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Network Working Group                                       E. RescorlaRequest for Comments: 2818                                   RTFM, Inc.Category: Informational                                        May 2000HTTP Over TLSStatus 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.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.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 Contents1. Introduction  . . . . . . . . . . . . . . . . . . . . . .21.1. Requirements Terminology  . . . . . . . . . . . . . . .22. HTTP Over TLS . . . . . . . . . . . . . . . . . . . . . .22.1. Connection Initiation . . . . . . . . . . . . . . . . .22.2. Connection Closure  . . . . . . . . . . . . . . . . . .22.2.1. Client Behavior . . . . . . . . . . . . . . . . . . .32.2.2. Server Behavior . . . . . . . . . . . . . . . . . . .32.3. Port Number . . . . . . . . . . . . . . . . . . . . . .42.4. URI Format  . . . . . . . . . . . . . . . . . . . . . .43. Endpoint Identification . . . . . . . . . . . . . . . . .43.1. Server Identity . . . . . . . . . . . . . . . . . . . .43.2. Client Identity . . . . . . . . . . . . . . . . . . . .5   References . . . . . . . . . . . . . . . . . . . . . . . . .6   Security Considerations  . . . . . . . . . . . . . . . . . .6   Author's Address . . . . . . . . . . . . . . . . . . . . . .6   Full Copyright Statement . . . . . . . . . . . . . . . . . .7Rescorla                     Informational                      [Page 1]

RFC 2818                     HTTP Over TLS                      May 20001.  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 mightRescorla                     Informational                      [Page 2]

RFC 2818                     HTTP Over TLS                      May 2000   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 BehaviorRFC 2616 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.Rescorla                     Informational                      [Page 3]

RFC 2818                     HTTP Over TLS                      May 2000   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.html3.  Endpoint Identification3.1.  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 manRescorla                     Informational                      [Page 4]

RFC 2818                     HTTP Over TLS                      May 2000   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.Rescorla                     Informational                      [Page 5]

RFC 2818                     HTTP Over TLS                      May 2000References   [RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet             Public Key Infrastructure: Part I: X.509 Certificate and             CRL Profile",RFC 2459, 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",RFC 2616, June 1999.   [RFC2119] Bradner, S., "Key Words for use in RFCs to indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol",RFC 2246,             January 1999.   [RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within             HTTP/1.1",RFC 2817, 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.comRescorla                     Informational                      [Page 6]

RFC 2818                     HTTP Over TLS                      May 2000Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Rescorla                     Informational                      [Page 7]