Linux traffic Interception using DNAT

Outline

To Intercept IPv4 web requests transparently without any kind of client configuration. When web traffic is reaching the machine squid is run on.

NOTE: This configuration is given for use 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.

NOTE: DNAT is only available for IPv4 traffic on older kernel versions. For IPv6 interception use TPROXY version 4.

iptables configuration

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

/etc/sysctl.conf Configuration

# 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

CategoryConfigExample