I have installed Ubuntu 11.10 on a refurbished machine. I have also replaced a defective NIC card with a new NIC card, two NIC cards are recognized when the install occurs. I also have squid installed as a proxy server. Squid works well when all workstations are connected to the router. However ... .

When I connect one NIC to my router (live internet connection available), the other NIC to my switch (without any internet available), I can create two separate networks, but I can't figure out how to make the two NIC cards transfer data between themselves, as well as, across the two networks.

  1. I have tried to bridge both Ethernet cards ... no luck!
  2. I have tried to update iptables ... no luck!
  3. I have set both NIC cards to static addresses ... no luck!
  4. I have configured one NIC to use the other as a gateway ... no luck! All result in an error that the pinged address (network related to switch) is on an unreachable destination.

What am I missing?

  • Please reformulate your question in something like "How do I make two NIC cards transfer data between each other?" for example. You may have more answers then.
    – Agmenor
    Jan 14, 2012 at 17:53

3 Answers 3


The script described by @laurent is unnecessary, because there is a canonical way. All you need to do is edit /etc/sysctl.conf and uncomment (delete # at the beginning of) this line:

net.ipv4.ip_forward = 1

Then it will already be applied at boot.


Ubuntu has ip fowarding disabled by default and you need to enable it to route packets with your machine:

to enable, type in terminal as root (sudo su):

echo 1 > /proc/sys/net/ipv4/ip_forward

Obs: doesn't work with sudo

And if you want to route internet from this machine you may need to configure NAT also.


the 1st command echo 1 ... doesn't work with sudo. You have to change to root with sudo su before (because sudo will run echo 1 as root but will try to redirect to the file as your user and this won't work). Anyways, you allways can check if there is a 1 with cat /proc/sys/net/ipv4/ip_forward. Obs: this has to be done on every boot so you can write a script and use update-rc.d on it.

POSTROUTING rule looks OK if eth0 is your internet connection NIC.

FORWARD rules I use for established and related connections:

-A FORWARD -p tcp -m state -d your_network_ip.0/ --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -p udp -m state -d your_network_ip.0/ --state RELATED,ESTABLISHED -j ACCEPT

2nd UPDATE - Automatic script:

# turn ip_forward on/off

case "$1" in
        echo 1 > /proc/sys/net/ipv4/ip_forward
        echo 0 > /proc/sys/net/ipv4/ip_forward
        echo "Usage: $0 { start | stop }"
exit 0

You save this script in /etc/init.d with the name you want (router for example) and you make it executable (sudo chmod +x /etc/init.d/router).

To have it run on each reboot you need to make start links with update-rc.d:

sudo update-rc.d router defaults

Other things you have to check:

  • DHCP working on the 2nd network and sending your machine IP as default gateway to subnet
  • default gateway (your machine IP in the new subnet) is better with fixed IP
  • you commented you can't ping the 2nd NIC but from where? the subnet, your machine or machines connected directly on the router?
  • squid is installed on your machine? Did you change the config to include the new subnet? Do you need squid? It is not very easy to configure and you can very well share internet and network without it if you don't need its additional features.

Obs: the update-rc.d message is OK, no problem with it. You should now have always a 1 in ip_forward file.

  • edited answer above
    – laurent
    Jan 14, 2012 at 17:47
  • IF you changed a defective NIC, its eth# changed. Are you sure you still have the same eth0 for external and eth1 for internal? If not, you can change the rules, or better, rename the NICs
    – laurent
    Jan 14, 2012 at 17:56
  • Correct to change with vim (tbh safer than logging as root to echo the 1 to the file I was saying). I will update my answer with the way to make this automatic at each reboot and you don't need to bridge the cards.
    – laurent
    Jan 14, 2012 at 20:10
  • going to stop squid, install and do initial config of DHCP Server. In your bullet point-DHCP working on the 2nd network and sending your machine IP as default gateway to subnet, I take this to mean the built in card is the parent and the PCI card is the child. I cannot ping from built in card to PCI card. also new issue with eth1 not coming up after ifup cmd is issue and static address set.I receive this error ... RTNETLINK answers: No such process Failed to bring up eth1. Jan 15, 2012 at 10:02
  • 3
    This is how to use sudo to set forwarding on: echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward or if you prefer: sudo tee /proc/sys/net/ipv4/ip_forward <<< 1 (the latter requires a shell such as Bash which supports here-strings). Oct 10, 2013 at 19:51

I tried this advice and ran into a problem that isn't covered here: the next computer has to know how to get back to the last computer.

In my case, I have 2 networks: A WiFi network,, and an Ethernet network, My "router" is a laptop with both a WiFi transceiver on the and an Ethernet interface on the network. The household router also has a WiFi transceiver and an Ethernet interface, but this Ethernet transceiver connects to The Internet. The household router also has a Network Address Translator (NAT). The household router has a routing table, and that routing table has a default route entry, which normally points to the Internet Service Providers (ISP's) router.

When a computer on the network wished to send a packet to some computer on The Internet, it consulted its own routing table and knew that the default gateway was the laptop, When the packet arrived at the laptop, it consulted its own routing table and forwarded the packet to the household router. The household router consulted its routing table, went through Network Address Translation (a detail we can ignore for this discussion), and then to the ISP's router. At some point, the destination machine will want to send a packet to my computer. The packet will arrive at the household router, go through reverse NAT, and then have to be routed. When the packet comes out of NAT, it will have a destination IPv4 address in the network. It was at this point that I ran into a problem: the household router didn't know about the Ethernet network, So it did what it's supposed to do - forward that packet to its default router, i.e. the ISP's router.

To resolve the problem, I had to make an entry in the household router's routing table, which tells it that to get to any computer in the network, send the packet to the laptop's WiFi address, which is in Now, when the packet arrives at the laptop, the laptop knows about the network, and routes the packet there.

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