Tutorial

This tutorial walks through the integration of a simple TFTP analyzer into Zeek. This discussion continues the example from Spicy’s own tutorial that develops the TFTP grammar, now focusing on how to use it with Zeek. Please go through that Spicy tutorial first before continuing here.

To turn a Spicy-side grammar into a Zeek analyzer, we need to provide Zeek with a description of how to employ it. There are two parts to that: Telling Zeek when to activate the analyzer, and defining events to generate. In addition, we will need a Zeek-side script to do something with our new TFTP events. We will walk through this in the following, starting with the mechanics of compiling the Spicy analyzer for Zeek. While we will build up the files involved individually first, see the final section for how the Zeek package manager, zkg, can be used to bootstrap a new Zeek package with a skeleton of everything needed for an analyzer.

Before proceeding, make sure that your Zeek comes with Spicy support built-in—which is the default since Zeek version 5.0:

# zeek - N Zeek::Spicy
Zeek::Spicy - Support for Spicy parsers (*.hlto) (built-in)

You should also have spicyz in your PATH:

# which spicyz
/usr/local/zeek/bin/spicyz

Note

There are a number of pieces involved in creating a full Zeek analyzer, in particular if you want to distribute it as a Zeek package. To help you get started with that, Zeek’s package manager can create a skeleton Spicy package by running:

# zkg create --features=spicy-protocol-analyzer --packagedir <packagedir>

The generated files mark places that will need manual editing with TODO. See the tutorial for more on this.

Compiling the Analyzer

Zeek comes with a tool spicyz that compiles Spicy analyzers into binary code that Zeek can load through a Spicy plugin. The following command line produces a binary object file tftp.hlto containing the executable analyzer code:

# spicyz -o tftp.hlto tftp.spicy

Below, we will prepare an additional interface definition file tftp.evt that describes the analyzer’s integration into Zeek. We will need to give that to spicyz as well, and our full compilation command hence becomes:

# spicyz -o tftp.hlto tftp.spicy tftp.evt

When starting Zeek, we add tftp.hlto to its command line:

# zeek -r tftp_rrq.pcap tftp.hlto

Activating the Analyzer

In Getting Started, we already saw how to inform Zeek about a new protocol analyzer. We follow the same scheme here and put the following into tftp.evt, the analyzer definition file:

# Note: When line numbers change in this file, update the documentation that pulls it in.

protocol analyzer spicy::TFTP over UDP:

The first line provides our analyzer with a Zeek-side name (spicy::TFTP) and also tells Zeek that we are adding an application analyzer on top of UDP (over UDP). TFTP::Packet provides the top-level entry point for parsing both sides of a TFTP connection. Furthermore, we want Zeek to automatically activate our analyzer for all sessions on UDP port 69 (i.e., TFTP’s well known port). See Analyzer Setup for more details on defining such a protocol analyzer section.

With this in place, we can already employ the analyzer inside Zeek. It will not generate any events yet, but we can at least see the output of the on %done { print self; } hook that still remains part of the grammar from earlier:

# zeek -r tftp_rrq.pcap tftp.hlto Spicy::enable_print=T
[$opcode=Opcode::RRQ, $rrq=[$filename=b"rfc1350.txt", $mode=b"octet"], $wrq=(not set), $data=(not set), $ack=(not set), $error=(not set)]

As by default, the Zeek plugin does not show the output of Spicy-side print statements, we added Spicy::enable_print=T to the command line to turn that on. We see that Zeek took care of the lower network layers, extracted the UDP payload from the Read Request, and passed that into our Spicy parser. (If you want to view more about the internals of what is happening here, there are a couple kinds of debug output available.)

You might be wondering why there is only one line of output, even though there are multiple TFTP packets in our pcap trace. Shouldn’t the print execute multiple times? Yes, it should, but it does not currently: Due to some intricacies of the TFTP protocol, our analyzer gets to see only the first packet for now. We will fix this later. For now, we focus on the Read Request packet that the output above shows.

Defining Events

The core task of any Zeek analyzer is to generate events for Zeek scripts to process. For binary protocols, events will often correspond pretty directly to data units specified by their specifications—and TFTP is no exception. We start with an event for Read/Write Requests by adding this definition to tftp.evt:

import TFTP;

on TFTP::Request -> event tftp::request($conn);

The first line makes our Spicy TFTP grammar available to the rest of the file. The line on ... defines one event: Every time a Request unit will be parsed, we want to receive an event tftp::request with one parameter: the connection it belongs to. Here, $conn is a reserved identifier that will turn into the standard connection record record on the Zeek side.

Now we need a Zeek event handler for our new event. Let’s put this into tftp.zeek:

event tftp::request(c: connection)
	{
	print "TFTP request", c$id;
	}

Running Zeek then gives us:

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
TFTP request, [orig_h=192.168.0.253, orig_p=50618/udp, resp_h=192.168.0.10, resp_p=69/udp]

Let’s extend the event signature a bit by passing further arguments:

import TFTP;

on TFTP::Request -> event tftp::request($conn, $is_orig, self.filename, self.mode);

This shows how each parameter gets specified as a Spicy expression: self refers to the instance currently being parsed (self), and self.filename retrieves the value of its filename field. $is_orig is another reserved ID that turns into a boolean that will be true if the event has been triggered by originator-side traffic. On the Zeek side, our event now has the following signature:

event tftp::request(c: connection, is_orig: bool, filename: string, mode: string)
	{
	print "TFTP request", c$id, is_orig, filename, mode;
	}

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
TFTP request, [orig_h=192.168.0.253, orig_p=50618/udp, resp_h=192.168.0.10, resp_p=69/udp], T, rfc1350.txt, octet

Going back to our earlier discussion of Read vs Write Requests, we do not yet make that distinction with the request event that we are sending to Zeek-land. However, since we had introduced the is_read unit parameter, we can easily separate the two by gating event generation through an additional if condition:

import TFTP;

This now defines two separate events, each being generated only for the corresponding value of is_read. Let’s try it with a new tftp.zeek:

event tftp::read_request(c: connection, is_orig: bool, filename: string, mode: string)
	{
	print "TFTP read request", c$id, is_orig, filename, mode;
	}

event tftp::write_request(c: connection, is_orig: bool, filename: string, mode: string)
	{
	print "TFTP write request", c$id, is_orig, filename, mode;
	}

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
TFTP read request, [orig_h=192.168.0.253, orig_p=50618/udp, resp_h=192.168.0.10, resp_p=69/udp], T, rfc1350.txt, octet

If we look at the conn.log that Zeek produces during this run, we will see that the service field is not filled in yet. That’s because our analyzer does not yet confirm to Zeek that it has been successful in parsing the content. To do that, we can call a library function that Spicy makes available once we have successfully parsed a request: spicy::accept_input. That function signals the host application—i.e., Zeek in our case—–that the parser is processing the expected protocol. With that, our request looks like this now:

type Request = unit(is_read: bool) {
    filename: bytes &until=b"\x00";
    mode:     bytes &until=b"\x00";

    on %done { spicy::accept_input(); }
};

Let’s try it again:

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
TFTP read request, [orig_h=192.168.0.253, orig_p=50618/udp, resp_h=192.168.0.10, resp_p=69/udp], T, rfc1350.txt, octet
# cat conn.log
[...]
1367411051.972852  C1f7uj4uuv6zu2aKti  192.168.0.253  50618  192.168.0.10  69  udp  spicy_tftp  -  -  -  S0  -  -0  D  1  48  0  0  -
[...]

Now the service field says TFTP! (There will be a 2nd connection in the log that we are not showing here; see the next section on that).

Turning to the other TFTP packet types, it is straight-forward to add events for them as well. The following is our complete tftp.evt file:

# Note: When line numbers change in this file, update the documentation that pulls it in.

protocol analyzer spicy::TFTP over UDP:
    parse with TFTP::Packet,
    port 69/udp;

import TFTP;

on TFTP::Request if ( is_read )   -> event tftp::read_request($conn, $is_orig, self.filename, self.mode);
on TFTP::Request if ( ! is_read ) -> event tftp::write_request($conn, $is_orig, self.filename, self.mode);

on TFTP::Data            -> event tftp::data($conn, $is_orig, self.num, self.data);
on TFTP::Acknowledgement -> event tftp::ack($conn, $is_orig, self.num);
on TFTP::Error           -> event tftp::error($conn, $is_orig, self.code, self.msg);

Detour: Zeek vs. TFTP

We noticed above that Zeek seems to be seeing only a single TFTP packet from our input trace, even though tcpdump shows that the pcap file contains multiple different types of packets. The reason becomes clear once we look more closely at the UDP ports that are in use:

# tcpdump -ttnr tftp_rrq.pcap
1367411051.972852 IP 192.168.0.253.50618 > 192.168.0.10.69:  20 RRQ "rfc1350.txtoctet" [tftp]
1367411052.077243 IP 192.168.0.10.3445 > 192.168.0.253.50618: UDP, length 516
1367411052.081790 IP 192.168.0.253.50618 > 192.168.0.10.3445: UDP, length 4
1367411052.086300 IP 192.168.0.10.3445 > 192.168.0.253.50618: UDP, length 516
1367411052.088961 IP 192.168.0.253.50618 > 192.168.0.10.3445: UDP, length 4
1367411052.088995 IP 192.168.0.10.3445 > 192.168.0.253.50618: UDP, length 516
[...]

Turns out that only the first packet is using the well-known TFTP port 69/udp, whereas all the subsequent packets use ephemeral ports. Due to the port difference, Zeek believes it is seeing two independent network connections, and it does not associate TFTP with the second one at all due to its lack of the well-known port (neither does tcpdump!). Zeek’s connection log confirms this by showing two separate entries:

# cat conn.log
1367411051.972852  CH3xFz3U1nYI1Dp1Dk  192.168.0.253  50618  192.168.0.10  69  udp  spicy_tftp  -  -  -  S0  -  -  0  D  1  48  0  0  -
1367411052.077243  CfwsLw2TaTIeo3gE9g  192.168.0.10  3445  192.168.0.253  50618  udp  -  0.181558  24795  196  SF  -  -  0  Dd  49  26167  49  1568  -

Switching the ports for subsequent packets is a quirk in TFTP that resembles similar behaviour in standard FTP, where data connections get set up separately as well. Fortunately, Zeek provides a built-in function to designate a specific analyzer for an anticipated future connection. We can call that function when we see the initial request:

function schedule_tftp_analyzer(id: conn_id) 
	{
	# Schedule the TFTP analyzer for the expected next packet coming in on different 
        # ports. We know that it will be exchanged between same IPs and reuse the 
        # originator's port. "Spicy_TFTP" is the Zeek-side name of the TFTP analyzer 
        # (generated from "Spicy::TFTP" in tftp.evt).
	Analyzer::schedule_analyzer(id$resp_h, id$orig_h, id$orig_p, Analyzer::get_tag("Spicy_TFTP"), 1min);
	}

event tftp::read_request(c: connection, is_orig: bool, filename: string, mode: string) 
	{
	print "TFTP read request", c$id, filename, mode;
	schedule_tftp_analyzer(c$id);
	}

event tftp::write_request(c: connection, is_orig: bool, filename: string, mode: string)
	{
	print "TFTP write request", c$id, filename, mode;
	schedule_tftp_analyzer(c$id);
	}

# Add handlers for other packet types so that we see their events being generated.
event tftp::data(c: connection, is_orig: bool, block_num: count, data: string)
	{
	print "TFTP data", block_num, data;
	}

event tftp::ack(c: connection, is_orig: bool, block_num: count)
	{
	print "TFTP ack", block_num;
	}

event tftp::error(c: connection, is_orig: bool, code: count, msg: string)
	{
	print "TFTP error", code, msg;
	}

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
TFTP read request, [orig_h=192.168.0.253, orig_p=50618/udp, resp_h=192.168.0.10, resp_p=69/udp], rfc1350.txt, octet
TFTP data, 1, \x0a\x0a\x0a\x0a\x0a\x0aNetwork Working Group [...]
TFTP ack, 1
TFTP data, 2, B Official Protocol\x0a   Standards" for the  [...]
TFTP ack, 2
TFTP data, 3, protocol was originally designed by Noel Chia [...]
TFTP ack, 3
TFTP data, 4, r mechanism was suggested by\x0a   PARC's EFT [...]
TFTP ack, 4
[...]

Now we are seeing all the packets as we would expect.

Zeek Script

Analyzers normally come along with a Zeek-side script that implements a set of standard base functionality, such as recording activity into a protocol specific log file. These scripts provide handlers for the analyzers’ events, and collect and correlate their activity as desired. We have created such a script for TFTP, based on the events that our Spicy analyzer generates. Once we add that to the Zeek command line, we will see a new tftp.log:

# spicyz -o tftp.hlto tftp.spicy tftp.evt
# zeek -r tftp_rrq.pcap tftp.hlto tftp.zeek
# cat tftp.log
#fields     ts      uid     id.orig_h       id.orig_p       id.resp_h       id.resp_p       wrq     fname   mode    uid_data        size    block_sent      block_acked     error_code      error_msg
1367411051.972852   CKWH8L3AIekSHYzBU       192.168.0.253   50618   192.168.0.10    69      F       rfc1350.txt     octet   ClAr3P158Ei77Fql8h      24599   49      49      -       -

The TFTP script also labels the second session as TFTP data by adding a corresponding entry to the service field inside the Zeek-side connection record. With that, we are now seeing this in conn.log:

1367411051.972852  ChbSfq3QWKuNirt9Uh  192.168.0.253  50618  192.168.0.10  69  udp  spicy_tftp  -  -  -  S0  -  -0  D  1  48  0  0  -
1367411052.077243  CowFQj20FHHduhHSYk  192.168.0.10  3445  192.168.0.253  50618  udp  spicy_tftp_data  0.181558  24795  196  SF  --  0  Dd  49  26167  49  1568  -

The TFTP script ends up being a bit more complex than one would expect for such a simple protocol. That’s because it tracks the two related connections (initial request and follow-up traffic on a different port), and combines them into a single TFTP transaction for logging. Since there is nothing Spicy-specific in that Zeek script, we skip discussing it here in more detail.

Creating a Zeek Package

We have now assembled all the parts needed for providing a new analyzer to Zeek. By adding a few further pieces, we can wrap that analyzer into a full Zeek package for others to install easily through zkg. To help create that wrapping, zkg provides a template for instantiating a skeleton analyzer package as a starting point. The skeleton comes in three different flavors, depending on which kind of analyzer you want to create: protocol, file, or packet analyzer. In each case, it creates all the necessary files along with the appropriate directory layout, and even includes a couple of standard test cases.

To create the scaffolding for our TFTP analyzer, execute the following command and provide the requested information:

# zkg create --features spicy-protocol-analyzer --packagedir spicy-tftp
"package-template" requires a "name" value (the name of the package, e.g. "FooBar" or "spicy-http"):
name: spicy-tftp
"package-template" requires a "analyzer" value (name of the Spicy analyzer, which typically corresponds to the protocol/format being parsed (e.g. "HTTP", "PNG")):
analyzer: TFTP
"package-template" requires a "protocol" value (transport protocol for the analyzer to use: TCP or UDP):
protocol: UDP
"package-template" requires a "unit_orig" value (name of the top-level Spicy parsing unit for the originator side of the connection (e.g. "Request")):
unit_orig: Packet
"package-template" requires a "unit_resp" value (name of the top-level Spicy parsing unit for the responder side of the connection (e.g. "Reply"); may be the same as originator side):
unit_resp: Packet

The above creates the following files (skipping anything related to .git):

spicy-tftp/CMakeLists.txt
spicy-tftp/COPYING
spicy-tftp/README
spicy-tftp/analyzer/CMakeLists.txt
spicy-tftp/analyzer/tftp.evt
spicy-tftp/analyzer/tftp.spicy
spicy-tftp/cmake/FindSpicyPlugin.cmake
spicy-tftp/scripts/__load__.zeek
spicy-tftp/scripts/dpd.sig
spicy-tftp/scripts/main.zeek
spicy-tftp/testing/Baseline/tests.run-pcap/conn.log
spicy-tftp/testing/Baseline/tests.run-pcap/output
spicy-tftp/testing/Baseline/tests.standalone/
spicy-tftp/testing/Baseline/tests.standalone/output
spicy-tftp/testing/Baseline/tests.trace/output
spicy-tftp/testing/Baseline/tests.trace/tftp.log
spicy-tftp/testing/Files/random.seed
spicy-tftp/testing/Makefile
spicy-tftp/testing/Scripts/README
spicy-tftp/testing/Scripts/diff-remove-timestamps
spicy-tftp/testing/Scripts/get-zeek-env
spicy-tftp/testing/Traces/tcp-port-12345.pcap
spicy-tftp/testing/Traces/udp-port-12345.pcap
spicy-tftp/testing/btest.cfg
spicy-tftp/testing/tests/availability.zeek
spicy-tftp/testing/tests/standalone.spicy
spicy-tftp/testing/tests/trace.zeek
spicy-tftp/zkg.meta

Note the *.evt, *.spicy, *.zeek files: they correspond to the files we created for TFTP in the preceding sections; we can just move our versions in there. Furthermore, the generated scaffolding marks places with TODO that need manual editing: use git grep TODO inside the spicy-tftp directory to find them. We won’t go through all the specific customizations for TFTP here, but for reference you can find the full TFTP package as created from the zkg template on GitHub.

If instead of a protocol analyzer, you’d like to create a file or packet analyzer, run zkg with --features spicy-file-analyzer or --features spicy-packet-analyzer, respectively. The generated skeleton will be suitably adjusted then.