminates.
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 (implicit)
.Cm and
operators -- i.e. all must match in order for the
rule to match.
Individual patterns can be prefixed by the
.Cm not
operator to reverse the result of the match, as in
.Dl "ipfw add 100 allow ip from not to any"
Additionally, sets of alternative match patterns (
.Em or-blocks
) can be constructed by putting the patterns in
lists enclosed between parentheses ( ) or braces { }, and
using the
.Cm or
operator as follows:
.Dl "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 advisable to put a backslash \\ in front of them
to prevent such interpretations.
The body of a rule must in general include a source and destination
address specifier.
The keyword
.Ar 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:
.Bd -ragged -offset indent
.Op Ar proto Cm from Ar src Cm to Ar dst
.Op Ar options
The first part (proto from src to dst) is for backward
compatibility with
.Nm ipfw1 .
.Nm ipfw2
any match pattern (including MAC headers, IPv4 protocols,
addresses and ports) can be specified in the
.Ar options
Rule fields have the following meaning:
.Bl -tag -width indent
.It Ar proto : protocol | Cm { Ar protocol Cm or ... }
.It Ar protocol : Oo Cm not Oc Ar protocol-name | protocol-number
An IPv4 protocol specified by number or name
(for a complete list see
.Pa /etc/protocols ) .
.Cm ip
.Cm all
keywords mean any protocol will match.
.Cm { Ar protocol Cm or ... }
format (an
.Em or-block )
is provided for convenience only but its use is deprecated.
.It Ar src No and Ar dst : Bro Cm addr | Cm { Ar addr Cm or ... } Brc Op Oo Cm not Oc Ar ports
An address (or a list, see below)
optionally followed by
.Ar ports
The second format (
.Em or-block
with multiple addresses) is provided for convenience only and
its use is discouraged.
.It Ar addr : Oo Cm not Oc Brq Cm any | me | Ar addr-list | Ar addr-set
.It Cm any
matches any IP address.
.It Cm me
matches any IP address configured on an interface in the system.
The address list is evaluated at the time the packet is
.It Ar addr-list : ip-addr Ns Op Ns , Ns Ar addr-list
.It Ar ip-addr :
A host or subnet address specified in one of the following ways:
.Bl -tag -width indent
.It Ar 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.
.It Ar addr Ns / Ns Ar masklen
Matches all addresses with base
.Ar addr
(specified as a dotted quad or a hostname)
and mask width of
.Cm masklen
As an example, will match
all IP numbers from to .
.It Ar addr Ns : Ns Ar mask
Matches all addresses with base
.Ar addr
(specified as a dotted quad or a hostname)
and the mask of
.Ar mask ,
specified as a dotted quad.
As an example, will match
We suggest to use this form only for non-contiguous
masks, and resort to the
.Ar addr Ns / Ns Ar masklen
format for contiguous masks, which is more compact and less
.It Ar addr-set : addr Ns Oo Ns / Ns Ar masklen Oc Ns Cm { Ns Ar list Ns Cm }
.It Ar list : Bro Ar num | num-num Brc Ns Op Ns , Ns Ar list
Matches all addresses with base address
.Ar addr
(specified as a dotted quad or a hostname)
and whose last byte is in the list between braces { } .
Note that there must be no spaces between braces and
numbers (spaces after commas are allowed).
Elements of the list can be specified as single entries
or ranges.
.Ar masklen
field is used to limit the size of the set of addresses,
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 reduces
the complexity of rulesets.
As an example, an address specified as{128,35-55,89}
will match the following IP addresses:
.br, to, .
.It Ar ports : Bro Ar port | port Ns \&- Ns Ar port Ns Brc Ns Op , Ns Ar ports
For protocols which support port numbers (such as TCP and UDP), optional
.Cm ports
may be specified as one or more ports or port ranges, separated
by commas but no spaces, and an optional
.Cm not
.Ql \&-
notation specifies a range of ports (including boundaries).
Service names (from
.Pa /etc/services )
may be used instead 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
.Em or-block
in the
.Cm options
section of the rule.
A backslash
.Pq Ql \e
can be used to escape the dash
.Pq Ql -
character in a service name (from a shell, the backslash must be
typed twice to avoid the shell itself interpreting it as an escape
.Dl "ipfw add count tcp from any ftp\e\e-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
See the
.Cm frag
option for details on matching fragmented packets.
Additional match patterns can be used within
rules. Zero or more of these so-called
.Em options
can be present in a rule, optionally prefixed by the
.Cm not
operand, and possibly grouped into
.Em or-blocks .
The following match patterns can be used (listed in alphabetical order):
.Bl -tag -width indent
.It Cm // 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
.Cm count
action followed by the comment.
.It Cm bridged
Matches only bridged packets.
.It Cm dst-ip Ar ip-address
Matches IP packets whose destination IP is one of the address(es)
specified as argument.
.It Cm dst-port Ar ports
Matches IP packets whose destination port is one of the port(s)
specified as argument.
.It Cm established
Matches TCP packets that have the RST or ACK bits set.
.It Cm 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.
.It Cm gid Ar group
Matches all TCP or UDP packets sent by or received for a
.Ar group .
.Ar group
may be specified by name or number.
.It Cm icmptypes Ar types
Matches ICMP packets whose ICMP type is in the list
.Ar types .
The list may be specified as any combination of
individual types (numeric) separated by commas.
.Em Ranges are not allowed.
The supported ICMP types are:
echo reply
.Pq Cm 0 ,
destination unreachable
.Pq Cm 3 ,
source quench
.Pq Cm 4 ,
.Pq Cm 5 ,
echo request
.Pq Cm 8 ,
router advertisement
.Pq Cm 9 ,
router solicitation
.Pq Cm 10 ,
time-to-live exceeded
.Pq Cm 11 ,
IP header bad
.Pq Cm 12 ,
timestamp request
.Pq Cm 13 ,
timestamp reply
.Pq Cm 14 ,
information request
.Pq Cm 15 ,
information reply
.Pq Cm 16 ,
address mask request
.Pq Cm 17
and address mask reply
.Pq Cm 18 .
.It Cm in | out
Matches incoming or outgoing packets, respectively.
.Cm in
.Cm out
are mutually exclusive (in fact,
.Cm out
is implemented as
.Cm not in Ns No ).
.It Cm ipid Ar id-list
Matches IP packets whose
.Cm ip_id
field has value included in
.Ar id-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm iplen Ar len-list
Matches IP packets whose total length, including header and data, is
in the set
.Ar len-list ,
which is either a single value or a list of values or ranges 
specified in the same way as
.Ar ports .
.It Cm ipoptions Ar spec
Matches packets whose IP header contains the comma separated list of
options specified in
.Ar spec .
The supported IP options are:
.Cm ssrr
(strict source route),
.Cm lsrr
(loose source route),
.Cm rr
(record packet route) and
.Cm ts
The absence of a particular option may be denoted
with a
.Ql \&! .
.It Cm ipprecedence Ar precedence
Matches IP packets whose precedence field is equal to
.Ar precedence .
.It Cm 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
.Cm ipsec
is different from specifying
.Cm proto Ar 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.
.It Cm iptos Ar spec
Matches IP packets whose
.Cm tos
field contains the comma separated list of
service types specified in
.Ar spec .
The supported IP types of service are:
.Cm lowdelay
.Cm throughput
.Cm reliability
.Cm mincost
.Cm congestion
The absence of a particular type may be denoted
with a
.Ql \&! .
.It Cm ipttl Ar ttl-list
Matches IP packets whose time to live is included in
.Ar ttl-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm ipversion Ar ver
Matches IP packets whose IP version field is
.Ar ver .
.It Cm 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 protocol.
The rule has a limited lifetime (controlled by a set of
.Xr sysctl 8
variables), and the lifetime is refreshed every time a matching
packet is found.
.It Cm layer2
Matches only layer2 packets, i.e. those passed to
from ether_demux() and ether_output_frame().
.It Cm limit Bro Cm src-addr | src-port | dst-addr | dst-port Brc Ar N
The firewall will only allow
.Ar 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
.It Cm { MAC | mac } Ar dst-mac src-mac
Match packets with a given
.Ar dst-mac
.Ar src-mac
addresses, specified as the
.Cm any
keyword (matching any MAC address), or six groups of hex digits
separated by colons,
and optionally followed by a mask indicating how many bits are
significant, as in
.Dl "MAC 10:20:30:40:50:60/33 any"
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.
.It Cm mac-type Ar mac-type
Matches packets whose Ethernet Type field
corresponds to one of those specified as argument.
.Ar mac-type
is specified in the same way as
.Cm port numbers
(i.e. one or more comma-separated single values or ranges).
You can use symbolic names for known values such as
.Em vlan , ipv4, ipv6 .
Values can be entered as decimal or hexadecimal (if prefixed by 0x),
and they are always printed as hexadecimal (unless the
.Cm -N
option is used, in which case symbolic resolution will be attempted).
.It Cm proto Ar protocol
Matches packets with the corresponding IPv4 protocol.
.It Cm recv | xmit | via Brq Ar ifX | Ar if Ns Cm * | Ar ipno | Ar any
Matches packets received, transmitted or going through,
respectively, the interface specified by exact name
.Ns No ( Ar ifX Ns No ),
by device name
.Ns No ( Ar if Ns Ar * Ns No ),
by IP address, or through some interface.
.Cm via
keyword causes the interface to always be checked.
.Cm recv
.Cm xmit
is used instead of
.Cm 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.:
.Dl "ipfw add deny ip from any to any out recv ed0 xmit ed1"
.Cm recv
interface can be tested on either incoming or outgoing packets,
while the
.Cm xmit
interface can only be tested on outgoing packets.
.Cm out
is required (and
.Cm in
is invalid) whenever
.Cm xmit
is used.
A packet may not have a receive or transmit interface: packets
originating from the local host have no receive interface,
while packets destined for the local host have no transmit
.It Cm setup
Matches TCP packets that have the SYN bit set but no ACK bit.
This is the short form of
.Dq Li tcpflags\ syn,!ack .
.It Cm src-ip Ar ip-address
Matches IP packets whose source IP is one of the address(es)
specified as argument.
.It Cm src-port Ar ports
Matches IP packets whose source port is one of the port(s)
specified as argument.
.It Cm tcpack Ar ack
TCP packets only.
Match if the TCP header acknowledgment number field is set to
.Ar ack .
.It Cm tcpflags Ar spec
TCP packets only.
Match if the TCP header contains the comma separated list of
flags specified in
.Ar spec .
The supported TCP flags are:
.Cm fin ,
.Cm syn ,
.Cm rst ,
.Cm psh ,
.Cm ack
.Cm urg .
The absence of a particular flag may be denoted
with a
.Ql \&! .
A rule which contains a
.Cm tcpflags
specification can never match a fragmented packet which has
a non-zero offset.
See the
.Cm frag
option for details on matching fragmented packets.
.It Cm tcpseq Ar seq
TCP packets only.
Match if the TCP header sequence number field is set to
.Ar seq .
.It Cm tcpwin Ar win
TCP packets only.
Match if the TCP header window field is set to
.Ar win .
.It Cm tcpoptions Ar spec
TCP packets only.
Match if the TCP header contains the comma separated list of
options specified in
.Ar spec .
The supported TCP options are:
.Cm mss
(maximum segment size),
.Cm window
(tcp window advertisement),
.Cm sack
(selective ack),
.Cm ts
(rfc1323 timestamp) and
.Cm cc
(rfc1644 t/tcp connection count).
The absence of a particular option may be denoted
with a
.Ql \&! .
.It Cm uid Ar user
Match all TCP or UDP packets sent by or received for a
.Ar user .
.Ar user
may be matched by name or identification number.
.It Cm 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 intentionally similar to
the Cisco IOS command:
.Dl ip verify unicast reverse-path
This option can be used to make anti-spoofing rules.
Each rule belongs to one of 32 different
.Em 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
.Cm 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 atomically) switch between them.
The command to enable/disable sets is
.Bd -ragged -offset indent
.Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
where multiple
.Cm enable
.Cm 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:
.Bd -ragged -offset indent
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
.Bd -ragged -offset indent
.Cm set move
.Brq Cm rule Ar rule-number | old-set
.Cm to Ar new-set
Also, you can atomically swap two rulesets with the command
.Bd -ragged -offset indent
.Cm set swap Ar first-set second-set
See the
Section on some possible uses of sets of rules.
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
.Cm check-state , keep-state
.Cm limit
options of
.Nm rules.
Dynamic rules are created when a packet matches a
.Cm keep-state
.Cm limit
rule, causing the creation of a
.Em dynamic
rule which will match all and only packets with
a given
.Em protocol
between a
.Em src-ip/src-port dst-ip/dst-port
pair of addresses (
.Em src
.Em 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
.Cm check-state, keep-state
.Cm limit
occurrence, and the action performed upon 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 firewall configuration,
but let the first TCP SYN packet from the inside network install a
dynamic rule for the flow so that packets belonging to that session
will be allowed through the firewall:
.Dl "ipfw add check-state"
.Dl "ipfw add allow tcp from my-subnet to any setup keep-state"
.Dl "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:
.Dl "ipfw add check-state"
.Dl "ipfw add allow udp from my-subnet to any keep-state"
.Dl "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
.Cm sysctl
See Section
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
for more examples on how to use dynamic rules.
is also the user interface for the
.Xr dummynet 4
traffic shaper.
.Nm dummynet
operates by first using the firewall to classify packets and divide them into
.Em flows ,
using any match pattern that can be used in
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:
.Bl -hang -offset XXXX
.It Em 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.
.It Em 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
.Em weight
and a reference pipe to each flow, and then all backlogged (i.e.,
with packets 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,
.Em pipes
can be used to set hard limits to the bandwidth that a flow can use, whereas
.Em queues
can be used to determine how different flow share the available bandwidth.
.Em pipe
.Em queue
configuration commands are the following:
.Bd -ragged -offset indent
.Cm pipe Ar number Cm config Ar pipe-configuration
.Cm queue Ar number Cm config Ar queue-configuration
The following parameters can be configured for a pipe:
.Bl -tag -width indent -compact
.It Cm bw Ar bandwidth | device
Bandwidth, measured in
.Sm off
.Op Cm K | M
.Brq Cm bit/s | Byte/s .
.Sm on
A value of 0 (default) means unlimited bandwidth.
The unit must immediately follow the number, as in
.Dl "ipfw pipe 1 config bw 300Kbit/s"
If a device name is specified instead of a numeric value, as in
.Dl "ipfw pipe 1 config bw tun0"
then the transmit clock is supplied by the specified device.
At the moment only the
.Xr tun 4
device supports this
functionality, for use in conjunction with
.Xr ppp 8 .
.It Cm delay Ar ms-delay
Propagation delay, measured in milliseconds.
The value is rounded to the next multiple of the clock tick
(typically 10ms, but it is a good practice to run kernels
.Dq "options 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:
.Bl -tag -width indent -compact
.It Cm pipe Ar 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.
.It Cm weight Ar 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:
.Bl -tag -width XXXX -compact
.It Cm buckets Ar hash-table-size
Specifies the size of the hash table used for storing the
various queues.
Default value is 64 controlled by the
.Xr sysctl 8
.Em net.inet.ip.dummynet.hash_size ,
allowed range is 16 to 65536.
.It Cm mask Ar mask-specifier
Packets sent to a given pipe or queue by an
rule can be further classified into multiple flows, each of which is then
sent to a different
.Em dynamic
pipe or queue.
A flow identifier is constructed by masking the IP addresses,
ports and protocol types as specified with the
.Cm 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 object, and matching packets
are sent to it.
Thus, when
.Em dynamic pipes
are used, each flow will get the same bandwidth as defined by the pipe,
whereas when
.Em 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:
.Cm dst-ip Ar mask ,
.Cm src-ip Ar mask ,
.Cm dst-port Ar mask ,
.Cm src-port Ar mask ,
.Cm proto Ar mask
.Cm all ,
where the latter means all bits in all fields are significant.
.It Cm 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. Setting 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.
.It Cm plr Ar packet-loss-rate
Packet loss rate.
.Ar 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.
.It Cm queue Brq Ar slots | size Ns Cm Kbytes
Queue size, in
.Ar slots
.Cm 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 max-sized ethernet packets (1500 bytes) mean 600Kbit
or 20s of queue on a 30Kbit/s pipe.
Even worse effect can result if you get packets from an
interface with a much larger MTU, e.g. the loopback interface
with its 16KB packets.
.It Cm red | gred Ar w_q Ns / Ns Ar min_th Ns / Ns Ar max_th Ns / Ns Ar max_p
Make use of the RED (Random Early Detection) queue management algorithm.
.Ar w_q
.Ar max_p
are floating
point numbers between 0 and 1 (0 not included), while
.Ar min_th
.Ar 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).
.Xr dummynet 4
also supports the gentle RED variant (gred).
.Xr sysctl 8
variables can be used to control the RED behaviour:
.Bl -tag -width indent
.It Em net.inet.ip.dummynet.red_lookup_depth
specifies the accuracy in computing the average queue
when the link is idle (defaults to 256, must be greater than zero)
.It Em net.inet.ip.dummynet.red_avg_pkt_size
specifies the expected average packet size (defaults to 512, must be
greater than zero)
.It Em 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).
Here are some important points to consider when designing your
.Bl -bullet
Remember that you filter both packets going
.Cm in
.Cm out .
Most connections need packets going in both directions.
Remember to test very carefully.
It is a good idea to be near the console when doing this.
If you cannot be near the console,
use an auto-recovery script such as the one in
.Pa /usr/share/examples/ipfw/change_rules.sh .
Don't forget the loopback interface.
.Bl -bullet
There are circumstances where fragmented datagrams are unconditionally
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
These packets are simply logged as
.Dq pullup failed
since there may not be enough good data in the packet to produce a
meaningful log entry.
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.
If you are logged in over a network, loading the
.Xr kld 4
version of
is probably not as straightforward as you would think.
I recommend the following command line:
.Bd -literal -offset indent
kldload ipfw && \e
ipfw add 32000 allow ip from any to any
Along the same lines, doing an
.Bd -literal -offset indent
ipfw flush
in similar surroundings is also a bad idea.
filter list may not be modified if the system security level
is set to 3 or higher
.Xr init 8
for information on system security levels).
.Xr divert 4
socket bound to the specified port will receive all packets
diverted to that port.
If no socket is bound to the destination port, or if the kernel
wasn't compiled with divert socket support, the packets are
A set of
.Xr sysctl 8
variables controls the behaviour of the firewall and
associated modules (
.Nm dummynet, bridge
These are shown below together with their default value
(but always check with the
.Xr sysctl 8
command what value is actually in use) and meaning:
.Bl -tag -width indent
.It Em net.inet.ip.dummynet.expire : No 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.
.It Em net.inet.ip.dummynet.hash_size : No 64
Default size of the hash table used for dynamic pipes/queues.
This value is used when no
.Cm buckets
option is specified when configuring a pipe/queue.
.It Em net.inet.ip.dummynet.max_chain_len : No 16
Target value for the maximum number of pipes/queues in a hash bucket.
The product
.Cm max_chain_len*hash_size
is used to determine the threshold over which empty pipes/queues
will be expired even when
.Cm net.inet.ip.dummynet.expire=0 .
.It Em net.inet.ip.dummynet.red_lookup_depth : No 256
.It Em net.inet.ip.dummynet.red_avg_pkt_size : No 512
.It Em net.inet.ip.dummynet.red_max_pkt_size : No 1500
Parameters used in the computations of the drop probability
for the RED algorithm.
.It Em net.inet.ip.fw.autoinc_step : No 100
Delta between rule numbers when auto-generating them.
The value must be in the range 1..1000.
This variable is only present in
.Nm ipfw2 ,
the delta is hardwired to 100 in
.Nm ipfw1 .
.It Em net.inet.ip.fw.curr_dyn_buckets : Em net.inet.ip.fw.dyn_buckets
The current number of buckets in the hash table for dynamic rules
.It Em net.inet.ip.fw.debug : No 1
Controls debugging messages produced by
.Nm .
.It Em net.inet.ip.fw.dyn_buckets : No 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
.Cm flush
command to make sure that the hash table is resized.
.It Em net.inet.ip.fw.dyn_count : No 3
Current number of dynamic rules
.It Em net.inet.ip.fw.dyn_keepalive : No 1
Enables generation of keepalive packets for
.Cm 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.
.It Em net.inet.ip.fw.dyn_max : No 8192
Maximum number of dynamic rules.
When you hit this limit, no more dynamic rules can be
installed until old ones expire.
.It Em net.inet.ip.fw.dyn_ack_lifetime : No 300
.It Em net.inet.ip.fw.dyn_syn_lifetime : No 20
.It Em net.inet.ip.fw.dyn_fin_lifetime : No 1
.It Em net.inet.ip.fw.dyn_rst_lifetime : No 1
.It Em net.inet.ip.fw.dyn_udp_lifetime : No 5
.It Em net.inet.ip.fw.dyn_short_lifetime : No 30
These variables control the lifetime, in seconds, of dynamic
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.
.Em dyn_fin_lifetime
.Em dyn_rst_lifetime
must be strictly lower than 5 seconds, the period of
repetition of keepalives. The firewall enforces that.
.It Em net.inet.ip.fw.enable : No 1
Enables the firewall.
Setting this variable to 0 lets you run your machine without
firewall even if compiled in.
.It Em net.inet.ip.fw.one_pass : No 1
When set, the packet exiting from the
.Xr dummynet 4
pipe is not passed though the firewall again.
Otherwise, after a pipe action, the packet is
reinjected into the firewall at the next rule.
.It Em net.inet.ip.fw.verbose : No 1
Enables verbose messages.
.It Em net.inet.ip.fw.verbose_limit : No 0
Limits the number of messages produced by a verbose firewall.
.It Em net.link.ether.ipfw : No 0
Controls whether layer-2 packets are passed to
.Nm .
Default is no.
.It Em net.link.ether.bridge_ipfw : No 0
Controls whether bridged packets are passed to
.Nm .
Default is no.
This Section lists the features that have been introduced in
.Nm ipfw2
which were not present in
.Nm ipfw1 .
We list them in order of the potential impact that they can
have in writing your rulesets.
You might want to consider using these features in order to
write your rulesets in a more efficient way.
.Bl -tag -width indent
.It Syntax and flags
.Nm ipfw1
does not support the -n flag (only test syntax),
nor it allows spaces after commas or supports all
rule fields in a single argument.
.It Handling of non-IPv4 packets
.Nm ipfw1
will silently accept all non-IPv4 packets (which
.Nm ipfw1
will only see when
.Em net.link.ether.bridge_ipfw=1 Ns
.Nm ipfw2
will filter all packets (including non-IPv4 ones) according to the ruleset.
To achieve the same behaviour as
.Nm ipfw1
you can use the following as the very first rule in your ruleset:
.Dl "ipfw add 1 allow layer2 not mac-type ip"
.Cm layer2
option might seem redundant, but it is necessary -- packets
passed to the firewall from layer3 will not have a MAC header,
so the
.Cm mac-type ip
pattern will always fail on them, and the
.Cm not
operator will make this rule into a pass-all.
.It Addresses
.Nm ipfw1
does not supports address sets or lists of addresses.
.It Port specifications
.Nm ipfw1
only allows one port range when specifying TCP and UDP ports, and
is limited to 10 entries instead of the 15 allowed by
.Nm ipfw2 .
Also, in
.Nm ipfw1
you can only specify ports when the rule is requesting
.Cm tcp
.Cm udp
packets. With
.Nm ipfw2
you can put port specifications in rules matching all packets,
and the match will be attempted only on those packets carrying
protocols which include port identifiers.
.Nm ipfw1
allowed the first port entry to be specified as
.Ar port:mask
.Ar mask
can be an arbitrary 16-bit mask.
This syntax is of questionable usefulness and it is not
supported anymore in
.Nm ipfw2 .
.It Or-blocks
.Nm ipfw1
does not support Or-blocks.
.It keepalives
.Nm ipfw1
does not generate keepalives for stateful sessions.
As a consequence, it might cause idle sessions to drop because
the lifetime of the dynamic rules expires.
.It Sets of rules
.Nm ipfw1
does not implement sets of rules.
.It MAC header filtering and Layer-2 firewalling.
.Nm ipfw1
does not implement filtering on MAC header fields, nor is it
invoked on packets from
.Cm ether_demux()
.Cm ether_output_frame().
The sysctl variable
.Em net.link.ether.ipfw
has no effect there.
.It Options
.Nm ipfw1 ,
the following options only accept a single value as an argument:
.Cm ipid, iplen, ipttl
The following options are not implemented by
.Nm ipfw1 :
.Cm dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port.
Additionally, the RELENG_4 version of
.Nm ipfw1
does not implement the following options:
.Cm ipid, iplen, ipprecedence, iptos, ipttl,
.Cm ipversion, tcpack, tcpseq, tcpwin .
.It Dummynet options
The following option for
.Nm dummynet
pipes/queues is not supported:
.Cm noerror .
There are far too many possible uses of
so this Section will only give a small set of examples.
This command adds an entry which denies all tcp packets from
.Em cracker.evil.org
to the telnet port of
.Em wolf.tambov.su
from being forwarded by the host:
.Dl "ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet"
This one disallows any connection from the entire cracker's
network to my host:
.Dl "ipfw add deny ip from to my.host.org"
A first and efficient way to limit access (not using dynamic rules)
is the use of the following rules:
.Dl "ipfw add allow tcp from any to any established"
.Dl "ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup"
.Dl "ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup"
.Dl "..."
.Dl "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
.Cm setup
rules only for selected source/destination pairs.
All other SYN packets will be rejected by the final
.Cm deny
If you administer one or more subnets, you can take advantage of the
.Nm ipfw2
syntax to specify address sets and or-blocks and write extremely
compact rulesets which selectively enable services to blocks
of clients, as below:
.Dl "goodguys=\*q{{20,35,66,18} or{6,3,11} }\*q"
.Dl "badguys=\*q10.1.2.0/24{8,38,60}\*q"
.Dl ""
.Dl "ipfw add allow ip from ${goodguys} to any"
.Dl "ipfw add deny ip from ${badguys} to any"
.Dl "... normal policies ..."
.Nm ipfw1
syntax would require a separate rule for each IP in the above
.Cm verrevpath
option could be used to do automated anti-spoofing by adding the
following to the top of a ruleset:
.Dl "ipfw add deny ip from any to any not verrevpath in"
This rule drops all incoming packets that appear to be coming to the
sytem 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.
In order to protect a site from flood attacks involving fake
TCP packets, it is safer to use dynamic rules:
.Dl "ipfw add check-state"
.Dl "ipfw add deny tcp from any to any established"
.Dl "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
.Cm check-state
.Cm keep-state
.Cm 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:
.Dl "ipfw add allow tcp from my-net/24 to any setup limit src-addr 10"
.Dl "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.
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
.Xr sysctl 8
variables which control the operation of the firewall.
Here is a good usage of the
.Cm list
command to see accounting records and timestamp information:
.Dl ipfw -at list
or in short form without timestamps:
.Dl ipfw -a list
which is equivalent to:
.Dl ipfw show
Next rule diverts all incoming packets from
to divert port 5000:
.Dl ipfw divert 5000 ip from to any in
The following rules show some of the applications of
.Xr dummynet 4
for simulations and the like.
This rule drops random incoming packets with a probability
of 5%:
.Dl "ipfw add prob 0.05 deny ip from any to any in"
A similar effect can be achieved making use of dummynet pipes:
.Dl "ipfw add pipe 10 ip from any to any"
.Dl "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 we do:
.Dl "ipfw add pipe 1 ip from to any out"
.Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"
note that we use the
.Cm out
modifier so that the rule is not used twice.
Remember in fact that
rules are checked both on incoming and outgoing packets.
Should we want to simulate a bidirectional link with bandwidth
limitations, the correct way is the following:
.Dl "ipfw add pipe 1 ip from any to any out"
.Dl "ipfw add pipe 2 ip from any to any in"
.Dl "ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes"
.Dl "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 directions, 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:
.Dl "ipfw add pipe 1 ip from any to any"
.Dl "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
introduce 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
.Dl "ipfw add pipe 1 ip from any to any out"
.Dl "ipfw add pipe 2 ip from any to any in"
.Dl "ipfw pipe 1 config delay 250ms bw 1Mbit/s"
.Dl "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:
.Dl "ipfw add pipe 1 tcp from any to any"
.Dl "ipfw add pipe 1 udp from any to any"
.Dl "ipfw add pipe 1 ip from any to any"
.Dl "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 only effect is
collecting statistics.
Note that we need 3 rules, not just the last one, because
tries to match IP packets it will not consider ports, so we
would not see connections on separate ports as different
A more sophisticated example is limiting the outbound traffic
on a net with per-host limits, rather than per-network limits:
.Dl "ipfw add pipe 1 ip from to any out"
.Dl "ipfw add pipe 2 ip from any to in"
.Dl "ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
.Dl "ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
To add a set of rules atomically, e.g. set 18:
.Dl "ipfw set disable 18"
.Dl "ipfw add NN set 18 ...         # repeat as needed"
.Dl "ipfw set enable 18"
To delete a set of rules atomically the command is simply:
.Dl "ipfw delete set 18"
To test a ruleset and disable it and regain control if something goes wrong:
.Dl "ipfw set disable 18"
.Dl "ipfw add NN set 18 ...         # repeat as needed"
.Dl "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.
.Xr cpp 1 ,
.Xr m4 1 ,
.Xr bridge 4 ,
.Xr divert 4 ,
.Xr dummynet 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr protocols 5 ,
.Xr services 5 ,
.Xr init 8 ,
.Xr kldload 8 ,
.Xr reboot 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8
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.
.Em !!! WARNING !!!
Misconfiguring the firewall can put your computer in an unusable state,
possibly shutting down network services and requiring console access to
regain control of it.
Incoming packet fragments diverted by
.Cm divert
.Cm tee
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 that match a
.Cm tee
rule should not be immediately accepted, but should continue
going through the rule list.
This may be fixed in a later version.
Packets diverted to userland, and then reinserted by a userland process
may lose various packet attributes.
The packet source interface name
will be preserved if it is shorter than 8 bytes and the userland process
saves and reuses the sockaddr_in
(as does
.Xr natd 8 ) ;
otherwise, it may be lost.
If a packet is reinserted in this manner, later rules may be incorrectly
applied, making the order of
.Cm divert
rules in the rule sequence very important.
.An Ugen J. S. Antsilevich ,
.An Poul-Henning Kamp ,
.An Alex Nash ,
.An Archie Cobbs ,
.An Luigi Rizzo .
.An -nosplit
API based upon code written by
.An Daniel Boulet
for BSDI.
Work on
.Xr dummynet 4
traffic shaper supported by Akamba Corp.
utility first appeared in
.Fx 2.0 .
.Xr dummynet 4
was introduced in
.Fx 2.2.8 .
Stateful extensions were introduced in
.Fx 4.0 .
.Nm ipfw2
was introduced in Summer 2002.