[TLS] Server name extension extensibility problem

[Mike <mike-list at pobox.com>]

Hi,

The server_name extension contains a ServerNameList, where:

   struct {
       NameType name_type;
       select (name_type) {
           case host_name: HostName;
       } name;
   } ServerName;

   enum {
       host_name(0), (255)
   } NameType;

   opaque HostName<1..2^16-1>;

   struct {
       ServerName server_name_list<1..2^16-1>
   } ServerNameList;

The text states that this may be extended in the future to
other name types, but I don't think that would be possible.

Suppose you were to add a new NameType and define a new
structure to hold the name.  Software that doesn't know
about the new name type won't know how to skip past it
while parsing the server_name_list, since ServerName is
currently only fully specified for a host_name.

I suppose you could restrict server_name_list to list the
ServerNames in order (host_name first, then the next-
defined name type, and so on), so software could simply
stop decoding once it encounters a type it doesn't know
about, but it's probably too late to do that since there
is already an installed base.

Mike
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