how to ensure message sending with ssl - erlang

Question

I am implementing as exercise a gen_server which behaves as interface towards an ssl authentication server. The ssl server severes the connection if a packet received is wrong (e.g. wrong username and password). The connection must be persistent.
In my gen_server, I open the ssl connection towards the server with an handle_cast/2:
handle_cast(connect, State) ->
......
case ssl:connect(Address, Port, Options, Timeout) of
{ok, NewSocket} ->
{noreply, State#state{socket=NewSocket}};
{error, Reason} ->
gen_server:cast(?SERVER, connect),
{noreply, State#state{socket=undefined}};
and then I wait for other other messages in handle_cast/2 which can be sent for example using:
gen_server:cast(Pid, {authenticate, User, Password}).
Whenever I receive such a cast message I spawn a new function which recovers the SSL socket from the server state using a gen_server:call/3 and sends the authentication message to the SSL server. If the sending part returns an error I try to reconnect, otherwise I read for a while on the socket, to be sure that the socket does not go down, and if it does I reconnect.
send_auth(_, _, 0) ->
{error, max_num_reached};
send_auth(User, Password, Num) ->
Socket = gen_server:call(?SERVER, socket),
%% also a check that socket is not 'undefined'
case ssl:send(Socket, AuthMessage) of
ok ->
case ssl:recv(Socket, 0, 2000) of
{error, timeout} ->
ok;
_ ->
gen_server:cast(?SERVER, connect),
send_auth(User, Password, Num-1)
end,
{error, closed} ->
gen_server:cast(?SERVER, connect),
send_auth(User, Password, Num-1)
end.
I made many tests, but every time, if one message (not the last) is wrong, none of the following messages is actually delivered.
How can I grant that all valid authentication messages are delivered to the authentication server? Moreover how can be sure that the server will connect only if it is not already trying to do so? Otherwise that would be like DOS attack!

Answer 1

As I can understand there's only one authentication server in the system so why can't you connect to it right in the gen_server's init? If the connection is somewhat reliable, you can just kill gen_server if there are any issues with connection and let supervisor re-launch it. It probably will solve your DOS concern.
When you spawn additional processes with send_auth you can just pass socket connection in arguments, instead of doing gen_server call.
To guarantee the delivery you'll add some kind of acknowledgement into your protocol. Auth server should reply to send_auth with something confirming that it got it. And send_auth should retry until it receives that acknowledgement or have some other fallback behavior in case it never does.