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Versions: 00                                                            
INTERNET-DRAFT                                           M. VanHeyningen
<draft-ietf-aft-socks-ext-00>                       Aventail Corporation
Expires in six months                                       22 July 1997

  Feature Discovery: A Generic Extension Mechanism for SOCKS Version 5

Status of this Memo

      This document is an Internet-Draft.  Internet-Drafts are working
      documents of the Internet Engineering Task Force (IETF), its
      areas, and its working groups.  Note that other groups may also
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   This document specifies a command extension to the SOCKS Version 5
   protocol which enables compliant clients to discover features
   supported by the server.  After discovering the support of such
   features, the client may use them in subsequent connections to that
   server.  This mechanism does not provide for negotiation; it is a way
   of instructing the client what features the server supports, not
   establishing which features the client supports or wishes to use.


   LIST-FEATURES is a new SOCKS V5 command, with an identifier of X'10'
   (16 decimal.)  This command is formatted as a standard command, per
   section 4 of [SOCKS5].  Servers which do not support the LIST-
   FEATURES command should respond with the "Command not supported"

   The client may set DST.ADDR and DST.PORT to the destination host and
   port of interest, or may send an IPv4 address of to indicate
   the query is not for any specific host.  The server may use this
   information, along with the address of the client, to customize the

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   The client may request that the server hold the connection after the
   LIST-FEATURES command is completed to perform another SOCKS5 command
   by sending a FLAG field of X'01'.

   The reply to the command is also formatted as a standard reply
   [SOCKS5, sec 6.]  If the client has requested a persistent connection
   and the server chooses to grant that request, it returns a FLAG of
   X'01'.  The address returned should be an IPv4 address of

   After the reply to the command is sent, the server sends a structure
   called the Feature Description List described below.  After sending
   the FDL, the server holds the connection open for another command if
   it has granted a persistent connection; otherwise it closes it.

The Feature Description List

   The structure passed by the server which advertises its full set of
   features is called the Feature Description List (FDL).

 Terminology and syntax

   The FDL is a tag-length-value (TLV) structure.  Tags consist of a
   main tag and a subtag.  These are written as separated by a hyphen,
   with the value following parentheses.  For example:


   When encoded into the structure, the tag and subtag each map to one
   byte in the tag table.  The length of the value is a single byte,
   followed by the value itself.

   For example, if TAG is X'07' and SUBTAG is X'13, the above example
   would be encoded in hexidecimal as follows:

      07 13 06 48 65 6c 6c 6f 2e

   An FDL consists of zero or more of these associations concatenated
   together, and is terminated by a TVL with a TAG of END (X'FF').

 FDL Meta-information

   The tag FDL (X'00') is used to describe information about the FDL
   itself, rather than about the server.

   At this time only one subtag, SCOPE (X'10') is defined.  It defines
   the scope of the FDL, and advises whether a client which made a
   different FDL request would have been advertised the same feature
   set.  Servers may provide this information, and clients may use it to

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   determine when the FDL must be re-fetched.

   The following bytes may be included in the value field, and indicate
   the following restrictions on scope:

      Value   Scope restriction
      X'01'   Client address
      X'02'   Authentication method used
      X'03'   Destination host specified in request

 SOCKS version support

   The tag SOCKS (X'01') is used to advertise versions of the SOCKS
   protocol this server will support.  The subtags are versions, as
   defined by the VER field.  No semantics are currently defined for the
   value field.

 Address type support

   The tag ADDR (X'02') is used to advertise address types supported by
   this client.  The subtags are ATYP values as defined in [SOCKS5].  No
   semantics are currently defined for the value field.

 Authentication method support

   The tag AUTH (X'03') is used to advertise authentication methods the
   server will support.  The subtags are individual authentication
   METHOD identifiers.  The semantics associated with the value are
   authentication-method specific; none are defined at this time.

 Command support

   The tag COMMAND (X'04') is used to advertise commands the server will
   support.  The subtags are the CMD identifiers.

   The semantics associated with the value are command-specific.  For
   the LIST-FEATURES command, a value containing X'01' indicates the
   server supports persistent connections with this command.

 Server information

   The tag SERVER (X'10') is used to advertise information about the
   server.  Several subtags are defined:

      Subtag   Byte  Contents

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INTERNET-DRAFT       Feature Discovery for SOCKS V5         22 July 1997

      HOSTNAME X'01' Preferred hostname of the server
      URL      X'02' URL with info on this server (policy, config...)
      OPAQUE   X'03' Opaque identifier for this server
      LOAD     X'04' Current server load (in ASCII floating point)

Security Considerations

   Since LIST-FEATURES is a SOCKS5 command, it may be performed only
   after authentication has taken place.  Servers may wish to restrict
   access to this command to users who have already authenticated
   successfully, although that would also serve to limit its use in
   automating configuration for users who may not yet be able to
   authenticate successfully.

   Servers may prefer not to advertise all the features they support,
   particualrly with regard to authentication methods supported.


   [RFC 1928] Leech, M. et al, "SOCKS Protocol V5," April 1996

   [SOCKS5] Leech, M. et al, "SOCKS Protocl V5," Internet Draft draft-
   ietf-aft-socks-pro-v5-00, March 1997, work in progress.

Author's Address

   Marc VanHeyningen
   Aventail Corporation
   117 South Main Street; Suite 400
   Seattle, WA  98104  USA

   Phone: +1 (206) 777-5600
   Email: marcvh@aventail.com

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