TCP IP Illustrated

Next we type a valid name and the server responds

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Unformatted text preview: ped the wrong hostname on the HELO command: bsdi instead of sun. Most SMTP servers take the IP address of the client and perform a DNS pointer query (Section 14.5) and compare the hostnames. This allows the server to log the client connection based on the IP address, not the name that a user might have mistyped. file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (13 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol Some servers respond with humorous messages, such as "You are a charlatan," or "why do you call yourself...". We see in this example that this server just prints our real domain name from the pointer query along with our IP address. We then type a VRFY command for an invalid name, and the server responds with a 550 error. Next we type a valid name, and the server responds with the usemame on the local host. Next we try the EXPN command and get a different response. The EXPN command determines that the mail for this user is being forwarded, and prints the forwarding address. Many sites disable the VRFY and EXPN commands, sometimes for privacy, and sometimes in the belief that it's a security hole. For example, we can try these commands with the SMTP server at the White House: sun % telnet 25 220 SMTP/smap Ready. helo 250 ( pleased to meet you. vrfy Clinton 500 Command unrecognized expn Clinton 500 Command unrecognized 28.4 SMTP Futures Changes are taking place with Internet mail. Recall the three pieces that comprise Internet mail: the envelope, headers, and body. New SMTP commands are being added that affect the envelope, non-ASCII characters can be used in the headers, and structure is being added to the body (MIME). In this section we consider the extensions to each of these three pieces in order. Envelope Changes: Extended SMTP RFC 1425 [Klensin et al. 1993a] defines the framework for adding extensi...
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