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Unformatted text preview: records is to provide an alternative mail receiver when the destination
host is down. If we look at the DNS entry for our host sun we see that it has two MX
sun % host -a-v-t mx sun.tuc.noao.edu
A 0 sun.tuc.noao.edu
188.8.131.52 file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (11 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol noao.edu 86400 IN A 184.108.40.206 The MX record with the lowest preference indicates that direct delivery to the host itself
should be tried first, and the next preference is to deliver the mail to the host noao.edu.
In the following script we send mail to ourself at the host sun.tuc.noao.edu, from
the host vangogh.cs.berkeley.edu, after turning off the destination's SMTP
server. When a connection request arrives for port 25, TCP should respond with an RST,
since no process has a passive open pending for that port.
vangogh % mail -v [email protected]
A test to a host that's down.
[email protected].. Connecting to
[email protected].. Connecting to noao.edu.
remainder is normal SMTP mail transfer
We see that the MTA tries to contact sun.tuc.noao.edu and then gives up and
contacts noao.edu instead.
Figure 28.5 is the tcpdump output that shows that TCP responds to the incoming SYNs
with an RST.
(0.0004) vangogh.3873 > 220.127.116.11.25: S
18.104.22.168.25 > vangogh.3873: R 0:0(0)
ack 2358303746 win 0
vangogh.3874 > 22.214.171.124.25: S
126.96.36.199.25 > vangogh.3874; R 0:0(0)
ack 2358367746 win 0 Figure 28.5 Attempt to connect to an SMTP server that is not running.
In line 1 vangogh sends a SYN to port 25 at the primary IP address for sun:
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12