freebsd-skq/sbin/ipfw/ipfw.8

.Dd February 24, 1996
.Dt IPFW 8 SMM
.Os FreeBSD
.Sh NAME
.Nm ipfw
.Nd controlling utility for IP firewall
.Sh SYNOPSIS
.Nm ipfw
.Ar file
.Nm ipfw
flush
.Nm ipfw
zero
.Oo
.Ar number
.Oc
.Nm ipfw
delete
.Ar number
.Nm ipfw
.Oo
.Fl atN
.Oc
list
.Nm ipfw
add
.Oo
.Ar number
.Oc
.Ar action 
.Oo
log
.Oc
.Ar proto
from
.Ar src
to
.Ar dst
.Oo 
via
.Ar name|ipno
.Oc
.Oo 
.Ar options
.Oc
.Sh DESCRIPTION
If used as shown in the first synopsis line, the
.Ar file
will be read line by line and applied as arguments to the 
.Nm ipfw
command.
.Pp
The ipfw code works by going through the rule-list for each packet,
until a match is found.
All rules have two counters associated with them, a packet count and
a byte count.
These counters are updated when a packet matches the rule.
.Pp
The rules are ordered by a ``line-number'' that is used to order and
delete rules.
If a rule is added without a number, it is put at the end, just before
the terminal ``policy-rule'', and numbered 100 higher than the previous
rule.
.Pp
One rule is always present:
.Bd -literal -offset center
65535 deny all from any to any
.Ed

this rule is the default policy, ie. don't allow anything at all.
Your job in setting up rules is to modify this policy to match your
needs.
.Pp
The following options are available:
.Bl -tag -width flag
.It Fl a
While listing, show counter values. This option is the only way to see
accounting records.
.It Fl t
While listing, show last match timestamp.
.It Fl N
Try to resolve addresses and service names.
.El
.Pp
.Ar action :
.Bl -hang -offset flag -width 1234567890123456
.It Nm allow
Allow packets that match rule.
The search terminates.
.It Nm pass
Same as allow.
.It Nm accept
Same as allow.
.It Nm count
Update counters for all packets that match rule.
The search continues with next rule.
.It Nm deny
Discard packets that match this rule.
The search terminates.
.It Nm reject
Discard packets that match this rule, try to send ICMP notice.
The search terminates.
.El
.Pp
When a packet matches a rule with the
.Nm log
keyword, a message will be printed on the console.
If the kernel was compiled with the
.Nm IP_FIREWALL_VERBOSE_LIMIT
option, then logging will cease after the number of packets
specified by the option are recieved for that particular
chain entry.  Logging may then be re-enabled by clearing
the packet counter for that entry.
.Pp
.Ar proto :
.Bl -hang -offset flag -width 1234567890123456
.It Nm ip
All packets match.
.It Nm all
All packets match.
.It Nm tcp
Only TCP packets match.
.It Nm udp
Only UDP packets match.
.It Nm icmp
Only ICMP packets match.
.El
.Pp
.Ar src 
and
.Ar dst :
.Pp
.Bl -hang -offset flag
.It <address/mask> [ports]
.El
.Pp
The
.Em <address/mask>
may be specified as:
.Bl -hang -offset flag -width 1234567890123456
.It Ar ipno
An ipnumber of the form 1.2.3.4.
Only this exact ip number match the rule.
.It Ar ipno/bits
An ipnumber with a mask width of the form 1.2.3.4/24.
In this case all ip numbers from 1.2.3.0 to 1.2.3.255 will match.
.It Ar ipno:mask
An ipnumber with a mask width of the form 1.2.3.4:255.255.240.0
In this case all ip numbers from 1.2.0.0 to 1.2.15.255 will match.
.El
.Pp
With the TCP and UDP
.Em protocols ,
an optional
.Em port
may be specified as:
.Pp
.Bl -hang -offset flag
.It Ns {port|port:port} Ns Op ,port Ns Op ,...
.El
.Pp
Service names (from 
.Pa /etc/services )
may not be used instead of a numeric port value.
Also, note that a range may only be specified as the first value,
and the port list is limited to
.Nm IP_FW_MAX_PORTS
(as defined in /usr/src/sys/netinet/ip_fw.h)
ports.
.Pp
If ``via''
.Ar name
is specified, only packets received via or on their way out of an interface
matching
.Ar name
will match this rule.
.Pp
If ``via''
.Ar ipno
is specified, only packets received via or on their way out of an interface
having the address
.Ar ipno
will match this rule.
.Pp
.Ar options :
.Bl -hang -offset flag -width 1234567890123456
.It frag
Matches if the packet is a fragment and this is not the first fragment
of the datagram.
.It in
Matches if this packet was on the way in.
.It out
Matches if this packet was on the way out.
.It ipoptions Ar spec
Matches if the IP header contains the comma separated list of 
options specified in
.Ar spec .
The supported IP options are:
.Nm ssrr 
(strict source route),
.Nm lsrr 
(loose source route),
.Nm rr 
(record packet route), and
.Nm ts 
(timestamp).
The absence of a particular option may be denoted
with a ``!''.
.It established
Matches packets that do not have the SYN bit set.
TCP packets only.
.It setup
Matches packets that have the SYN bit set but no ACK bit.
TCP packets only.
.It tcpflags Ar spec
Matches if the TCP header contains the comma separated list of
flags specified in
.Ar spec .
The supported TCP flags are:
.Nm fin ,
.Nm syn ,
.Nm rst ,
.Nm psh ,
.Nm ack ,
and
.Nm urg .
The absence of a particular flag may be denoted
with a ``!''.
.It icmptypes Ar types
Matches if the ICMP type is in the list
.Ar types .
The list may be specified as any combination of ranges
or individual types separated by commas.
.El
.Sh CHECKLIST
Here are some important points to consider when designing your
rules:
.Bl -bullet -hang -offset flag -width 1234567890123456
.It 
Remember that you filter both packets going in and out.
Most connections need packets going in both directions.
.It
Remember to test very carefully.
It is a good idea to be near the console when doing this.
.It
Don't forget the loopback interface.
.It
Don't filter
.Nm all
if you are also specifying a port.
.El
.Sh FINE POINTS
There is one kind of packet that the firewall will always discard,
that is an IP fragment with a fragment offset of one.
This is a valid packet, but it only has one use, to try to circumvent
firewalls.
.Pp
If you are logged in over a network, loading the LKM version of
.Nm
is probably not as straightforward as you would think.
I recommend this command line:
.Bd -literal -offset center
modload /lkm/ipfw_mod.o && \e
ipfw add 32000 allow all from any to any
.Ed

Along the same lines, doing an
.Bd -literal -offset center
ipfw flush
.Ed

in similar surroundings is also a bad idea.
.Sh EXAMPLES
This command adds an entry which denies all tcp packets from
.Em hacker.evil.org
to the telnet port of
.Em wolf.tambov.su
from being forwarded by the host:
.Pp
.Dl ipfw add deny tcp from hacker.evil.org to wolf.tambov.su 23
.Pp 
This one disallows any connection from the entire hackers network to
my host:
.Pp
.Dl ipfw addf deny all from 123.45.67.0/24 to my.host.org
.Pp
Here is good usage of list command to see accounting records:
.Pp
.Dl ipfw -aT l
.Pp
or in short form
.Pp
.Dl ipfw -a l
.Pp
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr getservbyport 3 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr ipaccounting 4 ,
.Xr reboot 8 ,
.Xr syslogd 8
.Sh BUGS
.Pp
.Em WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!
.Pp
This program can put your computer in rather unusable state. When
using it for the first time, work on the console of the computer, and
do
.Em NOT
do anything you don't understand.
.Pp
When manipulating/adding chain entries, service names are
not accepted.
.Sh HISTORY
Initially this utility was written for BSDI by:
.Pp
.Dl Daniel Boulet <danny@BouletFermat.ab.ca>
.Pp
The FreeBSD version is written completely by:
.Pp
.Dl Ugen J.S.Antsilevich <ugen@FreeBSD.ORG>
.Pp
This has all been extensively rearranged by Poul-Henning Kamp and
Alex Nash.