Physical and virtual network interfaces with same MAC. Is it possible?

Question

First of all, I'm a newbie in networks, so, maybe somewhere there is an answer for my question, but I failed to find an answer.

Well, I have a lot of devices what has an Internet connection. I want to connet all of them into one network using VPN technologies. I use tinc-vpn to solve my needs and I want to use DHCP to configure my network.

The problem is here: I've successfully configure DHCP server and it serve all connected devices well, but when I reset device it obtain new address from DHCP.

As I understand when device become online it sends special broadcasting request into network what contains device's MAC. DHCP process what request and send response with network configurations back to device. Well, ifconfig up virtual interface for VPN on device and assign random MAC to it. Whats why I always get random IP. I think if I ask ifconfig to assign MAC address of physical network to the virtual interface I'll resolve my problem. Is it possible? If not how can I resolve my problem? I would not bother about this if DHCP IP pool was very big, but it serve only 250 devices, and device became offline very often.

Thanks in advance.

Answer 1

Thanks everyone for response.

I think I found an excellent solution, without DHCP. It's based on calculating unique static IP address using physical MAC address of the device.

In my solution I will use /16 subnet (10.1.0.0, for example). So, for all devices I have to calculate last 2 octets of IP. For my calculations I use CRC-16 hash function on real MAC address of the device. In the result I'll get 2-byte integer what contains IP address octets.

An example. The MAC address of device is 22-23-B2-72-99-3A. CRC-16 of the MAC will be 0x2e49. 0x2e == 46 and 0x49 = 73, so, the device IP will be 10.1.46.73.

Pros:

• No DHCP;
• Already known, permanent and unique IP of the device for all networks.

Cons:

• Collisions of the CRC-16 for MAC (two devices in one subnet with same IP but with different MAC). But the probability of the situation is negligible, I think.

Any suggestions and criticism are welcome.

Answer 2

To answer the title: It is possible to set same MAC on two interfaces, but it can lead to some networking issues or just unexpected behavior.

There are four ways (i'm aware of) to solve your problem:

1. As grawity suggested: reserve ip address on DHCP server for specific hostname.
2. After the virtual network device is created, assign it specific MAC address. On linux this can be automated with script.
3. Request specific IP address with your DHCP client. To make this reliable, you should make DHCP lease duration quite short (like 5 minutes or 1 hour, depending on your vpn login frequency), so the requested IP isn't still occupied by an active lease.
4. As your VPN gateway is unlikely to change often, you could use static IP setup. You want that ip at all times - why bother with DHCP then? Make the DHCP ip range smaller and use part of the freed space for static IPs...

Edit:

So the script is run multiple times to automate creation of new devices. In that case you can either compute the IP (or MAC) from device name. Example:

PREFIX='tap'
DEV_NUM=${INTERFACE#$PREFIX}
# it's good idea to increment, because devices usually number from 0
# and ip ending with .0 and .1 generally shouldn't be used by a client
DEV_NUM=$(($DEV_NUM+2))
ifconfig ${INTERFACE} 10.0.1.${DEV_NUM} netmask 255.255.0.0
MAC_NUM=`printf "%02d\n" ${DEV_NUM}`
ip link set dev ${INTERFACE} address ba:df:ace:00:00:${MAC_NUM}

Or mantain file assigning IP to device name. Example:

ADDRESS_MAP='/etc/vpn-ip-mapping.conf'
while IFS='='; read -ra line; do
  if [ "${line[0]}" == "${INTERFACE}" ]; then
    IP_ADDRESS=${line[1]}
    ifconfig ${INTERFACE} ${IP_ADDRESS} netmask 255.255.0.0
    break
  fi
done < ${ADDRESS_MAP}

Answer 3

One more.

I found a problem why I can't recive configurations from my DHCP on device when connection established. The reason is that tinc-up or other scripts that tinc executes after creating tunnel are synchronous with tinc services, and my dhcp client exits by timeout before tinc make handshake.

To resolve this problem, I run dhcp client in background inside tinc-up script.

tinc-up script now looks like this:

#!/bin/sh

ifconfig $INTERFACE hw ether fe:fd:00:00:00:00 # set interface hw address to get same IP from leases every time, connection established

dhcpcd -w $INTERFACE & # start dhcp client parralel with handshake proccess of tinc