sendmail, 4th Edition

srv_features

Alter settings after inbound connect Policy rule set

Immediately after an inbound host connects to the listening sendmail daemon, and before the daemon issues its initial greeting message, sendmail performs the following steps:

1. It does a PTR lookup of the connecting host’s address to find the hostname.

2. It clears its buffers and counters, and sets all its defaults, to ready itself for the upcoming SMTP dialog.

3. It presets key macros to their current values, such ${load_avg}.

4. It calls the srv_features rule set to tune features so that they match the requirements of the connecting host.

The srv_features rule set is declared like this:

LOCAL_SRV_FEATURES
... your rules here

The srv_features rule set must return a $# followed by one or more of the characters defined in Table 19-3. When more than one character is returned, each must be separated from the next by a space. Each character turns a feature on or off. If the character is lowercase, it turns the feature on. Uppercase turns the feature off. One character, the t, is special because it causes sendmail to temporarily fail the connection.

Table 19-3. Characters that set/clear server features

On	Off	Description
a	A	Offer the AUTH SMTP extension.
b	B	Offer use of the SMTP VERB command (V8.13 and later).
c	C	C is the equivalent of AuthOptions=p; i.e., it doesn’t permit mechanisms susceptible to simple passive attack (e.g., PLAIN, LOGIN), unless a security layer is active.
d	D	Offer the DSN SMTP extension (V8.13 and ...