日志：
Jun 24 20:16:49 localhost systemd: Starting Unbound recursive Domain Name Server...
Jun 24 20:16:49 localhost unbound-checkconf: unbound-checkconf: no errors in /etc/unbound/unbound.confJun 24 20:16:49 localhost systemd: Started Unbound recursive Domain Name Server.
Jun 24 20:16:49 localhost systemd: Reached target Host and Network Name Lookups.
Jun 24 20:16:49 localhost systemd: Starting Host and Network Name Lookups.
Jun 24 20:16:49 localhost unbound: Jun 24 20:16:49 unbound[2446:0] warning: increased limit(open files) from 1024 to 8266
Jun 24 20:16:49 localhost unbound: Jun 24 20:16:49 unbound[2446:0] error: bind: address already in use
Jun 24 20:16:49 localhost unbound: Jun 24 20:16:49 unbound[2446:0] fatal error: could not open ports
Jun 24 20:16:49 localhost systemd: unbound.service: main process exited, code=exited, status=1/FAILURE
Jun 24 20:16:49 localhost systemd: Unit unbound.service entered failed state.
Jun 24 20:16:49 localhost systemd: unbound.service failed.
Jun 24 20:16:50 localhost systemd-logind: Removed session c1.
Jun 24 20:16:50 localhost systemd: Removed slice User Slice of gdm.
Jun 24 20:16:50 localhost systemd: Stopping User Slice of gdm.
Jun 24 20:16:53 localhost systemd: Started The Apache HTTP Server.
Jun 24 20:16:53 localhost systemd: Reached target Multi-User System.
Jun 24 20:16:53 localhost systemd: Starting Multi-User System.
Jun 24 20:16:53 localhost systemd: Reached target Graphical Interface.
Jun 24 20:16:53 localhost systemd: Starting Graphical Interface.
Jun 24 20:16:53 localhost systemd: Starting Update UTMP about System Runlevel Changes...
Jun 24 20:16:53 localhost systemd: Started Update UTMP about System Runlevel Changes.
Jun 24 20:16:53 localhost systemd: Startup finished in 607ms (kernel) + 1.405s (initrd) + 53.140s (userspace) = 55.154s.
[root@localhost ~]# systemctl status unbound
● unbound.service - Unbound recursive Domain Name Server
Loaded: loaded (/usr/lib/systemd/system/unbound.service; disabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Sun 2018-06-24 20:16:49 CST; 1min 4s ago
Process: 2446 ExecStart=/usr/sbin/unbound -d $UNBOUND_OPTIONS (code=exited, status=1/FAILURE)
Process: 2442 ExecStartPre=/usr/sbin/unbound-anchor -a /var/lib/unbound/root.key -c /etc/unbound/icannbundle.pem (code=exited, status=0/SUCCESS)
Process: 2441 ExecStartPre=/usr/sbin/unbound-checkconf (code=exited, status=0/SUCCESS)
Main PID: 2446 (code=exited, status=1/FAILURE)
解决：
# netstat -ntulp |grep 53
tcp 0 0 192.168.122.1:53 0.0.0.0:* LISTEN 1481/dnsmasq
udp 0 0 192.168.122.1:53 0.0.0.0:* 1481/dnsmasq
udp 0 0 0.0.0.0:5353 0.0.0.0:* 778/avahi-daemon: r
[root@localhost ~]# kill -9 1481
[root@localhost ~]# netstat -ntulp |grep 53
udp 0 0 0.0.0.0:5353 0.0.0.0:* 778/avahi-daemon: r
[root@localhost ~]# systemctl restart unbound
[root@localhost ~]# systemctl status unbound
● unbound.service - Unbound recursive Domain Name Server
Loaded: loaded (/usr/lib/systemd/system/unbound.service; disabled; vendor preset: disabled)
Active: active (running) since Sun 2018-06-24 20:25:54 CST; 5s ago
Process: 2756 ExecStartPre=/usr/sbin/unbound-anchor -a /var/lib/unbound/root.key -c /etc/unbound/icannbundle.pem (code=exited, status=0/SUCCESS)
Process: 2755 ExecStartPre=/usr/sbin/unbound-checkconf (code=exited, status=0/SUCCESS)
Main PID: 2759 (unbound)
CGroup: /system.slice/unbound.service
└─2759 /usr/sbin/unbound -d