1xx Informational
HTTP 101 Switching Protocols
The server accepts the client’s Upgrade request and switches the connection to the protocol it names. Today that is almost always a WebSocket handshake over HTTP/1.1: after the 101, the same connection carries WebSocket messages instead of HTTP.
Defined in RFC 9110, section 15.2.2(opens in a new tab)
//_01 Uptime checks
Is 101 up or down for a monitor?
Uptimecheck HTTP monitor
Default settings
A monitor’s request doesn’t ask to switch protocols, so a normal server never answers it with 101. If one did, it would be outside the default 200–399 range and count as down.
Is it down? checker
Free, at /is-it-down/
Below 500, so it would count as the site answering.
//_02 Causes and fixes
Why you see 101 and what to do
Typical causes
- A WebSocket connection being opened (Upgrade: websocket).
If you run the site
- If WebSockets fail behind a reverse proxy, the proxy must pass the Upgrade and Connection headers on. nginx needs proxy_http_version 1.1 and both headers set explicitly.
- HTTP/2 and HTTP/3 don’t use 101; WebSockets over HTTP/2 use an extended CONNECT request instead.
If you’re a visitor
- Nothing to do: it is how live features such as chat and dashboards connect.
Know the moment your site stops answering as it should
An Uptimecheck monitor requests your page as often as every minute, checks the status code against the ones you expect, confirms a failure with a second check and then alerts you.
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Questions
What does HTTP 101 mean?
The server accepts the client’s Upgrade request and switches the connection to the protocol it names. Today that is almost always a WebSocket handshake over HTTP/1.1: after the 101, the same connection carries WebSocket messages instead of HTTP.
Does an uptime monitor count 101 as down?
A monitor’s request doesn’t ask to switch protocols, so a normal server never answers it with 101. If one did, it would be outside the default 200–399 range and count as down.
Downtime is obsolete.
Set up your first monitor in under a minute, or look around the live demo first.
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