Best Practices How Do Firewall DLP Rules Prevent Data Leaks

Sanjay Seth, P J Networks Pvt Ltd Real Experiences

Well alright, here I am — third cup of coffee kicked in, the blue hue of my monitor a little too strong for a sunshine-drenched morning but hey, that’s the life of a cybersecurity consultant whose been around since those glorious days of PSTN and dial-up modems. Been a network admin since 1993 (there’s true old school) so I’ve seen firsthand the aftermath of data leaks when they’re not caught. I’m still buzzing from that DefCon hardware hacking village; I swear to god, every year that thing proves if there’s a way to get into your system, someone already pulled it.

But enough nerd talk. Down to brass tacks — how the hell do firewalls, in concert with Data Loss Prevention (DLP) rules, keep your crown jewels from taking a trip out the network window? Spoiler alert: It’s not magic. And no, it’s not simply a matter of slapping some AI-powered boilerplate on your firewall (I’m a little dubious of those buzzwords, in fact).

What is Data Loss Prevention

Let’s begin with the fundamental—because if you don’t already know what DLP is, you’re way behind the curve.

DLP = Data Loss Prevention and is focused on preventing sensitive data from leaving your network without authorization. It’s like that no-nonsense border patrol agent for your digital wealth. Now, you’re running a city, and DLP is the customs officer inspecting each suitcase before it boards the plane. Only, instead of suitcases, we have email, file transfers, web uploads and the like.

Why does this matter? Well, even as we head toward 2024, data leaks remain shockingly common—not just because of hackers, but also because of innocent mistakes, rogue employees or misconfigured systems.

I commonly configure DLP firewalls to combat these leaks at P J Networks. Believe me, it’s always a combination of tech and process, never just tech.

Identifying Sensitive Data

This is where many teams fall down — before you can stop leaks, you have to identify what to protect in the first place.

Sensitive data could be:

• Personally identifiable information (PII) of customers
• Banking information, financial documents
• Items like source code or trade secrets
• Compliance (e.g. GDPR, PCI-DSS, HIPAA) related data

When I was recently working with three large banks — yes, there was pressure — the first step I took was to sit down, roll up my sleeves and come to grips with what was sacred to them. That’s a rather important word choice. Because while not all data is sacred, the stuff that is bloody well had better remain inside.

What strategies do you have for finding sensitive data?

1. Classification tools — Some firewalls and security suites include built-in data classifiers, but don’t trust them unquestioningly.
2. Patterns and keywords — credit card numbers, social security numbers, proprietary project names, for example.
3. User — Get input from your constituency. The people at the frontline often understand what data matters most.

I had a rude awakening with the Slammer worm in the early 2000s, the way it propagated because of unprotected SQL servers. If firewalls at that time had the DLP capabilities, the damage probably would have been significantly less.

Configuring DLP Rules

Well, this is where the rubber meets the road. DLP rules on your firewall-non-wishful thinking. It requires finesse and ongoing adjustment.

Here’s my approach:

• Focus out from the easiest answers—first build a block on all outgoing flows matching anyone of your sensitive data patterns.
• Whitelist known safe traffic — you don’t want to block your whole business from being able to send email or files.
• Use layers – URL filtering, application control and content inspection all go hand in hand.
• Encrypt only if you’re careful—encrypted traffic is a double-edged sword. If your firewall can’t look at it, how do you know what’s in there? Some organizations deploy SSL inspection proxies, but that’s a whole other beast.

During a bank upgrade once, I saw them fight so many false positives that the IT guys wanted to just give up. The trick? Use granular rules for sensitive data to move toward mining test in-monitor mode, to finally work it, and getridd it like crazy.

A small rant:

Now I hate overly complicated password policies as much as the next person, but less is not more when it comes to DLP rules. Simplistic rules equal holes big enough for a truck to drive through. Firewalls are your digital gatekeepers—put them to work.

Monitoring File Transfers

Firewalls don’t sleep. But administrators have this ability — and this is where monitoring plays a role. It is critically important to have this oversight 24/7 — I cannot stress enough the importance of that.

Monitoring file transfers through your firewall means:

• Monitoring outbound FTP, HTTP, SMTP traffic for sensitive data strings
• Alerting on items that trigger DLP policies for two immediate actions
• Analytics on dashboards and logs to uncover repeat offenders or perhaps even insiders
• Integration with SIEM (Security Information and Event Management) systems for automatic correlation with other events

I think some clients underestimate the humans behind it — they get so tied to the tech that they don’t see the warning signs of bad training or lazy behavior.

Quick story: At a financial client once, their firewall logs logged multiple blocks of attempts to send patent documents via email. Turns out, a new employee was simply trying to share files externally with a partner — and wasn’t briefed on company policy. Monitoring is not only about security; it’s about education as well.

Incident Response

So your dlp rules for the firewall triggered on something suspicious. Now what? This is where the rubber meets the road: your incident response (IR) plan comes into play.

And no, having a firewall doesn’t provide a get-out-of-jail-free card.

When an incident is detected:

• Contain the suspected leak. Immediately block the offending device or user.
• Assess which data was at risk. Was it stolen, merely tried to escape, or was its sending a mistake?
• Inform affected individuals as per your compliance requirements (if any).
• Remediate: That is, fix the root cause — patch misconfigurations, retrain users, update policies.
• Modify your DLP rules and internal controls to ensure the incident does not happen again.

In all my years, I’ve learned that the greatest firewall in the world won’t save you if your incident response is trash.

The Fundamentals of Firewall DLP — Quick Take

• DLP: Prevents data leaks by inspecting traffic that leaves your firewall.
• Find out what sensitive data is important to you—not every enterprise will have the same rule.
• Set DLP rules carefully, test before enforcing, and iterate endlessly.
• Keep watch, because vulnerabilities take the form of internal threats as much as external ones.
• Implement a prolific incident response process to take action on what alerting your DLP uncovers.

Final Thoughts from My Desk

Look, I’ve gone down so many rabbit holes on networking and security—from the chit-chat on multiplexers in the ‘90s through the havoc wrought by Slammer, to the zero trust architecture of today—it all comes down to one simple fact:

Firewalls and DLP rules are not just toys on your network. They need to be part of a complete security mindset.

You can sprinkle some trendy AI-powered label on your product and call it good. But here’s the catch: In the absence of experience, vigilance, and human insight, your data remains at risk.

And keep in mind: a firewall is the gatekeeper at an estate. Good at keeping out uninvited guests yet knows nothing of a family member bringing the secret into the house. That’s why DLP is your guard dog—and you need to know how to train it.

If you’re looking for a concrete, proven solution to keep all your sensitive stuff in-house — and a few war stories in the process — then drop us a line at P J Networks. Because we’ve been doing this for decades, and we do it well.

Until next time, keep your firewalls high and your coffee higher.

— Sanjay Seth
P J Networks Pvt Ltd
Cyber Security Consulting Since the dawn of the Internet