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Unformatted text preview: e by itself to
Sending letter ...
Connecting to mlfarm.com via
... mail exchanger is
the MX records are found
First tries the one with lower preference
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
(0.0602) 2 4 0.985428
(0.7494) 5 sun.1624 > noao.edu.53: 2+ MX? mlfarm.com.
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
sun.1143 > mercury.hsi.com.25: . ack 1 win
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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