Which HTTP status codes count as down in uptime monitoring?

5xx codes almost always mean down. Unexpected 4xx codes on a page that should be public, such as 403 or 404 on the home page, usually mean something is broken too. 2xx and 3xx codes normally count as up.

What happens if a client receives a status code it does not recognize?

RFC 9110 says a client should treat an unrecognized code as equivalent to the x00 code of its class. An unknown 5xx code such as 599 is handled like a 500.

Where are HTTP status codes officially defined?

Most are defined in RFC 9110, HTTP Semantics. Some come from other RFCs, such as RFC 6585 for 429, 431 and 511. IANA maintains the official HTTP Status Code Registry that lists every registered code and its defining document.

Are codes like 520 or 499 official?

No. They are vendor-specific. Cloudflare uses 520 to 530 for problems between its edge and your origin, and nginx logs 499 when the client closed the connection before a response was sent. They are not in the IANA registry.

HTTP Status Codes - A Plain-English Reference for Monitoring

An HTTP status code is the three-digit number at the start of every HTTP response. The first digit tells you the class of result, and the remaining digits narrow it down. The meanings below follow RFC 9110 (HTTP Semantics, 2022), which defines most codes, and RFC 6585 for 429, 431 and 511; where another RFC defines a code, it is named. Each entry adds what the code means when an uptime monitor sees it.

Each code links to its own page with causes, fixes and what it means for monitoring; the full index of HTTP status codes lists them by class.

The five classes at a glance

Class	Meaning	Typical monitoring verdict
1xx Informational	Request received, still processing	Not a final answer; clients wait for the next response
2xx Success	Request received, understood and accepted	Up
3xx Redirection	Further action needed, usually following another URL	Up, if the redirect is expected
4xx Client error	The request has a problem, or is not allowed	Depends; unexpected 4xx on a public page usually means broken
5xx Server error	The server failed to handle a valid request	Down

A client that receives a code it does not know should treat it as the x00 code of the same class, so an unknown 5xx is handled like 500.

1xx Informational

Code	Name	Plain-English meaning	For monitoring
100	Continue	Go ahead and send the request body.	Interim only; monitors see the final code that follows.
101	Switching Protocols	The server is switching protocols, for example to WebSocket.	Expected on WebSocket upgrades; not used for normal page checks.
103	Early Hints	Preload these resources while the final response is prepared (RFC 8297).	Interim only; ignore it and judge the final code.

2xx Success

Code	Name	Plain-English meaning	For monitoring
200	OK	The request worked and here is the result.	Up. Pair it with a keyword check if error pages might also return 200.
201	Created	The request worked and created a new resource.	Up; common from API POST endpoints.
202	Accepted	Received and queued, but not finished yet.	Up; the work itself is not confirmed.
204	No Content	Success, with no body to return.	Up; common for health and API endpoints. Keyword checks cannot work on an empty body.
206	Partial Content	Here is the byte range you asked for.	Up; seen on downloads and video.

3xx Redirection

Code	Name	Plain-English meaning	For monitoring
301	Moved Permanently	This URL has moved for good; use the new one.	Up if expected, such as HTTP to HTTPS. Monitor the final URL directly when you can.
302	Found	Temporarily at another URL.	Up if expected. A surprise 302 to a login page can mean a session or auth problem.
303	See Other	Go fetch the result from another URL with GET.	Up; typical after a form submission.
304	Not Modified	Your cached copy is still valid.	Up; only appears on conditional requests.
307	Temporary Redirect	Temporarily elsewhere; repeat the same method and body there.	Up if expected.
308	Permanent Redirect	Moved for good; repeat the same method and body at the new URL.	Up if expected.

Watch for redirect loops: a URL that redirects to itself, directly or through a chain, never returns a page. A monitor that follows redirects with a hop limit reports this as a failure.

4xx Client errors

Code	Name	Plain-English meaning	For monitoring
400	Bad Request	The server cannot process this request as sent.	Usually a misconfigured check, not an outage. Verify the URL and headers.
401	Unauthorized	Authentication is required or failed.	Expected on protected endpoints. Monitor a public URL, or send credentials meant for monitoring.
403	Forbidden	Understood, but refused.	On a public page, often a firewall, WAF or bot rule blocking the monitor, or a permissions error after deploy.
404	Not Found	Nothing exists at this URL.	Down for that page; often a broken route, a bad deploy or a wrong document root.
405	Method Not Allowed	This URL does not accept this method.	Often a HEAD request that the app does not support. Check with GET.
408	Request Timeout	The client took too long to send the request.	Rare for monitors; investigate if it repeats.
409	Conflict	The request conflicts with the current state of the resource.	API-level; not an availability signal.
410	Gone	Removed on purpose and not coming back.	Update or remove the monitor.
413	Content Too Large	The request body is larger than the server allows.	API and upload checks; not usually availability.
414	URI Too Long	The URL is longer than the server accepts.	Rare; check the monitored URL.
415	Unsupported Media Type	The body's format is not accepted.	API checks; fix the Content-Type.
421	Misdirected Request	This server cannot answer for this hostname.	Can indicate TLS or virtual host misconfiguration.
422	Unprocessable Content	Well-formed, but the content is invalid.	API-level validation; not availability.
429	Too Many Requests	Rate limit hit; slow down (RFC 6585).	The site is up but throttling. Lower check frequency or allowlist the monitor.
431	Request Header Fields Too Large	Headers, often cookies, are too big (RFC 6585).	Rare for monitors; real users with large cookies may be affected.
451	Unavailable For Legal Reasons	Blocked for legal reasons (RFC 7725).	Can be regional; compare results from different regions.

5xx Server errors

Code	Name	Plain-English meaning	For monitoring
500	Internal Server Error	Something unexpected broke on the server.	Down. See HTTP 500.
501	Not Implemented	The server does not support this method at all.	Usually the check's method; switch HEAD to GET.
502	Bad Gateway	A proxy got an invalid response from the server behind it.	Down. See 502 Bad Gateway.
503	Service Unavailable	Temporarily overloaded or in maintenance.	Down, even if planned. See 503 Service Unavailable.
504	Gateway Timeout	A proxy gave up waiting for the server behind it.	Down. See 504 Gateway Timeout.
505	HTTP Version Not Supported	The server refuses this HTTP version.	Rare; usually a client or proxy configuration issue.
511	Network Authentication Required	Log in to the network first, as on hotel Wi-Fi (RFC 6585).	Indicates a captive portal between client and site, not a site failure.

Codes you will see that are not standard

Some codes appear in logs and error pages but are not in RFC 9110 or the IANA registry:

499 – logged by nginx when the client closed the connection before the response was sent. Users never receive it.
520–527 and 530 – Cloudflare-specific codes for problems between Cloudflare and your origin, such as 521 (origin refused the connection), 522 (connection timed out) and 524 (origin did not respond in time).
418 – listed in RFC 9110 as reserved and unused. It originated as an April Fools' joke and should not appear in production.

Treat vendor-specific 5xx codes as down, like any other 5xx.

Choosing which codes mean "up"

A sensible default for a public website is: 2xx and 3xx mean up, everything else means down, confirmed by a re-check before alerting. Two adjustments make that default more reliable:

Monitor the final URL. If http://example.com redirects to https://www.example.com/, monitor the HTTPS URL so a broken redirect and a broken site are not confused.
Check content, not just the code. A 200 response can still be a blank page, a database error rendered as HTML, or a parked domain. A keyword monitor that looks for text only present on a healthy page closes that gap.

PingRobot offers HTTP(S) and keyword monitors and re-checks every failure from a second region before alerting. If you are in the middle of an outage, use the website down checklist.

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