Flowsynth is a tool for rapidly modeling network traffic. Flowsynth can be used to generate text-based hexdumps of packets as well as native libpcap format packet captures.

Installation and Usage Overview

Flowsynth has been tested on Python 2.7 and Python 3.

pip install flowsynth to install the wheel.

Python Script


usage: flowsynth.py [-h] [-f OUTPUT_FORMAT] [-w OUTPUT_FILE] [-q] [-d]
                    [--display {text,json}] [--no-filecontent]

positional arguments:
  input                 input files

optional arguments:
  -h, --help            show this help message and exit
  -f OUTPUT_FORMAT      Output format. Valid output formats include: hex, pcap
  -w OUTPUT_FILE        Output file.
  -q                    Run silently
  -d                    Run in debug mode
  --display {text,json}
                        Display format
  --no-filecontent      Disable support for the filecontent attribute

Python Module

Example usage:

import flowsynth
fsmodel = flowsynth.Model(input="my.synth", output_file="out.pcap", output_format="pcap")

The Model class function build() executes flowsynth and the class constructor takes the same arguments as the script (see above):

class Model():
    def __init__(self, input, output_format="pcap", output_file="", quiet=False, debug=False, display="text", no_filecontent=False):

Note: Because of the current less-than-ideal use of global variables instead of class variables, if more than one Model object is used concurrently, there will be issues. Hopefully this limitation will be remedied in a future release.

If the module is installed, Flowsynth can be invoked from the command line and run like a script, e.g.:

python3 -m flowsynth -f pcap -w out.pcap my.synth

How it works

Flowsynth uses a syntax language to describe network flows. The syntax language is made up of individual instructions that are parsed by the application and are grouped into events, which are a logical representation of the instructions in the network domain. After all instructions have been parsed, the events are iterated over and converted into packets, which are the real-world representation of the traffic on the wire.

These three phases are referred to as the parsing phase, rendering phase, and the output phase.

Take the following synfile as an example:

flow default tcp myhost.corp.acme.net:12323 > google.com:80 (   tcp.initialize; );
default > ( content:"GET / HTTP/1.1\x0d\x0a"; content:"Host: google.com\x0d\x0a\x0d\x0a"; );
default < ( content:"HTTP/1.1 200 OK"; );

This sample contains two types of instructions: Flow declarations and event declarations. The first line (flow default tcp...) declares to Flowsynth that a flow is being tracked between myhost.corp.acme.net and google.com. The flow name is default. All events that apply to this flow will use this name (default) to identify which flow they apply to. The third argument specifies which protocol the flow will use. In this case it's tcp. Next we specify the source and destination addresses and ports. Finally, an optional attributes section is included at the end. The tcp.initialize attribute is included, which tells Flowsynth to automatically generate a three-way handshake for this flow. It's worth nothing that each attribute and line should be closed with a semicolon (;), as shown above. When this flow declaration instruction is parsed by Flowsynth the application will automatically generate event entries in the compiler timeline to establish a three way handshake.

Next, Flowsynth will parse the event declaration default > ( content .... Flowsynth will immediately identify that this event declaration belongs to the 'default' flow that was just declared. Once this event is associated with the flow any protocol specific values (like TCP SEQ and ACK numbers) will automatically be applied to the event. The directionality for this specific event is '>', or TO_SERVER. Once the parent flow and directionality have been established Flowsynth will parse the optional attributes section. Just like the flow declaration, each optional attribute must be closed with a semicolon (;). The two 'content' attributes are used to specify the packet's payload. In this case, a HTTP request is being rendered. Flowsynth will read these instructions and generate an entry in the compiler timeline for this event.

The last event declaration that is parsed by the application shows the server's response to the client. Using the same methods described above, Flowsynth will parse the event declaration and add it to the compiler timeline.

Once all the instructions have been parsed and processed, Flowsynth iterates over the compiler timeline and renders any events to native packets. In this phase of the application several important things happen:

  1. Protocol-specific intelligence, like TCP SEQ/ACK calculations, and ACK generation take place.
  2. Specific features of attributes, like converting '\x3A' to ':' take place.

Once all of the events have been rendered to native pcaps the output phase occurs. During the output phase the native packets are delivered to the user in one of the two output formats, as a hexdump, or as a native PCAP file.


flowsynth.py input.syn

In this most basic example, Flowsynth will read input.syn and output the resulting hexdump to the screen. By default Flowsynth will use 'hex' format.

flowsynth.py input.syn -f pcap -w /tmp/test.pcap

In this example, Flowsynth reads input.syn and outputs a libpcap formatted .pcap file to /tmp/test.pcap


All Flowsynth syntax files are plain-text files. Currently three types of instructions are supported.

As new features are added, this syntax reference will be updated.


Comments are supported using the # symbol.

# This is a synfile comment


Declaring a Flow

You can declare a flow using the following syntax:

flow [flow name] [proto] [src]:[srcport] [directionality] [dst]:[dstport] ([flow options]);

src and dst can be IPv4 addresses, IPv6 addresses, or resolvable domain names. For IPv6, the address(es) must be enclosed in square brackets ('[' and ']').

The following flow declaration would describe a flow going from a computer to google.com:

flow my_connection tcp mydesktop.corp.acme.com:44123 > google.com:80 (tcp.initialize;);

The following flow declaration would describe a flow going from a computer to a DNS server:

flow dns_request udp  mydesktop.corp.acme.com:11234 >;

The following flow declaration would describe a flow using IPv6 addresses:

flow default tcp [2600:1337:2800:1:248:1893:25c8:d1]:31337 > [2600:1337:2800::f1]:80 (tcp.initialize;);

For the interim, directionality should always be specified as to server: >

If a DNS record is specified in the flow declaration (instead of an explicit IP address) then Flowsynth will resolve the DNS entry at the time of the flow's declaration. The first A record returned for DNS entry will be used as the IP address throughout the session. The DNS query and response is not included in the output.

Flow Attributes

The following flow attributes are currently supported:


The tcp.initialize attribute informs Flowsynth that the flow should have an autogenerated TCP three-way handshake included in the output. The handshake is always added relative to the location of the flow declaration in the synfile.


(tcp.initialize; );


The src_mac attribute explicitly sets the MAC address for packets from the flow source. If no MAC is supplied, a random one is chosen.

usage: (tcp.initialize; src_mac: 37:16:3a:4e:6a:12; );


The dst_mac attribute explicitly sets the MAC address for packets from the flow destination. If no MAC is supplied, a random one is chosen.

usage: (tcp.initialize; dst_mac: 37:16:3a:4e:6a:13; );


Transferring Data

Data can be transferred between hosts using two methods. The example below outlines a data exchange between a client and a webserver:

my_connection > (content:"GET / HTTP/1.1\x0d\x0aHost:google.com\x0d\x0aUser-Agent: DogBot\x0d\x0a\x0d\x0a";);
my_connection < (content:"HTTP/1.1 200 OK\x0d\x0aContent-Length: 300\x0d\x0a\x0d\x0aWelcome to Google.com!";);

In this example, the flow my_connection must have been previously declared. A single packet with the content specified will be transmitted from the client to the server. The following method is also accepted, however, this may change in the future as the syntax is formalized.:

my_connection.to_server (content:"GET / HTTP/1.1\x0d\x0aHost:google.com\x0d\x0aUser-Agent: DogBot\x0d\x0a\x0d\x0a";);
my_connection.to_client (content:"HTTP/1.1 200 OK\x0d\x0aContent-Length: 300\x0d\x0a\x0d\x0aWelcome to Google.com!";);

Each content keyword within the () should be closed by a semicolon. Each line should also be closed with a semicolon. Failure to do so will generate a lexer error. Multiple content matches can also be used to logically seperate parts of the response, for example:

# the commands below describe a simple HTTP request
my_connection > (content:"GET / HTTP/1.1\x0d\x0aHost:google.com\x0d\x0a\x0d\x0a";);
my_connection < (content:"HTTP/1.1 200 OK\x0d\x0aContent-Type: text/html\x0d\x0a\x0d\x0a"; content:"This is my response body.";);

Event Attributes

The following event attributes are currently supported:

Content Attribute

The content attribute is used to specify the payload of a packet. Content attributes must be enclosed in double quotes. UTF-8 is supported and arbitrary bytes can be expressed with the "\xHH" notation where "HH" is the hexidecimal representation of the byte. For example, a carriage return (ASCII 0x0D) followed by a line feed (ASCII 0x0A) can be defined like this: \x0D\x0A. This translation takes place during the render phase.


default > ( content: "GET / HTTP/1.1\x0d\x0a"; );
Filecontent Attribute

The filecontent attribute is used to specify a file that can be used as the payload of a packet. The value of a filecontent attribute is the file that will be read into the payload.


default > ( content: "HTTP/1.1 200 OK\x0d\x0a\x0d\x0a"; filecontent: "index.html"; );
tcp.seq Attribute

The tcp.seq attribute lets you set the sequence number for the event's packet.

tcp.ack Attribute

The tcp.ack attribute lets you set the acknowledgement number for the event's packet.

tcp.noack Attribute

The tcp.noack attribute tells Flowsynth to not generate an ACK for this event.

tcp.flags.syn Attribute

The tcp.flags.syn attribute tells Flowsynth to force the packet to be a SYN packet.

tcp.flags.ack Attribute

The tcp.flags.ack attribute tells Flowsynth to force the packet to be an ACK packet.

tcp.flags.rst Attribute

The tcp.flags.rst attribute tells Flowsynth to force the packet to be a RST packet.