sendmail, 4th Edition

-d8.1

DNS name resolution Debug command-line switch

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 How sendmail Uses DNS on page 325.

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 15-6.

Table 15-6. Resolver errors from <netdb.h>

Value	Mnemonic	Description
−1	NETDB_INTERNAL	Error in the lookup code, see errno=
0	NETDB_SUCCESS	Success
1	HOST_NOT_FOUND	Host not found
2	TRY_AGAIN	Temporary DNS server failure
3	NO_RECOVERY	Nonrecoverable errors and refusals
4	NO_DATA	Valid name but no record of requested type