Sendmail, 3rd Edition

Name

${client_resolve}

Synopsis

When sendmail first assigns a value to the ${client_name} macro (${client_name}), it also looks up the hostname of that connecting client with DNS. Table 21-9 shows the possible results of that lookup.

Table 21-9. Possible values for the ${client_resolve} macro

Value	Meaning
OK	The address lookup was successful.
FAIL	The lookup resulted in a permanent failure.
FORGED	The forward lookup doesn’t match the reverse lookup.
TEMP	The lookup resulted in a temporary failure.

The ${client_resolve} macro is useful in the Local_check_rcpt (Section 7.1.3) and Local_check_mail (Section 7.1.2) rule sets. It can, for example, be used to accept mail only from machines whose hostname can be successfully looked up with DNS.

LOCAL_RULESETS
SLocal_check_mail
R$*                    $: $&{client_resolve}
ROK                    $@ Okay
RTEMP                  $#error $@ 4.7.1 $: "450 Can't resolve hostname just now."
R$*                    $#error $@ 5.7.1 $: "550 Sending hostname must resolve!"

Here, the first rule transfers the value of ${client_resolve} into the workspace. The $& prefix (Section 21.5.3) prevents that macro from wrongly being expanded when the configuration file is read. The second rule accepts the address if it can be looked up. The third rule defers acceptance of any sender address that results in a temporary lookup error. The last rule bounces mail from any host that cannot be looked up, or that appears to be a forged address.

${client_resolve} is transient. If it is defined in the configuration file or in the command ...