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I want to close non used ports in my server, after doing a port scan I found service urd running on port 465. Running lsof I see a "master" process running on that port (also 25, 587,etc..). Question is what is that master process? can I disable it?

lsof -Pnl +M -i4

(...)
master     2508        0   12u  IPv4   21914      0t0  TCP *:25 (LISTEN)
master     2508        0   17u  IPv4   21920      0t0  TCP *:587 (LISTEN)
master     2508        0   21u  IPv4   21926      0t0  TCP *:465 (LISTEN)
master     2508        0  120u  IPv4   22026      0t0  TCP 127.0.0.1:10025 (LISTEN)
master     2508        0  123u  IPv4   22030      0t0  TCP 127.0.0.1:10027 (LISTEN)
(...)
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2 Answers 2

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It is Postfix.

Postfix is a very popular open source Mail Transfer Agent (MTA) that can be used to route and deliver email on a Linux system.

Port 25: SMTP

Port 465: SMTPS

Port 587: mail submission agents (MSAs)

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  • then why does it show master instead of postfix in the process column?
    – spacebiker
    Jul 21, 2016 at 11:28
  • 2
    The master daemon is the resident process that runs other Postfix daemons on demand: daemons to send or receive messages via the network, daemons to deliver mail locally, etc. These daemons are created on demand up to a configurable maximum number per service.
    – jcbermu
    Jul 21, 2016 at 11:40
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The settings for master process is in /etc/postfix/master.cf

You can make it listen on 127.0.0.1:smtp instead of smtp which defaults to all interfaces. That will let it process emails coming from localhost. Your system might still want to deliver some alert messages to you, so it might make more sense to keep postfix running.

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