On 30/07/2003 at 03:56:20, Daniel Donnelly wrote: > So, how do you know there were control codes sent (did you use a > sniffer)? Is there any way to tell what codes telnet will send in that > first line (some configuration, maybe?)?
A packet sniffer will show them up. Because Telnet was created primarily as a way for users to log in and run terminal sessions on a machine, it supports a whole bunch of commands and configuration options (eg whether the server echoes text that the user types, whether the user will be logged out if the Telnet session is dropped). Those commands are sent as the byte 0xFF (called IAC, or Interpret As Command) followed by one or more bytes representing the option. The exact string sent on startup will depend on the Telnet client and its configuration. I just connected to IMail's SMTP server with PuTTY in Telnet mode, and in response to the 220 message the client sent the following bytes: FF FB 1F FF FB 20 FF FB 18 FF FB 27 FF FD 01 FF FB 03 FF FD 03 If you wanted to interpret those, FF is IAC, FB through FE represent "will, won't, do, don't", and the byte following FB through FE is a specific Telnet option (each of which is usually defined in its own RFC). None of that meaning is carried across to SMTP - IMail treats it as normal text and returns a 502 error because it saw 21 bytes of garbage before the command. > Just had the thought of sending a blank line as first line, to see > what that might do... It'll give you a 502, no different to if you typed a command. The client will send those options as soon as it knows there's a server on the other end. Cheers, Evan To Unsubscribe: http://www.ipswitch.com/support/mailing-lists.html List Archive: http://www.mail-archive.com/imail_forum%40list.ipswitch.com/ Knowledge Base/FAQ: http://www.ipswitch.com/support/IMail/
