UNIX System Administration Handbook - Evi Nemeth [368]
8. We have been inconsistent with terminology in this chapter, sometimes calling a returned message a “bounce” and sometimes calling it an “error.” They both mean the same thing: a message that is undeliverable and so is being returned, usually to the sender.
9. If the NFS filesystem is mounted “hard” and NFS fails, sendmail will block, with several file handles open and several waiting processes. You may eventually run out of process IDs or file handles and have to reboot the machine to clear things.
10 A white lie. Only postmaster and MAILER-DAEMON are really required (by the RFCs), but it is conventional to include hostmaster, abuse, and webmaster as well.
11. You may have to use two or more backslashes to get one of them past the shell and into sendmail.
12. LISTSERV Lite is a free version of the commercial package LISTSERV.
13. Lennart was a student in the computer science department, which in Swedish is the Institutionen för Datavetenskap; hence the name IDA.
14. Well, it’s not quite totally true yet that all files are kept under /etc/mail. The sendmail.pid file and sometimes the statistics file are still kept elsewhere.
15. The ports that sendmail listens on are determined by DAEMON_OPTIONS; port 25 is the default.
16 If the recipient of a mail message is a csh script, the spool directory must have mode 711 or the D= mailer clause must specify a directory with (at least) execute permission in which the script can be run.
17 UNIX directories are an efficient storage mechanism if they do not contain too many files. If you have a busy mail server with lots of mailing lists that get out of date, the queue directory can easily get so large that it is inefficient to deal with.
18. A few years ago, Sun Microsystems decided to change its mail routing from direct-to-workstation addresses to division-wide gateways. The queue lengths on these gateways became so long that mail to the employee across the hall took more than a day to arrive. Unclogging these queues required hardware upgrades of all the gateway machines on a very tight schedule.
19. The sendmail config language is “Turing complete,” which means that it can be used to write any possible computer program. Readers who have experienced the raw config file will realize what a frightening concept this is...
20. The quote characters can actually be changed with the changequote macro, but it’s better not to tamper with the syntax. You’ll just confuse the next person that maintains the file.
21. So where is the OSTYPE macro itself defined? In a file in the cf/m4 directory that is magically prepended to your config file when you run the Build script.
22 The header addresses are the To, From, Cc, and Bcc addresses that appear in the header of a message. The envelope addresses are the addresses to which the mail is actually delivered. The envelope addresses are originally built from the header addresses by the user agent, but they are processed separately by sendmail. Many of sendmail’s masquerading and redirection features would be impossible to implement if the distinction between header and envelope addresses was not maintained.
23. If you configure a client this way and then test the configuration with sendmail -bt, the client will appear to locally deliver local mail. The reason is that the nullclient directive is processed later, in ruleset 5 of the raw config file.
24. The TRUSTED_USERS feature is typically used to support mailing list software. For example, if you use Majordomo, you must add the “majordom” user to the TRUSTED_USERS class. The users root and daemon are the default members of the class.
25. The promiscuous version of chown has led to numerous security problems over the years. We consider it a design flaw. Some systems allow you to disable it; do so if you can.
26. Don’t put programs such as procmail that can spawn a shell in sm.bin. Instead, specify procmail as the local mailer. It’s not secure, but if that’s what your users need...
19.14 RECOMMENDED READING