Sending SSH_MSG_DISCONNECT before dropping connections

[olle ollesson]

Hi.

>From my understanding the MaxStartups option can be set to limit the number 
of concurrent sessions the OpenSSH server opens. My concern is how OpenSSH 
handles the case where this number is reached.

>From the code it looks like it simply closes the socket:

sshd.c:1440
if (drop_connection(startups) == 1) {
  debug("drop connection #%d", startups);
  close(newsock);
  continue;
}

Why is there no disconnect message sent that explains to the client why the 
socket was closed?

>From draft-ietf-secsh-transport-24.txt, chapter 11:
----------------------------------------------------------
11.  Additional Messages

   Either party may send any of the following messages at any time.

11.1  Disconnection Message
      byte      SSH_MSG_DISCONNECT
      uint32    reason code
      string    description [RFC3629]
      string    language tag [RFC3066]

   This message causes immediate termination of the connection.  All
   implementations MUST be able to process this message; they SHOULD be
   able to send this message.

   The sender MUST NOT send or receive any data after this message, and
   the recipient MUST NOT accept any data after receiving this message.
   The Disconnection Message 'description' string gives a more specific
   explanation in a human-readable form.  The Disconnection Message
   'reason code' gives the reason in a more machine-readable format
   (suitable for localization), and can have the values as displayed in
   the table below.  Note that the decimal representation is displayed
   in this table for readability but that the values are actually uint32
   values.

          Symbolic name                                reason code
          -------------                                -----------
     SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT             1
     SSH_DISCONNECT_PROTOCOL_ERROR                          2
     SSH_DISCONNECT_KEY_EXCHANGE_FAILED                     3
     SSH_DISCONNECT_RESERVED                                4
     SSH_DISCONNECT_MAC_ERROR                               5
     SSH_DISCONNECT_COMPRESSION_ERROR                       6
     SSH_DISCONNECT_SERVICE_NOT_AVAILABLE                   7
     SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED          8
     SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE                 9
     SSH_DISCONNECT_CONNECTION_LOST                        10
     SSH_DISCONNECT_BY_APPLICATION                         11
     SSH_DISCONNECT_TOO_MANY_CONNECTIONS                   12
     SSH_DISCONNECT_AUTH_CANCELLED_BY_USER                 13
     SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE         14
     SSH_DISCONNECT_ILLEGAL_USER_NAME                      15
----------------------------------------------------------

If a SSH_DISCONNECT_TOO_MANY_CONNECTIONS disconnect message was to be sent 
before the connection is closed how far would the SSH connection setup need 
to go before this could be done?

The draft says it can be sent at any time. Can we send it before the 
"Protocol Version Exchange"? Probably not. Before the Key Exchange? I'm not 
sure. Question is, how far would the ssh negotiation/connection setup need 
to go in order for the ssh server to refuse the connection in a more 
controlled way, that is sending the SSH_DISCONNECT_TOO_MANY_CONNECTIONS 
disconnect message, and still be compliant with the draft standard?

Best Regards,
Olle

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