Name resolution is the process of determining a machine's IP address based on its fully qualified domain name. This is done by using the Domain Name System (DNS). The process that sendmail uses to resolve a name is described in Section 9.2. When sendmail finds that a hostname is really an MX record, it attempts to look up the address (which can be an A or AAAA record) for the host that handles mail receipt. That request can fail for a variety of reasons. If the -d8.1 (a.k.a. -d8 ) debugging switch is specified, sendmail produces the following message: getmxrr: res_search( host ) failed (errno= errornum , h_errno= herrornum ) Here, host is the hostname that was looked up, errornum is the system error number (if any) from <errno.h >, and herrornum is the resolver-specific error number from <netdb.h >, as shown in Table 16-7. Table 16-7. Resolver errors from <netdb.h>
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