TCP IP Illustrated

Tucnoaoedu from a

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Unformatted text preview: TP server on the host mercury.hsi.com. The server's initial 220 response is shown in line 6. Somehow the host mercury.hsi.com must deliver this mail message to the destination, mlfarm.com. The UUCP protocols are a popular way for a system not connected to the Internet to exchange mail with its MX site. In this example the MTA asks for an MX record, gets a positive result, and sends the mail. Unfortunately the interaction between an MTA and the DNS can differ between implementations. RFC 974 specifies that an MTA should ask for MX records first, and if none are found, attempt delivery to the destination host (i.e., ask the DNS for an A record for the host, for its IP address). MTAs must also deal with CNAME records in the DNS (canonical names). As an example, if we send mail to rstevens@mailhost.tuc.noao.edu from a file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (10 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol BSD/386 host, the following steps are executed by the MTA (Sendmail). 1. Sendmail asks the DNS for CNAME records for mailhost.tuc.noao.edu. We see that a CNAME record exists: sun % host -t cname mailhost.tuc.noao.edu mailhost.tuc.noao.edu CNAME noao.edu 2. A DNS query is issued for CNAME records for noao.edu and the response says none exist. 3. Sendmail then asks the DNS for MX records for noao.edu and gets one MX record: sun % host -t mx noao.edu noao.edu MX noao.edu 4. Sendmail queries the DNS for an A record (IP address) for noao.edu and gets back the value 140.252.1.54. (This A record was probably returned by the name server for noao.edu as an additional RR with the MX reply in step 3.) 5. An SMTP connection is initiated to 140.252.1.54 and the mail is sent. A CNAME query is not tried for the data returned in the MX record (noao.edu). The data in the MX record cannot be an alias-it must be the name of a host that has an A record. The version of Sendmail distributed with SunOS 4.1.3 that uses the DNS only queries for MX records, and gives up if an MX record isn't found. MX Records: Hosts That Are Down Another use of MX...
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