TCP IP Illustrated

It was also configured to use the name server on the

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Unformatted text preview: e by itself to terminate message Sending letter ... ron@mlfarm.com... Connecting to mlfarm.com via tcp ... mail exchanger is the MX records are found mercury.hsi.com Trying 143.122.1.91... First tries the one with lower preference connected. remainder is normal SMTP mail transfer 220 mercury.hsi.com ... We can see in this output that the MTA discovered that the destination host had an MX record and used the MX record with the lowest preference value. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (9 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol Before running this example from the host sun, it was configured not to use its normal relay host, so we could see the mail exchange with the destination host. It was also configured to use the name server on the host noao.edu (which is across its dialup SLIP link), so we could capture both the mail transfer and the DNS traffic using tcpdump on the SLIP link. Figure 28.4 shows the starting portion of the tcpdump output. 1 0.0 0.445572 (0.4456) 0.505739 3 (0.0602) 2 4 0.985428 (0.4797) 0.986003 (0.0006) 1.735360 6 (0.7494) 5 sun.1624 > noao.edu.53: 2+ MX? mlfarm.com. (28) noao.edu.53 > sun.1624: 2* 2/0/2 MX mercury.hsi.com. 10 (113) sun.1143 > mercury.hsi.com.25: S 1617536000:1617536000(0) win 4096 mercury.hsi.com.25 > sun.1143: S 1832064000:1832064000(0) ack 1617536001 win 16384 sun.1143 > mercury.hsi.com.25: . ack 1 win 4096 mercury.hsi.com.25 > sun.1143: P 1:90(89) ack 1 win 16384 Figure 28.4 Sending mail to a host that uses MX records. In line 1 the MTA queries its name server for an MX record for mlfarm.com. The plus sign following the 2 means the recursion-desired flag is set. The response in line 2 has the authoritative bit set (the asterisk following the 2) and contains 2 answer RRs (the two MX host names), 0 authority RRs, and 2 additional RRs (the IP addresses of the two hosts). In lines 3-5 a TCP connection is established with the SM...
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