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Re: Ping second NIC over wire(ignoring loopback)

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Hello,

Giro a écrit :
> 
> Is there any way to let two local network interfaces communicate with
> each other without using the loopback device?

Not easily.
First, the kernel forces packets with a local destination address
through the loopback interface. Second, the kernel discards packets
received on a non-loopback interface with a local source address.

> On the same computer, I have interfaces eth1 (192.168.0.11) and eth2
> (192.168.0.12).  I want to
> ping from eth1 to eth2, and make the packet physically hit the wire,
> instead of going through the loopback device.

The above restrictions can be worked around with iptables NAT rules and
static ARP entries, tricking the kernel into believing that it is
communicating with a remote host. Sensitive people beware : what follows
is ugly.

Say that "remote" eth1 will be seen as 192.168.0.21 and "remote" eth2 as
192.168.0.22.

# make sure packets to "remote" addresses are routed through the correct
# interface
ip route add 192.168.0.21 dev eth2
ip route add 192.168.0.22 dev eth1

# create static ARP entries for the remote addresses
arp -s 192.168.0.21 $eth1_mac_address
arp -s 192.168.0.22 $eth2_mac_address

# replace the source address of outgoing packets with the "remote"
# addresses
iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to 192.168.0.21
iptables -t nat -A POSTROUTING -o eth2 -j SNAT --to 192.168.0.22

# replace the destination address of incoming packets with the real
# local addresses
iptables -t nat -A PREROUTING -i eth1 -j DNAT --to 192.168.0.11
iptables -t nat -A PREROUTING -i eth2 -j DNAT --to 192.168.0.12

With this a packet sent to 192.168.0.21 or 192.168.0.22 and its reply
should go on the wire.

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Thread

Ping second NIC over wire(ignoring loopback) Giro <giray.curgunlu@gmail.com> - 2011-12-06 00:47 -0800
  Re: Ping second NIC over wire(ignoring loopback) Pascal Hambourg <boite-a-spam@plouf.fr.eu.org> - 2011-12-06 23:36 +0100
  Re: Ping second NIC over wire(ignoring loopback) Rick Jones <rick.jones2@hp.com> - 2011-12-06 23:47 +0000
  Re: Ping second NIC over wire(ignoring loopback) David Schwartz <davids@webmaster.com> - 2011-12-12 18:37 -0800
    Re: Ping second NIC over wire(ignoring loopback) Mikhail Reznikov <mirekez@gmail.com> - 2024-02-07 04:18 -0800

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