sendmail, 4th Edition

Probe Network Interfaces

After the canonical name, and any other names for the local machine, have been placed in $=w, sendmail then searches (probes) all the network interfaces to find any additional names and addresses that might also need to be added to $=w. But note that if the DontProbeInterfaces option (DontProbeInterfaces on page 1023) is defined as true, this additional step is skipped. Note also that if the DontProbeInterfaces option is defined as the literal value localhost, only the loopback interface is skipped, and all the other network interfaces are included.

The list of network interfaces is obtained from your kernel using a system call appropriate for your operating system. The kernel generally returns a list composed of interface and IP address pairs. If you defined NETINET6 (NET... on page 126) when compiling, the list might contain IPv6 addresses. If you defined NETINET (NET... on page 126) when compiling, the list might contain IPv4 addresses.

For each address that is found, sendmail performs a reverse lookup using gethostbyaddr(3) or getipnodebyaddr(3). Each lookup (if successful) will return the hostname associated with the address.

Each address and hostname is appended to the class $=w. The names and addresses added can be viewed with the -d0.4 debugging command-line switch (-d0.4 on page 542), which also allows errors in this process to be printed.