fw monitor command reference CheckPoint firewall

Updated: 07 July 2020
This is a quick reference sheet of all usable options for the fw monitor tool .At the end I put a list of fw monitor examples. 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.

Note: Before R80.20 you would have to disable SecureXL acceleration with fwaccel off for fw monitor to see ALL the connection packets and filter them as needed. Starting with R80.20 you don't have to do it anymore, fw monitor sees accelerated traffic as well. BUT ... in such case the fw monitor filters given with -e do NOT work - that is, even if you set a filter, fw monitor will show ALL accelerated packets ignoring the filter. So turning off fwaccel off before sniffing, then turning back on fwaccel on might still be a good idea. As alternative for such a case, Checkpoint offer -F filter that DOES filter accelerated packets without fwaccel off, but it can filter only on IP address and ports, a fraction of capabilities of the -e filters. Also it is not implemented on R80.30.

For IPv6 traffic you would use fw6 monitor.

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 –pi|I|O|o position

Frequently, for debug purposes, you need to insert in ALL positions, be aware causes CPU load:

fw monitor -p all

The easiest way to specify where to insert is to first see the chain:

fw ctl chain

Then give position relative to any module you see there:

[Expert@CP80.30T200:0]# fw ctl chain
in chain (11):
        0: -7fffffff (0000000000000000) (00000000) SecureXL inbound (sxl_in)
        1: -7ffffffe (0000000000000000) (00000000) SecureXL inbound CT (sxl_ct)
        2: - 1fffff8 (ffffffff89f8de30) (00000001) Stateless verifications (in) (asm)
        3: - 1fffff7 (ffffffff89f91ef0) (00000001) fw multik misc proto forwarding
        4:         0 (ffffffff8a4233e0) (00000001) fw VM inbound  (fw)
        5:         2 (ffffffff89f92720) (00000001) fw SCV inbound (scv)
        6:         5 (ffffffff89b454f0) (00000003) fw offload inbound (offload_in)
        7:        10 (ffffffff8a4152a0) (00000001) fw post VM inbound  (post_vm)
        8:    100000 (ffffffff8a3cb780) (00000001) fw accounting inbound (acct)
        9:  7f730000 (ffffffff89523010) (00000001) passive streaming (in) (pass_str)
        10:  7f750000 (ffffffff8a171d10) (00000001) TCP streaming (in) (cpas)
out chain (9):
        0: - 1fffff0 (ffffffff8a167e70) (00000001) TCP streaming (out) (cpas)
        1: - 1ffff50 (ffffffff89523010) (00000001) passive streaming (out) (pass_str)
        2: - 1f00000 (ffffffff89f8de30) (00000001) Stateless verifications (out) (asm)
        3:         0 (ffffffff8a4233e0) (00000001) fw VM outbound (fw)
        4:        10 (ffffffff8a4152a0) (00000001) fw post VM outbound  (post_vm)
        5:  7f000000 (ffffffff8a3cb780) (00000001) fw accounting outbound (acct)
        6:  7f700000 (ffffffff8a168820) (00000001) TCP streaming post VM (cpas)
        7:  7f900000 (0000000000000000) (00000000) SecureXL outbound (sxl_out)
        8:  7fa00000 (0000000000000000) (00000000) SecureXL deliver (sxl_deliver)

E.g. to insert pre-inbound before the asm module: fw monitor -pi asm
Note: you can insert the sniffer where the Checkpoint will allow, e.g. trying to insert before SecureXL chain point sxl_in will give warning and will insert before the asm point anyway.

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 values: 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
-e expr expression to filter the packets (in detail later)
-f filter-file the same as above but read filter expression from file
-l len packet length to capture

Expressions
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]=8.8.8.8;'

Where:
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 predefined and kindly provided us in every Splat/Gaia 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 reference.

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 () to group.

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

fw monitor -m i -e 'accept host(208.44.108.136) ;'

packets where the source or destination IP is 208.44.108.136, show before "i" chain point.

fw monitor -e 'accept src=216.12.145.20 ;'

packets where source ip = 216.12.145.20

fw monitor -e 'accept src=216.12.145.20 or dst= 216.12.145.20;'

packets where source or destination ip = 216.12.145.20

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(216.163.137.68,24);'

packets having source IP in the network 216.163.137.0/24

fw monitor -e 'accept from_net(216.163.137.68,24) and port(25) and dst=8.8.8.8 ;'

packets coming from network 216.163.137.0/24 that are destined to the host 8.8.8.8 and having 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(66.240.206.90);'

incoming packets from/to host 66.240.206.90 , insert sniffer before module named ipopt_strip

fw monitor -D -m i -pi -ipopt_strip -e 'accept host(66.240.206.90);'

same as above but add debug info

Resources:

• PDF version of fw monitor command reference
• using tables in Checkpoint fw monitor capture tool

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