Internet sharing with iptables: choosing which connection to share

Question

I have a computer with two network devices (eth0 and wlan0), both connected to the internet (two different connections/isp). I'm trying to share the connection of wlan0 to another computer connected via ethernet to eth0.

What I'm doing is:

# sysctl net.ipv4.ip_forward=1
# iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

From the client computer I can then connect to this one, but the internet connection that gets shared is the one on eth0 and not the one on wlan0.

If I disable the internet connection on eth0 (by setting no gateway), then the connection to be shared is the one on wlan0. However, I'd like to have both internet connections enabled and specify to iptables which one to share. Is this possible?

What am I missing? Do I need some forwarding rule?

Answer 1

Seem like your default gw is on eth0 and client is redirected to it (via a icmp redirect).

To fix your setup you need to add a routing rule stating that all packets incoming from client_ip should be routed to wlanO_gw.

Try adding a new routing table:

• Edit /etc/iproute2/rt_tables and add a line for a new table, for example 252 masq where 252 is the table id and masq is the new table name.

• Add a rule to route ip_client packets with table masq

  ip rule add from ip_client/32 table masq

• add a default gw to the masq table

  ip ro add default via wlan0_gw table masq