LDAP (Lightweight Directory Access Protocol) powerful enterprise class directory service by default listen on UDP port 389 for queries. Following iptable rules allows incoming client request (open port UDP port 389) for server IP address 202.54.1.20 :
iptables -A INPUT -p udp -s 0/0 –sport 1024:65535 -d 202.54.1.20 –dport 389 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p udp -s 202.54.1.20 –sport 389 -d 0/0 –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

Restrict access to LDAP database server from your network is essential, following iptables allows incoming LDAP client request from IP address 202.54.1.0/24 network only:
iptables -A INPUT -p udp -s 202.54.1.0/24 –sport 1024:65535 -d 202.54.1.20 –dport 389 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p udp -s 202.54.1.20 –sport 389 -d 202.54.1.0/24 –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT