Multiple Internet Connections with split access

TATA_INTIP="192.168.15.2"
AIRTEL_INTIP="192.168.1.2"
TATA_LANIP="192.168.20.1"
AIRTEL_LANIP="192.168.2.1"
# Interface connected to Internet
BHARTI="eth0"
TATA="eth1"
AIRTEL="eth3"
# Interface connected to LAN
BHARTI_LAN="eth4"
TATA_LAN="eth4:0"
AIRTEL_LAN="eth4:1"
# Squid port
SQUID_PORT="3128"
SSL_PORT="3129"
iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
# Load IPTABLES modules for NAT and IP conntrack support
modprobe ip_conntrack
modprobe ip_conntrack_ftp
# For win xp ftp client
#modprobe ip_nat_ftp
echo 1 > /proc/sys/net/ipv4/ip_forward
# Setting default filter policy
iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT
# Unlimited access to loop back
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT


# Allow UDP, DNS and Passive FTP
#iptables -A INPUT -i $BHARTI -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i $TATA -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i $AIRTEL -m state --state ESTABLISHED,RELATED -j ACCEPT


# set this system as a router for Rest of LAN
iptables -t mangle -A PREROUTING -p tcp 192.168.20.0/24 -j MARK --set-mark 4
iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to-source 192.168.15.2
iptables -t nat -A POSTROUTING -o eth3 -j SNAT --to-source 192.168.1.2


# unlimited access to LAN
iptables -A INPUT -i $BHARTI_LAN -j ACCEPT
iptables -A OUTPUT -o $BHARTI -j ACCEPT

iptables -A INPUT -i $TATA_LAN -j ACCEPT
iptables -A OUTPUT -o $TATA -j ACCEPT

iptables -A INPUT -i $AIRTEL_LAN -j ACCEPT
iptables -A OUTPUT -o $AIRTEL -j ACCEPT



# DROP everything and Log it
iptables -A INPUT -j LOG
iptables -A INPUT -j DROP