To allow one trusted IP address to access all incoming packets add following line in iptables-precursor:
iptables -A INPUT -s 192.168.2.2 -j ACCEPT
where 192.168.2.2 is the IP address I would like to allow, change the IP address as per your requirement.
After adding above line run following commands to get it updated:
$ source iptables-precursor $ iptables-save > iptables
In above commands I used relative pats for iptables-precursor and iptables.