4. Tutorial: A Real Analyzer¶

In this chapter we will develop a simple protocol analyzer from scratch, including full Zeek integration. Our analyzer will parse the Trivial File Transfer Protocol (TFTP) in its original incarnation, as described in RFC 1350. TFTP provides a small protocol for copying files from a server to a client system. It is most commonly used these days for providing boot images to devices during initialization. The protocol is sufficiently simple that we can walk through it end to end. See its Wikipedia page for more background.

Contents

Creating a Spicy Grammar
Zeek Integration
Next Steps

4.1. Creating a Spicy Grammar ¶

We start by developing Spicy grammar for TFTP. The protocol is packet-based, and our grammar will parse the content of one TFTP packet at a time. While TFTP is running on top of UDP, we will leave the lower layers to Zeek and have Spicy parse just the actual UDP application-layer payload, as described in Section 5 of the protocol standard.

4.1.1. Parsing One Packet Type ¶

TFTP is a binary protocol that uses a set of standardized, numerical opcodes to distinguish between different types of packets—a common idiom with such protocols. Each packet contains the opcode inside the first two bytes of the UDP payload, followed by further fields that then differ by type. For example, the following is the format of a TFTP “Read Request” (RRQ) that initiates a download from a server:

 2 bytes     string    1 byte     string   1 byte    (from RFC 1350)
 ------------------------------------------------
| Opcode |  Filename  |   0  |    Mode    |   0  |
 ------------------------------------------------

A Read Request uses an opcode of 1. The filename is a sequence of ASCII bytes terminated by a null byte. The mode is another null-terminated byte sequence that usually is either netascii, octet, or mail, describing the desired encoding for data that will be received.

Let’s stay with the Read Request for a little bit and write a Spicy parser just for this one packet type. The following is a minimal Spicy unit to parse the three fields:

module TFTP;                          # [1]

public type ReadRequest = unit {      # [2]
    opcode:   uint16;                 # [3]
    filename: bytes &until=b"\x00";   # [4]
    mode:     bytes &until=b"\x00";   # [5]

    on %done { print self; }          # [6]
};

Let’s walk through:

[1] All Spicy source files must start with a module line defining a namespace for their content. By convention, the namespace should match what is being parsed, so we call ours TFTP. Naming our module TFTP also implies saving it under the name tftp.spicy, so that other modules can find it through import TFTP;. See Modules for more on all of this.

[2] In Spicy, one will typically create a unit type for each of the main data units that a protocol defines. We want to parse a Read Request, so we call our type accordingly. We declare it as public because we want to use this unit as the starting point for parsing data. The following lines then lay out the elements of such a request in the same order as the protocol defines them.

[3] Per the TFTP specification, the first field contains the opcode as an integer value encoded over two bytes. For multi-byte integer values, it is important to consider the byte order for parsing. TFTP uses network byte order which matches Spicy’s default, so there is nothing else for us to do here. (If we had to specify the order, we would add the &byte-order attribute).

[4] The filename is a null-terminated byte sequence, which we can express directly as such in Spicy: The filename field will accumulate bytes until a null byte is encountered. Note that even though the specification of a Read Request shows the 0 as separate element inside the packet, we don’t create a field for it, but rather exploit it as a terminator for the file name (which will not be included into the filename stored).

[5] The mode operates just the same as the filename.

[6] Once we are done parsing a Read Request, we print out the result for debugging.

We should now be able to parse a Read Request. To try it, we need the actual payload of a corresponding packet. With TFTP, the format is simple enough that we can start by faking data with printf and pipe that into the Spicy tool spicy-driver:

# printf '\000\001rfc1350.txt\000octet\000' | spicy-driver tftp.spicy
[$opcode=1, $filename=b"rfc1350.txt", $mode=b"octet"]

Here, spicy-driver compiles our ReadRequest unit into an executable parser and then feeds it with the data it is receiving on standard input. The output of spicy-driver is the result of our print statement executing at the end.

What would we do with a more complex protocol where we cannot easily use printf to create some dummy payload? We would probably have access to some protocol traffic in pcap traces, however we can’t just feed those into spicy-driver directly as they will contain all the other network layers as well that our grammar does not handle (e.g., IP and UDP). One way to test with a trace would be proceeding with Zeek integration at this point, so that we could let Zeek strip off the base layers and then feed our parser only the TFTP payload. However, during development it is often easier at first to extract application-layer protocol data from the traces ourselves, write it into files, and then feed those files into spicy-driver.

We can leverage Zeek for doing this extraction into files. If we had a TCP-based protocol, doing so would be trivial because Zeek has that functionality built in: When you run Zeek on a pcap trace and add Conn::default_extract=T to the command line, it will write out all the TCP streams into individual files. As TFTP is UDP-based, however, we will use a custom script, udp-contents.zeek. When you run Zeek with that script on trace, you will get one file per UDP packet each containing the corresponding application-layer UDP payload (make sure to use this with small traces only …).

Let’s use the UDP script with tftp_rrq.pcap, a tiny TFTP trace containing a single file download from Wireshark’s pcap archive. tcpdump shows us that the first packet indeed contains a Read Request:

# 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
[...]

Running Zeek on the trace with the udp-contents scripts produces the expected content files:

# zeek -r tftp_rrq.pcap udp-contents
# ls udp-contents.orig.*
udp-contents.orig.1367411051.972852.dat
udp-contents.orig.1367411052.077243.dat
udp-contents.orig.1367411052.086300.dat
udp-contents.orig.1367411052.088995.dat
udp-contents.orig.1367411052.091675.dat
[...]

Per the timestamps included with the names, the first file is the one containing our Read Request. We can pass that into our Spicy parser:

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
[$opcode=1, $filename=b"rfc1350.txt", $mode=b"octet"]

That gives us an easy way to test our TFTP parser.

4.1.2. Generalizing to More Packet Types ¶

So far we can parse a Read Request, but nothing else. In fact, we are not even examining the opcode yet at all to see if our input actually is a Read Request. To generalize our grammar to other TFTP packet types, we will need to parse the opcode on its own first, and then use the value to decide how to handle subsequent data. Let’s start over with a minimal version of our TFTP grammar that looks at just the opcode:

module TFTP;

public type Packet = unit {
    opcode: uint16;

    on %done { print self; }
};

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
[$opcode=1]

Next we create a separate type to parse the fields that are specific to a Read Request:

type ReadRequest = unit {
    filename: bytes &until=b"\x00";
    mode:     bytes &until=b"\x00";
};

We do not declare this type as public because we will use it only internally inside our grammar; it is not a top-level entry point for parsing (that’s Packet now).

Now we need to tie the two units together. We can do that by adding the ReadRequest as a field to the Packet, which will let Spicy parse it as a sub-unit:

module TFTP;

public type Packet = unit {
    opcode: uint16;
    rrq:    ReadRequest;

    on %done { print self; }
};

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
[$opcode=1, $rrq=[$filename=b"rfc1350.txt", $mode=b"octet"]]

However, this does not help us much yet: it still resembles our original version in that it continues to hardcode one specific packet type. But the direction of using sub-units is promising, we only need to instruct the parser to leverage the opcode to decide what particular sub-unit to use. Spicy provides a switch construct for such dispatching:

module TFTP;

public type Packet = unit {
    opcode: uint16;

    switch ( self.opcode ) {
        1 -> rrq: ReadRequest;
    };

    on %done { print self; }
};

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
[$opcode=1, $rrq=[$filename=b"rfc1350.txt", $mode=b"octet"]]

The self keyword always refers to the unit instance currently being parsed, and we use that to get to the opcode for switching on. If it is 1, we descend down into a Read Request.

What happens if it is something other than 1? Let’s try it with the first server-side packet, which contains a TFTP acknowledgment (opcode 4):

# cat udp-contents.resp.1367411052.081790.dat | spicy-driver tftp.spicy
[fatal error] terminating with uncaught exception of type spicy::rt::ParseError: parse error: no matching case in switch statement (:7:5-9:7)

Of course it is now easy to add another unit type for handling such acknowledgments:

public type Packet = unit {
    opcode: uint16;

    switch ( self.opcode ) {
        1 -> rrq: ReadRequest;
        4 -> ack: Acknowledgement;
    };

    on %done { print self; }
};

type Acknowledgement = unit {
    num: uint16; # block number being acknowledged
};

# cat udp-contents.resp.1367411052.081790.dat | spicy-driver tftp.spicy
[$opcode=4, $rrq=(not set), $ack=[$num=1]]

As expected, the output shows that our TFTP parser now descended into the ack sub-unit while leaving rrq unset.

TFTP defines three more opcodes for other packet types: 2 is a Write Request, 3 is file data being sent, and 5 is an error. We will add these to our grammar as well, so that we get the whole protocol covered (please refer to the RFC for specifics of each packet type):

module TFTP;

public type Packet = unit {
    opcode: uint16;

    switch ( self.opcode ) {
        1 -> rrq:   ReadRequest;
        2 -> wrq:   WriteRequest;
        3 -> data:  Data;
        4 -> ack:   Acknowledgement;
        5 -> error: Error;
    };

    on %done { print self; }
};

type ReadRequest = unit {
    filename: bytes &until=b"\x00";
    mode:     bytes &until=b"\x00";
};

type WriteRequest = unit {
    filename: bytes &until=b"\x00";
    mode:     bytes &until=b"\x00";
};

type Data = unit {
    num:  uint16;
    data: bytes &eod; # parse until end of data (i.e., packet) is reached
};

type Acknowledgement = unit {
    num: uint16;
};

type Error = unit {
    code: uint16;
    msg:  bytes &until=b"\x00";
};

This grammar works well already, but we can improve it a bit more.

4.1.3. Using Enums ¶

The use of integer values inside the switch construct is not exactly pretty: they are hard to read and maintain. We can improve our grammar by using an enumerator type with descriptive labels instead. We first declare an enum type that provides one label for each possible opcode:

type Opcode = enum { RRQ = 1, WRQ = 2, DATA = 3, ACK = 4, ERROR = 5 };

Now we can change the switch to look like this:

switch ( self.opcode ) {
        Opcode::RRQ   -> rrq:   ReadRequest;
        Opcode::WRQ   -> wrq:   WriteRequest;
        Opcode::DATA  -> data:  Data;
        Opcode::ACK   -> ack:   Acknowledgement;
        Opcode::ERROR -> error: Error;
        };

Much better, but there is a catch still: this will not compile because of a type mismatch. The switch cases’ expressions have type Opcode, but self.opcode remains of type uint16. That is because Spicy cannot know on its own that the integers we parse into opcode match the numerical values of the Opcode labels. But we can convert the former into the latter explicitly by adding a &convert attribute to the opcode field:

public type Packet = unit {
    opcode: uint16 &convert=Opcode($$);
    ...
};

This does two things:

Each time an uint16 gets parsed for this field, it is not directly stored in opcode, but instead first passed through the expression that &convert specifies. Spicy then stores the result of that expression, potentially adapting the field’s type accordingly. Inside the &convert expression, the parsed value is accessible through the special identifier $$.
Our &convert expression passes the parsed integer into the constructor for the Opcode enumerator type, which lets Spicy create an Opcode value with the label that corresponds to the integer value.

With this transformation, the opcode field now has type Opcode and hence can be used with our updated switch statement. You can see the new type for opcode in the output as well:

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
[$opcode=Opcode::RRQ, $rrq=[$filename=b"rfc1350.txt", $mode=b"octet"], $wrq=(not set), $data=(not set), $ack=(not set), $error=(not set)]

See On-the-fly Type Conversion with &convert for more on &convert, and Enum for more on the enum type.

Note

What happens when Opcode($$) receives an integer that does not correspond to any of the labels? Spicy permits that and will substitute an implicitly defined Opcode::Undef label. It will also retain the actual integer value, which can be recovered by converting the enum value back to an integer.

4.1.4. Using Unit Parameters ¶

Looking at the two types ReadRequest and WriteRequest, we see that both are using exactly the same fields. That means we do not really need two separate types here, and could instead define a single Request unit to cover both cases. Doing so is straight-forward, except for one issue: when parsing such a Request, we would now lose the information whether we are seeing read or a write operation. For our Zeek integration later it will be useful to retain that distinction, so let us leverage a Spicy capability that allows passing state into a sub-unit: unit parameters. Here’s the corresponding excerpt after that refactoring:

public type Packet = unit {
    opcode: uint16 &convert=Opcode($$);

    switch ( self.opcode ) {
        Opcode::RRQ   -> rrq:   Request(True);
        Opcode::WRQ   -> wrq:   Request(False);
        # ...
        };

    on %done { print self; }
};

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

    on %done { print "We got a %s request." % (is_read ? "read" : "write"); }
};

We see that the switch now passes either True or False into the Request type, depending on whether it is a Read Request or Write Request. For demonstration, we added another print statement, so that we can see how that boolean becomes available through the is_read unit parameter:

# cat udp-contents.orig.1367411051.972852.dat | spicy-driver tftp.spicy
We got a read request.
[$opcode=Opcode::RRQ, $rrq=[$filename=b"rfc1350.txt", $mode=b"octet"], $wrq=(not set), $data=(not set), $ack=(not set), $error=(not set)]

Admittedly, the unit parameter is almost overkill in this example, but it proves very useful in more complex grammars where one needs access to state information, in particular also from higher-level units. For example, if the Packet type stored additional state that sub-units needed access to, they could receive the Packet itself as a parameter.

4.1.5. Complete Grammar ¶

Combining everything discussed so far, this leaves us with the following complete grammar for TFTP, including the packet formats in comments as well:

# Copyright (c) 2021 by the Zeek Project. See LICENSE for details.
#
# Trivial File Transfer Protocol
#
# Specs from https://tools.ietf.org/html/rfc1350

module TFTP;

# Common header for all messages:
#
#      2 bytes
# ---------------
# |  TFTP Opcode  |
#  ---------------

public type Packet = unit {    # public top-level entry point for parsing
    op: uint16 &convert=Opcode($$);
    switch ( self.op ) {
        Opcode::RRQ   -> rrq:   Request(True);
        Opcode::WRQ   -> wrq:   Request(False);
        Opcode::DATA  -> data:  Data;
        Opcode::ACK   -> ack:   Acknowledgement;
        Opcode::ERROR -> error: Error;
        };
};

# TFTP supports five types of packets [...]:
#
# opcode  operation
#   1     Read request (RRQ)
#   2     Write request (WRQ)
#   3     Data (DATA)
#   4     Acknowledgment (ACK)
#   5     Error (ERROR)
type Opcode = enum {
    RRQ = 0x01,
    WRQ = 0x02,
    DATA = 0x03,
    ACK = 0x04,
    ERROR = 0x05
};

# Figure 5-1: RRQ/WRQ packet
#
#  2 bytes     string    1 byte     string   1 byte
#  ------------------------------------------------
# | Opcode |  Filename  |   0  |    Mode    |   0  |
#  ------------------------------------------------

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

# Figure 5-2: DATA packet
#
#  2 bytes     2 bytes      n bytes
#   ----------------------------------
#  | Opcode |   Block #  |   Data     |
#   ----------------------------------

type Data = unit {
    num:  uint16;
    data: bytes &eod;
};

# Figure 5-3: ACK packet
#
#  2 bytes     2 bytes
#  ---------------------
# | Opcode |   Block #  |
#  ---------------------

type Acknowledgement = unit {
    num: uint16;
};

#  Figure 5-4: ERROR packet
#
#  2 bytes     2 bytes      string    1 byte
#  -----------------------------------------
# | Opcode |  ErrorCode |   ErrMsg   |   0  |
#  -----------------------------------------

type Error = unit {
    code: uint16;
    msg:  bytes &until=b"\x00";
};

4.2. Zeek Integration ¶

To turn the Spicy-side grammar into a Zeek analyzer, we need to provide Spicy’s Zeek plugin 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.

Before proceeding, follow the instructions to install the Zeek plugin. You should now be seeing output similar to this:

# zeek -NN _Zeek::Spicy
_Zeek::Spicy - Support for Spicy parsers (*.spicy, *.evt, *.hlto) (dynamic, version x.y.z)

You should also have spicyz in your PATH:

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

Note that you need a very recent version of zkg to get spicyz into your PATH automatically; refer to the plugin instructions plugin for more.

4.2.1. Compiling the Analyzer ¶

While the Spicy plugin for Zeek can compile Spicy code on the fly, it is usually more convenient to compile an analyzer once upfront. Spicy comes with a tool spicyz for that. 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

Note

If you get an error from Zeek here, see Installation to make sure the Spicy plugin is installed correctly.

4.2.2. 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 number changes 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.

4.2.3. 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 extend our Spicy TFTP grammar to call two helper functions that the Spicy plugin makes available: zeek::confirm_protocol once we have successfully parsed a request, and zeek::reject_protocol in case we encounter a parsing error. While we could put this code right into tftp.spicy, we prefer to store it inside separate Spicy file (zeek_tftp.spicy) because this is Zeek-specific logic:

module Zeek_TFTP;

import zeek;   # Library module provided by the Spicy plugin for Zeek.
import TFTP;

on TFTP::Request::%done {
    zeek::confirm_protocol();
}

on TFTP::Request::%error {
    zeek::reject_protocol("error while parsing TFTP request");
}

# spicyz -o tftp.hlto tftp.spicy zeek_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 number changes 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);

4.2.4. 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 zeek_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.

4.2.5. 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 zeek_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.

4.3. Next Steps ¶

This tutorial provides an introduction to the Spicy language and toolchain. Spicy’s capabilities go much further than what we could show here. Some pointers for what to look at next:

Programming in Spicy provides an in-depth discussion of the Spicy language, including in particular all the constructs for parsing data and a reference of language elements. Note that most of Spicy’s types come with operators and methods for operating on values. The Debugging section helps understanding Spicy’s operation if results do not match what you would expect.
Examples summarizes grammars coming with the Spicy distribution.
Zeek Integration discusses Spicy’s integration into Zeek.