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I'm having a problem with redirecting pc's dns requests to a local dns server. I want to block access to other dns services except my own. I've found different methods of doing this, none work here. So I must be doing something wrong.

A lot of people suggest using this:

 iptables -t nat -A PREROUTING -p udp --dport 53 -j DNAT --to 192.168.2.70:53

But in my config this blocks access to DNS. Also I cannot find why this should work in the first place. Since prerouting is for external users that want access to an internal service right?

I also tried specifying the internal lan interface -i eth1 but, this just cannot work since the internal servers are also on the same interface. That would make a loop right? Servers sends out packets for port 53 and the gateway sends it back to the same server.

Then I would also like to ONLY allow the dns servers on my internal network, to contact other external dns servers. Like:

iptables -A OUTPUT -p udp -s 192.168.2.70 -d 208.67.222.222 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s 192.168.2.70 -d 208.67.222.220 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT 
iptables -A OUTPUT -p udp -s 192.168.2.72 -d 208.67.222.222 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT 
iptables -A OUTPUT -p tcp -s 192.168.2.72 -d 208.67.222.220 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT

But I'm guessing that the OUTPUT table is not correct. Since from what I learned, INPUT and OUTPUT are only for the local linux router no?

Information:

Internal dns servers: 192.168.2.70 and 192.168.2.72 
External dns server: 208.67.222.220 and 208.67.222.222
LAN interface: ETH1
WAN interface: ETH0

My current firewall config without prerouting&forward rules except one to show how it's configed.

 #!/bin/sh -e
ifconfig eth1 192.168.2.1/24

echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

        #Flush table's
        iptables -F INPUT
        iptables -F OUTPUT
        iptables -F FORWARD
        iptables -t nat -F

        #toestaan SSH verkeer
        iptables -t nat -A PREROUTING -p tcp --dport 22 -i eth0 -j DNAT --to 192.168.2.1
        iptables -A INPUT -p tcp --dport 22 -j ACCEPT

        #Drop traffic and accept
        iptables -P FORWARD DROP
        iptables -P INPUT DROP
        iptables -P OUTPUT ACCEPT

        #verkeer naar buiten toe laten en nat aanzetten
        iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
        iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
        iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

        iptables -A INPUT -p tcp -m state --syn --state NEW --dport 22 -m limit --limit 1/minute --limit-burst 1 -j ACCEPT
        iptables -A INPUT -p tcp -m state --syn --state NEW --dport 22 -j DROP

         #toestaan verkeer loopback
        iptables -A INPUT -i lo -j ACCEPT

        #toestaan lokaal netwerk
        iptables -A INPUT -i eth1 -j ACCEPT

        #accepteren established traffic
        iptables -A INPUT -i eth0 --match state --state RELATED,ESTABLISHED -j ACCEPT

        #droppen ICMP boodschappen
        iptables -A INPUT -p icmp -i eth0 -m limit --limit 10/minute -j ACCEPT
        iptables -A INPUT -p icmp -i eth0 -j REJECT

        #RDP forward voor windows servers
        iptables -t nat -A PREROUTING -p tcp --dport 3389 -i eth0 -j DNAT --to 192.168.2.73
        iptables -A FORWARD -p tcp --dport 3389 -j ACCEPT

2 Answers 2

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You need to put these rules in FORWARD chain:

iptables -A FORWARD -p udp -s 192.168.2.70 -d 208.67.222.222 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A FORWARD -p tcp -s 192.168.2.70 -d 208.67.222.220 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT 
iptables -A FORWARD -p udp -s 192.168.2.72 -d 208.67.222.222 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT 
iptables -A FORWARD -p tcp -s 192.168.2.72 -d 208.67.222.220 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT

Give users local DNS servers addresses via DHCP and do not bother to redirect their DNS traffic to local servers - if they manipulate with their network configuration (DNS servers configuration), they will just fail.

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  • Omg.. That is indeed correct. Totally forgot about the forward chain. DNS request to other dns servers indeed do not work anymore. Thanks! Apr 15, 2013 at 10:17
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I used these rules to force all DNS lookups to be done by dnsmasq on my DD-WRT router:

iptables -t nat -A PREROUTING -p udp --dport 53 -j REDIRECT
iptables -t nat -A PREROUTING -p tcp --dport 53 -j REDIRECT

Those can be stored in DD-WRT by going to Administration - Commands and then entering them in the box and hitting "Save Firewall".

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