The Safe Response Header Field
Telling browsers when it is safe to retry an HTTP request.
Holtman, K., "The Safe Response Header Field", RFC 2310, DOI 10.17487/RFC2310, April 1998, <https://www.rfc-editor.org/info/rfc2310>.
@misc{rfc2310,
series = {Request for Comments},
number = 2310,
howpublished = {RFC 2310},
publisher = {RFC Editor},
doi = {10.17487/RFC2310},
url = {https://www.rfc-editor.org/info/rfc2310},
author = {K. Holtman},
title = {{The Safe Response Header Field}},
pagetotal = 5,
year = 1998,
month = apr,
} RFC text
Contents
Network Working Group K. Holtman
Request for Comments: 2310 TUE
Category: Experimental April 1998
The Safe Response Header Field
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document defines a HTTP response header field called Safe, which
can be used to indicate that repeating a HTTP request is safe. Such
an indication will allow user agents to handle retries of some safe
requests, in particular safe POST requests, in a more user-friendly
way.
1 Introduction
This document defines a HTTP response header field called Safe, which
can be used to indicate that repeating a HTTP request is safe. Such
an indication will allow user agents to handle retries of some safe
requests, in particular safe POST requests, in a more user-friendly
way.
2 Terminology and Notation
This document uses the HTTP terminology and BNF notation defined in
[1]. It uses the key words in RFC 2119 for defining the significance
of each particular requirement.
3 Rationale
According to Section 9.1.1 (Safe Methods) of the HTTP/1.1
specification [1], POST requests are assumed to be `unsafe' by
default. `Unsafe' means `causes side effects for which the user will
be held accountable'.
It is sometimes necessary for a user agent to repeat a POST request.
Examples of such cases are
- when retrying a POST request which gave an indeterminate error
result in the previous attempt
- when the user presses the RELOAD button while a POST result is
displayed
- when the history function is used to redisplay a POST result
which is no longer in the history buffer.
If the POST request is unsafe, HTTP requires explicit user
confirmation is before the request is repeated. The confirmation
dialog often takes the form of a `repost form data?' dialog box.
This dialog is confusing to many users, and slows down navigation in
any case.
If the repeated POST request is safe, the user-unfriendly
confirmation dialog can be omitted. However plain HTTP/1.1 [1] has
no mechanism by which agents can tell if POST requests are safe, and
they must be assumed unsafe by default. This document adds a
mechanism to HTTP, the Safe header field, for telling if a POST
request is safe.
Using the Safe header field, web applications which require the use
of a safe POST request, rather than a GET request, for the submission
of web forms, can be made more user-friendly. The use of a POST
request may be required for a number of reasons, including
- the contents of the form are potentially very large
- the form is used to upload a file (see [2])
- the application needs some internationalization features
(see [3]) which are only available if the form contents are
transmitted in a request body the information in the form cannot
be encoded in a GET request URL because of security
considerations.
4 The Safe response header field
The Safe response header field is defined as an addition to the
HTTP/1.x protocol suite.
The Safe response header field is used by origin servers to indicate
whether repeating the received HTTP request is safe in the sense of
Section 9.1.1 (Safe Methods) of the HTTP/1.1 specification [1]. For
the purpose of this specification, a HTTP request is considered to be
a repetition of a previous request if both requests
- are issued by the same user agent, and
- apply to the same resource, and
- have the same request method, and
- both have no request body, or both have request bodies which are
byte-wise identical after decoding of any content and transfer
codings.
The Safe header field has the following syntax.
Safe = "Safe" ":" safe-nature
safe-nature = "yes" | "no"
An example of the header field is:
Safe: yes
If a Safe header field is absent in the response, the corresponding
request MUST be considered unsafe, unless it is a GET or HEAD
request. As GET and HEAD requests are safe by definition, user
agents SHOULD ignore a `Safe: no' header field in GET and HEAD
responses.
If, according to a received Safe header field, the repeating of a
request is safe, the request MAY be repeated automatically without
asking for user confirmation.
5 Security Considerations
For a discussion of the security considerations connected to HTTP
form submission, see [1]. The Safe header field introduces no new
security risks.
The use of GET requests for form submission has some security risks
which are absent for submission with other HTTP methods. By taking
away a counter-incentive to the use of GET requests for form
submission, the Safe header field may improve overall security.
6 References
[1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and
T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2068, January 1997.
[2] Nebel, E., and L. Masinter, "Form-based File Upload in HTML",
RFC 1867, November 1995.
[3] Yergeau, F., Nicol, G., Adams, G., and M. Duerst,
"Internationalization of the Hypertext Markup Language", RFC
2070, January 1997.
7 Author's Address
Koen Holtman
Technische Universiteit Eindhoven
Postbus 513
Kamer HG 6.57
5600 MB Eindhoven (The Netherlands)
EMail: koen@win.tue.nl
8. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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The limited permissions granted above are perpetual and will not be
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AI ELI5
On this page
§1Introduction
RFC 2310 defines the Safe HTTP response header field, which origin servers use to indicate whether repeating an HTTP request is safe. A request is safe when it does not cause side effects for which the user will be held accountable.
Repeating a POST request requires explicit user confirmation in HTTP/1.1 by default, but when the POST is actually safe, servers can signal this with the Safe header to let browsers retry automatically without the confusing confirmation dialog.
The Safe header tells a browser: this request will not cause trouble if you do it again, so you do not need to ask the user.
§2When Retries Happen
User agents need to repeat requests in several situations: when a network error leaves the result indeterminate, when the user presses reload while viewing a response, or when the browser history function redisplays a past response. In these cases, if the original POST request is unsafe, HTTP requires explicit user confirmation before repeating it.
If the POST is safe, the confirmation dialog can be omitted entirely. Web applications that need POST for large form data, file uploads, or security reasons can benefit from signaling safety to the browser.
Sometimes you need to ask the browser to send the same request again—maybe the network broke, or you clicked back. If the server says it is safe, the browser just does it instead of bugging you.
§3The Safe Header
The Safe response header field uses simple syntax: the field name followed by either yes or no. For example, a server sends Safe: yes to indicate the request is safe to repeat.
A request is considered a repetition of a previous request if issued by the same user agent, applied to the same resource, using the same method, and with identical request bodies (after decoding). If the Safe header is absent, the request is assumed unsafe unless it is GET or HEAD, which are safe by definition.
The header is just a tiny label: Safe: yes or Safe: no. It tells the browser which requests are really safe to try again.
§4Security
The RFC notes that the Safe header introduces no new security risks beyond those of HTTP form submission. The specification refers to RFC 2068 (HTTP/1.1) for a full discussion of security considerations with HTTP form submission. Using GET requests for form submission has security risks absent in POST submission, and the Safe header may improve overall security by reducing incentives to misuse GET for forms.
The Safe header itself does not create new dangers. It actually helps because it makes it safer to use POST instead of GET for sending form data.