Summary

OpenBSD has its own FTP server called ftpd.

ftpd, the FTP daemon, is installed and disabled by default.
All what to do in order to use it is just run rcctl enable and rcctl start.

Tutorial

Before Configuration

Let's see it's disabled by default:

$ doas rcctl check ftpd
ftpd(failed)

Step 1: Enable Daemon

First, enable it:

$ doas rcctl enable ftpd

* Note: Here, /etc/rc.conf.local will be updated like this:

$ cat /etc/rc.conf.local
ftpd_flags=

Step 2: Start Daemon

Then start it:

$ doas rcctl start ftpd
ftpd(ok)

The FTP server is now running.
You can establish ftp connection to the server 😉 (as long as there is no network issues.)

* Note: Accessing as root is forbidden.

Conclusion

OpenBSD provides not only the daemon program but also several configuration files. /etc/ftpchroot and /etc/ftpusers must be sometimes useful.