Linux traffic Interception using DNAT
Warning: Any example presented here is provided "as-is" with no support or guarantee of suitability. If you have any further questions about these examples please email the squid-users mailing list.
To Intercept IPv4 web requests transparently without any kind of client configuration. When web traffic is reaching the machine squid is run on.
NOTE: NAT configuration will only work when used on the squid box. This is required to perform intercept accurately and securely. To intercept from a gateway machine and direct traffic at a separate squid box use policy routing.
Replace SQUIDIP with the public IP which squid may use for its listening port and outbound connections. Replace SQUIDPORT with the port in squid.conf set with intercept flag.
Due to the NAT security vulnerabilities it is also a very good idea to block external access to the internal receiving port. This has to be done in the mangle part of iptables before DNAT happens so that intercepted traffic does not get dropped.
Without the first iptables line here being first, your setup may encounter problems with forwarding loops.
# your proxy IP SQUIDIP=192.168.0.2 # your proxy listening port SQUIDPORT=3129 iptables -t nat -A PREROUTING -s $SQUIDIP -p tcp --dport 80 -j ACCEPT iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination $SQUIDIP:$SQUIDPORT iptables -t nat -A POSTROUTING -j MASQUERADE iptables -t mangle -A PREROUTING -p tcp --dport $SQUIDPORT -j DROP
NOTE: DNAT is only available for IPv4 traffic on older kernel versions. For IPv6 interception use TPROXY version 4.
# Controls IP packet forwarding net.ipv4.ip_forward = 1 # Controls source route verification net.ipv4.conf.default.rp_filter = 0 # Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
Squid Configuration File
You will need to configure squid to know the IP is being intercepted like so:
http_port 3129 transparent
In Squid 3.1+ the transparent option has been split. Use 'intercept to catch DNAT packets.
http_port 3129 intercept