Companies Using Kestrel

Most of the servers I profile are infrastructure someone chose years ago. Kestrel is different: it ships inside ASP.NET Core, so a Kestrel header tells you a .NET team built the application that answered. A domain counts here when its Server header begins with Kestrel. As of our October 2026 crawl that's 26,519 domains, 9th of the 70 web servers we track, and 10,989 of them match a company. Two cohorts distort that count, and I'd rather name them before anything else.

A template network and a recruitment platform

The first is a network of "cash for cars" lead sites. 1,930 of the 26,735 live Kestrel domains we checked (7.22%) carry titles such as "Plano Car Buyer" or "Spokane Cash for Cars", and the eight I loaded share one build: the same Bootstrap 3 layout, a bundle served from a random six-character folder, a local phone number. Only 71 of them match a company, so they swell the domain count but barely touch the charts. A second automotive group, 975 dealer sites, most of them on one platform's build, adds 3.65%. The second distortion is in the company list. 39 of our top 50 are careers sites built on Radancy's recruitment platform, which loads its assets from tbcdn.talentbrew.com, yet Radancy's script turns up on only 212 Kestrel domains in all.

Sales signal: strip the cash-for-cars titles before you count accounts, and treat a careers-subdomain hit as Radancy's decision, not the employer's.

Growth trajectory

The chart reads 11,889 in June 2026, then 22,467, 24,494, 25,827 and 26,519 in October. The July jump of 10,578 is not a wave of new .NET apps. The domain set we crawl turns over every month, with dead sites dropping out and new ones added, and our detection signals widen over time; both moved that month. Since then the monthly gain has shrunk from 2,027 to 692. Since March 2026, 2,366 domains have dropped Kestrel on a hostname, and about one in ten of those still shows it on another.

"Most server headers tell you who runs the machine. A Kestrel header tells you a .NET team wrote the application." - David Thomson, CTO at TechnologyChecker.io

Sales signal: this is a small, steady pool of about 26,500 domains, not a fast-growing one. Its value is precision: every row is an ASP.NET Core application.

Who runs Kestrel

48.59% of matched companies have 1 to 10 employees, below the 69.2% on Apache's profile, and 4.84% have more than 1,000 staff, about five times Apache's 0.94%. The enterprise end is mostly Radancy careers sites (Unilever, Ford, Mayo Clinic, British Airways). The exceptions are more telling. S&P Global runs its data Marketplace on Kestrel, Air Products its customer login, the University of Washington its continuing-education site and Young Living its main site. The companies are older than the server: 36.09% were founded before 2000.

Sales signal: outside the careers sites, a Kestrel subdomain at a large company is usually a customer-facing application with an engineering team behind it. That team buys developer tooling and cloud capacity.

Countries and industries

The United States leads with 3,844 matched companies, then the United Kingdom (1,028), Brazil (557) and the Netherlands (400). The comparison with IIS is the useful part. On our IIS profile India and Italy are 2nd and 3rd; here they're 9th (228) and 10th (214), while Denmark, Sweden and Norway add 540. Software Development leads industries at 7.77%. Motor Vehicle Manufacturing is fourth at 4.44%, but 214 of its 427 companies are car dealers or cash-for-cars sites under a manufacturing label.

Sales signal: for .NET tooling, the Netherlands and the Nordics are worth more here than their size suggests. Skip the automotive rows unless you sell to dealers.

Mixed setups, not migrations

We test every server pair for switching, and none of Kestrel's passed. The header is per hostname, and Kestrel often has a partner: 1,248 domains show Kestrel and Nginx today, 946 Kestrel and Apache, 805 Kestrel and IIS and 215 Kestrel and Phusion Passenger. Most domains ever seen with both still run both. Microsoft Learn explains the IIS case: hosted in-process, an ASP.NET Core app answers through IIS's own server, and only out of process does IIS hand requests to Kestrel. So an app can move between IIS and Kestrel headers without moving anywhere.

"A header that flips from IIS to Kestrel usually means someone changed a hosting setting, not a vendor. Count the application, not the header." - David Thomson, CTO at TechnologyChecker.io

Sales signal: the 805 domains on both Kestrel and IIS look like Windows shops part-way through modernising. That's a better hosting or .NET consulting list than either server's alone.

The stack: Azure and .NET

Of the 26,721 live Kestrel domains left after we removed for-sale and gambling pages, 17.29% carry Azure markers (App Service cookies and headers or Azure DNS), and with Azure's CDN and Front Door the family reaches 5,022 of 26,735 live domains. Amazon Web Services is on 6.83%. Our ASP.NET rule fires on only 19.91%, because it looks for cookies and Web Forms markup that many ASP.NET Core apps never send, and Blazor shows on 8.64%. Umbraco, the .NET CMS, is on 2.69%, and Google Analytics on 45.36%. 122 domains still show Azure's default "up and running" or welcome page.

Sales signal: about one Kestrel domain in six is visibly on Azure. Azure partners, .NET consultancies and Blazor component vendors have the clearest fit.

The .NET support deadline

Microsoft's .NET support policy lists .NET 8 and .NET 9 as ending support on 10 November 2026, a month after this crawl, and .NET 6 as out of support since 12 November 2024. Kestrel's header carries no version, so we can't say which release any domain runs. Unless a domain already runs .NET 10, its release is out of support or about to be.

Sales signal: open with the date. An upgrade conversation with a .NET team in November 2026 doesn't need a pitch.

Key takeaways

1. 26,519 domains, 9th of 70 web servers, each one an ASP.NET Core application.

2. 1,930 of the live domains we checked (7.22%) are one cash-for-cars template network.

3. 39 of the top 50 companies are Radancy-built careers sites.

4. 4.84% of matched companies have more than 1,000 staff; Apache's profile shows 0.94%.

5. 17.29% carry Azure markers, and .NET 8 and 9 leave support on 10 November 2026.

How to use this list

Filter out the automotive titles, then split careers subdomains from the rest. Our technology detection methods post explains how header signals are matched. For the full list of 26,519 Kestrel domains with company, size and stack data, explore TechnologyChecker.io.