Yuri Slobodyanyuk's blog on IT Security and Networking sharing experience and expertise

fw monitor command reference

This is a quick reference sheet of all usable options for the fw monitor tool provided by Check Point on all their security gateway products.At the end I put a list of fw monitor examples, also look for more examples here: http://yurisk.info/2010/02/13/fw-monitor-add-on/. The previous experience with the tool is assumed, i’ll just say that if you are serious about debugging Checkpoint products learn it and learn it well.
By default the fw monitor sniffing driver is inserted into the 4 locations on
the Firewall kernel chain .
Here they are:

 i (PREIN) – inbound direction before firewall Virtual
Machine (VM, and it is CP terminology) . Most important fact to know about that
is that this packet capturing location shows packets BEFORE any security rule
in the policy is applied. That is, no matter what rules say a packet should at
least be seen here, this would prove that packets actually reach the firewall
at all.
 I (POSTIN) – inbound direction after firewall VM.
 o (PREOUT) – outbound direction before firewall VM,
 O (POSTOUT) – outbound direction after firewall VM.

You can change point of insertion within the fw chain with :

# fw monitor –p<i|I|O|o> <where to

easiest way to specify where to insert is to first see the chain:
# fw ctl chain
then give relative to any module you see there <+|->module_name

Now the usage itself:

# fw monitor
Usage: fw monitor [- u|s] [-i] [-d] [-T] <{-e
expression}+|-f <filter-file|->> [-l len] [-m mask] [-x offset[,len]]
[-o <file>] <[-pi pos] [-pI pos] [-po pos] [-pO pos] | -p all [-a
]> [-ci count] [-co count]

Round up of options:

-m mask , which point of capture is to be displayed, possible: i,I,o,O
-d/-D debug output from fw monitor itself, not very useful IMO.
-u|s print also connection/session Universal ID
– i after writing each packet flush stdout
-T add timestamp, not interesting
-e expr expression to filter the packets (in detail later)
-f filter_file the same as above but read expression from file
-l <len> packet length to capture

On the very low level fw monitor understands byte offsets from the header
start. So to specify for example 20th byte of the IP packet (that is source IP)
you can just use:

# fw monitor -e 'accept [12,b]=;'

12 – offset in bytes from the beginning of the packet
b – mandatory, means big endian order.
4 – not seen here but size (in bytes) of how many bytes to look for from the
starting offset (default is 4 )

To look for source port 53 (UDP/TCP) in raw packet:

# fw monitor -m i -e 'accept [20:2,b]=53;'
Here I say to fw monitor to look at 2 bytes at offset 20.

While this way of looking at packets is the most general and therefore includes
all cases, you rarely have the need for such a granular looking glass. In 99%
of the cases you will be doing alright with a limited known set of expressions.
Just for that Checkpoint defined and kindly provided us in every Splat
installation with definition files that give meaningful synonyms to the most
used patterns. There are few definition files but they circularly reference
each other providing multiple synonyms for the same pattern.
I put all those predefined patterns in the list below for the easy to use

Summary table of possible expressions to be fed to the fw
 host(IP_address)  to or from this host
 src=IP_address  where source ip = IP_address
 dst=IP_address  where destination ip = IP_address
 net(network_address,netmask)  to or from this network
 to_net(network_address,netmask)  to this network
 from_net(network_address,netmask)  from this network
 Specifying ports
 port(port_number)  having this source or destination port
 sport=port_number  having this source port
 dport=port_number  having this destination port
 tcpport(port_number)  having this source or destination port that is also TCP
 udpport(port_number)  having this source or destination port that is also UDP
 Specifying protocols  
 ip_p=<protocol_number_as_per_IANA>  this way you can specifiy any known protocol by its registered
number in IANAFor detailed list of protocol numbers see www.iana.org/assignments/protocol-numbers
 icmp  what it says , icmp protocol
 tcp  TCP
 udp  UDP
 Protocol specific oprions  
 ip_tos = <value>  TOS field of the IP packet
 ip_len = <length_in_bytes>  Length of the IP packet in bytes
 ip_src/ ip_dst = <IP_address>  Source or destination IP address of the packet
 ip_p =<protocol_number_as_per_IANA>  See above
  echo_reply  ICMP reply packets
  echo_req  Echo requests
  ping  Echo requests and echo replies
  icmp_error  ICMP error messages (Redirect,Unreachables,Time exceeded,Source
quench,Parameter problem)
  traceroute  Traceroute as implemented in Unix (UDP packets to high ports)
  tracert  Traceroute as implemented in Windows (ICMP packets , TTL
  icmp_type = <ICMP types as per RFC>  catch packets of certain type
  icmp_code = <ICMP type as per RFC>  catch packets of certain code
  ICMP types and where applicable respective codes:ICMP_ECHOREPLY
  icmp_ip_len = <length>  Length of ICMP packet
 icmp_ip_ttl = <TTL>  TTL of ICMP packet, use with icmp protocol otherwise will catch ANY
packet with TTL given
 < cut here—-bunch of other icmp-related fields
like ID ,sequence I don’t see any value in bringing here–>
 syn  SYN flag set
 fin  FIN flag set
 rst  RST flag set
 ack  ACK flag set
 first  first packet (means SYN is set but ACK is not)
 not_first  not first packet (SYN is not set)
 established  established connection (means ACK is set but SYN is not)
 last  last packet in stream (ACK and FIN are set)
 tcpdone  RST or FIN are set
 th_flags – more general way to match the flags inside
TCP packets
 th_flags = TH_PUSH  Push flag set
 th_flags = TH_URG  Urgent flag set
 uh_ulen = <length_in_bytes>  Length of the UDP header (doesnt include IP header)


And the last thing to remember before we move to examples – expressions support logical operators and numerical values support relative operators:

and – logical AND
or – logical OR
not – logical NOT
> MORE than
< LESS than
>= MORE than or EQUAL to
<= LESS than or EQUAL to
You can combine logical expressions and influence order by using ()

Below is laundry list of examples to showcase the reference table above.

# fw monitor -m i -e 'accept host( ;'
# fw monitor -e 'accept src= ;'  packets where source ip =
# fw monitor -e 'accept src= or dst=;'  packets where source or destination ip =
# fw monitor -e 'accept port(25) ;'  packets where destination or source port = 25
# fw monitor -e 'accept dport=80 ;'  packets where destination port = 80
#fw monitor -e 'accept sport>22 and dport>22 ; '  packets with source and destination ports greater than 22
# fw monitor -e 'accept ip_len = 1477;'  packets where their length equals exactly 1477 bytes
# fw monitor -e 'accept icmp_type=ICMP_UNREACH;'  ICMP packets of Unreachable type
# fw monitor -e 'accept from_net(,24);'  packets having source IP in the network
# fw monitor -e 'accept from_net(,24) and port(25) and dst= ;'  packets coming from network that are destined to the host and hving source or destination port = 25
# fw monitor -m i -x 40,450 -e 'accept port(80);'  incoming packets before any rules are applied also
display contents of the packet starting at 40th byte of 450 bytes length

# fw monitor -m i -pi -ipopt_strip -e 'accept host(;'  incoming packets from/to host , insert sniffer before module named ipopt_strip
# fw monitor -D -m i -pi -ipopt_strip -e 'accept host(;'  same as above but add debug info

PDF version of fw monitor command reference


  1. Yuri,

    Excellent! I have been waiting for something like this. Just one suggestions (maybe a request). How bout a simple guide of an actual debug which will show output so it will be clearer the use of the commands.

    thanks again.

  2. Yuri

    December 14, 2009 at 11:56 am

    there will be follow up , including debug of various problems and how it looks real-time.

  3. michael endrizzi

    November 11, 2012 at 1:13 am

    THis is awesome thanks.

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