6.16 on page 876).
??? When sendmail first starts, it looks up the IP address or addresses assigned to
eachnetwork interface. For eachaddress it finds, it uses DNS to look up the
hostname associated with that address.
??? When another host connects to the local host to transfer mail, the local sendmail
looks up the other host with DNS to find the other host??™s canonical name.
??? Before accepting mail, sendmail can look up the IP address of the connecting
host on various blacklist sites (?§7.2 on page 260). If that address is listed, the
message is rejected.
??? To relay based on MX records (?§7.4.4 on page 271), sendmail does a lookup to
determine whether the connecting host is listed as an MX server for the local
domain.
??? When delivering network SMTP mail, sendmail uses DNS to find the address (or
addresses) to which it should connect.
??? When sendmail expands $[ and $] in the RHS of a rule, it looks up the hostname
(or IP address) between them.
We discuss each of these uses later in this chapter.
9.2.1 Determine the Local Canonical Name
All versions of sendmail use more or less the same logical process to obtain the
canonical name of the local host. As illustrated in the following sample program,
sendmail first calls gethostname(3) to obtain the local host??™s name within its domain.
That name can be either a short name or a fully qualified one depending on how
your system is set up.
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