What process/service occupies a certain port like port 53 ?

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