Как сделать шлюз для инета на компьютере, где есть локальная сеть eth0 и ppp в инет (через диал-ап)?

ключевые слова masquerading simple в google

разрешаешь форвардинг пакетов и включаешь маскарадинг , например через ipchains , iptables

поиши ipchains how-to на этом сайте , или доку по iptables .

считается что iptables мошнее ipchains

вот так приблизительно:
# Generated by iptables-save v1.2.7a on Mon Jan 20 13:10:32 2003
*mangle
:PREROUTING ACCEPT [4:192]
:INPUT ACCEPT [4:192]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [4:160]
:POSTROUTING ACCEPT [4:160]
[0:0] -A FORWARD -i ppp0 -o ppp0 -j DROP
[0:0] -A FORWARD -i eth0 -o eth0 -j DROP
[0:0] -A FORWARD -i eth1 -o eth1 -j DROP
[0:0] -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
COMMIT
# Completed on Mon Jan 20 13:10:32 2003
# Generated by iptables-save v1.2.7a on Mon Jan 20 13:10:32 2003
*filter
:INPUT DROP [4:192]
:FORWARD ACCEPT [0:0]
:OUTPUT DROP [4:160]
[0:0] -A INPUT -i lo -j ACCEPT
[0:0] -A INPUT -i eth0 -j ACCEPT
[0:0] -A INPUT -i eth1 -j ACCEPT
[0:0] -A INPUT -i ppp0 -m state --state ESTABLISHED,RELATED -j ACCEPT
[0:0] -A INPUT -i ppp0 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
[0:0] -A OUTPUT -o lo -j ACCEPT
[0:0] -A OUTPUT -o eth0 -j ACCEPT
[0:0] -A OUTPUT -o eth1 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 21 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 20 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --sport 20 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 3128 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 80 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 110 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p tcp -m tcp --dport 25 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p udp -m udp --dport 53 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -p udp -m udp --dport 123 -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -m state --state ESTABLISHED,RELATED -j ACCEPT
[0:0] -A OUTPUT -o ppp0 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
#[0:0] -A OUTPUT -o ppp0 -p icmp -m icmp -j ACCEPT
COMMIT
# Completed on Mon Jan 20 13:10:32 2003
# Generated by iptables-save v1.2.7a on Mon Jan 20 13:10:32 2003
*nat
:OUTPUT ACCEPT [0:0]
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A PREROUTING -p tcp -m tcp -i eth0 --dport 80 -j REDIRECT --to-ports 3128
-A PREROUTING -p tcp -m tcp -i eth1 --dport 80 -j REDIRECT --to-ports 3128
-A POSTROUTING -o ppp0 -j MASQUERADE
COMMIT