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IPFW(8) FreeBSD System Manager's Manual 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 | one_pass | debug | verbose | dyn_keepalive}
ipfw disable {firewall | 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 {pipe | queue} number config config-options
ipfw [-s [field]] {pipe | queue} {delete | list | show} [number ...]
ipfw [-cnNqS] [-p preproc [preproc-flags]] pathname
DESCRIPTION
The ipfw utility is the user interface for controlling the ipfw(4) fire-
wall and the dummynet(4) traffic shaper in FreeBSD.
NOTE: this manual page documents the newer version of ipfw introduced
in FreeBSD CURRENT in July 2002, also known as ipfw2. ipfw2 is a
superset of the old firewall, ipfw1. The differences between the two
are listed in Section IPFW2 ENHANCEMENTS, which you are encouraged to
read to revise older rulesets and possibly write them more effi-
ciently. See Section USING IPFW2 IN FreeBSD-STABLE for instructions
on how to run ipfw2 on FreeBSD STABLE.
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 rein-
jected 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 asso-
ciated with the default rule can be either deny or allow depending 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 (addresses 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 typ-
ically used to open the firewall on-demand to legitimate traffic only.
See the STATEFUL FIREWALL and EXAMPLES Sections below for more informa-
tion 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 indicating the
time of the last match. Counters can be displayed or reset with ipfw
commands.
Rules can be added with the add command; deleted individually or in
groups with the delete command, and globally (except those in set 31)
with the flush command; displayed, optionally with the content of the
counters, using the show and list commands. Finally, counters 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, 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 SETS OF RULES for more information on sets.
The following options are available:
-a While listing, show counter values. The show command just
implies this option.
-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.
-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 Don't ask for confirmation for commands that can cause problems
if misused, i.e. flush. If there is no tty associated 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 output.
-q While adding, zeroing, resetlogging or flushing, be quiet about
actions (implies -f). This is useful for adjusting rules by exe-
cuting 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 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.
-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 postprocessing 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 include cpp(1) and
m4(1). If preproc doesn't start with a slash (`/') as its first charac-
ter, 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 pre-
processor 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.
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, pre-
venting you from being able to add any rules. This can adversely effect
the booting process. You can use ipfw disable firewall to temporarily
disable the firewall to regain access to the network, allowing 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_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 ipfw is invoked
from ether_demux(), but the same packets will have the MAC header
stripped off when ipfw is invoked from ip_input().
Also note that each packet is always checked against the complete rule-
set, 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 ip_input() ), the match pattern will not match, but a not operator
in front of such patterns will cause the pattern 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 example:
# 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 com-
mand 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]] body
where the body of the rule specifies which information is used for fil-
tering packets, among the following:
Layer-2 header fields When available
IPv4 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, identification,
fragment flag (non-zero IP off-
set), Time To Live
IP options
Misc. TCP header fields TCP flags (SYN, FIN, ACK, RST,
etc.), sequence number, acknowl-
edgment number, window
TCP options
ICMP types for ICMP packets
User/group ID When the packet can be associ-
ated 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.
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 non-
default rule number by the value of the sysctl variable
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 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 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 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 dummynet(4)) to simulate the effect of multiple paths lead-
ing 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 facility. The log-
ging 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 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
resetlog command.
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.
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 terminates.
check-state
Checks the packet against the dynamic ruleset. If a match is
found, execute the action associated with the rule which gener-
ated 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 in dotted quad format or a host name. The search
terminates if this rule matches.
If ipaddr is a local address, then matching packets will be for-
warded 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 number (if speci-
fied) 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.
A fwd rule will not match layer-2 packets (those received 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 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 netstat(1) entry look rather weird
but is intended for use with transparent proxy servers.
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 terminates; 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 bandwidth 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 termi-
nates.
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 divert(4) socket
bound to port port. The search terminates and the original
packet is accepted (but see Section BUGS below).
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.
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 (implicit) and
operators -- i.e. all must match in order for the rule to match. Indi-
vidual patterns can be prefixed by the not operator to reverse 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 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 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 compatibility with
ipfw1. In ipfw2 any match pattern (including MAC headers, IPv4 proto-
cols, 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 IPv4 protocol specified by number or name (for a complete list
see /etc/protocols). The ip or all keywords mean any protocol
will match.
The { protocol or ... } format (an or-block) is provided for con-
venience only but its use is deprecated.
src and dst: {addr | { addr or ... }} [[not] ports]
An address (or a list, see below) optionally followed 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 | addr-list | addr-set}
any matches any IP address.
me matches any IP address configured on an interface in the system.
The address list is evaluated at the time the packet is analysed.
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 (specified as a dot-
ted quad 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 (specified as a dot-
ted quad 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.*. We suggest to use this form only for
non-contiguous masks, and resort to the addr/masklen for-
mat 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 (specified as a dot-
ted 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. The
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 bit-
mask, it takes constant time and dramatically reduces the com-
plexity 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 .
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 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
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 details 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 alphabetical 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
Matches only bridged packets.
dst-ip ip-address
Matches IP 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.
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.
icmptypes types
Matches ICMP packets whose ICMP type is in the list types. The
list may be specified as any combination of individual types
(numeric) separated by commas. Ranges are not allowed. The sup-
ported 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).
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 IP packets whose ip_id field has value included 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 IP 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 IP 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 specifying 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.
iptos spec
Matches IP packets whose tos field contains the comma separated
list of service types specified in spec. The supported IP types
of service are:
lowdelay (IPTOS_LOWDELAY), throughput (IPTOS_THROUGHPUT),
reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST),
congestion (IPTOS_CE). The absence of a particular type may be
denoted with a `!'.
ipttl ttl-list
Matches IP 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 protocol. The rule
has a limited lifetime (controlled by a set of sysctl(8) vari-
ables), 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.
{ MAC | mac } dst-mac src-mac
Match packets with a given dst-mac and src-mac addresses, speci-
fied as the 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
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.
mac-type mac-type
Matches packets whose Ethernet Type field corresponds to one of
those specified as argument. mac-type is specified in the same
way as port numbers (i.e. one or more comma-separated single val-
ues or ranges). You can use symbolic names for known values such
as vlan, ipv4, ipv6. Values can be entered as decimal or hexa-
decimal (if prefixed by 0x), and they are always printed as hexa-
decimal (unless the -N option is used, in which case symbolic
resolution will be attempted).
proto protocol
Matches packets with the corresponding IPv4 protocol.
recv | xmit | via {ifX | if* | ipno | any}
Matches packets received, transmitted or going through, respec-
tively, the interface specified by exact name (ifX), by device
name (if*), by IP address, or through some interface.
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 interface: packets
originating from the local host have no receive interface, 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 IP packets whose source IP is one of the address(es)
specified as argument.
src-port ports
Matches IP packets whose source port is one of the port(s) speci-
fied as argument.
tcpack ack
TCP packets only. Match if the TCP header acknowledgment number
field is set to ack.
tcpflags spec
TCP packets only. Match if the TCP header contains the comma
separated list of flags specified in spec. The supported TCP
flags are:
fin, syn, rst, psh, ack and urg. The absence of a particular
flag may be denoted with a `!'. A rule which contains 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 advertisement),
sack (selective ack), ts (rfc1323 timestamp) and cc (rfc1644
t/tcp connection count). The absence of a particular 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 number.
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:
ip verify unicast reverse-path
This option can be used to make anti-spoofing rules.
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 mul-
tiple configurations of the firewall and quickly (and atomically) 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 exe-
cution 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 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 configura-
tion, 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:
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 dif-
ferent 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 effi-
cient variant of the WFQ policy.
The queue associates a 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 pro-
portionally to their weights. Note that weights are not pri-
orities; 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 (typically 10ms,
but it is a good practice to run kernels with ``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:
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 fur-
ther classified into multiple flows, each of which is then sent to
a different dynamic pipe or queue. A flow identifier is con-
structed by masking the IP addresses, ports and protocol 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 object, and match-
ing 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, src-ip mask, dst-port mask, src-port mask, proto mask
or all,
where the latter means all bits in all fields are significant.
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.
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 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 inter-
face with its 16KB packets.
red | gred w_q/min_th/max_th/max_p
Make use of the RED (Random Early Detection) queue management algo-
rithm. w_q and max_p are floating point numbers between 0 and 1 (0
not included), while min_th and max_th are integer numbers specify-
ing 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 behaviour:
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)
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).
CHECKLIST
Here are some important points to consider when designing your rules:
Remember that you filter both packets going in and out. Most connec-
tions 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
/usr/share/examples/ipfw/change_rules.sh.
Don't forget the loopback interface.
FINE POINTS
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 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 kld(4) version of
ipfw is probably not as straightforward as you would think. I recom-
mend the following command line:
kldload ipfw && \
ipfw add 32000 allow ip from any to any
Along the same lines, doing an
ipfw flush
in similar surroundings is also a bad idea.
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 destination port, or
if the kernel wasn't 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 below 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 traf-
fic. 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 specified when con-
figuring 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. This variable is only present in
ipfw2, the delta is hardwired to 100 in ipfw1.
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 con-
nection 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 fire-
wall 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 is not
passed though the firewall again. Otherwise, after a pipe
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.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.
USING IPFW2 IN FreeBSD-STABLE
ipfw2 is standard in FreeBSD CURRENT, whereas FreeBSD STABLE still uses
ipfw1 unless the kernel is compiled with options IPFW2, and /sbin/ipfw
and /usr/lib/libalias are recompiled with -DIPFW2 and reinstalled (the
same effect can be achieved by adding IPFW2=TRUE to /etc/make.conf before
a buildworld).
IPFW2 ENHANCEMENTS
This Section lists the features that have been introduced in ipfw2 which
were not present in 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.
Syntax and flags
ipfw1 does not support the -n flag (only test syntax), nor it
allows spaces after commas or supports all rule fields in a sin-
gle argument.
Handling of non-IPv4 packets
ipfw1 will silently accept all non-IPv4 packets (which ipfw1 will
only see when net.link.ether.bridge_ipfw=1). ipfw2 will filter
all packets (including non-IPv4 ones) according to the ruleset.
To achieve the same behaviour as ipfw1 you can use the following
as the very first rule in your ruleset:
ipfw add 1 allow layer2 not mac-type ip
The layer2 option might seem redundant, but it is necessary --
packets passed to the firewall from layer3 will not have a MAC
header, so the mac-type ip pattern will always fail on them, and
the not operator will make this rule into a pass-all.
Addresses
ipfw1 does not supports address sets or lists of addresses.
Port specifications
ipfw1 only allows one port range when specifying TCP and UDP
ports, and is limited to 10 entries instead of the 15 allowed by
ipfw2. Also, in ipfw1 you can only specify ports when the rule
is requesting tcp or udp packets. With 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.
Finally, ipfw1 allowed the first port entry to be specified as
port:mask where mask can be an arbitrary 16-bit mask. This syn-
tax is of questionable usefulness and it is not supported anymore
in ipfw2.
Or-blocks
ipfw1 does not support Or-blocks.
keepalives
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.
Sets of rules
ipfw1 does not implement sets of rules.
MAC header filtering and Layer-2 firewalling.
ipfw1 does not implement filtering on MAC header fields, nor is
it invoked on packets from ether_demux() and
ether_output_frame(). The sysctl variable net.link.ether.ipfw has
no effect there.
Options
In ipfw1, the following options only accept a single value as an
argument:
ipid, iplen, ipttl
The following options are not implemented by ipfw1:
dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port.
Additionally, the RELENG_4 version of ipfw1 does not implement
the following options:
ipid, iplen, ipprecedence, iptos, ipttl, ipversion, tcpack,
tcpseq, tcpwin.
Dummynet options
The following option for dummynet pipes/queues is not supported:
noerror.
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 for-
warded by the host:
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:
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 advantage of the
ipfw2 syntax to specify address sets and or-blocks and write extremely
compact rulesets 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 ipfw1 syntax would require a separate rule for each IP in the above
example.
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 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.
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 connec-
tion 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 fol-
lowing 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 act-
ing 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 incoming and outgo-
ing packets.
Should we want to simulate a bidirectional link with bandwidth limita-
tions, 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 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 manage-
ment 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 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 band-
width:
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 only 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 consider ports,
so we would not see connections on separate ports as different ones.
A more sophisticated example is limiting the outbound traffic 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
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 something 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" ter-
minates, and your ruleset will be left active. Otherwise, e.g. if you
cannot access your box, the ruleset will be disabled after the sleep ter-
minates thus restoring the previous situation.
SEE ALSO
cpp(1), m4(1), bridge(4), divert(4), dummynet(4), ip(4), ipfirewall(4),
protocols(5), services(5), init(8), kldload(8), reboot(8), sysctl(8),
syslogd(8)
BUGS
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 unusable state,
possibly shutting down network services and requiring console access to
regain control of it.
Incoming packet fragments diverted by divert or 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 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 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 divert rules in the rule
sequence very important.
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.
HISTORY
The ipfw utility first appeared in FreeBSD 2.0. dummynet(4) was intro-
duced in FreeBSD 2.2.8. Stateful extensions were introduced in
FreeBSD 4.0. ipfw2 was introduced in Summer 2002.
FreeBSD 4.9 August 13, 2002 FreeBSD 4.9
Федотов А.М.
Введение в Internet