2021-10-22 2026-03-06 Expert Review Mark Nottingham New Proxy-Status parameters and proxy error types, along with changes to existing ones, can be requested using the registry interface or the ietf-http-wg@w3.org mailing list. error The error parameter's value is a Token that is a proxy error type. When present, it indicates that the intermediary encountered an issue when obtaining this response. next-hop The next-hop parameter's value is a String or Token that identifies the intermediary or origin server selected (and used, if contacted) to obtain this response. It might be a hostname, IP address, or alias. next-protocol The next-protocol parameter's value indicates the Application-Layer Protocol Negotiation (ALPN) protocol identifier of the protocol used by the intermediary to connect to the next hop when obtaining this response. received-status The received-status parameter's value indicates the HTTP status code that the intermediary received from the next-hop server when obtaining this response. details The details parameter's value is a String containing additional information not captured anywhere else. This can include implementation-specific or deployment-specific information. next-hop-aliases A string containing one or more DNS aliases or canonical names used to establish a proxied connection to the next hop. Expert Review Mark Nottingham New Proxy-Status parameters and proxy error types, along with changes to existing ones, can be requested using the registry interface or the ietf-http-wg@w3.org mailing list. dns_timeout The intermediary encountered a timeout when trying to find an IP address for the next-hop hostname. None 504 true dns_error The intermediary encountered a DNS error when trying to find an IP address for the next-hop hostname. - rcode: A String conveying the DNS RCODE that indicates the error type. See , Section 3. - info-code: An Integer conveying the Extended DNS Error Code INFO-CODE. See . 502 true destination_not_found 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. None 500 true destination_unavailable 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. None 503 true destination_ip_prohibited The intermediary is configured to prohibit connections to the next-hop IP address. None 502 true destination_ip_unroutable The intermediary cannot find a route to the next-hop IP address. None 502 true connection_refused The intermediary's connection to the next hop was refused. None 502 true connection_terminated The intermediary's connection to the next hop was closed before a complete response was received. None 502 false connection_timeout The intermediary's attempt to open a connection to the next hop timed out. None 504 true connection_read_timeout 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. None 504 false connection_write_timeout The intermediary was attempting to write data to a connection but was not able to (e.g., because its buffers were full). None 504 false connection_limit_reached The intermediary is configured to limit the number of connections it has to the next hop, and that limit has been exceeded. None 503 true tls_protocol_error The intermediary encountered a TLS error when communicating with the next hop, either during the handshake or afterwards. None 502 false Not appropriate when a TLS alert is received; see tls_alert_received. tls_certificate_error The intermediary encountered an error when verifying the certificate presented by the next hop. None 502 true tls_alert_received The intermediary received a TLS alert from the next hop. - alert-id: An Integer containing the applicable value from the TLS Alerts registry. See . - alert-message: A Token or String containing the applicable description string from the TLS Alerts registry. See . 502 false http_request_error 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, and 429. - status-code: An Integer containing the generated status code. - status-phrase: A String containing the generated status phrase. The applicable 4xx status code true This type helps distinguish between responses generated by intermediaries from those generated by the origin. http_request_denied The intermediary rejected the HTTP request based on its configuration and/or policy settings. The request wasn't forwarded to the next hop. None 403 true http_response_incomplete The intermediary received an incomplete response to the request from the next hop. None 502 false http_response_header_section_size The intermediary received a response to the request whose header section was considered too large. - header-section-size: An Integer indicating how large the received headers were. Note that they might not be complete; i.e., the intermediary may have discarded or refused additional data. 502 false http_response_header_size The intermediary received a response to the request containing an individual header field line that was considered too large. - header-name: A String indicating the name of the header field that triggered the error. - header-size: An Integer indicating the size of the header field that triggered the error. 502 false http_response_body_size The intermediary received a response to the request whose body was considered too large. - body-size: An Integer indicating how large the received body was. Note that it may not have been complete; i.e., the intermediary may have discarded or refused additional data. 502 false http_response_trailer_section_size The intermediary received a response to the request whose trailer section was considered too large. - trailer-section-size: An Integer indicating how large the received trailers were. Note that they might not be complete; i.e., the intermediary may have discarded or refused additional data. 502 false http_response_trailer_size The intermediary received a response to the request containing an individual trailer field line that was considered too large. - trailer-name: A String indicating the name of the trailer field that triggered the error. - trailer-size: An Integer indicating the size of the trailer field that triggered the error. 502 false http_response_transfer_coding The intermediary encountered an error decoding the transfer coding of the response. - coding: A Token containing the specific coding (from the HTTP Transfer Coding Registry) that caused the error. 502 false http_response_content_coding The intermediary encountered an error decoding the content coding of the response. - coding: A Token containing the specific coding (from the HTTP Content Coding Registry) that caused the error. 502 false http_response_timeout The intermediary reached a configured time limit waiting for the complete response. None 504 false http_upgrade_failed The process of negotiating an upgrade of the HTTP version between the intermediary and the next hop failed. None 502 true http_protocol_error The intermediary encountered an HTTP protocol error when communicating with the next hop. This error should only be used when a more specific one is not defined. None 502 false proxy_internal_response The intermediary generated the response itself without attempting to connect to the next hop. None The most appropriate status code for the response true proxy_internal_error The intermediary encountered an internal error unrelated to the origin. None 500 true proxy_configuration_error The intermediary encountered an error regarding its configuration. None 500 true proxy_loop_detected The intermediary tried to forward the request to itself, or a loop has been detected using different means (e.g., ). None 502 true incremental_refused The HTTP message contained the Incremental HTTP header field, but the intermediary refused to forward the message incrementally. None 501 true