The Proxy-Status HTTP Header Field

HTTP intermediaries – including both forward proxies and gateways (also known as “reverse proxies”) – have become an increasingly significant part of HTTP deployments. In particular, reverse proxies and Content Delivery Networks (CDNs) form part of the critical infrastructure of many Web sites. Typically, HTTP intermediaries forward requests towards the origin server and then forward their responses back to clients. However, if an error occurs, the response is generated by the intermediary itself. HTTP accommodates these types of errors with a few status codes; for example, 502 Bad Gateway and 504 Gateway Timeout. However, experience has shown that more information is necessary to aid debugging and communicate what’s happened to the client. To address this, defines a new HTTP response header field to convey such information, using the Proxy Status Types defined in . explains how to define new Proxy Status Types.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This specification uses Structured Headers to specify syntax. The terms sh-param-list, sh-item, sh-string, sh-token and sh-integer refer to the structured types defined therein. Note that in this specification, “proxy” is used to indicate both forward and reverse proxies, otherwise known as gateways. “Next hop” indicates the connection in the direction leading to the origin server for the request.

The Proxy-Status HTTP response header field allows an intermediary to indicate the nature and details of an error condition it encounters when servicing a request. It is a Structured Headers Parameterised List, where each item in the list indicates an error condition. Typically, it will have only one param-item (the error condition that triggered generation of the response it occurs within), but more than one value is not prohibited. Each param-item’s primary-id is a Proxy Status Type, a registered value that indicates the nature of the error. Each param-item can have zero to many parameters. lists parameters that can be used with all Proxy Status Types; individual types can define additional parameters to use with them. All parameters are optional; see for their potential security impact. For example:

indicates the specific nature of the timeout as a connect timeout to the origin with the identifier “abc”, and that is was generated by the intermediary that identifies itself as “FooCDN.” Furthermore, three connection attempts were made. Or:

indicates that this 429 Too Many Requests response was generated by the intermediary, not the origin. Each Proxy Status Type has a Recommended HTTP Status Code. When generating a HTTP response containing Proxy-Status, 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) when another status code might be used. lists the Proxy Status Types defined in this document; new ones can be defined using the procedure outlined in . Proxy-Status MAY be sent in HTTP trailers, but – as with all trailers – it might be silently discarded along the path to the user agent, this SHOULD NOT be done unless it is not possible to send it in headers. For example, if an intermediary is streaming a response and the upstream connection suddenly terminates, Proxy-Status can be appended to the trailers of the outgoing message (since the headers have already been sent). Note that there are various security considerations for intermediaries using the Proxy-Status header field; see . Origin servers MUST NOT generate the Proxy-Status header field.

This section lists parameters that are potentially applicable to most Proxy Status Types. proxy - a sh-token identifying the HTTP intermediary generating this response. origin - a sh-token identifying the origin server whose behaviour triggered this response. protocol - a sh-token indicating the ALPN protocol identifier used to connect to the next hop. This is only applicable when that connection was actually established. tries - a sh-integer indicating the number of times that the error has occurred before this response. details - a sh-string containing additional information not captured anywhere else. This can include implementation-specific or deployment-specific information.

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

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

Name: dns_error Description: The intermediary encountered a DNS error when trying to find an IP address for the destination hostname. Extra Parameters: rcode: A sh-string conveying the DNS RCODE that indicates the error type. See , Section 3. Recommended HTTP status code: 502

Name: destination_not_found Description: The intermediary cannot determine the appropriate destination 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

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: Recommended HTTP status code: 503

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

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

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

Name: connection_terminated Description: The intermediary’s connection to the next hop was closed before any part of the response was received. If some part was received, see http_response_incomplete. Extra Parameters: None. Recommended HTTP status code: 502

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

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

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

Name: connnection_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:

Name: http_response_status Description: The intermediary has received a 4xx or 5xx status code from the next hop and forwarded it to the client. Extra Parameters: None. Recommended HTTP status code:

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

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: details: a sh-string containing details about the error condition. For example, this might be the HTTP/2 error code or free-form text describing the condition. Recommended HTTP status code: 502

Name: http_response_header_block_size Description: The intermediary received a response to the request whose header block was considered too large. Extra Parameters: header_block_size: a sh-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

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: a sh-string indicating the name of the header that triggered the error. Recommended HTTP status code: 502

Name: http_response_body_size Description: The intermediary received a response to the request whose body was considered too large. Extra Parameters: body_size: a sh-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

Name: http_response_transfer_coding Description: The intermediary encountered an error decoding the transfer-coding of the response. Extra Parameters: coding: a sh-token containing the specific coding that caused the error. details: a sh-string containing details about the error condition. Recommended HTTP status code: 502

Name: http_response_content_coding Description: The intermediary encountered an error decoding the content-coding of the response. Extra Parameters: coding: a sh-token containing the specific coding that caused the error. details: a sh-string containing details about the error condition. Recommended HTTP status code: 502

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

Name: tls_handshake_error Description: The intermediary encountered an error during TLS handshake with the next hop. Extra Parameters: alert_message: a sh-token containing the applicable description string from the TLS Alerts registry. Recommended HTTP status code: 502

Name: tls_untrusted_peer_certificate Description: The intermediary received untrusted peer certificate during TLS handshake with the next hop. Extra Parameters: None. Recommended HTTP status code: 502

Name: tls_expired_peer_certificate Description: The intermediary received expired peer certificate during TLS handshake with the next hop. Extra Parameters: None. Recommended HTTP status code: 502

Name: tls_unexpected_peer_certificate Description: The intermediary received unexpected peer certificate (e.g., SPKI doesn’t match) during TLS handshake with the next hop. Extra Parameters: details: a sh-string containing the checksum or SPKI of the certificate received from the next hop. Recommended HTTP status code: 502

Name: tls_unexpected_peer_identity Description: The intermediary received peer certificate with unexpected identity (e.g., Subject Alternative Name doesn’t match) during TLS handshake with the next hop. Extra Parameters: details: a sh-string containing the identity of the next hop. Recommended HTTP status code: 502

Name: tls_missing_proxy_certificate Description: The next hop requested client certificate from the intermediary during TLS handshake, but it wasn’t configured with one. Extra Parameters: None. Recommended HTTP status code: 500

Name: tls_rejected_proxy_certificate Description: The next hop rejected client certificate provided by the intermediary during TLS handshake. Extra Parameters: None. Recommended HTTP status code: 500

Name: tls_error Description: The intermediary encountered a TLS error when communicating with the next hop. Extra Parameters: alert_message: a sh-token containing the applicable description string from the TLS Alerts registry. Recommended HTTP status code: 502

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. This proxy status type helps distinguish between responses generated by intermediaries from those generated by the origin. Extra Parameters: None. Recommended HTTP status code: The applicable 4xx status code

Name: http_request_denied Description: The intermediary rejected 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: 400

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

Name: proxy_internal_error Description: The intermediary encountered an internal error unrelated to the origin. Extra Parameters: details: a sh-string containing details about the error condition. Recommended HTTP status code: 500

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

New Proxy Status Types can be defined by registering them in the HTTP Proxy Status Types registry. Registration requests are reviewed and approved by a Designated Expert, as per , Section 4.5. 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 If the value is generic; vendor-specific, application-specific and deployment-specific values are discouraged Registration requests should use the following template: Name: [a name for the Proxy Status Type that is allowable as a sh-param-list key] Description: [a description of the conditions that generate the Proxy Status Types] Extra Parameters: [zero or more optional parameters, typed using one of the types available in sh-item] Recommended HTTP status code: [the appropriate HTTP status code for this entry] See the registry at https://iana.org/assignments/http-proxy-statuses for details on where to send registration requests.

Upon publication, please create the HTTP Proxy Status Types registry at https://iana.org/assignments/http-proxy-statuses and populate it with the types defined in ; see for its associated procedures.

One of the primary security concerns when using Proxy-Status is leaking information that might aid an attacker. As a result, care needs to be taken when deciding to generate a Proxy-Status header. Note that intermediaries are not required to generate a Proxy-Status header field in any response, and can conditionally generate them based upon request attributes (e.g., authentication tokens, IP address). Likewise, generation of all parameters is optional. Special care needs to be taken in generating proxy and origin parameters, as they can expose information about the intermediary’s configuration and back-end topology.