Suricata Lua scripting flowvar access
Funded by Emerging Threats, I’ve been working on giving the lua scripts access to flowvars.
Currently only “flowvars” are done, “flowints” will be next. Please review the code at: https://github.com/inliniac/suricata/tree/dev-lua-flowvar
Pcre based flowvar capturing is done in a post-match fashion. If the rule containing the “capture” matches, the var is stored in the flow.
For lua scripting, this wasn’t what the rule writers wanted. In this case, the flowvars are stored in the flow regardless of a rule match.
The way a script can start using flowvars is by first registering which one it needs access to:
function init (args)
local needs = {}
needs["http.request_headers.raw"] = tostring(true)
needs["flowvar"] = {"cnt"}
return needs
end
More than one can be registered, e.g.:
needs["flowvar"] = {"cnt", "somevar", "anothervar" }
The maximum is 15 per script. The order of the vars matters. As Suricata uses id’s internally, to use the vars you have to use id’s as well. The first registered var has id 0, 2nd 1 and so on:
function match(args)
a = ScFlowvarGet(0);
if a then
print ("We have an A: " .. (a))
a = tostring(tonumber(a)+1)
print ("A incremented to: " .. (a))
ScFlowvarSet(0, a, #a)
else
print "Init A to 1"
a = tostring(1)
ScFlowvarSet(0, a, #a)
end
print ("A is " .. (a))
if tonumber(a) == 23 then
print "Match!"
return 1
end
return 0
end
You can also use a var:
function init (args)
local needs = {}
needs["http.request_headers.raw"] = tostring(true)
needs["flowvar"] = {"blah", "cnt"}
return needs
end
local var_cnt = 1
function match(args)
a = ScFlowvarGet(var_cnt);
if a then
print ("We have an A: " .. (a))
a = tostring(tonumber(a)+1)
print ("A incremented to: " .. (a))
ScFlowvarSet(var_cnt, a, #a)
else
print "Init A to 1"
a = tostring(1)
ScFlowvarSet(var_cnt, a, #a)
end
print ("A is " .. (a))
if tonumber(a) == 23 then
print "Match!"
return 1
end
return 0
end
Flowvars are set at the end of the rule’s inspection, so after the script has run.
When multiple stores are done from the script and/or pcre, the last match will win. So if order matters, rule priority can be used to control inspection order.
Thoughts, comments, and code review highly welcomed at the oisf-devel list.