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





 
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.





 
RFC 2818                     HTTP Over TLS                      May 2000


References

   [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.