Expressions¶

antonmedv/expr - Expression evaluation engine for Go: fast, non-Turing complete, dynamic typing, static typing

Several places of Crowdsec's configuration use expr, notably :

Filter that are used to determine events eligibility in parsers and scenarios or profiles
Statics use expr in the expression directive, to compute complex values
Whitelists rely on expression directive to allow more complex whitelists filters

When Crowdsec relies on expr, a context is provided to let the expression access relevant objects :

evt. is the representation of the current event and is the most relevant object
in profiles, signal is accessible via the sig. object

If the debug is enabled (in the scenario or parser where expr is used), additional debug will be displayed regarding evaluated expressions.

Helpers¶

In order to makes its use in Crowdsec more efficient, we added a few helpers that are documented bellow.

Parses a string representation of a float number to an actual float number (binding on strconv.ParseFloat)

Atof(evt.Parsed.tcp_port)

Extract the FieldName from the JsonBlob and returns it as a string. (binding on jsonparser)

JsonExtract(evt.Parsed.some_json_blob, "foo.bar[0].one_item")

Returns the content of FileName as an array of string, while providing cache mechanism.

evt.Parsed.some_field in File('some_patterns.txt') any(File('rdns_seo_bots.txt'), { evt.Enriched.reverse_dns endsWith #})

Returns true if the StringToMatch is matched by one of the expressions contained in FileName (uses RE2 regexp engine).

RegexpInFile( evt.Enriched.reverse_dns, 'my_legit_seo_whitelists.txt')

Returns the uppercase version of the string

Upper("yop")

Returns true if the IP IPStr is contained in the IP range RangeStr (uses net.ParseCIDR)

IpInRange("1.2.3.4", "1.2.3.0/24")