Missing eth0 Ethernet interface in Ubuntu — can't connect to router
First of all, your ethernet isn't being managed by Ubuntu. Try ifconfig -a instead of just ifconfig, so you can see all your networking devices, managed or not. If you do see ethX in the ifconfig -a list, the solution should be straightforward, and you seemed to have gotten half of it. The following needs to go into your /etc/network/interfaces file:
auto ethX
iface ethX inet dhcp
The first line "activates" management of the interface and the second line sets it to DHCP and IP.
However, if you don't even see any ethX interfaces when you do ifconfig -a, it's a driver issue (Ubuntu isn't even seeing the interface). To solve this, either check from Windows' device manager for the PCI Vendor ID and Device ID of your ethernet card, which you can cross-reference here and see if you can find a driver for that (Vendor ID is the manufacturer, Device ID is the acutal model of the ethernet card). An alternative in-linux way to do this is via lspci.
In Windows 7, getting the Vendor/Device IDs is through the Device Manager -- open up the Network Interfaces node, double click on your network card, click on 'detail', and select "Hardware IDs" from the drop-down list. The Vendor ID are the 4 hexadecimal digits after the VEN_ prefix, and the device id is the 4 hex digits after the &DEV_ immediately following the vendor.
I had a similar problem. By running ifconfig -a, I determined that the only network devices on my computer were p4p1 and lo. There was no eth0.
So I edited /etc/network/interfaces, replacing all instances of eth0 with p4p1. The file's contents are now:
auto lo
iface lo inet loopback
auto p4p1
iface p4p1 inet dhcp
After rebooting, the networking was working fine.
If it matters, I was using Ubuntu 12.04.5 Server edition.
I dont see anything in the comments suggesting that you look into udev and its rules which should install eth0. On my system, in /etc/udev/rules.d/75-network-devices.rules, I have the following;
# Local network rules to name your network cards.
#
# These rules were generated by nethelper.sh, but you can
# customize them.
#
# You may edit them as needed.
# (If, for example, your machine has more than one network
# card and you need to be sure they will always be given
# the same name, like eth0, based on the MAC address)
#
# If you delete this file, /lib/udev/nethelper.sh will try to
# generate it again the next time udev is started.
KERNEL=="eth?", ATTR{address}=="original has my MAC address here", NAME="eth0"
Also, make sure you have udevd running. This is what creates ethx on a Slackware based system.
You may also want to try ls /etc/init.d | grep eth and see if there is anything in the init as an artifact that ETH0 was ever there. The above is what use in Gentoo, I think Ubuntu uses a different mechanism but its worth a shot.
Back in Ubuntu 6.06 I had an issue like this, I ended clearing the CMOS on the MB and somehow it solved my problem. That's a stab in the dark though.