How to debug a website with passive packet sniffing

This is a guest blog for the Computer Weekly Developer Network from John Thompson, CEO of UserReplay.

The firm specialises in “session replay technology” designed to allow web developers to record, re-run and analyse a visitor’s journeys through a website. For ecommerce companies this service is intended to find and fix site bugs, resolve disputes, recover abandoned baskets and prevent fraudulent transactions online.

Not glamorous, not exciting, just frustrating

Bug fixing is an inevitable part of a developer’s life. It is not normally considered the most glamorous or exciting part of the job. The most frustrating bugs to fix are the intermittent, hard to replicate problems.

The bug is there, it is damaging, but it just can’t be replicated in the lab and so it can’t be fixed. The problem is either invisible in the log files, or they confirm the bug is there, but do not provide enough information to diagnose or fix.

On ecommerce websites such bugs cost money, a lot of money.

Eight out of nine users (source: who encounter a technical problem immediately abandon their transaction.

Every hour the bug stays out in the wild, money is lost.

The real problem with bugs is…

However, few end users phone in and discuss their bugs and those that do are not software testers.

THE TECHNICAL BIG CHALLENGE: The problem here is that your average user can not normally describe their own technical environment, or the detailed steps needed to reproduce the bug. Even if they could, they would be talking to a customer services representative who would not be able to utilise the information in the first place. Vague accusations start to filter through the development team from customer services, but with nothing useful for diagnosis or resolution.

So to server-side session replay solutions?

According to, session replay technology is now utilised on 28% of transactional websites and customers estimate that it can reduce the replicate and fix cycle for bugs by a factor of 20.

1UR Schematic 1.png


The firm’s “passive packet sniffing” and tri-state mutex technology records every interaction between the client and the server — this means that the entire http(s) conversation is captured including every step leading up to the technical problem. Every user journey is recorded and indexed in a database, so impacted journeys can be found and analysed.

UserReplay isn’t the only firm in the passive packet sniffing market by any means and there are open source alternatives in the shape of tools such as Wireshark, Ettercap or NetworkMiner that are reckoned to be able to be able to give anyone the ability to sniff network traffic (should you have ambitions to do so) with just a little practice or training.

Given the all-consuming rise of the web and the need to refine our networks and ecommerce channels, this is a technology on the ascendancy.