/usr/local/bin/sshd-keygen

#!/bin/bash
 
# Create the host keys for the OpenSSH server.
KEYTYPE=$1
case $KEYTYPE in
	"dsa") ;& # disabled in FIPS
	"ed25519")
		FIPS=/proc/sys/crypto/fips_enabled
		if [[ -r "$FIPS" && $(cat $FIPS) == "1" ]]; then
			exit 0
		fi ;;
	"rsa") ;; # always ok
	"ecdsa") ;;
	*) # wrong argument
		exit 12 ;;
esac
KEY=/etc/ssh/ssh_host_${KEYTYPE}_key
 
KEYGEN=/usr/bin/ssh-keygen
if [[ ! -x $KEYGEN ]]; then
	exit 13
fi
 
# remove old keys
rm -f $KEY{,.pub}
 
# create new keys
if ! $KEYGEN -q -t $KEYTYPE -f $KEY -C '' -N '' >&/dev/null; then
	exit 1
fi
 
# sanitize permissions
chgrp ssh_keys $KEY
chmod 640 $KEY
chmod 644 $KEY.pub
if [[ -x /usr/sbin/restorecon ]]; then
	/usr/sbin/restorecon $KEY{,.pub}
fi
 
exit 0

chmod +x /usr/local/bin/sshd-keygen

systemctl edit --full --force sshd-keygen@.service

/etc/systemd/system/sshd-keygen@.service

[Unit]
Description=OpenSSH %i Server Key Generation
ConditionFileNotEmpty=|!/etc/ssh/ssh_host_%i_key
 
[Service]
Type=oneshot
EnvironmentFile=-/etc/sysconfig/sshd
ExecStart=/usr/local/bin/sshd-keygen %i
 
[Install]
WantedBy=sshd-keygen.target

systemctl edit --full --force sshd-keygen.target

/etc/systemd/system/sshd-keygen.target

[Unit]
Wants=sshd-keygen@rsa.service
Wants=sshd-keygen@ecdsa.service
Wants=sshd-keygen@ed25519.service
PartOf=sshd.service

systemctl edit ssh.service

/etc/systemd/system/ssh.service.d/override.conf

[Unit]
…
After=network.target sshd-keygen.target
Wants=sshd-keygen.target