Using lsof to get the command and PID about the process which occupied port 53 [root@ractw21 ~]# lsof -P -i:53 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME dnsmasq 4038 nobody 5u IPv4 27642 0t0 UDP ractw21.example.com:53 dnsmasq 4038 nobody 6u IPv4 27643 0t0 TCP ractw21.example.com:53 (LISTEN) Find full command path using ps [root@ractw21 ~]# ps -elf | grep dnsmasq 5 S nobody 4038 1 0 80 0 - 3888 poll_s 18:08 ? 00:00:00 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelper 1 S root 4040 4038 0 80 0 - 3881 pipe_w 18:08 ? 00:00:00 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelper What service starts /sbin/dnsmasq ? [root@ractw21 ~]# systemctl status --all | grep -B 10 4038 | more libvirtd.service - Virtualization daemon Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; disabled; vendor preset: enabled) Active: active (running) since Sat 2017-04-15 18:08:06 CEST; 21h ago Docs: man:libvirtd(8) http://libvirt.org Main PID: 3952 (libvirtd) CGroup: /system.slice/libvirtd.service ├─3952 /usr/sbin/libvirtd ├─4038 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_ leaseshelper └─4040 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_ leaseshelper In this case libvirtd service started /sbin/dnsmasq. Check,stop and disable libvirtd service [root@ractw21 ~]# systemctl stop libvirtd.service [root@ractw21 ~]# systemctl disable libvirtd.service [root@ractw21 ~]# systemctl status libvirtd.service ● libvirtd.service - Virtualization daemon Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; disabled; vendor preset: enabled) Active: inactive (dead) since Sun 2017-04-16 16:12:21 CEST; 1min 44s ago Docs: man:libvirtd(8) http://libvirt.org Main PID: 3952 (code=exited, status=0/SUCCESS) CGroup: /system.slice/libvirtd.service ├─4038 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvi... └─4040 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvi... Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: started, version 2.66 cachesize 150 Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: compile time options: IPv6 GNU-getopt DBus no-i18n IDN DHCP DHCPv6 no-Lu...t auth Apr 15 18:08:07 ractw21.example.com dnsmasq-dhcp[4038]: DHCP, IP range 192.168.122.2 -- 192.168.122.254, lease time 1h Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: reading /etc/resolv.conf Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: using nameserver 192.168.5.50#53 Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: read /etc/hosts - 2 addresses Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses Apr 15 18:08:07 ractw21.example.com dnsmasq-dhcp[4038]: read /var/lib/libvirt/dnsmasq/default.hostsfile Apr 16 16:12:21 ractw21.example.com systemd[1]: Stopping Virtualization daemon... Apr 16 16:12:21 ractw21.example.com systemd[1]: Stopped Virtualization daemon. Hint: Some lines were ellipsized, use -l to show in full. After a reboot port 53 should be free and service libvirtd.service should be disabled [root@ractw21 ~]# lsof -P -i:53 [root@ractw21 ~]# systemctl status libvirtd.service ● libvirtd.service - Virtualization daemon Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; disabled; vendor preset: enabled) Active: inactive (dead) Docs: man:libvirtd(8) http://libvirt.org Check,stop and disable libvirtd service which occupies Port #53 [root@ractw21 ~]# systemctl stop libvirtd.service [root@ractw21 ~]# systemctl disable libvirtd.service [root@ractw21 ~]# systemctl status libvirtd.service ● libvirtd.service - Virtualization daemon Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; disabled; vendor preset: enabled) Active: inactive (dead) since Sun 2017-04-16 16:12:21 CEST; 1min 44s ago Docs: man:libvirtd(8) http://libvirt.org Main PID: 3952 (code=exited, status=0/SUCCESS) CGroup: /system.slice/libvirtd.service ├─4038 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvi... └─4040 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvi... Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: started, version 2.66 cachesize 150 Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: compile time options: IPv6 GNU-getopt DBus no-i18n IDN DHCP DHCPv6 no-Lu...t auth Apr 15 18:08:07 ractw21.example.com dnsmasq-dhcp[4038]: DHCP, IP range 192.168.122.2 -- 192.168.122.254, lease time 1h Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: reading /etc/resolv.conf Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: using nameserver 192.168.5.50#53 Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: read /etc/hosts - 2 addresses Apr 15 18:08:07 ractw21.example.com dnsmasq[4038]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses Apr 15 18:08:07 ractw21.example.com dnsmasq-dhcp[4038]: read /var/lib/libvirt/dnsmasq/default.hostsfile Apr 16 16:12:21 ractw21.example.com systemd[1]: Stopping Virtualization daemon... Apr 16 16:12:21 ractw21.example.com systemd[1]: Stopped Virtualization daemon. Hint: Some lines were ellipsized, use -l to show in full. After reboot prot #53 should be unused and service libvirtd should be inactive [grid@ractw21 linuxx64_12201_grid_home]$ lsof -P -i:53 [grid@ractw21 linuxx64_12201_grid_home]$ systemctl status libvirtd.service ● libvirtd.service - Virtualization daemon Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; disabled; vendor preset: enabled) Active: inactive (dead) Docs: man:libvirtd(8) http://libvirt.org