DNS resolution issue after successful VPN(OPENVPN) connection

We have set up a VPN server on centos(6.8). Whenever we connect the VPN client to the VPN server, DNS resolution doesn’t work it gets timed out. Routing rules are as below.

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
17871 3685K ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:1194
42713 4002K ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
   44  3432 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
 1793  102K ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           state NEW tcp dpt:22
25371 5579K REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           reject-with icmp-host-prohibited
    0     0 ACCEPT     all  --  tun0   *       0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  *      *       10.8.0.0/24          0.0.0.0/0
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
15563 2440K REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           reject-with icmp-host-prohibited
    0     0 ACCEPT     all  --  tun0   *       0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  tun0   eth0    0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
    0     0 ACCEPT     all  --  eth0   tun0    0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
    0     0 ACCEPT     all  --  *      *       10.255.0.0/16        0.0.0.0/0

Chain OUTPUT (policy ACCEPT 42772 packets, 5449K bytes)
 pkts bytes target     prot opt in     out     source               destination
 2453  331K ACCEPT     all  --  *      tun0    0.0.0.0/0            0.0.0.0/0
Chain PREROUTING (policy ACCEPT 2454 packets, 395K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 62 packets, 4475 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 MASQUERADE  all  --  *      eth0    10.255.0.0/16        0.0.0.0/0

Chain OUTPUT (policy ACCEPT 62 packets, 4475 bytes)
 pkts bytes target     prot opt in     out     source               destination