The Proxy-Status HTTP Response Header Field

The Proxy-Status HTTP Field The Proxy-Status HTTP response field allows an intermediary to convey additional information about its handling of a response and its associated request. It is a List : Each member of the list represents an intermediary that has handled the response. The first member of the list represents the intermediary closest to the origin server, and the last member of the list represents the intermediary closest to the user agent. For example: indicates that this response was handled first by FooProxy and then ExampleCDN. Intermediaries determine when it is appropriate to add the Proxy-Status field to a response. Some might decide to append to it to all responses, whereas others might only do so when specifically configured to, or when the request contains a header that activates a debugging mode. Each member of the list identifies the intermediary that inserted the value, and MUST have a type of either sf-string or sf-token. Depending on the deployment, this might be a product or service name (e.g., ExampleProxy or "Example CDN"), a hostname ("proxy-3.example.com"), an IP address, or a generated string. Parameters on each member convey additional information about that intermediary's handling of the response and its associated request; see . While all of these parameters are OPTIONAL, intermediaries are encouraged to provide as much information as possible (but see for security considerations in doing so). When adding a value to the Proxy-Status field, intermediaries SHOULD preserve the existing members of the field, to allow debugging of the entire chain of intermediaries handling the request. Proxy-Status MAY be sent in HTTP trailers. For example, if an intermediary is streaming a response and the upstream connection suddenly terminates, Proxy-Status can only be appended to the trailers of the outgoing message, since the headers have already been sent. As with all trailers, it might be silently discarded along the path to the user agent, so this SHOULD NOT be done unless it is not possible to send it in headers, and an intermediary MUST NOT send Proxy-Status as a trailer field unless it has also sent a corresponding Proxy-Status header field in the same message, so that the trailer value's ordering relative to other intermediaries is preserved. Origin servers MUST NOT generate the Proxy-Status field.

Proxy-Status Parameters This section lists parameters that can be used on the members of the Proxy-Status field. Unrecognised parameters SHOULD be ignored.

error The error parameter's value is an sf-token that is a Proxy Error Type. When present, it indicates that the proxy encountered an issue when obtaining a response. Unless a Proxy Error Type specifies otherwise, the presences of error often, but not always, indicates that response was generated by the proxy, not the origin server or any other upstream server. For example, a proxy might attempt to correct an error, or part of a response might be forwarded before the error is encountered. lists the Proxy Error Types defined in this document; new ones can be defined using the procedure outlined in . For example: indicates that this 504 response was generated by SomeCDN, due to a connection timeout when going forward. Or: indicates that this 429 Too Many Requests response was generated by the intermediary, not the origin. When sending the error parameter, the most specific Proxy Error Type SHOULD be sent, provided that it accurately represents the error condition. If an appropriate Proxy Error Type is not defined, there are a number of generic error types (e.g., proxy_internal_error, http_protocol_error) that can be used. If they are not suitable, consider registering a new Proxy Error Type (see ). Each Proxy Error Type has a Recommended HTTP Status Code. When generating a HTTP response containing error, its HTTP status code SHOULD be set to the Recommended HTTP Status Code. However, there may be circumstances (e.g., for backwards compatibility with previous behaviours, a status code has already been sent) when another status code might be used. Proxy Error Types can also define any number of extra parameters for use with that type. Their use, like all parameters, is optional. As a result, if an extra parameter is used with a Proxy Error Type for which it is not defined, it will be ignored.

next-hop The next-hop parameter's value is an sf-string or sf-token that identifies the intermediary or origin server selected (and used, if contacted) for this response. It might be a hostname, IP address, or alias. For example:

next-protocol The next-protocol parameter's value indicates the ALPN protocol identifier used by the intermediary to connect to the next hop. This is only applicable when that connection was actually established. The value MUST be either an sf-token or sf-binary. If the protocol identifier is able to be expressed as an sf-token using UTF-8 encoding, that form MUST be used. For example:

received-status The received-status parameter's value indicates the HTTP status code that the intermediary received from the next hop server. The value MUST be an sf-integer. For example:

details The details parameter's value is an sf-string containing additional information not captured anywhere else. This can include implementation-specific or deployment-specific information. For example:

Defining New Proxy-Status Parameters New Proxy-Status Parameters can be defined by registering them in the HTTP Proxy-Status Parameters registry. Registration requests are reviewed and approved by a Designated Expert, as per . A specification document is appreciated, but not required. The Expert(s) should consider the following factors when evaluating requests:

Community feedback
If the value is sufficiently well-defined
Generic parameters are preferred over vendor-specific, application-specific or deployment-specific values. If a generic value cannot be agreed upon in the community, the parameter's name should be correspondingly specific (e.g., with a prefix that identifies the vendor, application or deployment).
Parameter names should not conflict with registered extra parameters in the Proxy Error Type Registry.

Registration requests should use the following template:

Name: [a name for the Proxy-Status Parameter that matches key]
Description: [a description of the parameter semantics and value]
Reference: [to a specification defining this parameter]

See the registry at https://iana.org/assignments/http-proxy-status for details on where to send registration requests.

Proxy Error Types This section lists the Proxy Error Types defined by this document. See for information about defining new Proxy Error Types.

DNS Timeout

Name: dns_timeout
Description: The intermediary encountered a timeout when trying to find an IP address for the next hop hostname.
Extra Parameters: None.
Recommended HTTP status code: 504

DNS Error

Name: dns_error
Description: The intermediary encountered a DNS error when trying to find an IP address for the next hop hostname.
Extra Parameters:
- rcode: A sf-string conveying the DNS RCODE that indicates the error type. See .
Recommended HTTP status code: 502

Destination Not Found

Name: destination_not_found
Description: The intermediary cannot determine the appropriate next hop to use for this request; for example, it may not be configured. Note that this error is specific to gateways, which typically require specific configuration to identify the "backend" server; forward proxies use in-band information to identify the origin server.
Extra Parameters: None.
Recommended HTTP status code: 500

Destination Unavailable

Name: destination_unavailable
Description: The intermediary considers the next hop to be unavailable; e.g., recent attempts to communicate with it may have failed, or a health check may indicate that it is down.
Extra Parameters: None.
Recommended HTTP status code: 503

Destination IP Prohibited

Name: destination_ip_prohibited
Description: The intermediary is configured to prohibit connections to the next hop IP address.
Extra Parameters: None.
Recommended HTTP status code: 502

Destination IP Unroutable

Name: destination_ip_unroutable
Description: The intermediary cannot find a route to the next hop IP address.
Extra Parameters: None.
Recommended HTTP status code: 502

Connection Refused

Name: connection_refused
Description: The intermediary's connection to the next hop was refused.
Extra Parameters: None.
Recommended HTTP status code: 502

Connection Terminated

Name: connection_terminated
Description: The intermediary's connection to the next hop was closed before complete response was received.
Extra Parameters: None.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

Connection Timeout

Name: connection_timeout
Description: The intermediary's attempt to open a connection to the next hop timed out.
Extra Parameters: None.
Recommended HTTP status code: 504

Connection Read Timeout

Name: connection_read_timeout
Description: The intermediary was expecting data on a connection (e.g., part of a response), but did not receive any new data in a configured time limit.
Extra Parameters: None.
Recommended HTTP status code: 504
Notes: Responses with this error type might not have been generated by the intermediary.

Connection Write Timeout

Name: connection_write_timeout
Description: The intermediary was attempting to write data to a connection, but was not able to (e.g., because its buffers were full).
Extra Parameters: None.
Recommended HTTP status code: 504
Notes: Responses with this error type might not have been generated by the intermediary.

Connection Limit Reached

Name: connection_limit_reached
Description: The intermediary is configured to limit the number of connections it has to the next hop, and that limit has been passed.
Extra Parameters: None.
Recommended HTTP status code: 503

TLS Protocol Error

Name: tls_protocol_error
Description: The intermediary encountered a TLS error when communicating with the next hop, either during handshake or afterwards.
Extra Parameters: None.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).

TLS Certificate Error

Name: tls_certificate_error
Description: The intermediary encountered an error when verifying the certificate presented by the next hop.
Extra Parameters: None.
Recommended HTTP status code: 502

Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).

TLS Alert Received

Name: tls_alert_received
Description: The intermediary received a TLS alert from the next hop.
Extra Parameters:
- alert-message: an sf-token containing the applicable description string from the TLS Alerts registry.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Request Error

Name: http_request_error
Description: The intermediary is generating a client (4xx) response on the origin's behalf. Applicable status codes include (but are not limited to) 400, 403, 405, 406, 408, 411, 413, 414, 415, 416, 417, 429.
Extra Parameters:
- status-code: an sf-integer containing the generated status code.
- status-phrase: an sf-string containing the generated status phrase.
Recommended HTTP status code: The applicable 4xx status code
Notes: This type helps distinguish between responses generated by intermediaries from those generated by the origin.

HTTP Request Denied

Name: http_request_denied
Description: The intermediary rejected the HTTP request based on its configuration and/or policy settings. The request wasn't forwarded to the next hop.
Extra Parameters: None.
Recommended HTTP status code: 403

HTTP Incomplete Response

Name: http_response_incomplete
Description: The intermediary received an incomplete response to the request from the next hop.
Extra Parameters: None.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Header Section Too Large

Name: http_response_header_section_size
Description: The intermediary received a response to the request whose header section was considered too large.
Extra Parameters:
- header-section-size: an sf-integer indicating how large the headers received were. Note that they might not be complete; i.e., the intermediary may have discarded or refused additional data.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Header Too Large

Name: http_response_header_size
Description: The intermediary received a response to the request containing an individual header line that was considered too large.
Extra Parameters:
- header-name: an sf-string indicating the name of the header that triggered the error.
- header-size: an sf-integer indicating the size of the header that triggered the error.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Body Too Large

Name: http_response_body_size
Description: The intermediary received a response to the request whose body was considered too large.
Extra Parameters:
- body-size: an sf-integer indicating how large the body received was. Note that it may not have been complete; i.e., the intermediary may have discarded or refused additional data.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Trailer Section Too Large

Name: http_response_trailer_section_size
Description: The intermediary received a response to the request whose trailer section was considered too large.
Extra Parameters:
- trailer-section-size: an sf-integer indicating how large the trailers received were. Note that they might not be complete; i.e., the intermediary may have discarded or refused additional data.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Trailer Too Large

Name: http_response_trailer_size
Description: The intermediary received a response to the request containing an individual trailer line that was considered too large.
Extra Parameters:
- trailer-name: an sf-string indicating the name of the trailer that triggered the error.
- trailer-size: an sf-integer indicating the size of the trailer that triggered the error.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Transfer-Coding Error

Name: http_response_transfer_coding
Description: The intermediary encountered an error decoding the transfer-coding of the response.
Extra Parameters:
- coding: an sf-token containing the specific coding that caused the error.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Content-Coding Error

Name: http_response_content_coding
Description: The intermediary encountered an error decoding the content-coding of the response.
Extra Parameters:
- coding: an sf-token containing the specific coding that caused the error.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Response Timeout

Name: http_response_timeout
Description: The intermediary reached a configured time limit waiting for the complete response.
Extra Parameters: None.
Recommended HTTP status code: 504
Notes: Responses with this error type might not have been generated by the intermediary.

HTTP Upgrade Failed

Name: http_upgrade_failed
Description: The HTTP Upgrade between the intermediary and the next hop failed.
Extra Parameters: None.
Recommended HTTP status code: 502

HTTP Protocol Error

Name: http_protocol_error
Description: The intermediary encountered a HTTP protocol error when communicating with the next hop. This error should only be used when a more specific one is not defined.
Extra Parameters: None.
Recommended HTTP status code: 502
Notes: Responses with this error type might not have been generated by the intermediary.

Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).

Proxy Internal Response

Name: proxy_internal_response
Description: The intermediary generated the response locally, without attempting to connect to the next hop (e.g. in response to a request to a debug endpoint terminated at the intermediary).
Extra Parameters: None.
Recommended HTTP status code:

Proxy Internal Error

Name: proxy_internal_error
Description: The intermediary encountered an internal error unrelated to the origin.
Extra Parameters: None
Recommended HTTP status code: 500

Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).

Proxy Configuration Error

Name: proxy_configuration_error
Description: The intermediary encountered an error regarding its configuration.
Extra Parameters: None
Recommended HTTP status code: 500

Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).

Proxy Loop Detected

Name: proxy_loop_detected
Description: The intermediary tried to forward the request to itself, or a loop has been detected using different means (e.g. ).
Extra Parameters: None.
Recommended HTTP status code: 502

Defining New Proxy Error Types New Proxy Error Types can be defined by registering them in the HTTP Proxy Error Types registry. Registration requests are reviewed and approved by a Designated Expert, as per . A specification document is appreciated, but not required. The Expert(s) should consider the following factors when evaluating requests:

Community feedback
If the value is sufficiently well-defined
Generic types are preferred over vendor-specific, application-specific or deployment-specific values. If a generic value cannot be agreed upon in the community, the types's name should be correspondingly specific (e.g., with a prefix that identifies the vendor, application or deployment).
Extra Parameters should not conflict with registered Proxy-Status parameters.

Registration requests should use the following template:

Name: [a name for the Proxy Error Type that matches sf-token]
Description: [a description of the conditions that generate the Proxy Error Type]
Extra Parameters: [zero or more optional parameters, along with their allowable type(s)]
Recommended HTTP status code: [the appropriate HTTP status code for this entry]
Notes: [optional]

If the Proxy Error Type might occur in responses that are not generated by the intermediary -- for example, when the error is detected during response content processing and a Proxy-Status trailer field is appended -- that SHOULD be explained in the Notes. See the registry at https://iana.org/assignments/http-proxy-status for details on where to send registration requests.