[Netkit.users] Interface problem.-

Massimo Rimondini rimondin a dia.uniroma3.it
Ven 17 Mar 2006 17:00:00 CET

Dear Michael,

virtual machine PC1 does not seem to be essential for the experiences 
you have proposed (ping PC2->Vrouter1, ping Vrouter1->Vrouter2, ping 
PC2->Vrouter2), hence I have been experimenting on the following 

+------------+  +----------+  +----------+
| PC2 (real) |  | vrouter1 |  | vrouter2 |
+------+-----+  +--+----+--+  +-----+----+
       |          1|    |0          |0
       +-----------+    +-----A-----+

IP addresses:       
  eth1: (guest side), (host side)

I report below the outcome of the varoius ping commands, together with a 
brief explanation.

PC2 -> (Vrouter1, eth1): succeeds

PC2 -> (Vrouter1, eth0): fails because there is no 
corresponding entry in the host's routing table; you can fix this by 
issuing the following command on the host machine (as administrator):
route add -net dev nk_tap_USERNAME

Vrouter1 -> (Vrouter2, eth0): succeeds

PC2 -> (Vrouter2, eth0): fails even if the host has the 
correct entry in the routing table; this happens because Vrouter1 is not 
forwarding arp requests on subnet; in order to fix this, 
you can enable proxy_arp inside Vrouter1 by using the following command:
echo 1 > /proc/sys/net/ipv4/conf/all/proxy_arp
This is not enough, though. Vrouter2 is now correctly receiving ICMP 
Echo Request packets but is unable to answer because subnet is not in its routing table; you can fix this by issuing 
the following command inside Vrouter2:
route add default gw
Now everything should work as expected.

Hope this helps.

Massimo Rimondini.

maruiz a ec.gba.gov.ar wrote:

>Hi Massimo,
>  Yes, i have duplicate addresses, thanks, but i try many times, and cannot
>reach with ping from RealHosts to Vmachines (Pc2 to Vrouter1, and Vrouter1
>to Vrouter2 can, but cannot from Pc2 to Vrouter2)
> Please, if you can sendme any configuration thas has proven.
>>Hi Miguel,
>>you seem to have duplicate IP addresses in your configuration. In fact,
>> is assigned to both vrouter1's tap interface and to pc1 (or
>>When you configure a tap interface, you have to specify two IP addresses:
>>- The "tap" address, which is the address of a fictitious network
>>interface on the real host (you can even see it by using ifconfig on the
>>real host). This interface serves as a connection point between vrouter1
>>and the real network. Since this is the address of a new network
>>interface, it cannot overlap with other existing addresses.
>>- The "guest" address, which is the address of a network interface
>>showing up inside the virtual machine (vrouter1).
>>Uniqueness of IP addresses should however still be guaranteed.
>>I suggest to remove this problem before further analyzing the issue.
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