Multi physical interface with same subnet

jaf

So let me get this right… There are more than one machine, and they both have 192.168.10.1 and you want to give them both internet access. And you think it would be easier to run 2 pfsense boxes vs just having them change the IP of the one of these machines?

At a total loss here..
"the owner doesn't want to"

Then tell him he can not be on your network ;)

What is the relationship here - are you the customer, he is the customer? Do you need this on your network, or does the owner?

Yes, he is my customer, and customer is king…

johnpoz

He is your customer.. Why would he think he could connect to a network and not change the IP on multiple machines all having the same IP? So while he is king, he is also mentally challenged when it comes to networking..

Why is the box not just dhcp? I mean really set it to dhcp and there you go done.. You can have as many boxes then without any issue.

JKnott

Yes, he is my customer, and customer is king…

And if what he wants is impossible? Assuming you set up NAT. You will have 2 devices with the same IP address, but on opposite sides of it. When the device behind NAT has traffic for it's twin on the outside, it will never leave that device, as it will assume it for itself. When some other device has traffic for the device behind NAT, NAT will then have to decide which way to send the traffic. It does this by comparing the destination address with the network addresses. If both networks have the same address, there is absolutely no way for NAT to know which way to send that traffic. What that customer wants is impossible. King or not he has to be told and hopefully you won't lose your head! ;)

johnpoz

Its not impossible just stupid… You would have to it like he showed with different nat device for very box that wants to use 192.168.10.1 so you could nat it to a different network..

Each box would have to be isolated from each other on their own network with a different nat box. Utterly moronic...

jaf

@johnpoz:

He is your customer.. Why would he think he could connect to a network and not change the IP on multiple machines all having the same IP? So while he is king, he is also mentally challenged when it comes to networking..

Why is the box not just dhcp? I mean really set it to dhcp and there you go done.. You can have as many boxes then without any issue.

Sorry, I can't give here the full context of my customer. But believe me, if I ask this question, it's because I really need this configuration. If I can do it differently, I will do!

What that customer wants is impossible. King or not he has to be told and hopefully you won't lose your head! ;)

Sorry, I don't understand why it's impossible. Do you agree it's possible at least with 2 routers?
If not, I don't understand what's the purpose of lan with NAT if all lan must have strictly different subnet and address?

johnpoz

You can do it - you would just need a box to nat every single box that wants to use the same address so you can put them all on different networks.

JKnott

Sorry, I don't understand why it's impossible. Do you agree it's possible at least with 2 routers?
If not, I don't understand what's the purpose of lan with NAT if all lan must have strictly different subnet and address?

You need to understand how a device knows whether to use a router or direct connection to another device. It is based on the IP address, the network address and the subnet mask. The network address is determined by doing a logical AND of the host address and subnet mask. This leaves only the network address. Then, when the device wants to send traffic to another, it again uses the subnet mask to determine whether the other device is on the same network or reachable through a router. If the other device has the same network address, it is assumed to be on the local network and the router, NAT or otherwise, will not be used. This means that it is impossible for anything from that device to ever reach the network on the other side of the router, since both sides have the same network address.

You'll have to advise your customer that what he wants is not possible. There's no way around it.

BTW, NAT was created to get around the IPv4 address shortage, though it's sometimes used to merge networks.

johnpoz

where did he say he was using same address on both sides?

JKnott

@johnpoz:

where did he say he was using same address on both sides?

In the first post.

I have a configuration with 2 lan on 2 physical interfaces because my 2 lan have the same subnet and machines on LAN 1 and LAN 2 can have the same IP address.

johnpoz

Agreed he can not do that with 1 pfsense. But he can do it with two - the picture he posted..

As long as the common wan network is different than 192.168.10 then he can attach multiple 192.168.10 devices to this common network via different nat boxes.. See attached example

2samenetworks.png_thumb

jaf

So it seems I have my answer of my question asked on the first post.
It's impossible with 1 pfsense, I need 2.
Thanks.

johnpoz

I said you could do it back n post #3
https://forum.pfsense.org/index.php?topic=144526.msg786761#msg786761

But you would need multiple nat boxes to do it.

JKnott

@johnpoz:

Agreed he can not do that with 1 pfsense. But he can do it with two - the picture he posted..

As long as the common wan network is different than 192.168.10 then he can attach multiple 192.168.10 devices to this common network via different nat boxes.. See attached example

Will the computer on the left ever have to communicate with the one on the right? Depending on what else is on the network, there could be some real FUN with port forwarding. Regardless, it seems like a lot of trouble to avoid changing an IP address.

jaf

@JKnott:

Will the computer on the left ever have to communicate with the one on the right?

No I say that on this post : https://forum.pfsense.org/index.php?topic=144526.msg786831#msg786831
@johnpoz:

I said you could do it back n post #3
https://forum.pfsense.org/index.php?topic=144526.msg786761#msg786761

But you would need multiple nat boxes to do it.

My initial question was to know if it's possible with only 1 pfsense.
I didn't know if this "strange case" can be solve by 1 pfsense, it seems it's too strange to be implemented. :-\

johnpoz

You can not use 1 pfsense and have interfaces on the same network, and also have devices on this network having the same IP..

Now if the IPs were different and you just needed to leverage connections to pfsense that could be done via bridge the ports..

So if your machines/devices had IPs say 192.168.10.1 and 192.168.10.2 then you could bridge 2 interfaces on pfsense and that would work. But in such a case its easier to just use a switch if you need multiple connections, etc. ;)

kpa

It's all down to routing. Imagine if you had the same subnet, for example 192.168.1.0/24, on two different interfaces. The routing table would have two entries for the same subnet like:


Destination     Gateway
192.168.1.0/24	link#1 ....
192.168.1.0/24	link#2 ....

If the system would be now asked to forward a packet to host 192.168.1.100 that is on the first (link#1) network, would it be able to decide which link (interface) to use? The answer is an absolute no because both entries in the routing table are equally wide (/24) so there is no way to decide which link to use.

johnpoz

In such a scenario it "could" (in theory) be possible if the routing device also looked in its arp table and said own 192.168.1.100 is connected to link2.. But where you for sure have a problem is in his scenario both devices have the same IP… So there are 2 192.168.1.100..

His proposed setup is borked for sure.. The correct solution would be to change 1 of the segments to a different L3 scheme. Or if you want single L2 to make sure devices all have different IPs in the L3 addressing your using on that L2

JKnott

@kpa:

It's all down to routing. Imagine if you had the same subnet, for example 192.168.1.0/24, on two different interfaces. The routing table would have two entries for the same subnet like:
Destination     Gateway
192.168.1.0/24	link#1 ....
192.168.1.0/24	link#2 ....
If the system would be now asked to forward a packet to host 192.168.1.100 that is on the first (link#1) network, would it be able to decide which link (interface) to use? The answer is an absolute no because both entries in the routing table are equally wide (/24) so there is no way to decide which link to use.

The only possible solution would still require at least a different IP address. Then there could be a specific route to the device and then the routing longest path match would work. The device would also need to be configured with a suitable subnet mask, so that it knew other devices in the same network were elsewhere. Even then it's still a messy solution to a problem that could be avoided entirely by changing the address to a different subnet.

Derelict

or a metric, etc.

jahonix

@jaf:

if it's possible with only 1 pfsense.

Definitely no.

@jaf:

…it seems it's too strange to be implemented.

It's technically not possible to do something like that with one device.

Imagine putting two green apples on one table. Now tell your customer to "grab the green apple".
He just doesn't know which one.
Same for an IP packet. It just doesn't know if it should take the left or the right route to one of two devices with the exact same address. Routing doesn't work this way.