title: The Proxy-Status HTTP Response Header Field abbrev: Proxy-Status docname: draft-ietf-httpbis-proxy-status-latest date: {DATE} category: std ipr: trust200902 area: Applications and Real-Time workgroup: HTTP keyword: Internet-Draft stand_alone: yes github-issue-label: proxy-status pi: [toc, sortrefs, symrefs]
ins: M. Nottingham
name: Mark Nottingham
organization: Fastly
city: Prahran
region: VIC
country: Australia
email: [email protected]
uri: https://www.mnot.net/
- ins: P. Sikora name: Piotr Sikora organization: Google email: [email protected]
normative: RFC2119:
informative:
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This document defines the Proxy-Status HTTP field to convey the details of intermediary handling of responses, including generated errors.
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RFC EDITOR: please remove this section before publication
Discussion of this draft takes place on the HTTP working group mailing list ([email protected]), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
Working Group information can be found at https://httpwg.org/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/proxy-status.
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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 before a response is obtained from upstream, 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.
Additionally, intermediaries sometimes want to convey additional information about their handling of a response, even if they did not generate it.
To enable these uses, {{header}} defines a new HTTP response field to allow intermediaries to convey details of their handling of a response, {{params}} enumerates the kind of information taht can be conveyed, and {{error-types}} defines a set of error types for use when a proxy generates the response.
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 {{!RFC2119}} {{!RFC8174}} when, and only when, they appear in all capitals, as shown here.
This specification uses Structured Headers {{!I-D.ietf-httpbis-header-structure}} to specify syntax. The terms sf-list, sf-item, sf-string, sf-token, sf-integer and key 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 field allows an intermediary to convey additional information about its handling of a response and its associated request.
It is a List {{!I-D.ietf-httpbis-header-structure}}:
Cache-Status = sf-listEach 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:
Proxy-Status: FooProxy, ExampleCDN
indicates that this response was handled first by FooAccelerator and then ExampleCDN.
Intermediaries determine when it is appropriate to add the Proxy-Status field to a response. Some might decide to add 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.
The list members identify 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"), and IP address, or a generated string.
Parameters on each member convey additional information about that intermediary's handling of the response; see {{params}} for defined parameters. While all of these parameters are OPTIONAL, intermediaries are encouraged to provide as much information as possible.
When adding a value to the Proxy-Status field, intermediaries SHOULD preserve the existing contents of the field, to allow debugging of the entire chain of intermediaries handling the request.
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, so 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 field; see {{security}}.
Origin servers MUST NOT generate the Proxy-Status field.
This section lists parameters that can be used on the members of Proxy-Status.
The origin parameter's value is a sf-string or sf-token that identifies the origin server selected (and used, if contacted) for this response. Its contents might be a hostname, IP address, or alias.
This is most useful for gateways (also known as "reverse proxies"), since they are often configured to use an origin server other than that which appears in the URL, and sometimes they use several origins to serve a given site.
For example:
Proxy-Status: cdn.example.org; origin=backend.example.org
The fwd-protocol parameter's value indicates the ALPN protocol identifier {{!RFC7301}} 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 a sf-token or sf-binary. If the protocol identifier is able to be expressed as a sf-token using UTF-8 encoding, that form MUST be used.
For example:
Proxy-Status: "proxy.example.org"; fwd-protocol=h2
The error parameter's value is a sf-token that is a Proxy Error Type. When present, it indicates that the response was generated by the proxy, not the origin server or any other upstream server.
{{error-types}} lists the Proxy Error Types defined in this document; new ones can be defined using the procedure outlined in {{register-error}}.
For example:
HTTP/1.1 504 Gateway Timeout
Proxy-Status: SomeCDN; error=connection_timeout
indicates that this 504 response was generated by SomeCDN, due to a connection timeout when going forward.
Or:
HTTP/1.1 429 Too Many Requests
Proxy-Status: SomeReverseProxy; error=http_request_error
indicates that this 429 Too Many Requests response was generated by the intermediary, not the origin.
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) when another status code might be used.
The details parameter's value is a sf-string containing additional information not captured anywhere else. This can include implementation-specific or deployment-specific information.
For example:
Proxy-Status: ExampleProxy; error="http_protocol_error";
details="Malformed response header - space before colon"
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 {{!RFC8126}}, 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 Parameter that matches key]
- Description: [a description of the
- 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.
This section lists the Proxy Error Types defined by this document. See {{register-error}} for information about defining new Proxy Error Types.
- 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
- 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 {{!RFC8499}}, Section 3.
- Recommended HTTP status code: 502
- 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
- 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
- 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
- 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
- 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: 503
- 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: None.
- 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 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
- 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 sf-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 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
- Name: http_response_transfer_coding
- Description: The intermediary encountered an error decoding the transfer-coding of the response.
- Extra Parameters:
- coding: a sf-token containing the specific coding that caused the error.
- 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 sf-token containing the specific coding that caused the error.
- 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 sf-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 an 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 an 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 an unexpected peer certificate (e.g., SPKI doesn't match) during the TLS handshake with the next hop.
- Extra Parameters:
- identity: a sf-string containing a comma-separated list of Subject Alternative Names from the certificate received from the next hop.
- sha256: a sf-string containing the hex-encoded SHA-256 of the certificate received from the next hop.
- spki: a sf-string containing the base64-encoded SHA-256 of the Subject Public Key Info (SPKI) from the certificate received from the next hop.
- Recommended HTTP status code: 502
- Name: tls_missing_proxy_certificate
- Description: The next hop requested a 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 the 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 sf-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.
- Extra Parameters:
- status_code: a sf-integer containing the generated status code.
- status_phrase: a sf-string containing the generated status phrase.
- Recommended HTTP status code: The applicable 4xx status code
This type helps distinguish between responses generated by intermediaries from those generated by the origin.
- 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: 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_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:
- Name: proxy_internal_error
- Description: The intermediary encountered an internal error unrelated to the origin.
- Extra Parameters:
- error: a sf-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. {{?RFC8586}}).
- Extra Parameters: None.
- Recommended HTTP status code: 502
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 {{!RFC8126}}, 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 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]
See the registry at https://iana.org/assignments/http-proxy-status for details on where to send registration requests.
Upon publication, please create the HTTP Proxy-Status Parameters registry and the HTTP Proxy Error Types registry at https://iana.org/assignments/http-proxy-statuses and populate them with the types defined in {{params}} and {{error-types}} respectively; see {{register-param}} and {{register-error}} for its associated procedures.
One of the primary security concerns when using Proxy-Status is leaking information that might aid an attacker. For example, information about the intermediary's configuration and back-end topology can be exposed.
As a result, care needs to be taken when deciding to generate a Proxy-Status field. Note that intermediaries are not required to generate a Proxy-Status 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.
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