RFC2818 - HTTP Over TLS

Network Working Group E. Rescorla

Request for Comments: 2818 RTFM, Inc.

Category: Informational May 2000

HTTP Over TLS

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

This memo describes how to use TLS to secure HTTP connections over

the Internet. Current practice is to layer HTTP over SSL (the

predecessor to TLS), distinguishing secured traffic from insecure

traffic by the use of a different server port. This document

documents that practice using TLS. A companion document describes a

method for using HTTP/TLS over the same port as normal HTTP

[RFC2817].

Table of Contents

1. IntrodUCtion . . . . . . . . . . . . . . . . . . . . . . 2

1.1. Requirements Terminology . . . . . . . . . . . . . . . 2

2. HTTP Over TLS . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Connection Initiation . . . . . . . . . . . . . . . . . 2

2.2. Connection Closure . . . . . . . . . . . . . . . . . . 2

2.2.1. Client Behavior . . . . . . . . . . . . . . . . . . . 3

2.2.2. Server Behavior . . . . . . . . . . . . . . . . . . . 3

2.3. Port Number . . . . . . . . . . . . . . . . . . . . . . 4

2.4. URI Format . . . . . . . . . . . . . . . . . . . . . . 4

3. Endpoint Identification . . . . . . . . . . . . . . . . . 4

3.1. Server Identity . . . . . . . . . . . . . . . . . . . . 4

3.2. Client Identity . . . . . . . . . . . . . . . . . . . . 5

References . . . . . . . . . . . . . . . . . . . . . . . . . 6

Security Considerations . . . . . . . . . . . . . . . . . . 6

Author's Address . . . . . . . . . . . . . . . . . . . . . . 6

Full Copyright Statement . . . . . . . . . . . . . . . . . . 7

1. Introduction

HTTP [RFC2616] was originally used in the clear on the Internet.

However, increased use of HTTP for sensitive applications has

required security measures. SSL, and its successor TLS [RFC2246] were

designed to provide channel-oriented security. This document

describes how to use HTTP over TLS.

1.1. Requirements Terminology

KeyWords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and

"MAY" that appear in this document are to be interpreted as described

in [RFC2119].

2. HTTP Over TLS

Conceptually, HTTP/TLS is very simple. Simply use HTTP over TLS

precisely as you would use HTTP over TCP.

2.1. Connection Initiation

The agent acting as the HTTP client should also act as the TLS

client. It should initiate a connection to the server on the

appropriate port and then send the TLS ClientHello to begin the TLS

handshake. When the TLS handshake has finished. The client may then

initiate the first HTTP request. All HTTP data MUST be sent as TLS

"application data". Normal HTTP behavior, including retained

connections should be followed.

2.2. Connection Closure

TLS provides a facility for secure connection closure. When a valid

closure alert is received, an implementation can be assured that no

further data will be received on that connection. TLS

implementations MUST initiate an exchange of closure alerts before

closing a connection. A TLS implementation MAY, after sending a

closure alert, close the connection without waiting for the peer to

send its closure alert, generating an "incomplete close". Note that

an implementation which does this MAY choose to reuse the session.

This SHOULD only be done when the application knows (typically

through detecting HTTP message boundaries) that it has received all

the message data that it cares about.

As specified in [RFC2246], any implementation which receives a

connection close without first receiving a valid closure alert (a

"premature close") MUST NOT reuse that session. Note that a

premature close does not call into question the security of the data

already received, but simply indicates that subsequent data might

have been truncated. Because TLS is oblivious to HTTP

request/response boundaries, it is necessary to examine the HTTP data

itself (specifically the Content-Length header) to determine whether

the truncation occurred inside a message or between messages.

2.2.1. Client Behavior

Because HTTP uses connection closure to signal end of server data,

client implementations MUST treat any premature closes as errors and

the data received as potentially truncated. While in some cases the

HTTP protocol allows the client to find out whether truncation took

place so that, if it received the complete reply, it may tolerate

such errors following the principle to "[be] strict when sending and

tolerant when receiving" [RFC1958], often truncation does not show in

the HTTP protocol data; two cases in particular deserve special note:

A HTTP response without a Content-Length header. Since data length

in this situation is signalled by connection close a premature

close generated by the server cannot be distinguished from a

spurious close generated by an attacker.

A HTTP response with a valid Content-Length header closed before

all data has been read. Because TLS does not provide document

oriented protection, it is impossible to determine whether the

server has miscomputed the Content-Length or an attacker has

truncated the connection.

There is one exception to the above rule. When encountering a

premature close, a client SHOULD treat as completed all requests for

which it has received as much data as specified in the Content-Length

header.

A client detecting an incomplete close SHOULD recover gracefully. It

MAY resume a TLS session closed in this fashion.

Clients MUST send a closure alert before closing the connection.

Clients which are unprepared to receive any more data MAY choose not

to wait for the server's closure alert and simply close the

connection, thus generating an incomplete close on the server side.

2.2.2. Server Behavior

RFC2616 permits an HTTP client to close the connection at any time,

and requires servers to recover gracefully. In particular, servers

SHOULD be prepared to receive an incomplete close from the client,

since the client can often determine when the end of server data is.

Servers SHOULD be willing to resume TLS sessions closed in this

fashion.

Implementation note: In HTTP implementations which do not use

persistent connections, the server ordinarily eXPects to be able to

signal end of data by closing the connection. When Content-Length is

used, however, the client may have already sent the closure alert and

dropped the connection.

Servers MUST attempt to initiate an exchange of closure alerts with

the client before closing the connection. Servers MAY close the

connection after sending the closure alert, thus generating an

incomplete close on the client side.

2.3. Port Number

The first data that an HTTP server expects to receive from the client

is the Request-Line production. The first data that a TLS server (and

hence an HTTP/TLS server) expects to receive is the ClientHello.

Consequently, common practice has been to run HTTP/TLS over a

separate port in order to distinguish which protocol is being used.

When HTTP/TLS is being run over a TCP/IP connection, the default port

is 443. This does not preclude HTTP/TLS from being run over another

transport. TLS only presumes a reliable connection-oriented data

stream.

2.4. URI Format

HTTP/TLS is differentiated from HTTP URIs by using the 'https'

protocol identifier in place of the 'http' protocol identifier. An

example URI specifying HTTP/TLS is:

https://www.example.com/~smith/home.Html

3. Endpoint Identification

3.1. Server Identity

In general, HTTP/TLS requests are generated by dereferencing a URI.

As a consequence, the hostname for the server is known to the client.

If the hostname is available, the client MUST check it against the

server's identity as presented in the server's Certificate message,

in order to prevent man-in-the-middle attacks.

If the client has external information as to the expected identity of

the server, the hostname check MAY be omitted. (For instance, a

client may be connecting to a machine whose address and hostname are

dynamic but the client knows the certificate that the server will

present.) In such cases, it is important to narrow the scope of

acceptable certificates as much as possible in order to prevent man

in the middle attacks. In special cases, it may be appropriate for

the client to simply ignore the server's identity, but it must be

understood that this leaves the connection open to active attack.

If a subjectAltName extension of type dNSName is present, that MUST

be used as the identity. Otherwise, the (most specific) Common Name

field in the Subject field of the certificate MUST be used. Although

the use of the Common Name is existing practice, it is deprecated and

Certification Authorities are encouraged to use the dNSName instead.

Matching is performed using the matching rules specified by

[RFC2459]. If more than one identity of a given type is present in

the certificate (e.g., more than one dNSName name, a match in any one

of the set is considered acceptable.) Names may contain the wildcard

character * which is considered to match any single domain name

component or component fragment. E.g., *.a.com matches foo.a.com but

not bar.foo.a.com. f*.com matches foo.com but not bar.com.

In some cases, the URI is specified as an IP address rather than a

hostname. In this case, the iPAddress subjectAltName must be present

in the certificate and must exactly match the IP in the URI.

If the hostname does not match the identity in the certificate, user

oriented clients MUST either notify the user (clients MAY give the

user the opportunity to continue with the connection in any case) or

terminate the connection with a bad certificate error. Automated

clients MUST log the error to an appropriate audit log (if available)

and SHOULD terminate the connection (with a bad certificate error).

Automated clients MAY provide a configuration setting that disables

this check, but MUST provide a setting which enables it.

Note that in many cases the URI itself comes from an untrusted

source. The above-described check provides no protection against

attacks where this source is compromised. For example, if the URI was

oBTained by clicking on an HTML page which was itself obtained

without using HTTP/TLS, a man in the middle could have replaced the

URI. In order to prevent this form of attack, users should carefully

examine the certificate presented by the server to determine if it

meets their expectations.

3.2. Client Identity

Typically, the server has no external knowledge of what the client's

identity ought to be and so checks (other than that the client has a

certificate chain rooted in an appropriate CA) are not possible. If a

server has such knowledge (typically from some source external to

HTTP or TLS) it SHOULD check the identity as described above.

References

[RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet

Public Key Infrastructure: Part I: X.509 Certificate and

CRL Profile", RFC2459, January 1999.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,

L., Leach, P. and T. Berners-Lee, "Hypertext Transfer

Protocol, HTTP/1.1", RFC2616, June 1999.

[RFC2119] Bradner, S., "Key Words for use in RFCs to indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol", RFC2246,

January 1999.

[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within

HTTP/1.1", RFC2817, May 2000.

Security Considerations

This entire document is about security.

Author's Address

Eric Rescorla

RTFM, Inc.

30 Newell Road, #16

East Palo Alto, CA 94303

Phone: (650) 328-8631

EMail: ekr@rtfm.com

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