18.2. Lua functions
18.2.1. Differences between output and detect:
Currently, the needs
key initialization varies, depending on what is the goal of the script: output or detection.
If the script is for detection, the needs
initialization should be as seen in the example below (see Lua Scripting for Detection for a complete example of a detection script):
function init (args)
local needs = {}
needs["packet"] = tostring(true)
return needs
end
For output logs, follow the pattern below. (The complete script structure can be seen at Lua Output:)
function init (args)
local needs = {}
needs["protocol"] = "http"
return needs
end
Do notice that the functions and protocols available for log
and match
may also vary. DNP3, for instance, is not
available for logging.
18.2.2. packet
Initialize with:
function init (args)
local needs = {}
needs["type"] = "packet"
return needs
end
18.2.3. flow
function init (args)
local needs = {}
needs["type"] = "flow"
return needs
end
18.2.4. http
For output, init with:
function init (args)
local needs = {}
needs["protocol"] = "http"
return needs
end
For detection, use the specific buffer (cf Lua Scripting for Detection for a complete list), as with:
function init (args)
local needs = {}
needs["http.uri"] = tostring(true)
return needs
end
18.2.4.1. HttpGetRequestBody and HttpGetResponseBody.
Make normalized body data available to the script through HttpGetRequestBody and HttpGetResponseBody.
There no guarantees that all of the body will be available.
Example:
function log(args)
a, o, e = HttpGetResponseBody();
--print("offset " .. o .. " end " .. e)
for n, v in ipairs(a) do
print(v)
end
end
18.2.4.2. HttpGetRequestHost
Get the host from libhtp's htp_tx_request_hostname(tx), which can either be the host portion of the url or the host portion of the Host header.
Example:
http_host = HttpGetRequestHost()
if http_host == nil then
http_host = "<hostname unknown>"
end
18.2.4.3. HttpGetRequestHeader
http_ua = HttpGetRequestHeader("User-Agent")
if http_ua == nil then
http_ua = "<useragent unknown>"
end
18.2.4.4. HttpGetResponseHeader
server = HttpGetResponseHeader("Server");
print ("Server: " .. server);
18.2.4.5. HttpGetRequestLine
rl = HttpGetRequestLine();
print ("Request Line: " .. rl);
18.2.4.6. HttpGetResponseLine
rsl = HttpGetResponseLine();
print ("Response Line: " .. rsl);
18.2.4.7. HttpGetRawRequestHeaders
rh = HttpGetRawRequestHeaders();
print ("Raw Request Headers: " .. rh);
18.2.4.8. HttpGetRawResponseHeaders
rh = HttpGetRawResponseHeaders();
print ("Raw Response Headers: " .. rh);
18.2.4.9. HttpGetRequestUriRaw
http_uri = HttpGetRequestUriRaw()
if http_uri == nil then
http_uri = "<unknown>"
end
18.2.4.10. HttpGetRequestUriNormalized
http_uri = HttpGetRequestUriNormalized()
if http_uri == nil then
http_uri = "<unknown>"
end
18.2.4.11. HttpGetRequestHeaders
a = HttpGetRequestHeaders();
for n, v in pairs(a) do
print(n,v)
end
18.2.4.12. HttpGetResponseHeaders
a = HttpGetResponseHeaders();
for n, v in pairs(a) do
print(n,v)
end
18.2.5. TLS
For log output, initialize with:
function init (args)
local needs = {}
needs["protocol"] = "tls"
return needs
end
For detection, initialization is as follows:
function init (args)
local needs = {}
needs["tls"] = tostring(true)
return needs
end
18.2.5.1. TlsGetVersion
Get the negotiated version in a TLS session as a string through TlsGetVersion.
Example:
function log (args)
version = TlsGetVersion()
if version then
-- do something
end
end
18.2.5.2. TlsGetCertInfo
Make certificate information available to the script through TlsGetCertInfo.
Example:
function log (args)
version, subject, issuer, fingerprint = TlsGetCertInfo()
if version == nil then
return 0
end
end
18.2.5.3. TlsGetCertChain
Make certificate chain available to the script through TlsGetCertChain.
The output is an array of certificate with each certificate being an hash with data and length keys.
Example:
-- Use debian lua-luaossl coming from https://github.com/wahern/luaossl
local x509 = require"openssl.x509"
chain = TlsGetCertChain()
for k, v in pairs(chain) do
-- v.length is length of data
-- v.data is raw binary data of certificate
cert = x509.new(v["data"], "DER")
print(cert:text() .. "\n")
end
18.2.5.4. TlsGetCertNotAfter
Get the Unix timestamp of end of validity of certificate.
Example:
function log (args)
notafter = TlsGetCertNotAfter()
if notafter < os.time() then
-- expired certificate
end
end
18.2.5.5. TlsGetCertNotBefore
Get the Unix timestamp of beginning of validity of certificate.
Example:
function log (args)
notbefore = TlsGetCertNotBefore()
if notbefore > os.time() then
-- not yet valid certificate
end
end
18.2.5.6. TlsGetCertSerial
Get TLS certificate serial number through TlsGetCertSerial.
Example:
function log (args)
serial = TlsGetCertSerial()
if serial then
-- do something
end
end
18.2.5.7. TlsGetSNI
Get the Server name Indication from a TLS connection.
Example:
function log (args)
asked_domain = TlsGetSNI()
if string.find(asked_domain, "badguys") then
-- ok connection to bad guys let's do something
end
end
18.2.6. Streaming Data
Streaming data can currently log out reassembled TCP data and normalized HTTP data. The script will be invoked for each consecutive data chunk.
In case of TCP reassembled data, all possible overlaps are removed according to the host OS settings.
function init (args)
local needs = {}
needs["type"] = "streaming"
needs["filter"] = "tcp"
return needs
end
In case of HTTP body data, the bodies are unzipped and dechunked if applicable.
function init (args)
local needs = {}
needs["type"] = "streaming"
needs["protocol"] = "http"
return needs
end
18.2.6.1. SCStreamingBuffer
function log(args)
-- sb_ts and sb_tc are bools indicating the direction of the data
data, sb_open, sb_close, sb_ts, sb_tc = SCStreamingBuffer()
if sb_ts then
print("->")
else
print("<-")
end
hex_dump(data)
end
18.2.7. Flow variables
It is possible to access, define and modify Flow variables from Lua. To do so, you must use the functions described in this section and declare the counter in init function:
function init(args)
local needs = {}
needs["tls"] tostring(true)
needs["flowint"] = {"tls-cnt"}
return needs
end
Here we define a tls-cnt Flowint that can now be used in output or in a signature via dedicated functions. The access to the Flow variable is done by index so in our case we need to use 0.
function match(args)
a = SCFlowintGet(0);
if a then
SCFlowintSet(0, a + 1)
else
SCFlowintSet(0, 1)
end
18.2.7.1. SCFlowintGet
Get the Flowint at index given by the parameter.
18.2.7.2. SCFlowintSet
Set the Flowint at index given by the first parameter. The second parameter is the value.
18.2.7.3. SCFlowintIncr
Increment Flowint at index given by the first parameter.
18.2.7.4. SCFlowintDecr
Decrement Flowint at index given by the first parameter.
18.2.8. Misc
18.2.8.1. SCThreadInfo
tid, tname, tgroup = SCThreadInfo()
It gives: tid (integer), tname (string), tgroup (string)
18.2.8.2. SCLogError, SCLogWarning, SCLogNotice, SCLogInfo, SCLogDebug
Print a message. It will go into the outputs defined in the yaml. Whether it will be printed depends on the log level.
Example:
SCLogError("some error message")
18.2.8.3. SCLogPath
Expose the log path.
name = "fast_lua.log"
function setup (args)
filename = SCLogPath() .. "/" .. name
file = assert(io.open(filename, "a"))
end
18.2.8.4. SCByteVarGet
Get the ByteVar at index given by the parameter. These variables are defined by byte_extract or byte_math in Suricata rules. Only callable from match scripts.
function init(args)
local needs = {}
needs["bytevar"] = {"var1", "var2"}
return needs
end
Here we define a register that we will be using variables var1 and var2. The access to the Byte variables is done by index.
function match(args)
var1 = SCByteVarGet(0)
var2 = SCByteVarGet(1)