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Edit Comment Close Premium member Presentation Transcript Email : Netprog: Email Protocols 1 Email SMTP - Simple Mail Transfer Protocol RFC 821 POP - Post Office Protocol RFC 1939 Also: RFC 822 Standard for the Format of ARPA Internet Text Messages RFCs 1521, 1522 Mime Terminology : Netprog: Email Protocols 2 Terminology User Agent: end-user mail program Message Transfer Agent: responsible for communicating with remote hosts and transmitting/receiving email (both a client and server). Mail Exchanger: host that takes care of email for a domain. SMTP : Netprog: Email Protocols 3 SMTP Used to exchange mail messages between mail servers (Message Transfer Agents). MTA MTA UA SMTP File System MTA SMTP UA SMTP Protocol : Netprog: Email Protocols 4 SMTP Protocol SMTP sender is the client SMTP receiver is the server. Alternating dialogue: client sends command and server responds with command status message. Order of the commands is important! Status messages include ascii encoded numeric status code (like HTTP,FTP) and text string. SMTP Commands : Netprog: Email Protocols 5 SMTP Commands HELO - identifies sender MAIL FROM: - starts a mail transaction and identifies the mail originator RCPT TO: - identifies individual recipient. There may be multiple RCPT TO: commands. DATA - sender ready to transmit a series of lines of text, each ends with \r\n. A line containing only a period ‘.’ indicates the end of the data. Data Format : Netprog: Email Protocols 6 Data Format ASCII only- must convert binary to an ASCII representation to send via email. What if we want to send a line containing only a period? Sender prepends a period to any line staring with a period (in the message). Receiver strips the leading period in any line that starts with a period and has more stuff. Typical Exchange : Netprog: Email Protocols 7 Typical Exchange z3.cs.rpi.edu> telnet mail.cs.rpi.edu 25 Trying 188.8.131.52... Connected to fred.cs.rpi.edu. Escape character is '^]'. 220 cs.rpi.edu ESMTP Sendmail 8.9.3/8.9.3; Tue, 14 Mar 2000 19:41:16 –0500 HELO z3.cs.rpi.edu 250 cs.rpi.edu Hello email@example.com [184.108.40.206], pleased to meet you MAIL FROM: firstname.lastname@example.org 250 Regis@millionaire.com... Sender ok RCPT TO: hollingd 250 hollingd... Recipient ok DATA 354 Enter mail, end with "." on a line by itself Hi Dave – What is buffer overflow? . 250 VAA07541 Message accepted for delivery Leading Period : Netprog: Email Protocols 8 Leading Period DATA 354 Enter mail, end with "." on a line by itself Hi dave - this message is a test of SMTP .. ..foo .. . 250 VAA0771 Message accepted for delivery Resulting Message: Hi dave - this message is a test of SMTP . .foo . Other SMTP Commands : Netprog: Email Protocols 9 Other SMTP Commands VRFY - confirm that a name is a valid recipient. EXPN - expand an alias (group email address). TURN - switch roles (sender <=> receiver). more Commands : Netprog: Email Protocols 10 more Commands SOML - Send Or Mail if recipient is logged in, display message on terminal, otherwise email. SAML - Send and Mail NOOP - send back a positive reply code. RSET - abort current transaction. Mail Headers : Netprog: Email Protocols 11 Mail Headers Email messages contain many headers, some headers are created by the UA, some are automatically added by the MTA. Every MTA adds (at least) a “Received:” header. Some of the headers are read by (parsed) intermediate MTAs, but the content is ignored and passed on transparently. POP – Post Office Protocol : Netprog: Email Protocols 12 POP – Post Office Protocol Used to transfer mail from a mail server to a UA. Mail Server UA File System POP POP (version 3) : Netprog: Email Protocols 13 POP (version 3) Similar to SMTP command/reply lockstep protocol. Used to retrieve mail for a single user requires authentication Commands and replies are ASCII lines. Replies start with “+OK” or “-ERR”. Replies may contain multiple lines. POP-3 Commands : Netprog: Email Protocols 14 POP-3 Commands USER - specify username PASS - specify password STAT - get mailbox status number of messages in the mailbox. LIST - get a list of messages and sizes. One per line, termination line contains ‘.’ only. RETR - retrieve a message More POP-3 Commands : Netprog: Email Protocols 15 More POP-3 Commands DELE - mark a message for deletion from the mailbox. NOOP - send back positive reply RSET - reset. All deletion marks are unmarked. QUIT - remove marked messages and close the (TCP) connection. Optional Commands : Netprog: Email Protocols 16 Optional Commands TOP - send header lines from messages. APOP - alternative authentication message digest based on opening greeting sent from POP server. Requires shared secret! No cleartext password on the network. Does not authenticate the server!!!! A Pop3 Exchange : Netprog: Email Protocols 17 A Pop3 Exchange > telnet monte pop3 Trying 220.127.116.11... Connected to monte.cs.rpi.edu (18.104.22.168). Escape character is '^]'. +OK POP3 monte.cs.rpi.edu v7.59 server ready user joe +OK User name accepted, password please pass joepw +OK Mailbox open, 1 messages stat +OK 1 412 list +OK Mailbox scan listing follows 1 412 . Pop3 Example Continued : Netprog: Email Protocols 18 Pop3 Example Continued retr 1 +OK 412 octets Return-Path: <hollingd> Received: (from hollingd@localhost) by monte.cs.rpi.edu (8.9.3/8.9.3) id NAA06943 for joe; Mon, 20 Mar 2000 13:49:54 -0500 Date: Mon, 20 Mar 2000 13:49:54 -0500 From: Dave Hollinger <email@example.com> Message-Id: <200003201849.NAA06943@monte.cs.rpi.edu> To: firstname.lastname@example.org Status: O blah . You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.