From MikroTik Wiki
layer7-protocol is a method of searching for patterns in ICMP/TCP/UDP streams.
L7 matcher collects the first 10 packets of a connection or the first 2KB of a connection and searches for the pattern in the collected data. If the pattern is not found in the collected data, the matcher stops inspecting further. Allocated memory is freed and the protocol is considered as unknown. You should take into account that a lot of connections will significantly increase memory and CPU usage. To avoid this, add regular firewall matchers to reduce amount of data passed to layer-7 filters repeatedly.
Additional requirement is that layer7 matcher must see both directions of traffic (incoming and outgoing). To satisfy this requirement l7 rules should be set in forward chain. If rule is set in input/prerouting chain then the same rule must be also set in output/postrouting chain, otherwise the collected data may not be complete resulting in an incorrectly matched pattern.
Example L7 patterns compatible with RouterOS can found in l7-filter project page.
You can also download a script with a list of common protocols here (only for RouterOS v3), just run Import command with this file.
/ip firewall layer7-protocol
|name (string; Default: )||Descriptive name of l7 pattern used by configuration in firewall rules. See example >>.|
|regexp (string; Default: )||POSIX compliant regular expression used to match pattern.|
Simple L7 usage example
First, add Regexp strings to the protocols menu, to define strings you will be looking for. In this example we will use pattern to match rdp packets.
/ip firewall layer7-protocol add name=rdp regexp="rdpdr.*cliprdr.*rdpsnd"
Then, use the defined protocols in firewall.
/ip firewall filter # add few known protocols to reduce mem usage add action=accept chain=forward comment="" disabled=no port=80 protocol=tcp add action=accept chain=forward comment="" disabled=no port=443 protocol=tcp # add l7 matcher add action=accept chain=forward comment="" disabled=no layer7-protocol=\ rdp protocol=tcp
As you can see before l7 rule we added several regular rules that will match known traffic thus reducing memory usage.
L7 in input chain
In this example we will try to match telnet protocol connecting to our router.
/ip firewall layer7-protocol add comment="" name=telnet regexp="^\\xff[\\xfb-\\xfe].\\xff[\\xfb-\\xfe].\\xff[\\xfb-\\xfe]"
Note that we need both directions that is why we need also l7 rule in output chain that sees outgoing packets.
/ip firewall filter add action=accept chain=input comment="" disabled=no layer7-protocol=telnet \ protocol=tcp add action=passthrough chain=output comment="" disabled=no layer7-protocol=telnet \ protocol=tcp
/ip firewall layer7-protocol add name=youtube regexp="(GET \\/videoplayback\\\?|GET \\/crossdomain\\.xml)"
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