-bd

-bd

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Run as a daemon

All versions

The -bd command-line switch causes sendmail to become a daemon, running in the background, listening for and handling incoming SMTP connections. ^[5]

^[5] In its classic invocation, -bd is usually combined with a -q1h .

To become a daemon, sendmail first performs a fork (2). The parent then exits, and the child becomes the daemon by disconnecting itself from its controlling terminal. The -bD command-line switch can be used to prevent the fork (2) and the detachment and allows the sendmail program's behavior to be observed while it runs in daemon mode.

As a daemon, sendmail does a listen (2) on TCP port 25 by default for incoming SMTP messages. ^[6] When another site connects to the listening daemon, the daemon performs a fork (2), and the child handles receipt of the incoming mail message.

^[6] Beginning with V8.10, sendmail also listens on port 587 for message submissions via MUAs. This default behavior can be turned off with the no_default_msa mc feature (FEATURE(no_default_msa)).