Проблема решена. Сделал запись следующего содержания:
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -F
iptables -F -t nat
iptables -F -t mangle
iptables -X
iptables -X -t nat
iptables -X -t mangle
iptables -A INPUT -p all -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -p all -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -p all -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
iptables -A INPUT -i lo -p all -j ACCEPT
iptables -A OUTPUT -o lo -p all -j ACCEPT
iptables -A INPUT -i eth1 -j ACCEPT
iptables -A OUTPUT -o eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -s 192.168.1.0/24 -p tcp -m multiport --dports 21,22,25,110,143,443 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth1 -s 192.168.1.0/24 -p udp -m multiport --dports 53 -j ACCEPT
iptables -A FORWARD -i eth1 -s 192.168.1.0/24 -p tcp -m multiport --dports 53 -j ACCEPT
iptables -A INPUT -i eth1 -p ICMP -j ACCEPT
iptables -A OUTPUT -o eth1 -p ICMP -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -m iprange --src-range 192.168.1.50-192.168.1.255 -p tcp -m multiport --dports 80 -j DNAT --to-destination 192.168.1.11:3128