networking - How to allow access to web only through proxy?

I have a Linux machine kernel 3.7.0 with Squid proxy server and a direct Internet connection. Browsers and Squid reside on the same machine. Is it possible to allow access to web only through Squid? Maybe use SELinux?

You can use the owner module in iptables (-m owner --uid-owner $SQUID_UID) to setup allow rules for Squid and then deny other traffic.

Maybe something like this?

# Your debian machine (gateway)
LAN_IP="192.168.0.1"

# Your network
LAN_IP_RANGE="192.168.0.0/24"                                                               

# Your squid machine
PROXY_IP="192.168.0.254"                                                                   
PROXY_PORT="3128"

iptables -t mangle -A PREROUTING -s $LAN_IP_RANGE ! -d $LAN_IP_RANGE -p TCP --destination-port 80 -j MARK --set-mark 11                                     
iptables -t nat -A PREROUTING -m mark --mark 11 -p TCP -j DNAT --to-destination ${PROXY_IP}:${PROXY_PORT}                                                          
iptables -t nat -A POSTROUTING -m mark --mark 11 -p TCP -j SNAT --to-source $LAN_IP

Source

Here is the code:

iptables -P INPUT DROP
iptables -P FORWARD DROP
#loopback interface
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
#DNS
iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
iptables -A INPUT -p udp --sport 53 -j ACCEPT
#http,https traffic only through Squid - nobody user
iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -m state -m owner --uid-owner nobody --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp -m multiport --sports 80,443 -m state  --state ESTABLISHED -j ACCEPT