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Nornir/Netmiko ASA Rule Checker

This simple program uses the Cisco ASA packet-tracer utility to test traffic flows across the firewall. On a per-ASA basis, users specify a list of simulations in YAML or JSON format. This program returns the result of those tests in a variety of formats. Both IPv4 and IPv6 are supported.

This project has limited functionality on Cisco Firepower Threat Defense (FTD) as the packet-tracer utility is available in FTD. FTD support is experimental and is not particularly stable nor extensively tested.

Contact information:
Twitter: @nickrusso42518


Follows these steps to get started. This assumes you have Python 3.6 or newer already installed, along with pip for Python package management:

  1. Clone the repository: git clone
  2. Create a new virtual environment (venv): python3.6 -m venv ~/narc
  3. Activate the new venv: source ~/narc/bin/activate
  4. Install packages into the venv: pip install -r requirements.txt
  5. Type make and ensure all tests pass. See "Testing" for more details.


This project is built on two powerful Python-based tools:

  1. Nornir, a task execution framework with concurrency support
  2. Netmiko, a library for accessing network device command lines

Below is a diagram of the high-level architecture. The text of this README explains all the individual components, including those not shown. High-level Architecture


The host_vars/ directory contains individual YAML files, one per ASA, that contain a list of dictionaries named checks. Using Nornir for concurrency and Netmiko for SSH-based device access, the tool issues a packet-tracer command for each check. The data is returned as XML which is easily converted into Python data structures for further processing. To keep things simple for users, XML is never exposed outside of the program (see "Output Formats"). An example file might be host_vars/asa1.yaml, which is shown below.

  - id: "DNS OUTBOUND"
    in_intf: "inside"
    proto: "udp"
    src_ip: ""
    src_port: 5000
    dst_ip: ""
    dst_port: 53
    should: "allow"
    in_intf: "inside"
    proto: "tcp"
    src_ip: ""
    src_port: 5000
    dst_ip: ""
    dst_port: 443
    should: "allow"
  - id: "SSH INBOUND"
    in_intf: "management"
    proto: "tcp"
    src_ip: "fc00:192:0:2::2"
    src_port: 5000
    dst_ip: "fc00:8:8:8::8"
    dst_port: 22
    should: "drop"
    in_intf: "inside"
    proto: "icmp"
    src_ip: ""
    icmp_type: 8
    icmp_code: 0
    dst_ip: ""
    should: "allow"
  - id: "L2TP OUTBOUND"
    in_intf: "inside"
    proto: 115
    src_ip: ""
    dst_ip: ""
    should: "drop"

You can also use JSON format, which may bring better performance when checks is very large. If both .json and .yaml files exist for a given host, the JSON file is given preference and the YAML file is ignored. If neither file is specified, the Nornir task raises a FileNotFoundError.

You can use the IP protocol number (1-255) or the protocol name, assuming the ASA supports it. Currently, only the names icmp, tcp, and udp are supported. For icmp, you must specify the icmp_type and icmp_code. For tcp or udp, you must specify the src_port and dst_port. For all other protocols, you only need to specify the src_ip and dst_ip. If you specify protocol number of 6 instead of tcp, this script will treat it as a rawip packet, which can be desirable for generalized TCP testing if you want to omit ports. The same is true for UDP (17) and ICMP (1).

Note that it is uncommon for firewalls to filter traffic based on source port. The packet-tracer utility requires specifying a value. Additionally, the id key is useful for documentation to describe each check. This string is also displayed in some of the output format styles.


Each individual check dictionary is checked for validity. The following checks are performed on each check:

  • All required keys are present: id in_intf proto should src_ip dst_ip. These keys are required regardless of the check type.
  • should is "allow" or "drop"
  • All conditional keys are present
    • src_port and dst_port for TCP/UDP
    • icmp_type and icmp_code for ICMP
  • src_ip and dst_ip are valid IPv4 or IPv6 addresses
  • src_ip and dst_ip are using the same IP version (both v4 or both v6)
  • src_port and dst_port are integers in the range 0-65535 when present
  • icmp_type and icmp_code are integers in the range 0-255 when present
  • proto is one of the following:
    • A string equal to "tcp", "udp", or "icmp"
    • An integer in the range 0-255 (uses the rawip ASA protocol option)

Once the individual checks are validated, one final test ensures that there are no duplicate id fields across any checks.

Output Formats

In the past, this program gave the user many options regarding formats. Those options are gone, replaced by the following actions.

  1. A terse, text-based output is written to a file named outputs/result.txt The format is: {host} {check id} -> {PASS or FAIL} The final field measures whether the actual result matched the intended result. Rules that have should: allow that actually result in ALLOW receive a PASS. Rules with should: drop that actually DROP also get a PASS. Any other combination receives a FAIL, indicating a mismatch between intent and reality.
  2. To provide additional detail, a CSV-formatted string is written to a file name outputs/result.csv. This option displays a superset of the data in the "terse" format. The output includes column headers as well, simplifying shell redirection to output files. The command below is a handy way to view CSV files from the shell (use arrows to pan): column -s, -t outputs/result.csv | less -S
  3. Finally, the program returns the results as structured data in JSON format to outputs/result.json. This data is largely unchanged, with the exception of wrapping all of a given host's results under a subdictionary with a key equal to the check id field. Note that this output is verbose and explains every processing phase of the firewall for a given simulation.

Other Options

To improve usability, the tool offers some command-line options:

  • To reduce the output generated, users may opt to only see the failures. Use -f or --failonly to apply this filter, which is handy on large check lists. Note that this affects the terse, CSV, and JSON formats.
  • Some users prefer to see status updates as the script runs. Use -s or --status to enable logging to stdout in the following format: {hostname}@{utc_timestamp}: {msg}

Here are some example outputs to demonstrate these options.

$ python && cat outputs/result.txt
ASAV1        DNS OUTBOUND             -> FAIL
ASAV1        HTTPS OUTBOUND           -> PASS
ASAV1        SSH INBOUND              -> PASS
ASAV1        PING OUTBOUND            -> PASS
ASAV1        L2TP OUTBOUND            -> PASS
ASAV2        DNS OUTBOUND             -> PASS
ASAV2        HTTPS OUTBOUND           -> FAIL
ASAV2        SSH INBOUND              -> PASS
ASAV2        PING OUTBOUND            -> PASS
ASAV2        L2TP OUTBOUND            -> PASS

$ python --failonly && cat outputs/result.txt
ASAV1        DNS OUTBOUND             -> FAIL
ASAV2        HTTPS OUTBOUND           -> FAIL

$ python --status
ASAV1@2019-12-31T18:37:57.975656: loading YAML vars
ASAV1@2019-12-31T18:37:57.978265: loading vars succeeded
ASAV1@2019-12-31T18:37:57.978427: starting  check DNS OUTBOUND (1/5)
ASAV2@2019-12-31T18:37:57.978814: loading JSON vars
ASAV2@2019-12-31T18:37:57.978965: loading vars succeeded
ASAV2@2019-12-31T18:37:57.979099: starting  check DNS OUTBOUND (1/5)
ASAV1@2019-12-31T18:38:03.976345: completed check DNS OUTBOUND (1/5)
ASAV1@2019-12-31T18:38:03.976397: starting  check HTTPS OUTBOUND (2/5)
ASAV2@2019-12-31T18:38:04.076485: completed check DNS OUTBOUND (1/5)
ASAV2@2019-12-31T18:38:04.076538: starting  check HTTPS OUTBOUND (2/5)
ASAV1@2019-12-31T18:38:04.578326: completed check HTTPS OUTBOUND (2/5)
ASAV1@2019-12-31T18:38:04.578377: starting  check SSH INBOUND (3/5)
ASAV2@2019-12-31T18:38:04.678691: completed check HTTPS OUTBOUND (2/5)
ASAV2@2019-12-31T18:38:04.678740: starting  check SSH INBOUND (3/5)
ASAV1@2019-12-31T18:38:05.180719: completed check SSH INBOUND (3/5)
ASAV1@2019-12-31T18:38:05.180774: starting  check PING OUTBOUND (4/5)
ASAV2@2019-12-31T18:38:05.280919: completed check SSH INBOUND (3/5)
ASAV2@2019-12-31T18:38:05.280967: starting  check PING OUTBOUND (4/5)
ASAV1@2019-12-31T18:38:05.782703: completed check PING OUTBOUND (4/5)
ASAV1@2019-12-31T18:38:05.782752: starting  check L2TP OUTBOUND (5/5)
ASAV2@2019-12-31T18:38:05.883009: completed check PING OUTBOUND (4/5)
ASAV2@2019-12-31T18:38:05.883055: starting  check L2TP OUTBOUND (5/5)
ASAV1@2019-12-31T18:38:06.384632: completed check L2TP OUTBOUND (5/5)
ASAV2@2019-12-31T18:38:06.485029: completed check L2TP OUTBOUND (5/5)


To keep things simple (for now), the tool has some limitations:

  1. Only source and destination IP matches are supported.
  2. All YAML var files must use .yaml, not .yml, as their file extensions. This minimizes Nornir modifications.


This project is extensively tested. All testing conducted on the following ASAv virtual appliance:

ASAV1# show version

Cisco Adaptive Security Appliance Software Version 9.12(2)
Firepower Extensible Operating System Version 2.6(1.141)

Hardware:   ASAv, 8192 MB RAM, CPU Xeon E5 series 2300 MHz, 1 CPU (2 cores)
Model Id:   ASAv10


A GNU Makefile is used to automate testing with the following targets:

  • lint: Runs yamllint and pylint linters, a custom JSON linter, and the black formatter
  • unit: Runs unit tests on helper functions via pytest.
  • dry: Runs a series of local tests to ensure the code works. These do not communicate with any ASAs and are handy for regression testing
  • clean: Deletes any artifacts, such as .pyc, .log, and output/ files
  • all: Default target that runs the sequence clean lint unit dry


It is unlikely that this project will be run on a large number of inventory devices. That is, the number of ASAs in scope is likely to be small. However, the length of the checks list for each ASA is likely to be large, especially for more complex rulesets. The outputs below provide some "wall clock" completion times for a variety of checks list lengths. In general, the intial netmiko connectivity and issuance of the first command takes about 5 seconds. Each subsequent command takes about 750 ms. Both of these estimates assume very low latency (less than 5 ms) and only using local authentication/authorization (no RADIUS/TACACS).

10 checks, 1 host

Completed in approximately 10 seconds.

$ python --status
ASAV1@2019-12-31T18:44:27.589602: loading YAML vars
ASAV1@2019-12-31T18:44:27.594586: loading vars succeeded
ASAV1@2019-12-31T18:44:27.594787: starting  check test0 (1/10)
ASAV1@2019-12-31T18:44:32.559337: completed check test0 (1/10)
ASAV1@2019-12-31T18:44:37.377834: starting  check test9 (10/10)
ASAV1@2019-12-31T18:44:37.979797: completed check test9 (10/10)

100 checks, 1 host

Completed in approximately 65 seconds (1 minute).

$ python --status
ASAV1@2019-12-31T18:46:32.466578: loading YAML vars
ASAV1@2019-12-31T18:46:32.493279: loading vars succeeded
ASAV1@2019-12-31T18:46:32.494623: starting  check test0 (1/100)
ASAV1@2019-12-31T18:46:37.458609: completed check test0 (1/100)
ASAV1@2019-12-31T18:47:36.459852: starting  check test99 (100/100)
ASAV1@2019-12-31T18:47:37.061772: completed check test99 (100/100)

1000 checks, 1 host

Completed in approximately 607 seconds (10 minutes).

$ python --status
ASAV1@2019-12-31T18:51:55.426311: loading YAML vars
ASAV1@2019-12-31T18:51:55.667978: loading vars succeeded
ASAV1@2019-12-31T18:51:55.681037: starting  check test0 (1/1000)
ASAV1@2019-12-31T18:52:00.645887: completed check test0 (1/1000)
ASAV1@2019-12-31T19:02:01.469790: starting  check test999 (1000/1000)
ASAV1@2019-12-31T19:02:02.071796: completed check test999 (1000/1000)

1000 checks, 2 hosts

Completed in approximately 607 seconds (10 minutes). Also notice that the JSON variable loading with ASAV2 took approximately 3 ms while the YAML variable lodaing with ASAV1 took approximately 300 ms. For rulesets with thousands of items in the checks list, using JSON is recommended. Last, so long as your Linux machine has enough CPU cores/sockets for each host, additional hosts will not meaningfully impact completion times.

$ python --status
ASAV1@2019-12-31T19:05:24.926371: loading YAML vars
ASAV2@2019-12-31T19:05:24.947493: loading JSON vars
ASAV2@2019-12-31T19:05:24.950366: loading vars succeeded
ASAV2@2019-12-31T19:05:24.962976: starting  check test0 (1/1000)
ASAV1@2019-12-31T19:05:25.223925: loading vars succeeded
ASAV1@2019-12-31T19:05:25.236867: starting  check test0 (1/1000)
ASAV2@2019-12-31T19:05:30.148560: completed check test0 (1/1000)
ASAV1@2019-12-31T19:05:30.199834: completed check test0 (1/1000)
ASAV2@2019-12-31T19:15:30.996224: starting  check test999 (1000/1000)
ASAV1@2019-12-31T19:15:31.009734: starting  check test999 (1000/1000)
ASAV2@2019-12-31T19:15:31.598462: completed check test999 (1000/1000)
ASAV1@2019-12-31T19:15:31.623626: completed check test999 (1000/1000)


Nornir/Netmiko ASA Rule Checker - Validate firewall rule compliance







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