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  • Luciana Obregon

Firewall Lockdown Gory Details: The Nightmare Continues

In the previous blog post, we introduced a common problem facing firewall engineers: locking down a production firewall configured with an “allow-all-by-default” policy. We presented a simple firewall lockdown process (illustrated below) and discussed the challenge of using Microsoft Excel to open and analyze large spreadsheets containing hundreds of thousands to millions of rows.

Firewall Lockdown Steps

In this blog post, we will continue on the discussion by presenting the next challenge: analyzing firewall logs to identify unique network connections to then translate into configurable rulesets.

When analyzing firewall logs, there really isn’t a single recipe that you can follow each and every time to identify unique network connections. In my opinion, analyzing firewall logs is more an art than it is a science, but repeatability is key. Whatever technique you use, it has to be repeatable to reduce the number of errors and inconsistencies. In its simplest form, a firewall rule contains four elements:

  • Source IP address/subnet

  • Destination IP address/subnet

  • Destination port

  • Protocol

Whatever technique you use must be able to identify all unique combinations of these four elements which you would then use to write the firewall rules.

In a future post, we’ll show you how you can complete this task in just a few clicks using Gigasheet across tens of millions or even hundreds of millions of rows. Want to help test it out? Join our beta!

What I am going to describe next is a technique that I have used in the past to analyze firewall logs and I will highlight some of my pain points along the way. In the interest of simplification, I will focus the discussion on analyzing firewall logs generated by connection-oriented protocols, such as TCP. I will also use a sample of Cisco ASA firewall logs generated in a test environment.

A Cisco ASA firewall generates at least two log messages per TCP connection: one when a connection is built, and another one when the same connection is torn down. An example of both logs can be seen below for a connection initiated by host to host on TCP port 443:

  • Built outbound TCP connection 2 for inside: ( to outside: (

  • Teardown TCP connection 2 for inside: ( to outside: ( duration 0:03:00 bytes 819 TCP FINs from outside

Both logs contain similar information but the log generated when a connection is torn down contains the most valuable details for analysis because it reveals whether a connection is fully established (i.e., TCP three-way handshake) as well as the amount of data transmitted. Ultimately, the analysis should focus on identifying network connections where data is transmitted, and we can do that by analyzing the “bytes” and “TCP flags” fields in the “teardown” log, as illustrated below:

Teardown TCP connection 2 for inside: ( to outside: ( duration 0:03:00 bytes 819 TCP FINs from outside

The technique that I’ve used in the past includes the following steps:

  1. Eliminate logs generated when TCP connections are built

  2. From the remaining logs (or the logs generated when connections are torn down):

  3. Eliminate logs generated by half-open connections or logs containing a TCP SYN flag, and

  4. Eliminate logs for fully established connections where no data is transmitted, or logs with zero bytes

  5. Identify all unique connections, or unique combinations of source IP, destination IP, and destination port, and protocol

To put this technique into practice, I start by opening a CSV export of the firewall logs in Microsoft Excel:

I then convert all the text to columns using a space and a forward slash (/) as delimiters (I use / to separate the IP address from the port and place each value in a different column):

Next, I enable the automatic filter and deselect the word “Built” from column A to remove all logs generated at connection-built time: