iptables rules for DNS/Transparent proxy with ip exceptions

Posted by SlimSCSI on Server Fault See other posts from Server Fault or by SlimSCSI
Published on 2011-09-01T02:59:04Z Indexed on 2012/10/29 17:05 UTC
Read the original article Hit count: 193

Filed under:

iptables

I am running a router (A Netgear WNDR3700 if that matters) with dd-wrt. For content filtering I am using OpenDNS. I wanted to make sure a user could not bypass OpenDNS by putting in their own name servers, so I have a rule to catch all DNS traffic.

iptables -t nat -A PREROUTING -i br0 -p all --dport 53 -j DNAT --to $LAN_IP

I did have one computer on the network I wanted to allow past OpenDNS filters. On that machine I manually set the name servers, and created another rule to allow it to pass

iptables -t nat -I PREROUTING -i br0 -s 192.168.1.2 -j ACCEPT

This worked well.

Today, I installed a transparent proxy (squid) on the router and added these rules:

iptables -t nat -A PREROUTING -i br0 -s $LAN_NET -d $LAN_NET -p tcp --dport 80 -j ACCEPT
iptables -t nat -A PREROUTING -i br0 -s ! $PROXY_IP -p tcp --dport 80 -j DNAT --to $PROXY_IP:$PROXY_PORT
iptables -t nat -I POSTROUTING -o br0 -s $LAN_NET -d $PROXY_IP -p tcp -j SNAT --to $LAN_IP
iptables -I FORWARD -i br0 -o br0 -s $LAN_NET -d $PROXY_IP -p tcp --dport $PROXY_PORT -j ACCEPT

This also works, however the 192.168.1.2 address does not get routed through squid. How can I have 192.168.1.2 (and maybe others in the future) by-pass the port 53 rules, but not the port 80 rules?

Related posts about iptables

Sometimes this script fails to update the iptables

as seen on Server Fault - Search for 'Server Fault'
It does not happen often, but sometimes after running the below script, checking the iptables with service iptables status shows that they weren't updated and the script doesn't output any error. The iptables is structured as look-up tree (long repeated sections snipped): #!/bin/sh iptables -t… >>> More
My current iptable configuration doesn't work [on hold]

as seen on Server Fault - Search for 'Server Fault'
sudo chkconfig iptables off /etc/init.d/iptables on ### Clear/flush iptables sudo iptables -F sudo iptables -P INPUT ACCEPT sudo iptables -P OUTPUT ACCEPT sudo iptables -P FORWARD ACCEPT ### Allow SSH iptables -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT iptables… >>> More
L2TP iptables port forward

as seen on Server Fault - Search for 'Server Fault'
Hi all, I'm setting up port forwarding for an L2TP VPN connection to the local Windows 2003 VPN server. The router is a simpel Debian machine with iptables. The VPN server works perfect. But I cannot log in from the WAN. I'm missing something. The VPN server is using a pre-shared key (L2TP) and… >>> More
iptables -P FORWARD DROP makes port forwarding slow

as seen on Server Fault - Search for 'Server Fault'
I have three computers, linked like this: box1 (ubuntu) box2 router & gateway (debian) box3 (opensuse) [10.0.1.1] ---- [10.0.1.18,10.0.2.18,10.0.3.18] ---- [10.0.3.15] | box4, www [10.0.2.1] Among other… >>> More
IPtables AWS EC2 NAT/Reverse NAT - For Reverse Proxy style setup but with IPtables

as seen on Server Fault - Search for 'Server Fault'
I was thinking initially needing to do a reverse proxy or something so I could get some SSL/TLS traffic look like it is being terminated at a server and IP address in the AWS cloud, and then that traffic is forwarded onto our actual web servers that aren't in the cloud... I've not done much iptables… >>> More

Developer IT