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I'm trying to better understand a behavior I see - I took control of a server that had TFTP running and am trying to turn it off. I've already ran:

systemctl stop tftp

systemctl stop xinetd

However, when stopping the tftp service it says

Warning: Stopping tftp.service, but it can still be activated by: tftp.socket

ss -lnpu afterwards gives me

UNCONN      0      0                             :::69                                        
:::*                   users:(("systemd",pid=1,fd=60))

I checked and the port is indeed still listening, but the PID is listed as systemd? I don't understand the behavior - how is TFTP being run directly by systemd?

Edit: I figured out there was another "service" called tftp.socket running that kept the socket open after I killed the tftp.service. What I still don't understand is why this shows up as being part of the systmed pid.

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The tftp-server is started to handle a tftp request. As log as there is no such request, systemd does not start the tftp server. But there needs to be a process that listens to the incoming request on port 69, otherwise that request lands in the bitbucket.

That process is systemd.

When a requests on port 69 comes in, systemd forks-off a tftp server to handle the request. When the tftp server is finished, it just dies.

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  • In other words, systemd behaves just like the xinetd service which OP mentioned... – user1686 Jul 26 '19 at 16:43
  • Did he? The only mentioning of xinetd in the question that I have seen is that he used systemctl stop xinetd. But then, the OP edited the question, so he might have taken it out before I read it. – Ljm Dullaart Jul 26 '19 at 17:52
  • You're correct - I'm not using xinetd. I just stopped it. The only change I made with the edit is the indicated. – Grant Curell Jul 27 '19 at 16:13

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