ypserv, yppasswdd or ypbind fails to start with timeout on system boot when rpcbind using hosts.allow is accessed from a remote host

Issue

• ypserv, yppasswdd or ypbind fails to start with timeout on system boot when rpcbind using hosts.allow is accessed from a remote host

• ypserv.service error in messages sample:

systemd[1]: ypserv.service start operation timed out. Terminating.
systemd[1]: ypserv.service stop-final-sigterm timed out. Killing.
systemd[1]: ypserv.service: main process exited, code=killed, status=9/KILL
systemd[1]: Failed to start NIS/YP (Network Information Service) Server.

• Reproducer sample:

(1) On a NIS server(hostname: ypsrv1):

# systemctl list-unit-files
UNIT FILE                                     STATE  
...
rpcbind.service                               enabled 
ypbind.service                                enabled 
ypserv.service                                enabled 

# vim /etc/nsswitch.conf
...
hosts:      files nis

# vim /etc/yp.conf
ypserver 127.0.0.1

# /var/yp/ypservers
 ypsrv1.example.com

# vim /etc/hosts.allow
rpcbind:    LOCAL

# vim /etc/hosts.deny
rpcbind: ALL

(2) On a remote host:
This host entry is not in ypsrv1's /etc/hosts.
For example, access to rpcbind on ypsrv1 as follows:

# for i in $(seq 1 100); do sleep 1; (rpcinfo -u ypsrv1 portmapper &) ; done 

(3) Reboot ypsrv1:
It might be needed several or many times.