HTTP Cheat Sheet

Below is a collection of useful things to know about the HTTP protocol including methods response codes etc etc.

HTTP Request Types

Whenever a request is sent by your web client of choice the supplied HTTP client header needs to contain a Method for the request type been initiated this lets the server know what request the client is making below is a breakdown of request types:

GET – This is normally used to request retrieval of data from a server.
POST – This is normally used to send data to a server often causing a change of state on the server side application such as submitting details which then get stored in a DB
OPTIONS – This is used to determine which request types the server supports
PUT – This replaces the target resource with the supplied payload.
DELETE – This is used to delete the specified resource
HEAD – This is similar to the GET request however only the HTTP header is returned.
TRACE – performs a loop-back test to the target resource

Client side HTTP Header fields

Here is a collection of common client side HTTP header fields and what there intended function is:

Field nameDescriptionExample
A-IMAcceptable instance-manipulations for the request.[10]A-IM: feed
AcceptMedia type(s) that is/are acceptable for the response. See Content negotiation.Accept: text/html
Accept-CharsetCharacter sets that are acceptable.Accept-Charset: utf-8
Accept-DatetimeAcceptable version in time.Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT
Accept-EncodingList of acceptable encodings. See HTTP compression.Accept-Encoding: gzip, deflate
Accept-LanguageList of acceptable human languages for response. See Content negotiation.Accept-Language: en-US
Access-Control-Request-Method,
Access-Control-Request-Headers[11]
Initiates a request for cross-origin resource sharing with Origin (below).Access-Control-Request-Method: GET
AuthorizationAuthentication credentials for HTTP authentication.Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
Cache-ControlUsed to specify directives that must be obeyed by all caching mechanisms along the request-response chain.Cache-Control: no-cache
ConnectionControl options for the current connection and list of hop-by-hop request fields.[12] Must not be used with HTTP/2.[13]Connection: keep-alive Connection: Upgrade
Content-EncodingThe type of encoding used on the data. See HTTP compression.Content-Encoding: gzip
Content-LengthThe length of the request body in octets (8-bit bytes).Content-Length: 348
Content-MD5A Base64-encoded binary MD5 sum of the content of the request body.Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
Content-TypeThe Media type of the body of the request (used with POST and PUT requests).Content-Type: application/x-www-form-urlencoded
CookieAn HTTP cookie previously sent by the server with Set-Cookie (below).Cookie: $Version=1; Skin=new;
DateThe date and time at which the message was originated (in “HTTP-date” format as defined by RFC 7231 Date/Time Formats).Date: Tue, 15 Nov 1994 08:12:31 GMT
ExpectIndicates that particular server behaviors are required by the client.Expect: 100-continue
ForwardedDisclose original information of a client connecting to a web server through an HTTP proxy.[15]Forwarded: for=192.0.2.60;proto=http;by=203.0.113.43 Forwarded: for=192.0.2.43, for=198.51.100.17
FromThe email address of the user making the request.From: user@example.com
HostThe domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested. Mandatory since HTTP/1.1.[16] If the request is generated directly in HTTP/2, it should not be used.[17]Host: en.wikipedia.org:8080 Host: en.wikipedia.org
HTTP2-SettingsA request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Setting header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.[18][19]HTTP2-Settings: token64
If-MatchOnly perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.If-Match: "737060cd8c284d8af7ad3082f209582d"
If-Modified-SinceAllows a 304 Not Modified to be returned if content is unchanged.If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If-None-MatchAllows a 304 Not Modified to be returned if content is unchanged, see HTTP ETag.If-None-Match: "737060cd8c284d8af7ad3082f209582d"
If-RangeIf the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.If-Range: "737060cd8c284d8af7ad3082f209582d"
If-Unmodified-SinceOnly send the response if the entity has not been modified since a specific time.If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
Max-ForwardsLimit the number of times the message can be forwarded through proxies or gateways.Max-Forwards: 10
Origin[11]Initiates a request for cross-origin resource sharing (asks server for Access-Control-* response fields).Origin: http://www.example-social-network.com
PragmaImplementation-specific fields that may have various effects anywhere along the request-response chain.Pragma: no-cache
Proxy-AuthorizationAuthorization credentials for connecting to a proxy.Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
RangeRequest only part of an entity. Bytes are numbered from 0. See Byte serving.Range: bytes=500-999
Referer [sic]This is the address of the previous web page from which a link to the currently requested page was followed. (The word “referrer” has been misspelled in the RFC as well as in most implementations to the point that it has become standard usage and is considered correct terminology)Referer: http://en.wikipedia.org/wiki/Main_Page
TEThe transfer encodings the user agent is willing to accept: the same values as for the response header field Transfer-Encoding can be used, plus the “trailers” value (related to the “chunked” transfer method) to notify the server it expects to receive additional fields in the trailer after the last, zero-sized, chunk. Only trailers is supported in HTTP/2.[13]TE: trailers, deflate
TrailerThe Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer coding.Trailer: Max-Forwards
Transfer-EncodingThe form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity. Must not be used with HTTP/2.[13]Transfer-Encoding: chunked
User-AgentThe user agent string of the user agent.User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0
UpgradeAsk the server to upgrade to another protocol. Must not be used in HTTP/2.[13]Upgrade: h2c, HTTPS/1.3, IRC/6.9, RTA/x11, websocket
ViaInforms the server of proxies through which the request was sent.Via: 1.0 fred, 1.1 example.com (Apache/1.1)
WarningA general warning about possible problems with the entity body.Warning: 199 Miscellaneous warning
Field nameDescriptionExample
Upgrade-Insecure-Requests[20]Tells a server which (presumably in the middle of a HTTP -> HTTPS migration) hosts mixed content that the client would prefer redirection to HTTPS and can handle Content-Security-Policy: upgrade-insecure-requests Must not be used with HTTP/2[13]Upgrade-Insecure-Requests: 1
X-Requested-WithMainly used to identify Ajax requests (most JavaScript frameworks send this field with value of XMLHttpRequest); also identifies Android apps using WebView[21]X-Requested-With: XMLHttpRequest
DNT[22]Requests a web application to disable their tracking of a user. This is Mozilla’s version of the X-Do-Not-Track header field (since Firefox 4.0 Beta 11). Safari and IE9 also have support for this field.[23] On March 7, 2011, a draft proposal was submitted to IETF.[24] The W3C Tracking Protection Working Group is producing a specification.[25]DNT: 1 (Do Not Track Enabled) DNT: 0 (Do Not Track Disabled)
X-Forwarded-For[26]A de facto standard for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer. Superseded by Forwarded header.X-Forwarded-For: client1, proxy1, proxy2 X-Forwarded-For: 129.78.138.66, 129.78.64.103
X-Forwarded-Host[27]A de facto standard for identifying the original host requested by the client in the Host HTTP request header, since the host name and/or port of the reverse proxy (load balancer) may differ from the origin server handling the request. Superseded by Forwarded header.X-Forwarded-Host: en.wikipedia.org:8080 X-Forwarded-Host: en.wikipedia.org
X-Forwarded-Proto[28]A de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (or a load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS. An alternative form of the header (X-ProxyUser-Ip) is used by Google clients talking to Google servers. Superseded by Forwarded header.X-Forwarded-Proto: https
Front-End-Https[29]Non-standard header field used by Microsoft applications and load-balancersFront-End-Https: on
X-Http-Method-Override[30]Requests a web application to override the method specified in the request (typically POST) with the method given in the header field (typically PUT or DELETE). This can be used when a user agent or firewall prevents PUT or DELETE methods from being sent directly (note that this is either a bug in the software component, which ought to be fixed, or an intentional configuration, in which case bypassing it may be the wrong thing to do).X-HTTP-Method-Override: DELETE
X-ATT-DeviceId[31]Allows easier parsing of the MakeModel/Firmware that is usually found in the User-Agent String of AT&T DevicesX-Att-Deviceid: GT-P7320/P7320XXLPG
X-Wap-Profile[32]Links to an XML file on the Internet with a full description and details about the device currently connecting. In the example to the right is an XML file for an AT&T Samsung Galaxy S2.x-wap-profile: http://wap.samsungmobile.com/uaprof/SGH-I777.xml
Proxy-Connection[33]Implemented as a misunderstanding of the HTTP specifications. Common because of mistakes in implementations of early HTTP versions. Has exactly the same functionality as standard Connection field. Must not be used with HTTP/2.[13]Proxy-Connection: keep-alive
X-UIDH[34][35][36]Server-side deep packet insertion of a unique ID identifying customers of Verizon Wireless; also known as “perma-cookie” or “supercookie”X-UIDH: ...
X-Csrf-Token[37]Used to prevent cross-site request forgery. Alternative header names are: X-CSRFToken[38] and X-XSRF-TOKEN[39]X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql
X-Request-ID[40][41], X-Correlation-ID[42][43]Correlates HTTP requests between a client and server.X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5
Save-DataThe Save-Data client hint request header available in Chrome, Opera, and Yandex browsers lets developers deliver lighter, faster applications to users who opt-in to data saving mode in their browser.Save-Data: on

Server Side HTTP Header Fields

Here is a collection of common client side HTTP header fields and what there intended function is:

Field nameDescriptionExample
Access-Control-Allow-Origin,
Access-Control-Allow-Credentials,
Access-Control-Expose-Headers,
Access-Control-Max-Age,
Access-Control-Allow-Methods,
Access-Control-Allow-Headers[11]
Specifying which web sites can participate in cross-origin resource sharingAccess-Control-Allow-Origin: *
Accept-Patch[44]Specifies which patch document formats this server supportsAccept-Patch: text/example;charset=utf-8
Accept-RangesWhat partial content range types this server supports via byte servingAccept-Ranges: bytes
AgeThe age the object has been in a proxy cache in secondsAge: 12
AllowValid methods for a specified resource. To be used for a 405 Method not allowedAllow: GET, HEAD
Alt-Svc[45]A server uses “Alt-Svc” header (meaning Alternative Services) to indicate that its resources can also be accessed at a different network location (host or port) or using a different protocol When using HTTP/2, servers should instead send an ALTSVC frame. [46]Alt-Svc: http/1.1="http2.example.com:8001"; ma=7200
Cache-ControlTells all caching mechanisms from server to client whether they may cache this object. It is measured in secondsCache-Control: max-age=3600
ConnectionControl options for the current connection and list of hop-by-hop response fields.[12] Must not be used with HTTP/2.[13]Connection: close
Content-Disposition[47]An opportunity to raise a “File Download” dialogue box for a known MIME type with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters.Content-Disposition: attachment; filename="fname.ext"
Content-EncodingThe type of encoding used on the data. See HTTP compression.Content-Encoding: gzip
Content-LanguageThe natural language or languages of the intended audience for the enclosed content[48]Content-Language: da
Content-LengthThe length of the response body in octets (8-bit bytes)Content-Length: 348
Content-LocationAn alternate location for the returned dataContent-Location: /index.htm
Content-MD5A Base64-encoded binary MD5 sum of the content of the responseContent-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
Content-RangeWhere in a full body message this partial message belongsContent-Range: bytes 21010-47021/47022
Content-TypeThe MIME type of this contentContent-Type: text/html; charset=utf-8
DateThe date and time that the message was sent (in “HTTP-date” format as defined by RFC 7231) [49]Date: Tue, 15 Nov 1994 08:12:31 GMT
Delta-BaseSpecifies the delta-encoding entity tag of the response.[10]Delta-Base: "abc"
ETagAn identifier for a specific version of a resource, often a message digestETag: "737060cd8c284d8af7ad3082f209582d"
ExpiresGives the date/time after which the response is considered stale (in “HTTP-date” format as defined by RFC 7231)Expires: Thu, 01 Dec 1994 16:00:00 GMT
IMInstance-manipulations applied to the response.[10]IM: feed
Last-ModifiedThe last modified date for the requested object (in “HTTP-date” format as defined by RFC 7231)Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
LinkUsed to express a typed relationship with another resource, where the relation type is defined by RFC 5988Link: </feed>; rel="alternate"[50]
LocationUsed in redirection, or when a new resource has been created.Example 1: Location: http://www.w3.org/pub/WWW/People.html Example 2: Location: /pub/WWW/People.html
P3PThis field is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[51] most browsers have never fully implemented it, a lot of websites set this field with fake policy text, that was enough to fool browsers the existence of P3P policy and grant permissions for third party cookies.P3P: CP="This is not a P3P policy! See https://en.wikipedia.org/wiki/Special:CentralAutoLogin/P3P for more info."
PragmaImplementation-specific fields that may have various effects anywhere along the request-response chain.Pragma: no-cache
Proxy-AuthenticateRequest authentication to access the proxy.Proxy-Authenticate: Basic
Public-Key-Pins[52]HTTP Public Key Pinning, announces hash of website’s authentic TLS certificatePublic-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
Retry-AfterIf an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.[53]Example 1: Retry-After: 120 Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT
ServerA name for the serverServer: Apache/2.4.1 (Unix)
Set-CookieAn HTTP cookieSet-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1
Strict-Transport-SecurityA HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains.Strict-Transport-Security: max-age=16070400; includeSubDomains
TrailerThe Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer coding.Trailer: Max-Forwards
Transfer-EncodingThe form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity. Must not be used with HTTP/2.[13]Transfer-Encoding: chunked
TkTracking Status header, value suggested to be sent in response to a DNT(do-not-track), possible values: “!” — under construction “?” — dynamic “G” — gateway to multiple parties “N” — not tracking “T” — tracking “C” — tracking with consent “P” — tracking only if consented “D” — disregarding DNT “U” — updatedTk: ?
UpgradeAsk the client to upgrade to another protocol. Must not be used in HTTP/2[13]Upgrade: h2c, HTTPS/1.3, IRC/6.9, RTA/x11, websocket
VaryTells downstream proxies how to match future request headers to decide whether the cached response can be used rather than requesting a fresh one from the origin server.Example 1: Vary: * Example 2: Vary: Accept-Language
ViaInforms the client of proxies through which the response was sent.Via: 1.0 fred, 1.1 example.com (Apache/1.1)
WarningA general warning about possible problems with the entity body.Warning: 199 Miscellaneous warning
WWW-AuthenticateIndicates the authentication scheme that should be used to access the requested entity.WWW-Authenticate: Basic
X-Frame-Options[54]Clickjacking protection: deny – no rendering within a frame, sameorigin – no rendering if origin mismatch, allow-from – allow from specified location, allowall – non-standard, allow from any locationX-Frame-Options: deny
Field nameDescriptionExample
Content-Security-Policy,
X-Content-Security-Policy,
X-WebKit-CSP[56]
Content Security Policy definition.X-WebKit-CSP: default-src 'self'
RefreshUsed in redirection, or when a new resource has been created. This refresh redirects after 5 seconds. Header extension introduced by Netscape and supported by most web browsers.Refresh: 5; url=http://www.w3.org/pub/WWW/People.html
StatusCGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate “Status-Line” instead, defined by RFC 7230.[57]Status: 200 OK
Timing-Allow-OriginThe Timing-Allow-Origin response header specifies origins that are allowed to see values of attributes retrieved via features of the Resource Timing API, which would otherwise be reported as zero due to cross-origin restrictions.[58]Timing-Allow-Origin: * Timing-Allow-Origin: <origin>[, <origin>]*
X-Content-Duration[59]Provide the duration of the audio or video in seconds; only supported by Gecko browsersX-Content-Duration: 42.666
X-Content-Type-Options[60]The only defined value, “nosniff”, prevents Internet Explorer from MIME-sniffing a response away from the declared content-type. This also applies to Google Chrome, when downloading extensions.[61]X-Content-Type-Options: nosniff[62]
X-Powered-By[63]Specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in X-Runtime, X-Version, or X-AspNet-Version)X-Powered-By: PHP/5.4.0
X-Request-ID,
X-Correlation-ID[40]
Correlates HTTP requests between a client and server.X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5
X-UA-Compatible[64]Recommends the preferred rendering engine (often a backward-compatibility mode) to use to display the content. Also used to activate Chrome Frame in Internet Explorer.X-UA-Compatible: IE=EmulateIE7
X-UA-Compatible: IE=edge
X-UA-Compatible: Chrome=1
X-XSS-Protection[65]Cross-site scripting (XSS) filterX-XSS-Protection: 1; mode=block

HTTP Response Codes

1xx: Informational - Request received, continuing process
2xx: Success - The action was successfully received, understood, and accepted
3xx: Redirection - Further action must be taken in order to complete the request
4xx: Client Error - The request contains bad syntax or cannot be fulfilled
5xx: Server Error - The server failed to fulfill an apparently valid request
    
Value Description Reference
100Continue[RFC7231, Section 6.2.1]
101Switching Protocols[RFC7231, Section 6.2.2]
102Processing[RFC2518]
103Early Hints[RFC8297]
104-199Unassigned
200OK[RFC7231, Section 6.3.1]
201Created[RFC7231, Section 6.3.2]
202Accepted[RFC7231, Section 6.3.3]
203Non-Authoritative Information[RFC7231, Section 6.3.4]
204No Content[RFC7231, Section 6.3.5]
205Reset Content[RFC7231, Section 6.3.6]
206Partial Content[RFC7233, Section 4.1]
207Multi-Status[RFC4918]
208Already Reported[RFC5842]
209-225Unassigned
226IM Used[RFC3229]
227-299Unassigned
300Multiple Choices[RFC7231, Section 6.4.1]
301Moved Permanently[RFC7231, Section 6.4.2]
302Found[RFC7231, Section 6.4.3]
303See Other[RFC7231, Section 6.4.4]
304Not Modified[RFC7232, Section 4.1]
305Use Proxy[RFC7231, Section 6.4.5]
306(Unused)[RFC7231, Section 6.4.6]
307Temporary Redirect[RFC7231, Section 6.4.7]
308Permanent Redirect[RFC7538]
309-399Unassigned
400Bad Request[RFC7231, Section 6.5.1]
401Unauthorized[RFC7235, Section 3.1]
402Payment Required[RFC7231, Section 6.5.2]
403Forbidden[RFC7231, Section 6.5.3]
404Not Found[RFC7231, Section 6.5.4]
405Method Not Allowed[RFC7231, Section 6.5.5]
406Not Acceptable[RFC7231, Section 6.5.6]
407Proxy Authentication Required[RFC7235, Section 3.2]
408Request Timeout[RFC7231, Section 6.5.7]
409Conflict[RFC7231, Section 6.5.8]
410Gone[RFC7231, Section 6.5.9]
411Length Required[RFC7231, Section 6.5.10]
412Precondition Failed[RFC7232, Section 4.2][RFC8144, Section 3.2]
413Payload Too Large[RFC7231, Section 6.5.11]
414URI Too Long[RFC7231, Section 6.5.12]
415Unsupported Media Type[RFC7231, Section 6.5.13][RFC7694, Section 3]
416Range Not Satisfiable[RFC7233, Section 4.4]
417Expectation Failed[RFC7231, Section 6.5.14]
418-420Unassigned
421Misdirected Request[RFC7540, Section 9.1.2]
422Unprocessable Entity[RFC4918]
423Locked[RFC4918]
424Failed Dependency[RFC4918]
425Too Early[RFC8470]
426Upgrade Required[RFC7231, Section 6.5.15]
427Unassigned
428Precondition Required[RFC6585]
429Too Many Requests[RFC6585]
430Unassigned
431Request Header Fields Too Large[RFC6585]
432-450Unassigned
451Unavailable For Legal Reasons[RFC7725]
452-499Unassigned
500Internal Server Error[RFC7231, Section 6.6.1]
501Not Implemented[RFC7231, Section 6.6.2]
502Bad Gateway[RFC7231, Section 6.6.3]
503Service Unavailable[RFC7231, Section 6.6.4]
504Gateway Timeout[RFC7231, Section 6.6.5]
505HTTP Version Not Supported[RFC7231, Section 6.6.6]
506Variant Also Negotiates[RFC2295]
507Insufficient Storage[RFC4918]
508Loop Detected[RFC5842]
509Unassigned
510Not Extended[RFC2774]
511Network Authentication Required[RFC6585]
512-599Unassigned

Vendor Specific Response codes

The following codes are not specified by any standard.

Internet Information Services

Microsoft’s Internet Information Services (IIS) web server expands the 4xx error space to signal errors with the client’s request.
440 Login Time-out The client’s session has expired and must log in again.[82]
449 Retry With The server cannot honour the request because the user has not provided the required information.[83]
451 Redirect Used in Exchange ActiveSync when either a more efficient server is available or the server cannot access the users’ mailbox.[84] The client is expected to re-run the HTTP AutoDiscover operation to find a more appropriate server.[85]

IIS sometimes uses additional decimal sub-codes for more specific information,[86] however these sub-codes only appear in the response payload and in documentation, not in the place of an actual HTTP status code.

nginx

The nginx web server software expands the 4xx error space to signal issues with the client’s request.[87][88] 444 No Response Used internally[89] to instruct the server to return no information to the client and close the connection immediately.
494 Request header too large Client sent too large request or too long header line.
495 SSL Certificate Error An expansion of the 400 Bad Request response code, used when the client has provided an invalid client certificate.
496 SSL Certificate Required An expansion of the 400 Bad Request response code, used when a client certificate is required but not provided.
497 HTTP Request Sent to HTTPS Port An expansion of the 400 Bad Request response code, used when the client has made a HTTP request to a port listening for HTTPS requests.
499 Client Closed Request Used when the client has closed the request before the server could send a response.

Cloudflare

Cloudflare‘s reverse proxy service expands the 5xx series of errors space to signal issues with the origin server.[90]
520 Web Server Returned an Unknown Error The origin server returned an empty, unknown, or unexplained response to Cloudflare.[91]
521 Web Server Is Down The origin server has refused the connection from Cloudflare.
522 Connection Timed Out Cloudflare could not negotiate a TCP handshake with the origin server.
523 Origin Is Unreachable Cloudflare could not reach the origin server; for example, if the DNS records for the origin server are incorrect.
524 A Timeout Occurred Cloudflare was able to complete a TCP connection to the origin server, but did not receive a timely HTTP response.
525 SSL Handshake Failed Cloudflare could not negotiate a SSL/TLS handshake with the origin server.
526 Invalid SSL Certificate Cloudflare could not validate the SSL certificate on the origin web server.
527 Railgun Error Error 527 indicates an interrupted connection between Cloudflare and the origin server’s Railgun server.[92]
530 Error 530 is returned along with a 1xxx error.[93]

AWS Elastic Load Balancer

Amazon‘s Elastic Load Balancing adds a few custom 4xx return codes

460 Client closed the connection with the load balancer before the idle timeout period elapsed. Typically when client timeout is sooner than the Elastic Load Balancer’s timeout.

463 The load balancer received an X-Forwarded-For request header with more than 30 IP addresses.

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