It’s important to know your HTTP error codes even if you’re a network engineer. You don’t need to memorize them but know about them. You’ll almost always get called into or blamed for them :). I’m actually shocked at times when developers and other engineers have no clue about them.
For example, the “new” setup is giving a 500 error. Network, please check your side and maybe run a packet capture. Ummm, no. A 500 error is a server side error. So I know the network is not the first place you look. If the end user is getting a 500, that means the client is actually communicating with the server so it’s not network. I know just based off the error. The same goes for 400 errors. If the end user is getting a 404-Not Found or 403-Forbidden, we know that the communication is fine. Those are errors coming from the client but giving the error because of what the client got back from the server. HTTP 404 errors indicate the network connection between client and server was made successfully.
You can see it’s important to know how things work to help make troubleshooting more efficient. If you have both sides that don’t know what they are doing, you could spend hours looking into the network waisting time. At least on the network side, knowing these errors will help your team not waist time.
Now I’m not saying it’s never the network but 100% of the time it was never the network when I got the 400/500 error. Usually, if it’s the network, you won’t get any response. Or more than one thing will be down, not just that server. Even with a Firewall in between, if the port is not open, it’s going to timeout. You won’t get back the detailed error messages and the message you do get will be something like ERR_CONNECTION_REFUSED.
Types of HTTP Error and Status Codes:
100-199: informational status
200-299: success status
300-399: redirection status
400-499: client errors
500-599: server errors
It’s always good to check out the RFC’s at https://tools.ietf.org/html/. They have a lot of great information on a wide range of topics. It’s a place used to develop a “standard” network protocol, a function of a network protocol or any feature which is related with network communication. A good example is EIGRP. We all know it’s a Cisco proprietary protocol. You could only use it with Cisco hardware/software. But the Functionality of EIGRP was converted to an open standard in 2013 and was published with informational status as RFC 7868 in 2016. It’s a stripped down version but you can see that IETF has detailed information about it.
Below is for Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content which lists all the HTTP Error and Status Codes with some good information about them.
6.2. Informational 1xx
The 1xx (Informational) class of status code indicates an interim
response for communicating connection status or request progress
prior to completing the requested action and sending a final
response. 1xx responses are terminated by the first empty line after
the status-line (the empty line signaling the end of the header
section). Since HTTP/1.0 did not define any 1xx status codes, a
server MUST NOT send a 1xx response to an HTTP/1.0 client.
A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one. A
user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself requested
the generation of the 1xx response. For example, if a proxy adds an
“Expect: 100-continue” field when it forwards a request, then it need
not forward the corresponding 100 (Continue) response(s).
6.2.1. 100 Continue
The 100 (Continue) status code indicates that the initial part of a
request has been received and has not yet been rejected by the
server. The server intends to send a final response after the
request has been fully received and acted upon.
When the request contains an Expect header field that includes a
100-continue expectation, the 100 response indicates that the server
wishes to receive the request payload body, as described in
Section 5.1.1. The client ought to continue sending the request and
discard the 100 response.
If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim
response.
6.2.2. 101 Switching Protocols
The 101 (Switching Protocols) status code indicates that the server
understands and is willing to comply with the client’s request, via
the Upgrade header field (Section 6.7 of [RFC7230]), for a change in
the application protocol being used on this connection. The server
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MUST generate an Upgrade header field in the response that indicates
which protocol(s) will be switched to immediately after the empty
line that terminates the 101 response.
It is assumed that the server will only agree to switch protocols
when it is advantageous to do so. For example, switching to a newer
version of HTTP might be advantageous over older versions, and
switching to a real-time, synchronous protocol might be advantageous
when delivering resources that use such features.
6.3. Successful 2xx
The 2xx (Successful) class of status code indicates that the client’s
request was successfully received, understood, and accepted.
6.3.1. 200 OK
The 200 (OK) status code indicates that the request has succeeded.
The payload sent in a 200 response depends on the request method.
For the methods defined by this specification, the intended meaning
of the payload can be summarized as:
GET a representation of the target resource;
HEAD the same representation as GET, but without the representation
data;
POST a representation of the status of, or results obtained from,
the action;
PUT, DELETE a representation of the status of the action;
OPTIONS a representation of the communications options;
TRACE a representation of the request message as received by the end
server.
Aside from responses to CONNECT, a 200 response always has a payload,
though an origin server MAY generate a payload body of zero length.
If no payload is desired, an origin server ought to send 204 (No
Content) instead. For CONNECT, no payload is allowed because the
successful result is a tunnel, which begins immediately after the 200
response header section.
A 200 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
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6.3.2. 201 Created
The 201 (Created) status code indicates that the request has been
fulfilled and has resulted in one or more new resources being
created. The primary resource created by the request is identified
by either a Location header field in the response or, if no Location
field is received, by the effective request URI.
The 201 response payload typically describes and links to the
resource(s) created. See Section 7.2 for a discussion of the meaning
and purpose of validator header fields, such as ETag and
Last-Modified, in a 201 response.
6.3.3. 202 Accepted
The 202 (Accepted) status code indicates that the request has been
accepted for processing, but the processing has not been completed.
The request might or might not eventually be acted upon, as it might
be disallowed when processing actually takes place. There is no
facility in HTTP for re-sending a status code from an asynchronous
operation.
The 202 response is intentionally noncommittal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent’s connection to the server persist
until the process is completed. The representation sent with this
response ought to describe the request’s current status and point to
(or embed) a status monitor that can provide the user with an
estimate of when the request will be fulfilled.
6.3.4. 203 Non-Authoritative Information
The 203 (Non-Authoritative Information) status code indicates that
the request was successful but the enclosed payload has been modified
from that of the origin server’s 200 (OK) response by a transforming
proxy (Section 5.7.2 of [RFC7230]). This status code allows the
proxy to notify recipients when a transformation has been applied,
since that knowledge might impact later decisions regarding the
content. For example, future cache validation requests for the
content might only be applicable along the same request path (through
the same proxies).
The 203 response is similar to the Warning code of 214 Transformation
Applied (Section 5.5 of [RFC7234]), which has the advantage of being
applicable to responses with any status code.
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A 203 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.3.5. 204 No Content
The 204 (No Content) status code indicates that the server has
successfully fulfilled the request and that there is no additional
content to send in the response payload body. Metadata in the
response header fields refer to the target resource and its selected
representation after the requested action was applied.
For example, if a 204 status code is received in response to a PUT
request and the response contains an ETag header field, then the PUT
was successful and the ETag field-value contains the entity-tag for
the new representation of that target resource.
The 204 response allows a server to indicate that the action has been
successfully applied to the target resource, while implying that the
user agent does not need to traverse away from its current “document
view” (if any). The server assumes that the user agent will provide
some indication of the success to its user, in accord with its own
interface, and apply any new or updated metadata in the response to
its active representation.
For example, a 204 status code is commonly used with document editing
interfaces corresponding to a “save” action, such that the document
being saved remains available to the user for editing. It is also
frequently used with interfaces that expect automated data transfers
to be prevalent, such as within distributed version control systems.
A 204 response is terminated by the first empty line after the header
fields because it cannot contain a message body.
A 204 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.3.6. 205 Reset Content
The 205 (Reset Content) status code indicates that the server has
fulfilled the request and desires that the user agent reset the
“document view”, which caused the request to be sent, to its original
state as received from the origin server.
This response is intended to support a common data entry use case
where the user receives content that supports data entry (a form,
notepad, canvas, etc.), enters or manipulates data in that space,
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causes the entered data to be submitted in a request, and then the
data entry mechanism is reset for the next entry so that the user can
easily initiate another input action.
Since the 205 status code implies that no additional content will be
provided, a server MUST NOT generate a payload in a 205 response. In
other words, a server MUST do one of the following for a 205
response: a) indicate a zero-length body for the response by
including a Content-Length header field with a value of 0; b)
indicate a zero-length payload for the response by including a
Transfer-Encoding header field with a value of chunked and a message
body consisting of a single chunk of zero-length; or, c) close the
connection immediately after sending the blank line terminating the
header section.
6.4. Redirection 3xx
The 3xx (Redirection) class of status code indicates that further
action needs to be taken by the user agent in order to fulfill the
request. If a Location header field (Section 7.1.2) is provided, the
user agent MAY automatically redirect its request to the URI
referenced by the Location field value, even if the specific status
code is not understood. Automatic redirection needs to done with
care for methods not known to be safe, as defined in Section 4.2.1,
since the user might not wish to redirect an unsafe request.
There are several types of redirects:
1. Redirects that indicate the resource might be available at a
different URI, as provided by the Location field, as in the
status codes 301 (Moved Permanently), 302 (Found), and 307
(Temporary Redirect).
2. Redirection that offers a choice of matching resources, each
capable of representing the original request target, as in the
300 (Multiple Choices) status code.
3. Redirection to a different resource, identified by the Location
field, that can represent an indirect response to the request, as
in the 303 (See Other) status code.
4. Redirection to a previously cached result, as in the 304 (Not
Modified) status code.
Note: In HTTP/1.0, the status codes 301 (Moved Permanently) and
302 (Found) were defined for the first type of redirect
([RFC1945], Section 9.3). Early user agents split on whether the
method applied to the redirect target would be the same as the
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original request or would be rewritten as GET. Although HTTP
originally defined the former semantics for 301 and 302 (to match
its original implementation at CERN), and defined 303 (See Other)
to match the latter semantics, prevailing practice gradually
converged on the latter semantics for 301 and 302 as well. The
first revision of HTTP/1.1 added 307 (Temporary Redirect) to
indicate the former semantics without being impacted by divergent
practice. Over 10 years later, most user agents still do method
rewriting for 301 and 302; therefore, this specification makes
that behavior conformant when the original request is POST.
A client SHOULD detect and intervene in cyclical redirections (i.e.,
“infinite” redirection loops).
Note: An earlier version of this specification recommended a
maximum of five redirections ([RFC2068], Section 10.3). Content
developers need to be aware that some clients might implement such
a fixed limitation.
6.4.1. 300 Multiple Choices
The 300 (Multiple Choices) status code indicates that the target
resource has more than one representation, each with its own more
specific identifier, and information about the alternatives is being
provided so that the user (or user agent) can select a preferred
representation by redirecting its request to one or more of those
identifiers. In other words, the server desires that the user agent
engage in reactive negotiation to select the most appropriate
representation(s) for its needs (Section 3.4).
If the server has a preferred choice, the server SHOULD generate a
Location header field containing a preferred choice’s URI reference.
The user agent MAY use the Location field value for automatic
redirection.
For request methods other than HEAD, the server SHOULD generate a
payload in the 300 response containing a list of representation
metadata and URI reference(s) from which the user or user agent can
choose the one most preferred. The user agent MAY make a selection
from that list automatically if it understands the provided media
type. A specific format for automatic selection is not defined by
this specification because HTTP tries to remain orthogonal to the
definition of its payloads. In practice, the representation is
provided in some easily parsed format believed to be acceptable to
the user agent, as determined by shared design or content
negotiation, or in some commonly accepted hypertext format.
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A 300 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
Note: The original proposal for the 300 status code defined the
URI header field as providing a list of alternative
representations, such that it would be usable for 200, 300, and
406 responses and be transferred in responses to the HEAD method.
However, lack of deployment and disagreement over syntax led to
both URI and Alternates (a subsequent proposal) being dropped from
this specification. It is possible to communicate the list using
a set of Link header fields [RFC5988], each with a relationship of
“alternate”, though deployment is a chicken-and-egg problem.
6.4.2. 301 Moved Permanently
The 301 (Moved Permanently) status code indicates that the target
resource has been assigned a new permanent URI and any future
references to this resource ought to use one of the enclosed URIs.
Clients with link-editing capabilities ought to automatically re-link
references to the effective request URI to one or more of the new
references sent by the server, where possible.
The server SHOULD generate a Location header field in the response
containing a preferred URI reference for the new permanent URI. The
user agent MAY use the Location field value for automatic
redirection. The server’s response payload usually contains a short
hypertext note with a hyperlink to the new URI(s).
Note: For historical reasons, a user agent MAY change the request
method from POST to GET for the subsequent request. If this
behavior is undesired, the 307 (Temporary Redirect) status code
can be used instead.
A 301 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.4.3. 302 Found
The 302 (Found) status code indicates that the target resource
resides temporarily under a different URI. Since the redirection
might be altered on occasion, the client ought to continue to use the
effective request URI for future requests.
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The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server’s
response payload usually contains a short hypertext note with a
hyperlink to the different URI(s).
Note: For historical reasons, a user agent MAY change the request
method from POST to GET for the subsequent request. If this
behavior is undesired, the 307 (Temporary Redirect) status code
can be used instead.
6.4.4. 303 See Other
The 303 (See Other) status code indicates that the server is
redirecting the user agent to a different resource, as indicated by a
URI in the Location header field, which is intended to provide an
indirect response to the original request. A user agent can perform
a retrieval request targeting that URI (a GET or HEAD request if
using HTTP), which might also be redirected, and present the eventual
result as an answer to the original request. Note that the new URI
in the Location header field is not considered equivalent to the
effective request URI.
This status code is applicable to any HTTP method. It is primarily
used to allow the output of a POST action to redirect the user agent
to a selected resource, since doing so provides the information
corresponding to the POST response in a form that can be separately
identified, bookmarked, and cached, independent of the original
request.
A 303 response to a GET request indicates that the origin server does
not have a representation of the target resource that can be
transferred by the server over HTTP. However, the Location field
value refers to a resource that is descriptive of the target
resource, such that making a retrieval request on that other resource
might result in a representation that is useful to recipients without
implying that it represents the original target resource. Note that
answers to the questions of what can be represented, what
representations are adequate, and what might be a useful description
are outside the scope of HTTP.
Except for responses to a HEAD request, the representation of a 303
response ought to contain a short hypertext note with a hyperlink to
the same URI reference provided in the Location header field.
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6.4.5. 305 Use Proxy
The 305 (Use Proxy) status code was defined in a previous version of
this specification and is now deprecated (Appendix B).
6.4.6. 306 (Unused)
The 306 status code was defined in a previous version of this
specification, is no longer used, and the code is reserved.
6.4.7. 307 Temporary Redirect
The 307 (Temporary Redirect) status code indicates that the target
resource resides temporarily under a different URI and the user agent
MUST NOT change the request method if it performs an automatic
redirection to that URI. Since the redirection can change over time,
the client ought to continue using the original effective request URI
for future requests.
The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server’s
response payload usually contains a short hypertext note with a
hyperlink to the different URI(s).
Note: This status code is similar to 302 (Found), except that it
does not allow changing the request method from POST to GET. This
specification defines no equivalent counterpart for 301 (Moved
Permanently) ([RFC7238], however, defines the status code 308
(Permanent Redirect) for this purpose).
6.5. Client Error 4xx
The 4xx (Client Error) class of status code indicates that the client
seems to have erred. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
User agents SHOULD display any included representation to the user.
6.5.1. 400 Bad Request
The 400 (Bad Request) status code indicates that the server cannot or
will not process the request due to something that is perceived to be
a client error (e.g., malformed request syntax, invalid request
message framing, or deceptive request routing).
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6.5.2. 402 Payment Required
The 402 (Payment Required) status code is reserved for future use.
6.5.3. 403 Forbidden
The 403 (Forbidden) status code indicates that the server understood
the request but refuses to authorize it. A server that wishes to
make public why the request has been forbidden can describe that
reason in the response payload (if any).
If authentication credentials were provided in the request, the
server considers them insufficient to grant access. The client
SHOULD NOT automatically repeat the request with the same
credentials. The client MAY repeat the request with new or different
credentials. However, a request might be forbidden for reasons
unrelated to the credentials.
An origin server that wishes to “hide” the current existence of a
forbidden target resource MAY instead respond with a status code of
404 (Not Found).
6.5.4. 404 Not Found
The 404 (Not Found) status code indicates that the origin server did
not find a current representation for the target resource or is not
willing to disclose that one exists. A 404 status code does not
indicate whether this lack of representation is temporary or
permanent; the 410 (Gone) status code is preferred over 404 if the
origin server knows, presumably through some configurable means, that
the condition is likely to be permanent.
A 404 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.5.5. 405 Method Not Allowed
The 405 (Method Not Allowed) status code indicates that the method
received in the request-line is known by the origin server but not
supported by the target resource. The origin server MUST generate an
Allow header field in a 405 response containing a list of the target
resource’s currently supported methods.
A 405 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
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6.5.6. 406 Not Acceptable
The 406 (Not Acceptable) status code indicates that the target
resource does not have a current representation that would be
acceptable to the user agent, according to the proactive negotiation
header fields received in the request (Section 5.3), and the server
is unwilling to supply a default representation.
The server SHOULD generate a payload containing a list of available
representation characteristics and corresponding resource identifiers
from which the user or user agent can choose the one most
appropriate. A user agent MAY automatically select the most
appropriate choice from that list. However, this specification does
not define any standard for such automatic selection, as described in
Section 6.4.1.
6.5.7. 408 Request Timeout
The 408 (Request Timeout) status code indicates that the server did
not receive a complete request message within the time that it was
prepared to wait. A server SHOULD send the “close” connection option
(Section 6.1 of [RFC7230]) in the response, since 408 implies that
the server has decided to close the connection rather than continue
waiting. If the client has an outstanding request in transit, the
client MAY repeat that request on a new connection.
6.5.8. 409 Conflict
The 409 (Conflict) status code indicates that the request could not
be completed due to a conflict with the current state of the target
resource. This code is used in situations where the user might be
able to resolve the conflict and resubmit the request. The server
SHOULD generate a payload that includes enough information for a user
to recognize the source of the conflict.
Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being
PUT included changes to a resource that conflict with those made by
an earlier (third-party) request, the origin server might use a 409
response to indicate that it can’t complete the request. In this
case, the response representation would likely contain information
useful for merging the differences based on the revision history.
6.5.9. 410 Gone
The 410 (Gone) status code indicates that access to the target
resource is no longer available at the origin server and that this
condition is likely to be permanent. If the origin server does not
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know, or has no facility to determine, whether or not the condition
is permanent, the status code 404 (Not Found) ought to be used
instead.
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common
for limited-time, promotional services and for resources belonging to
individuals no longer associated with the origin server’s site. It
is not necessary to mark all permanently unavailable resources as
“gone” or to keep the mark for any length of time — that is left to
the discretion of the server owner.
A 410 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.5.10. 411 Length Required
The 411 (Length Required) status code indicates that the server
refuses to accept the request without a defined Content-Length
(Section 3.3.2 of [RFC7230]). The client MAY repeat the request if
it adds a valid Content-Length header field containing the length of
the message body in the request message.
6.5.11. 413 Payload Too Large
The 413 (Payload Too Large) status code indicates that the server is
refusing to process a request because the request payload is larger
than the server is willing or able to process. The server MAY close
the connection to prevent the client from continuing the request.
If the condition is temporary, the server SHOULD generate a
Retry-After header field to indicate that it is temporary and after
what time the client MAY try again.
6.5.12. 414 URI Too Long
The 414 (URI Too Long) status code indicates that the server is
refusing to service the request because the request-target (Section
5.3 of [RFC7230]) is longer than the server is willing to interpret.
This rare condition is only likely to occur when a client has
improperly converted a POST request to a GET request with long query
information, when the client has descended into a “black hole” of
redirection (e.g., a redirected URI prefix that points to a suffix of
itself) or when the server is under attack by a client attempting to
exploit potential security holes.
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A 414 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.5.13. 415 Unsupported Media Type
The 415 (Unsupported Media Type) status code indicates that the
origin server is refusing to service the request because the payload
is in a format not supported by this method on the target resource.
The format problem might be due to the request’s indicated
Content-Type or Content-Encoding, or as a result of inspecting the
data directly.
6.5.14. 417 Expectation Failed
The 417 (Expectation Failed) status code indicates that the
expectation given in the request’s Expect header field
(Section 5.1.1) could not be met by at least one of the inbound
servers.
6.5.15. 426 Upgrade Required
The 426 (Upgrade Required) status code indicates that the server
refuses to perform the request using the current protocol but might
be willing to do so after the client upgrades to a different
protocol. The server MUST send an Upgrade header field in a 426
response to indicate the required protocol(s) (Section 6.7 of
[RFC7230]).
Example:
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol.
6.6. Server Error 5xx
The 5xx (Server Error) class of status code indicates that the server
is aware that it has erred or is incapable of performing the
requested method. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
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condition. A user agent SHOULD display any included representation
to the user. These response codes are applicable to any request
method.
6.6.1. 500 Internal Server Error
The 500 (Internal Server Error) status code indicates that the server
encountered an unexpected condition that prevented it from fulfilling
the request.
6.6.2. 501 Not Implemented
The 501 (Not Implemented) status code indicates that the server does
not support the functionality required to fulfill the request. This
is the appropriate response when the server does not recognize the
request method and is not capable of supporting it for any resource.
A 501 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
6.6.3. 502 Bad Gateway
The 502 (Bad Gateway) status code indicates that the server, while
acting as a gateway or proxy, received an invalid response from an
inbound server it accessed while attempting to fulfill the request.
6.6.4. 503 Service Unavailable
The 503 (Service Unavailable) status code indicates that the server
is currently unable to handle the request due to a temporary overload
or scheduled maintenance, which will likely be alleviated after some
delay. The server MAY send a Retry-After header field
(Section 7.1.3) to suggest an appropriate amount of time for the
client to wait before retrying the request.
Note: The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers might
simply refuse the connection.
6.6.5. 504 Gateway Timeout
The 504 (Gateway Timeout) status code indicates that the server,
while acting as a gateway or proxy, did not receive a timely response
from an upstream server it needed to access in order to complete the
request.