Есть Debian с eth0 и ppp0 интерфейсами.
root@debian:~# nano /etc/network/interfaces
.......
#this network
iface eth0 inet static
address 100.7.1.2
netmask 255.255.255.0
gateway 100.7.1.4
network 100.7.1.0
auto eth0
#for 3g modem
auto ppp0
iface ppp0 inet wvdial
up route add -net 10.0.31.0 netmask 255.255.255.0 gw 10.0.31.12
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 100.7.1.4 0.0.0.0 UG 0 0 0 eth0
0.0.0.0 172.17.4.111 0.0.0.0 UG 1024 0 0 ppp0
10.0.31.0 10.0.31.12 255.255.255.0 UG 0 0 0 ppp0
100.7.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 eth0
172.17.4.111 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT all -- localnet/24 anywhere
ACCEPT all -- anywhere localnet/24
Chain OUTPUT (policy ACCEPT)
target prot opt source destination