Extensibility of server_name

[pasi.eronen@…]

Mike 2008-05-07:

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.