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Guest OS acting as NAT

Source: microsoft.public.virtualpc
Sent: 05/23/2008
From: The Dark Free Soul <(email address - cut out)>
Message:

Hi guys!

I want to virtualize a linux box on virtual pc. Host os is Windows
2003 server and I managed to install GNU/Linux CentOS 5.1 on it. Host
os is connected to a class B lan where 172.16.0.1 is the gateway and
in the same LAN there are also DNSs.
I configured Virtual PC to offer 2 network cards to the guest os:
shared network (NAT) and Gigabit card connected.
On linux i get eth0 and eth1. If I automatically acquire an IP
address, on eth0 I get 192.168.131.65 from a DHCP server running on
192.168.131.255 but DHCP server does not specify any default gateway.
On eth1 I can set any address i like (I used 172.16.1.12).
As first attempt I set guest os to use as default gateway 172.16.0.1
and LAN DNSs as DNS. Guest os could surf the internet! (note: in all
this eth0 is not used so maybe I don't really need ip 192.168.131.65)

After this I set guest os to manage 4 class C LANs, and my guest os is
supposed to act a gateway for the LANs (I'm not using VLANs so
everything is going on the same network interface. I just declared 4
aliases on the same eth1 network interface in order to have a total of
5 IPs: 172.16.1.12/16, 192.168.10.1/24, 192.168.11.1/24,
192.168.12.1/24 and 192.168.13.1/24).
Clients connected to these LANs can work great, they can reach the
guest os and every host in the respective LAN but they can't reach
internet or LAN 172.16.0.0/16. This is a BIG problem for me!
I investigated why it's not working since the guest os is configured
to act as NAT for class C LANs clients.
I monitored the traffic going in and out from the guest os and I
discovered this:
- when clients from class C sent a request addressed to the outside
internet the request arrives to the guest os
- guest os forward it to class B DNSs
- DNSs reply to guest os ip 172.16.1.12
- guest os does NOT forward back the answer coming from DNSs to the
class C clients so they never get an answer and connections go in
timeout

At first it may sounds like a guest os bad configuration but it's not.
Same os is installed stand alone (not virtualized) with very same
configuration works great.
I tryed to use 192.168.13.255 as default gateway instead of 172.16.0.1
but with no result... (and also guest os could not reach internet
anymore).

I really hope You can help me, I'm sure I'm missing something...
Many thanks in advance to anyone for your time.

A

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Related Messages

Re: Guest OS acting as NAT

Sent: 05/23/2008
From: The Dark Free Soul <(email address - cut out)>
Message:

On May 23, 12:48=A0pm, "Bill Grant" <not.available@online> wrote:
ch has
he

Hi Bill and many thanks for answering me!
All pc are connected on the same fisical network, simply some pc use a
class B ip and some other use a class C ip.
Of course, there's a gateway serving class C LANs (to forward traffic
to class B) and a border gateway serving class B (to forward traffic
to internet).
Class C LANs are WLAN and the gateway (that I should replace with the
virtual machine) has one single network card with more IP aliases.
The idea is to have a big wired LAN and four separated wireless LANs.
Setting VPC to offer just one "normal" NIC seems not to solve the
problem, the situation is same as before... simply now I just don't
have the other network interface with ip 192.168.131.65 that I was not
using. Vm can still surf the internet but client are out.
Any other idea?
Save me :) it's third day I'm trying to work it out :)
Many thanks again!

Show quoted text

> =A0 =A0 =A0The second NIC in the vm should be connected to the network whi=
> the NAT clients on it. Where are these machines? How did they access the
> Internet before?
>
> =A0 =A0 If one NIC has an Internet connection, the machines connected to t=
> other NIC should be able to get to the Internet if this NIC is the default=
> gateway for them and you have NAT configured on the vm.

Re: Guest OS acting as NAT

Sent: 05/23/2008
From: "Bill Grant" <not.available@online>
Message:

"The Dark Free Soul" <(email address - cut out)> wrote in message
news:(email address - cut out)...

To use the vm as a router, one NIC should be linked to the Internet or to
a network which has Internet access. That sounds like your 172.16.0 network
since the guest can surf the Internet through that.

No NIC should be set to Shared Networking (NAT). This is not suitable
for your requirements.

The second NIC in the vm should be connected to the network which has
the NAT clients on it. Where are these machines? How did they access the
Internet before?

If one NIC has an Internet connection, the machines connected to the
other NIC should be able to get to the Internet if this NIC is the default
gateway for them and you have NAT configured on the vm.

Show quoted text

> Hi guys!
>
> I want to virtualize a linux box on virtual pc. Host os is Windows
> 2003 server and I managed to install GNU/Linux CentOS 5.1 on it. Host
> os is connected to a class B lan where 172.16.0.1 is the gateway and
> in the same LAN there are also DNSs.
> I configured Virtual PC to offer 2 network cards to the guest os:
> shared network (NAT) and Gigabit card connected.
> On linux i get eth0 and eth1. If I automatically acquire an IP
> address, on eth0 I get 192.168.131.65 from a DHCP server running on
> 192.168.131.255 but DHCP server does not specify any default gateway.
> On eth1 I can set any address i like (I used 172.16.1.12).
> As first attempt I set guest os to use as default gateway 172.16.0.1
> and LAN DNSs as DNS. Guest os could surf the internet! (note: in all
> this eth0 is not used so maybe I don't really need ip 192.168.131.65)
>
> After this I set guest os to manage 4 class C LANs, and my guest os is
> supposed to act a gateway for the LANs (I'm not using VLANs so
> everything is going on the same network interface. I just declared 4
> aliases on the same eth1 network interface in order to have a total of
> 5 IPs: 172.16.1.12/16, 192.168.10.1/24, 192.168.11.1/24,
> 192.168.12.1/24 and 192.168.13.1/24).
> Clients connected to these LANs can work great, they can reach the
> guest os and every host in the respective LAN but they can't reach
> internet or LAN 172.16.0.0/16. This is a BIG problem for me!
> I investigated why it's not working since the guest os is configured
> to act as NAT for class C LANs clients.
> I monitored the traffic going in and out from the guest os and I
> discovered this:
> - when clients from class C sent a request addressed to the outside
> internet the request arrives to the guest os
> - guest os forward it to class B DNSs
> - DNSs reply to guest os ip 172.16.1.12
> - guest os does NOT forward back the answer coming from DNSs to the
> class C clients so they never get an answer and connections go in
> timeout
>
> At first it may sounds like a guest os bad configuration but it's not.
> Same os is installed stand alone (not virtualized) with very same
> configuration works great.
> I tryed to use 192.168.13.255 as default gateway instead of 172.16.0.1
> but with no result... (and also guest os could not reach internet
> anymore).
>
> I really hope You can help me, I'm sure I'm missing something...
> Many thanks in advance to anyone for your time.
>
> A

Re: Guest OS acting as NAT

Sent: 05/24/2008
From: The Dark Free Soul <(email address - cut out)>
Message:

Well Bill I can only say you are right. A router would be best idea
but since it gotta offer radius services for wlan it's a computer (to
keep things cheap).
I know there's no need to virtualize anything here but it's not my
decision. Unfortunatly I gotta do it... it's supposed to be a kind of
demostation that it can be done.
Anyway I know it's a bad designe but I gotta do it. Do you have any
suggestion to give me? I can't understand why it wont work...
Many thanks again for your time. I really apriciate it.

Re: Guest OS acting as NAT

Sent: 05/24/2008
From: "Bill Grant" <not.available@online>
Message:

"The Dark Free Soul" <(email address - cut out)> wrote in message
news:(email address - cut out)...
On May 23, 12:48 pm, "Bill Grant" <not.available@online> wrote:

t>o class B) and a border gateway serving class B (to forward traffic
t>o internet).

I am a fan of virtual machines but I really can't see why you would want
to use a virtual machine to do that. I can't even see why you would use a
computer to do that rather than a wireless access point.

The only reason I run routers in vms is to route between virtual
networks or to route traffic from virtual networks on to a physical network.
If all machines are on wired or wireless network, why use a virtual machine
at all? If you must use a computer as a router why not use the host OS? What
is the host doing that prevents it being a router?

Show quoted text

> The second NIC in the vm should be connected to the network which has
> the NAT clients on it. Where are these machines? How did they access the
> Internet before?
>
> If one NIC has an Internet connection, the machines connected to the
> other NIC should be able to get to the Internet if this NIC is the default
> gateway for them and you have NAT configured on the vm.
>Hi Bill and many thanks for answering me!
>All pc are connected on the same fisical network, simply some pc use a
>class B ip and some other use a class C ip.
>Of course, there's a gateway serving class C LANs (to forward traffic
>Class C LANs are WLAN and the gateway (that I should replace with the
>virtual machine) has one single network card with more IP aliases.
>The idea is to have a big wired LAN and four separated wireless LANs.
>Setting VPC to offer just one "normal" NIC seems not to solve the
>problem, the situation is same as before... simply now I just don't
>have the other network interface with ip 192.168.131.65 that I was not
>using. Vm can still surf the internet but client are out.
>Any other idea?
>Save me :) it's third day I'm trying to work it out :)
>Many thanks again!

Re: Guest OS acting as NAT

Sent: 05/25/2008
From: "Bill Grant" <not.available@online>
Message:

"The Dark Free Soul" <(email address - cut out)> wrote in message
news:(email address - cut out)...

OK. First off, even if everything is on one physical network I would use
two NICs in the router. They will both need to be set to link to the
physical NIC in the host, since this is the only connection to the network.
Virtual Machine Network Services will make sure the traffic gets to the
correct NICs in the vm.

Set one NIC to get its network config automatically. It will get its
config from DHCP on the class B LAN and look just like an extra machine on
the LAN. You have done that before and you know that it works. This is the
"public" side of your NAT router.

Manually set the second NIC with the IP addresses which are the gateway
addresses for the Class C subnets. This is the "private" side of your NAT
router.

Now configure the NAT router using itables or whatever it is in Linux .

Show quoted text

> Well Bill I can only say you are right. A router would be best idea
> but since it gotta offer radius services for wlan it's a computer (to
> keep things cheap).
> I know there's no need to virtualize anything here but it's not my
> decision. Unfortunatly I gotta do it... it's supposed to be a kind of
> demostation that it can be done.
> Anyway I know it's a bad designe but I gotta do it. Do you have any
> suggestion to give me? I can't understand why it wont work...
> Many thanks again for your time. I really apriciate it.

Post Reply | This is Spam! | Mark as Spam

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