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IETF RFC 7838
Last modified on Friday, April 8th, 2016
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Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 7838 Akamai
Category: Standards Track P. McManus
ISSN: 2070-1721 Mozilla
J. Reschke
greenbytes
April 2016
HTTP Alternative Services
 Abstract
This document specifies "Alternative Services" for HTTP, which allow
an origin's resources to be authoritatively available at a separate
network location, possibly accessed with a different protocol
configuration.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7838.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Nottingham, et al. Standards Track PAGE 1 
RFC 7838 HTTP Alternative Services April 2016
Table of Contents
1. Introduction ....................................................2
1.1. Notational Conventions .....................................3
2. Alternative Services Concepts ...................................3
2.1. Host Authentication ........................................5
2.2. Alternative Service Caching ................................6
2.3. Requiring Server Name Indication ...........................6
2.4. Using Alternative Services .................................6
3. The Alt-Svc HTTP Header Field ...................................8
3.1. Caching Alt-Svc Header Field Values .......................10
4. The ALTSVC HTTP/2 Frame ........................................11
5. The Alt-Used HTTP Header Field .................................13
6. The 421 (Misdirected Request) HTTP Status Code .................13
7. IANA Considerations ............................................13
7.1. Header Field Registrations ................................13
7.2. The ALTSVC HTTP/2 Frame Type ..............................14
7.3. Alt-Svc Parameter Registry ................................14
7.3.1. Procedure ..........................................14
7.3.2. Registrations ......................................15
8. Internationalization Considerations ............................15
9. Security Considerations ........................................15
9.1. Changing Ports ............................................15
9.2. Changing Hosts ............................................15
9.3. Changing Protocols ........................................16
9.4. Tracking Clients Using Alternative Services ...............17
9.5. Confusion regarding Request Scheme ........................17
10. References ....................................................18
10.1. Normative References .....................................18
10.2. Informative References ...................................19
Acknowledgements ..................................................19
Authors' Addresses ................................................20
1. Introduction
HTTP [RFC 7230] conflates the identification of resources with their
location. In other words, "http://" and "https://" URIs are used to
both name and find things to interact with.
In some cases, it is desirable to separate identification and
location in HTTP; keeping the same identifier for a resource, but
interacting with it at a different location on the network.
For example:
o An origin server might wish to redirect a client to a different
server when it is under load, or it has found a server in a
location that is more local to the client.
Nottingham, et al. Standards Track PAGE 2
RFC 7838 HTTP Alternative Services April 2016
o An origin server might wish to offer access to its resources using
a new protocol, such as HTTP/2 [RFC 7540], or one using improved
security, such as Transport Layer Security (TLS) [RFC 5246].
o An origin server might wish to segment its clients into groups of
capabilities, such as those supporting Server Name Indication
(SNI) (Section 3 of [RFC 6066]), for operational purposes.
This specification defines a new concept in HTTP, "Alternative
Services", that allows an origin server to nominate additional means
of interacting with it on the network. It defines a general
framework for this in Section 2, along with specific mechanisms for
advertising their existence using HTTP header fields (Section 3) or
HTTP/2 frames (Section 4), plus a way to indicate that an alternative
service was used (Section 5).
It also endorses the status code 421 (Misdirected Request)
(Section 6) that origin servers or their nominated alternatives can
use to indicate that they are not authoritative for a given origin,
in cases where the wrong location is used.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
This document uses the Augmented BNF defined in [RFC 5234] and updated
by [RFC 7405] along with the "#rule" extension defined in Section 7 of
[RFC 7230]. The rules below are defined in [RFC 5234], [RFC 7230], and
[RFC 7234]:
OWS = <OWS, see [RFC 7230], Section 3.2.3>
delta-seconds = <delta-seconds; see [RFC 7234], Section 1.2.1>
port = <port, see [RFC 7230], Section 2.7>
quoted-string = <quoted-string, see [RFC 7230], Section 3.2.6>
token = <token, see [RFC 7230], Section 3.2.6>
uri-host = <uri-host, see [RFC 7230], Section 2.7>
2. Alternative Services Concepts
This specification defines a new concept in HTTP, the "Alternative
Service". When an origin [RFC 6454] has resources that are accessible
through a different protocol/host/port combination, it is said to
have an alternative service available.
Nottingham, et al. Standards Track PAGE 3
RFC 7838 HTTP Alternative Services April 2016
An alternative service can be used to interact with the resources on
an origin server at a separate location on the network, possibly
using a different protocol configuration. Alternative services are
considered authoritative for an origin's resources, in the sense of
[RFC 7230], Section 9.1.
For example, an origin:
("http", "www.example.com", "80")
might declare that its resources are also accessible at the
alternative service:
("h2", "new.example.com", "81")
By their nature, alternative services are explicitly at the
granularity of an origin; they cannot be selectively applied to
resources within an origin.
Alternative services do not replace or change the origin for any
given resource; in general, they are not visible to the software
"above" the access mechanism. The alternative service is essentially
alternative routing information that can also be used to reach the
origin in the same way that DNS CNAME or SRV records define routing
information at the name resolution level. Each origin maps to a set
of these routes -- the default route is derived from the origin
itself and the other routes are introduced based on alternative-
service information.
Furthermore, it is important to note that the first member of an
alternative service tuple is different from the "scheme" component of
an origin; it is more specific, identifying not only the major
version of the protocol being used, but potentially the communication
options for that protocol as well.
This means that clients using an alternative service can change the
host, port, and protocol that they are using to fetch resources, but
these changes MUST NOT be propagated to the application that is using
HTTP; from that standpoint, the URI being accessed and all
information derived from it (scheme, host, and port) are the same as
before.
Importantly, this includes its security context; in particular, when
TLS [RFC 5246] is used to authenticate, the alternative service will
need to present a certificate for the origin's host name, not that of
the alternative. Likewise, the Host header field ([RFC 7230],
Section 5.4) is still derived from the origin, not the alternative
service (just as it would if a CNAME were being used).
Nottingham, et al. Standards Track PAGE 4
RFC 7838 HTTP Alternative Services April 2016
The changes MAY, however, be made visible in debugging tools,
consoles, etc.
Formally, an alternative service is identified by the combination of:
o An Application Layer Protocol Negotiation (ALPN) protocol name, as
per [RFC 7301]
o A host, as per [RFC 3986], Section 3.2.2
o A port, as per [RFC 3986], Section 3.2.3
The ALPN protocol name is used to identify the application protocol
or suite of protocols used by the alternative service. Note that for
the purpose of this specification, an ALPN protocol name implicitly
includes TLS in the suite of protocols it identifies, unless
specified otherwise in its definition. In particular, the ALPN name
"http/1.1", registered by Section 6 of [RFC 7301], identifies HTTP/1.1
over TLS.
Additionally, each alternative service MUST have a freshness
lifetime, expressed in seconds (see Section 2.2).
There are many ways that a client could discover the alternative
service(s) associated with an origin. This document describes two
such mechanisms: the "Alt-Svc" HTTP header field (Section 3) and the
"ALTSVC" HTTP/2 frame type (Section 4).
The remainder of this section describes requirements that are common
to alternative services, regardless of how they are discovered.
2.1. Host Authentication
Clients MUST have reasonable assurances that the alternative service
is under control of and valid for the whole origin. This mitigates
the attack described in Section 9.2.
For the purposes of this document, "reasonable assurances" can be
established through use of a TLS-based protocol with the certificate
checks defined in [RFC 2818]. Clients MAY impose additional criteria
for establishing reasonable assurances.
For example, if the origin's host is "www.example.com" and an
alternative is offered on "other.example.com" with the "h2" protocol,
and the certificate offered is valid for "www.example.com", the
client can use the alternative. However, if either is offered with
the "h2c" protocol, the client cannot use it, because there is no
Nottingham, et al. Standards Track PAGE 5
RFC 7838 HTTP Alternative Services April 2016
mechanism (at the time of the publication of this specification) in
that protocol to establish the relationship between the origin and
the alternative.
2.2. Alternative Service Caching
Mechanisms for discovering alternative services also associate a
freshness lifetime with them; for example, the Alt-Svc header field
uses the "ma" parameter.
Clients can choose to use an alternative service instead of the
origin at any time when it is considered fresh; see Section 2.4 for
specific recommendations.
Clients with existing connections to an alternative service do not
need to stop using it when its freshness lifetime ends; the caching
mechanism is intended for limiting how long an alternative service
can be used for establishing new connections, not limiting the use of
existing ones.
Alternative services are fully authoritative for the origin in
question, including the ability to clear or update cached alternative
service entries, extend freshness lifetimes, and any other authority
the origin server would have.
When alternative services are used to send a client to the most
optimal server, a change in network configuration can result in
cached values becoming suboptimal. Therefore, clients SHOULD remove
from cache all alternative services that lack the "persist" flag with
the value "1" when they detect such a change, when information about
network state is available.
2.3. Requiring Server Name Indication
A client MUST NOT use a TLS-based alternative service unless the
client supports TLS Server Name Indication (SNI). This supports the
conservation of IP addresses on the alternative service host.
Note that the SNI information provided in TLS by the client will be
that of the origin, not the alternative (as will the Host HTTP header
field value).
2.4. Using Alternative Services
By their nature, alternative services are OPTIONAL: clients do not
need to use them. However, it is advantageous for clients to behave
in a predictable way when alternative services are used by servers,
to aid in purposes like load balancing.
Nottingham, et al. Standards Track PAGE 6
RFC 7838 HTTP Alternative Services April 2016
Therefore, if a client supporting this specification becomes aware of
an alternative service, the client SHOULD use that alternative
service for all requests to the associated origin as soon as it is
available, provided the alternative service information is fresh
(Section 2.2) and the security properties of the alternative service
protocol are desirable, as compared to the existing connection. A
viable alternative service is then treated in every way as the
origin; this includes the ability to advertise alternative services.
If a client becomes aware of multiple alternative services, it
chooses the most suitable according to its own criteria, keeping
security properties in mind. For example, an origin might advertise
multiple alternative services to notify clients of support for
multiple versions of HTTP.
A client configured to use a proxy for a given request SHOULD NOT
directly connect to an alternative service for this request, but
instead route it through that proxy.
When a client uses an alternative service for a request, it can
indicate this to the server using the Alt-Used header field
(Section 5).
The client does not need to block requests on any existing
connection; it can be used until the alternative connection is
established. However, if the security properties of the existing
connection are weak (for example, cleartext HTTP/1.1), then it might
make sense to block until the new connection is fully available in
order to avoid information leakage.
Furthermore, if the connection to the alternative service fails or is
unresponsive, the client MAY fall back to using the origin or another
alternative service. Note, however, that this could be the basis of
a downgrade attack, thus losing any enhanced security properties of
the alternative service. If the connection to the alternative
service does not negotiate the expected protocol (for example, ALPN
fails to negotiate h2, or an Upgrade request to h2c is not accepted),
the connection to the alternative service MUST be considered to have
failed.
Nottingham, et al. Standards Track PAGE 7
RFC 7838 HTTP Alternative Services April 2016
3. The Alt-Svc HTTP Header Field
An HTTP(S) origin server can advertise the availability of
alternative services to clients by adding an Alt-Svc header field to
responses.
Alt-Svc = clear / 1#alt-value
clear = %s"clear"; "clear", case-sensitive
alt-value = alternative *( OWS ";" OWS parameter )
alternative = protocol-id "=" alt-authority
protocol-id = token ; percent-encoded ALPN protocol name
alt-authority = quoted-string ; containing [ uri-host ] ":" port
parameter = token "=" ( token / quoted-string )
The field value consists either of a list of values, each of which
indicates one alternative service, or the keyword "clear".
A field value containing the special value "clear" indicates that the
origin requests all alternatives for that origin to be invalidated
(including those specified in the same response, in case of an
invalid reply containing both "clear" and alternative services).
ALPN protocol names are octet sequences with no additional
constraints on format. Octets not allowed in tokens ([RFC 7230],
Section 3.2.6) MUST be percent-encoded as per Section 2.1 of
[RFC 3986]. Consequently, the octet representing the percent
character "%" (hex 25) MUST be percent-encoded as well.
In order to have precisely one way to represent any ALPN protocol
name, the following additional constraints apply:
1. Octets in the ALPN protocol name MUST NOT be percent-encoded if
they are valid token characters except "%", and
2. When using percent-encoding, uppercase hex digits MUST be used.
With these constraints, recipients can apply simple string comparison
to match protocol identifiers.
The "alt-authority" component consists of an OPTIONAL uri-host
("host" in Section 3.2.2 of [RFC 3986]), a colon (":"), and a port
number.
For example:
Alt-Svc: h2=":8000"
Nottingham, et al. Standards Track PAGE 8
RFC 7838 HTTP Alternative Services April 2016
This indicates the "h2" protocol ([RFC 7540]) on the same host using
the indicated port 8000.
An example involving a change of host:
Alt-Svc: h2="new.example.org:80"
This indicates the "h2" protocol on the host "new.example.org",
running on port 80. Note that the "quoted-string" syntax needs to be
used because ":" is not an allowed character in "token".
Examples for protocol name escaping:
+--------------------+-------------+---------------------+
| ALPN protocol name | protocol-id | Note |
+--------------------+-------------+---------------------+
| h2 | h2 | No escaping needed |
+--------------------+-------------+---------------------+
| w=x:y#z | w%3Dx%3Ay#z | "=" and ":" escaped |
+--------------------+-------------+---------------------+
| x%y | x%25y | "%" needs escaping |
+--------------------+-------------+---------------------+
Alt-Svc MAY occur in any HTTP response message, regardless of the
status code. Note that recipients of Alt-Svc can ignore the header
field (and are required to in some situations; see Sections 2.1 and
6).
The Alt-Svc field value can have multiple values:
Alt-Svc: h2="alt.example.com:8000", h2=":443"
When multiple values are present, the order of the values reflects
the server's preference (with the first value being the most
preferred alternative).
The value(s) advertised by Alt-Svc can be used by clients to open a
new connection to an alternative service. Subsequent requests can
start using this new connection immediately or can continue using the
existing connection while the new connection is created.
When using HTTP/2 ([RFC 7540]), servers SHOULD instead send an ALTSVC
frame (Section 4). A single ALTSVC frame can be sent for a
connection; a new frame is not needed for every request. Note that,
despite this recommendation, Alt-Svc header fields remain valid in
responses delivered over HTTP/2.
Nottingham, et al. Standards Track PAGE 9
RFC 7838 HTTP Alternative Services April 2016
Each "alt-value" is followed by an OPTIONAL semicolon-separated list
of additional parameters, each such "parameter" comprising a name and
a value.
This specification defines two parameters: "ma" and "persist",
defined in Section 3.1. Unknown parameters MUST be ignored. That
is, the values (alt-value) they appear in MUST be processed as if the
unknown parameter was not present.
New parameters can be defined in extension specifications (see
Section 7.3 for registration details).
Note that all field elements that allow "quoted-string" syntax MUST
be processed as per Section 3.2.6 of [RFC 7230].
3.1. Caching Alt-Svc Header Field Values
When an alternative service is advertised using Alt-Svc, it is
considered fresh for 24 hours from generation of the message. This
can be modified with the "ma" (max-age) parameter.
Syntax:
ma = delta-seconds; see [RFC 7234], Section 1.2.1
The delta-seconds value indicates the number of seconds since the
response was generated for which the alternative service is
considered fresh.
Alt-Svc: h2=":443"; ma=3600
See Section 4.2.3 of [RFC 7234] for details on determining the
response age.
For example, a response:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=600
Age: 30
Alt-Svc: h2=":8000"; ma=60
indicates that an alternative service is available and usable for the
next 60 seconds. However, the response has already been cached for
30 seconds (as per the Age header field value); therefore, the
alternative service is only fresh for the 30 seconds from when this
response was received, minus estimated transit time.
Nottingham, et al. Standards Track PAGE 10
RFC 7838 HTTP Alternative Services April 2016
Note that the freshness lifetime for HTTP caching (here, 600 seconds)
does not affect caching of Alt-Svc values.
When an Alt-Svc response header field is received from an origin, its
value invalidates and replaces all cached alternative services for
that origin.
By default, cached alternative services will be cleared when the
client detects a network change. Alternative services that are
intended to be longer lived (such as those that are not specific to
the client access network) can carry the "persist" parameter with a
value "1" as a hint that the service is potentially useful beyond a
network configuration change.
Syntax:
persist = "1"
For example:
Alt-Svc: h2=":443"; ma=2592000; persist=1
This specification only defines a single value for "persist".
Clients MUST ignore "persist" parameters with values other than "1".
See Section 2.2 for general requirements on caching alternative
services.
4. The ALTSVC HTTP/2 Frame
The ALTSVC HTTP/2 frame ([RFC 7540], Section 4) advertises the
availability of an alternative service to an HTTP/2 client.
The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints
that do not support this frame will ignore it (as per the
extensibility rules defined in Section 4.1 of [RFC 7540]).
An ALTSVC frame from a server to a client on a stream other than
stream 0 indicates that the conveyed alternative service is
associated with the origin of that stream.
An ALTSVC frame from a server to a client on stream 0 indicates that
the conveyed alternative service is associated with the origin
contained in the Origin field of the frame. An association with an
origin that the client does not consider authoritative for the
current connection MUST be ignored.
The ALTSVC frame type is 0xa (decimal 10).
Nottingham, et al. Standards Track PAGE 11
RFC 7838 HTTP Alternative Services April 2016
+-------------------------------+-------------------------------+
| Origin-Len (16) | Origin? (*) ...
+-------------------------------+-------------------------------+
| Alt-Svc-Field-Value (*) ...
+---------------------------------------------------------------+
ALTSVC Frame Payload
The ALTSVC frame contains the following fields:
Origin-Len: An unsigned, 16-bit integer indicating the length, in
octets, of the Origin field.
Origin: An OPTIONAL sequence of characters containing the ASCII
serialization of an origin ([RFC 6454], Section 6.2) to which the
alternative service is applicable.
Alt-Svc-Field-Value: A sequence of octets (length determined by
subtracting the length of all preceding fields from the frame
length) containing a value identical to the Alt-Svc field value
defined in Section 3 (ABNF production "Alt-Svc").
The ALTSVC frame does not define any flags.
The ALTSVC frame is intended for receipt by clients. A device acting
as a server MUST ignore it.
An ALTSVC frame on stream 0 with empty (length 0) "Origin"
information is invalid and MUST be ignored. An ALTSVC frame on a
stream other than stream 0 containing non-empty "Origin" information
is invalid and MUST be ignored.
The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT
forward ALTSVC frames, though it can use the information contained in
ALTSVC frames in forming new ALTSVC frames to send to its own
clients.
Receiving an ALTSVC frame is semantically equivalent to receiving an
Alt-Svc header field. As a result, the ALTSVC frame causes
alternative services for the corresponding origin to be replaced.
Note that it would be unwise to mix the use of Alt-Svc header fields
with the use of ALTSVC frames, as the sequence of receipt might be
hard to predict.
Nottingham, et al. Standards Track PAGE 12
RFC 7838 HTTP Alternative Services April 2016
5. The Alt-Used HTTP Header Field
The Alt-Used header field is used in requests to identify the
alternative service in use, just as the Host header field
(Section 5.4 of [RFC 7230]) identifies the host and port of the
origin.
Alt-Used = uri-host [ ":" port ]
Alt-Used is intended to allow alternative services to detect loops,
differentiate traffic for purposes of load balancing, and generally
to ensure that it is possible to identify the intended destination of
traffic, since introducing this information after a protocol is in
use has proven to be problematic.
When using an alternative service, clients SHOULD include an Alt-Used
header field in all requests.
For example:
GET /thing HTTP/1.1
Host: origin.example.com
Alt-Used: alternate.example.net
6. The 421 (Misdirected Request) HTTP Status Code
The 421 (Misdirected Request) status code is defined in Section 9.1.2
of [RFC 7540] to indicate that the current server instance is not
authoritative for the requested resource. This can be used to
indicate that an alternative service is not authoritative; see
Section 2).
Clients receiving 421 (Misdirected Request) from an alternative
service MUST remove the corresponding entry from its alternative
service cache (see Section 2.2) for that origin. Regardless of the
idempotency of the request method, they MAY retry the request, either
at another alternative server, or at the origin.
An Alt-Svc header field in a 421 (Misdirected Request) response MUST
be ignored.
7. IANA Considerations
7.1. Header Field Registrations
HTTP header fields are registered within the "Message Headers"
registry maintained at <https://www.iana.org/assignments/message-
headers/>.
Nottingham, et al. Standards Track PAGE 13
RFC 7838 HTTP Alternative Services April 2016
This document defines the following HTTP header fields, so their
associated registry entries have been added according to the
permanent registrations below (see [BCP90]):
+-------------------+----------+----------+------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+------------+
| Alt-Svc | http | standard | Section 3 |
| Alt-Used | http | standard | Section 5 |
+-------------------+----------+----------+------------+
The change controller is: "IETF (iesg@ietf.org) -- Internet
Engineering Task Force".
7.2. The ALTSVC HTTP/2 Frame Type
This document registers the ALTSVC frame type in the "HTTP/2 Frame
Type" registry ([RFC 7540], Section 11.2).
Frame Type: ALTSVC
Code: 0xa
Specification: Section 4 of this document
7.3. Alt-Svc Parameter Registry
The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
defines the name space for parameters. It has been created and will
be maintained at <http://www.iana.org/assignments/http-alt-svc-
parameters>.
7.3.1. Procedure
A registration MUST include the following fields:
o Parameter Name
o Pointer to specification text
Values to be added to this name space require Expert Review (see
[RFC 5226], Section 4.1).
Nottingham, et al. Standards Track PAGE 14
RFC 7838 HTTP Alternative Services April 2016
7.3.2. Registrations
The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
has been populated with the registrations below:
+-------------------+--------------+
| Alt-Svc Parameter | Reference |
+-------------------+--------------+
| ma | Section 3.1 |
| persist | Section 3.1 |
+-------------------+--------------+
8. Internationalization Considerations
An internationalized domain name that appears in either the header
field (Section 3) or the HTTP/2 frame (Section 4) MUST be expressed
using A-labels ([RFC 5890], Section 2.3.2.1).
9. Security Considerations
9.1. Changing Ports
Using an alternative service implies accessing an origin's resources
on an alternative port, at a minimum. Therefore, an attacker that
can inject alternative services and listen at the advertised port is
able to hijack an origin. On certain servers, it is normal for users
to be able to control some personal pages available on a shared port
and also to accept requests on less-privileged ports.
For example, an attacker that can add HTTP response header fields to
some pages can redirect traffic for an entire origin to a different
port on the same host using the Alt-Svc header field; if that port is
under the attacker's control, they can thus masquerade as the HTTP
server.
This risk is mitigated by the requirements in Section 2.1.
On servers, this risk can also be reduced by restricting the ability
to advertise alternative services, and restricting who can open a
port for listening on that host.
9.2. Changing Hosts
When the host is changed due to the use of an alternative service,
this presents an opportunity for attackers to hijack communication to
an origin.
Nottingham, et al. Standards Track PAGE 15
RFC 7838 HTTP Alternative Services April 2016
For example, if an attacker can convince a user agent to send all
traffic for "innocent.example.org" to "evil.example.com" by
successfully associating it as an alternative service, they can
masquerade as that origin. This can be done locally (see mitigations
in Section 9.1) or remotely (e.g., by an intermediary as a man-in-
the-middle attack).
This is the reason for the requirement in Section 2.1 that clients
have reasonable assurances that the alternative service is under
control of and valid for the whole origin; for example, presenting a
certificate for the origin proves that the alternative service is
authorized to serve traffic for the origin.
Note that this assurance is only as strong as the method used to
authenticate the alternative service. In particular, when TLS
authentication is used to do so, there are well-known exploits to
make an attacker's certificate appear as legitimate.
Alternative services could be used to persist such an attack. For
example, an intermediary could man-in-the-middle TLS-protected
communication to a target and then direct all traffic to an
alternative service with a large freshness lifetime so that the user
agent still directs traffic to the attacker even when not using the
intermediary.
Implementations MUST perform any certificate-pinning validation (such
as [RFC 7469]) on alternative services just as they would on direct
connections to the origin. Implementations might also choose to add
other requirements around which certificates are acceptable for
alternative services.
9.3. Changing Protocols
When the ALPN protocol is changed due to the use of an alternative
service, the security properties of the new connection to the origin
can be different from that of the "normal" connection to the origin,
because the protocol identifier itself implies this.
For example, if an "https://" URI has a protocol advertised that does
not use some form of end-to-end encryption (most likely, TLS), this
violates the expectations for security that the URI scheme implies.
Therefore, clients cannot use alternative services blindly, but
instead evaluate the option(s) presented to ensure that security
requirements and expectations of specifications, implementations, and
end users are met.
Nottingham, et al. Standards Track PAGE 16
RFC 7838 HTTP Alternative Services April 2016
9.4. Tracking Clients Using Alternative Services
Choosing an alternative service implies connecting to a new, server-
supplied host name. By using unique names, servers could conceivably
track client requests. Such tracking could follow users across
multiple networks, when the "persist" flag is used.
Clients that wish to prevent requests from being correlated can
decide not to use alternative services for multiple requests that
would not otherwise be allowed to be correlated.
In a user agent, any alternative service information MUST be removed
when origin-specific data is cleared (typically, when cookies
[RFC 6265] are cleared).
9.5. Confusion regarding Request Scheme
Some server-side HTTP applications make assumptions about security
based upon connection context; for example, equating being served
upon port 443 with the use of an "https://" URI and the various
security properties that implies.
This affects not only the security properties of the connection
itself, but also the state of the client at the other end of it; for
example, a Web browser treats "https://" URIs differently than
"http://" URIs in many ways, not just for purposes of protocol
handling.
Since one of the uses of Alternative Services is to allow a
connection to be migrated to a different protocol and port, these
applications can become confused about the security properties of a
given connection, sending information (for example, cookies and
content) that is intended for a secure context (such as an "https://"
URI) to a client that is not treating it as one.
This risk can be mitigated in servers by using the URI scheme
explicitly carried by the protocol (such as ":scheme" in HTTP/2 or
the "absolute form" of the request target in HTTP/1.1) as an
indication of security context, instead of other connection
properties ([RFC 7540], Section 8.1.2.3 and [RFC 7230], Section 5.3.2).
When the protocol does not explicitly carry the scheme (as is usually
the case for HTTP/1.1 over TLS), servers can mitigate this risk by
either assuming that all requests have an insecure context, or by
refraining from advertising alternative services for insecure schemes
(for example, HTTP).
Nottingham, et al. Standards Track PAGE 17
RFC 7838 HTTP Alternative Services April 2016
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC 2818, May 2000,
<http://www.rfc-editor.org/info/RFC 2818>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<http://www.rfc-editor.org/info/RFC 3986>.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC 5226, May 2008,
<http://www.rfc-editor.org/info/RFC 5226>.
[RFC 5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC 5234, January 2008,
<http://www.rfc-editor.org/info/RFC 5234>.
[RFC 5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC 5890, August 2010,
<http://www.rfc-editor.org/info/RFC 5890>.
[RFC 6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, DOI 10.17487/RFC 6066,
January 2011, <http://www.rfc-editor.org/info/RFC 6066>.
[RFC 6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC 6454, December 2011,
<http://www.rfc-editor.org/info/RFC 6454>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
Nottingham, et al. Standards Track PAGE 18
RFC 7838 HTTP Alternative Services April 2016
[RFC 7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC 7234, June 2014,
<http://www.rfc-editor.org/info/RFC 7234>.
[RFC 7301] Friedl, S., Popov, A., Langley, A., and S. Emile,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC 7301,
July 2014, <http://www.rfc-editor.org/info/RFC 7301>.
[RFC 7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC 7405, December 2014,
<http://www.rfc-editor.org/info/RFC 7405>.
[RFC 7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", RFC 7540,
DOI 10.17487/RFC 7540, May 2015,
<http://www.rfc-editor.org/info/RFC 7540>.
10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004, <http://www.rfc-editor.org/info/bcp90>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC 6265, April 2011,
<http://www.rfc-editor.org/info/RFC 6265>.
[RFC 7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC 7469, April
2015, <http://www.rfc-editor.org/info/RFC 7469>.
Acknowledgements
Thanks to Adam Langley, Bence Beky, Chris Lonvick, Eliot Lear, Erik
Nygren, Guy Podjarny, Herve Ruellan, Lucas Pardue, Martin Thomson,
Matthew Kerwin, Mike Bishop, Paul Hoffman, Richard Barnes, Richard
Bradbury, Stephen Farrell, Stephen Ludin, and Will Chan for their
feedback and suggestions.
The Alt-Svc header field was influenced by the design of the
Alternate-Protocol header field in SPDY.
Nottingham, et al. Standards Track PAGE 19
RFC 7838 HTTP Alternative Services April 2016
Authors' Addresses
Mark Nottingham
Akamai
Email: mnot@mnot.net
URI: https://www.mnot.net/
Patrick McManus
Mozilla
Email: mcmanus@ducksong.com
URI: https://mozillians.org/u/pmcmanus/
Julian F. Reschke
greenbytes GmbH
Email: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/
Nottingham, et al. Standards Track PAGE 20
RFC TOTAL SIZE: 42681 bytes
PUBLICATION DATE: Friday, April 8th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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