Shulou(Shulou.com)06/02 Report--

1.SMTP works in two cases: one is the transfer of e-mail from the client to the server; the other is the transfer from one server to another server 2.SMTP is a request / response protocol, commands and responses are based on ASCII text and end with CR and LF characters. The response includes a three-digit code 3.SMTP indicating the return status to listen for connection request 4. 5 on TCP protocol port 25. Connection and delivery process:

a. Establish a TCP connection b. The client sends a HELO command to identify the sender, and then the client sends a MAIL command that the server wants to respond with OK, indicating that it is ready to receive c. The client sends a RCPT command to identify the intended recipient of the email, and multiple RCPT line servers indicate whether they are willing to accept the email for the recipient. When the negotiation is over, send an email and use the command DATA to send e. It means to end the input and send it out f. End this send and exit with the QUIT command.

5. Two other commands, VRFY---, are used to verify the existence of a given user's mailbox and to receive detailed information about that user. EXPN--- is used to expand the mailing list.

6. Mail routing process: the SMTP server routes e-mail based on the domain name of the planned recipient in the domain name service DNS. The SMTP server routes email based on the MX record in DNS, which registers the domain name and the associated SMTP relay host, and emails belonging to that domain should be sent to that host.

If the SMTP server mail.abc.com receives a letter to be sent to shuser@sh.abc.com:

A.Sendmail requests DNS to give the CNAME record of the host sh.abc.com. If so, if CNAME to shmail.abc.com, request the CNAME record of shmail.abc.com again until there is no CNAME record. Suppose it is CNAME to shmail.abc.com, then sendmail requests the DNS of @ abc.com domain to give the MX record of shmail.abc.com, and shmail MX 5 shmail.abc.com 10 shmail2.abc.com c. Sendmail finally requests DNS to give the A record of shmail.abc.com, that is, IP address, if the return value is 1.2.3.4 d. Sendmail connects with 1.2.3.4 Send this message to shuser@sh.abc.com to 1.2.3.4 the SMTP daemon for this server

7.SMTP basic command set:

Command description-HELO identifies to the server that the sender can cheat and lie, but in general the server can detect it.

MAIL initializes mail transmission mail from: RCPT identifies a single mail recipient; there can often be multiple rcpt to after MAIL commands: DATA indicates that all mail recipients have been identified and initializes data transfer after single or multiple RCPT commands. End. VRFY is used to verify that the specified user / mailbox exists For security reasons, the server often forbids this command EXPN to verify the existence of a given mailbox list and to expand the mailbox list. It is also often disabled to query what commands the server supports, NOOP no operation, and the server should respond to OK QUIT to end the session RSET to reset the session. The current transfer is canceled--

8. The address specified in the MAIL FROM command is called the envelope from address and does not need to be the same as the sender's own address. RCPT TO is equivalent, and the specified recipient address is called the envelope to address, regardless of what the actual to: line is. 9. Why is there no RCPT CC and RCPT BCC:? all receiver negotiations are implemented through the RCPT TO command? if it is BCC, the negotiation is sent and the envelope is deleted when the other party receives 10. E-mail is divided into envelope part, header part and body part envelope from, envelope to and message from:, message to: completely irrelevant. Evnelope is provided by the SMTP background between server hosts, while message from/to is provided by the user. With or without a colon is also the difference.

11. How to check whether a letter is forged by the envelope part? A. The relevance of received rows. In the current SMTP mail transmission system, in addition to the one handled by the internal host at both ends, consider the part between the two company firewalls. If the two firewall machines are An and B respectively, but the receiver checks the envelope received: line and finds that it has passed through C. It's a fake. B. Received: whether the host and IP address correspondence in the line corresponds to: Receibed: from galangal.org (turmeric.com [104.128.23.115] by mail .bieberdorf .edu.edu.... C. Manually added to the last received line: Received: from galangal.org ([104.128.23.115]) by mail .bieberdorf.edu (8.8.5) Received: from lemongrass.org by galangal.org (8.7.3) Received: from graprao.com by lemongrass.org (8.6.4)

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.