Here is my problem more simply explained ...
An user (IP address : 10.0.0.1) has a DNS server IP address specified in his configuration (we don't know the address, and maybe this address is invalid so we want to change it with a iptables rules).
His gateway is 10.0.0.254. The gateway has another interface at 192.168.100.254 where a DNS server is launch (by named).
At the gateway, a iptables rule send all his DNS packets at the DNS server (192.168.100.254)
The rule is :
iptables -A PREROUTING -t nat -s 10.0.0.1 -p UDP --dport 53 -j DNAT --to-destination 192.168.100.254
** BUT ** this rule is not effective immediately, and take fews minutes before to be active !!!
Same thing if you flush the nat table (iptables -F PREROUTING -t nat ) : the last DNS is active during some minutes (sometimes immediately but not systematically)...
To easely see that : just put an invalid DNS server in the configuration of the client, and a real one at the gateway (192.168.100.254). You will see that rules are not immediately active !!!
Is someone know why ???
Thanks. -- Mark
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