DHCP server stops right after system is ready for log-in

I am setting up Ubuntu server (16.10) as router with the following h/w config:
1. onboard lan port as external interface to web (enp7s0)
2. 4 lan ports (PCIE card) as internal interfaces to my PCs (enp4s0f0 enp4s0f1 enp5s0f0 enp5s0f1)

The problem:
- DHCP server seems to be partially working on initial system startup stage as my PC could have an IP address from it. But DHCP servers stops after the system is ready for log-in.
- "DHCP server partially works" means PC could have an IP address but not the DSN from my ISP.
Please help to solve the problem. Thanks in advance.

The configuration files are as follows:
[isc-dhcp-server.conf]
INTERFACES="br0"

[dhcpd.conf]
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.8.255;
option routers 192.168.8.3;

subnet 192.168.8.0
netmask 255.255.255.0
{
        range 192.168.8.100 192.168.8.199;
        }

[interfaces]
# The loopback network interface
auto lo
iface lo inet loopback

# The external network interface
auto enp7s0
iface enp7s0 inet dhcp

# The internal network interfaces
auto br0
iface br0 inet static
        address 192.168.8.3
        netmask 255.255.255.0
        broadcast 192.168.8.255
        network 192.168.8.0
        getway 192.168.8.3
        bridge_ports enp4s0f0 enp4s0f1 enp5s0f0 enp5s0f1
        bridge_stp off
        bridge_fd 0
        bridge_waitport 0