Making legacy Linux networking work in Hyper-V

1. Launch Virtual Machine Settings
2. Remove Network Adapter.
3. Add Legacy Network Adapter. 
4. Make sure Legacy Network Adapter is conected to a live network. 
5. Boot your Virtual Machine. A lot of linux systems will work at this stage or automatic script will kick in and help you configure new adapter. If not proceed further.
6. Ensure that your system detects Legacy network card. Login into the legacy system and run lspci . One of the lines should contain Ethernet controller: Digital Equipment Corporation DECchip 21140 [FasterNet] (rev 20) . If the line is not there then either you did not add the legacy network adapter or virtual OS system is extremely old. Lets assume the Legacy Network Adapter was found and we can proceed. If you don’t have lspci in your legacy system proceed anyway.
7. Login as root to the system and edit depending on linux kernel version (to check linux version use uname -a )
• /etc/modprobe.conf (kernel 2.6)
• /etc/modules.conf (kernel 2.4)
• (or for older systems: /etc/conf.modules)
8. Make sure you add a line alias eth0 tulip to this file and remove other references to eth0 device.  Save the file.
9. At this stage you can reboot a VM and your Hyper-V network card should be detected. In fact its network settings will be exactly the same you had on actual linux system before you imported it into Hyper-V . By the way, your imported VM might fail to start network if there is a device with the same IP address on the network where Legacy Network Adapter is connected to.
10. Editing and changing your Linux system settings will depend on particular version of OS as it is slightly different in case it is Debian/Ubuntu or RedHat based.

If everything goes well you will have a Linux VM with working network access.

Hopefully, this helps.