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I'm running a router with DNS server and I would like to make sure that everybody in my network is actually using my DNS. What are the necessary iptables (?) to redirect all DNS-related traffic to my local DNS server?

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  • So you are setting up a DNS server with recursion enabled ? Before going any further, read this urgently : serverfault.com/questions/634793/… – Xavier Lucas Oct 11 '14 at 15:57
  • This is somewhat of a lost cause. If your users don't want to use your DNS, there are plenty of ways they can tunnel around you. – Andrew B Oct 13 '14 at 13:30
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You probably mean a kind of transparent forwarding, something which iptables calls Destination NAT:

ethL=eth0     # internal network NIC
dnsip=1.2.3.4 # IP of your DNS
iptables -t nat -A PREROUTING -p udp --dport 53 -i $ethL -j DNAT --to $dnsip

However, there are potential problems with this approach to consider:

  • this is a variant of NAT with all the associated costs (pass-through performance, kernel memory &c);
  • this will only work for packets which cross the router - clients who speak to a DNS server on their local IP network will continue to.
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  • This is the answer that addresses the actual question: using iptables to force outbound queries onto the local network's DNS server. – Andrew B Oct 13 '14 at 13:32
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You'll want to block any outbound DNS traffic from your users except to your local DNS server, so something like this can get you started:

local_network=X.X.X.X/24 # IP address/subnet-mask of local network
dns_server=Y.Y.Y.Y # IP address of DNS server

iptables -A INPUT -s $local_network -d $dns_server -p udp --dport 53 -j ACCEPT
iptables -A INPUT -s $local_network ! -d $dns_server -p udp --dport 53 -j DROP
iptables -A OUTPUT -s $dns_server -d $local_network -p udp --sport 53 -j ACCEPT

If your DNS server is a caching/recursive DNS server, then you'll need to allow it to do external lookups as well:

iptables -A INPUT -d $dns_server -p udp --sport 53 -j ACCEPT
iptables -A OUTPUT -s $dns_server -p udp --dport 53 -j ACCEPT
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You need to allow only DNS traffic from you DNS server and deny all other requests

# Allow DNS from your ip

iptables -A INPUT -p udp --dport 53 -s <source IP> -j ACCEPT
iptables -A INPUT -p tcp --dport 53 -s <source IP> -j ACCEPT

# Deny all other requests

iptables -A INPUT -p udp --dport 53 -j DROP
iptables -A INPUT -p tcp --dport 53 -j DROP

Check the Answer here Block all incoming DNS requests EXCEPT from IPs x,y,

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