Hello!
> Many mailer clients are waiting for greeting message from sendmail,
> but some of them does not timeout though.
Examples?
I am sorry, but missing greeting is the most frequent error in SMTP.
Look at logs. Such bug in mail agent would be discovered next five
minutes after starting.
> Yes I know sending FIN is too difficult technicaly.
No, it is not a technical problem. To send FIN you have to create
socket for beginning. It is major difference between modern stacks
and RFCs. SYN-RECV state is not considered as synchronized more.
> Does "dangerous" mean weakness againt Dos?
Yes, sort of. See below.
> How about sending RST instead of FIN?
It is a technical problem. New code must be written to send active
RST without socket context.
Also, instant burst of RSTs to clients, which are still
did not autheniticate itself is suspicious.
> >> connection is closed silently and without notification.
>
> Not gentle I think.
> SYN_RECV connection might ignore many ACK from a client again
> and again when an ack_backlog is full.
1. We did not sent _anything_ for enough long time.
2. Connection is not synchronized.
Hence, we may die silently.
> It would be better to send RST than nothing.
Sorry, "nothing" is always better than "anything" in the Internet.
Unfortunately, we cannot do "nothing" in the most of cases. 8)
Alexey
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