When we type a search query or log onto a video game, many of us assume our internet connection is simply a conduit that allows the magic of the internet to flow freely into our lives. In 2009, Christian Kreibich, Vern Paxon, and other ICSI researchers drew back the veil and forever changed that assumption with the debut of a groundbreaking network monitoring system.
Their creation, called Netalyzr, quickly proved popular with everyday internet users and computer scientists alike. By measuring network properties—and communicating the results in a way that non-experts could understand—Netalyzr brought much-needed transparency to what was really happening behind the scenes when we use the internet. The service was released as a browser-based tool in 2009 and as an Android app in 2013, performing millions of tests for hundreds of thousands of users before being retired in 2019 once its developers had moved on to other projects or jobs.
Netalyzr’s tests provided information about a connection’s health and uncovered the degree to which internet service providers (ISPs) were interfering with users’ internet connections. Alarmingly, the system revealed that some ISPs were earning commissions by stealthily redirecting users to ads or brands’ content. For example, users who mistyped a URL may get directed to a page full of ads rather than a notice that the page doesn’t exist. ISPs even bypassed search engines for some keywords, redirecting users through affiliate marketing companies and ultimately to brands’ websites instead of returning normal search results. Bringing this activity to light helped drive attention to the policy issues around net neutrality and empowered everyday internet users to find out how their ISP might be constraining their internet activity. It also provided useful security insights to help users determine whether their device had been compromised, and offered a powerful tool for researchers to study broad trends in how people connect to the internet.
The system won the Federal Communications Commission’s Open Internet Research Challenge and was recognized as one of four end-user tests recommended by the Internet Society to predict problems with the Internet protocol address standard IPv6. The paper introducing the system was recognized with a prestigious ACM Internet Measurement Conference Test of Time Award over a decade after its publication. The work was funded primarily by the National Science Foundation, with additional support from Amazon, Google, and Comcast.
What made ICSI a good place to pursue these projects?
“First, ICSI is one of the very few places in the U.S. that could sustain such a project from inception to long-term maintenance, due to its focus on enabling fundamental research. In many other environments the staff would simply be stretched too thin, needing to rely on students with much greater turnover. Second, the Networking Group in particular has a long track record of building working systems—real, running code that people can use. Netalyzr exemplifies this culture. Third, the team’s long history in network measurement made it one of the (most likely *the*) ideal environments to undertake this project.”
Cristian Kreibich
Former Senior Research Scientist; now at Corelight
This story was published in January 2026 as part of a retrospective series highlighting ICSI’s accomplishments and impacts over the years. To learn about our ongoing work, explore our Core Research Themes.