Add logic to dynamically discover primary interface name

[liwenwu-amazon]

Issue #171 , if available:

Description of changes:

• use metadata service to find out the mac address on primary interface
• go through all the interfaces and find the interface which has same mac address as primary interface's mac.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mogren]

On what images have you tested this change? Ubuntu? CentOS?

[mogren]

This call to list all interfaces is probably the biggest change. 😄

[liwenwu-amazon]

I have only tested using amazon linux worker AMI. Since net.Interface() should works on all flavors of Linux, such as CentOS or Ubuntu, this change should work for CentOS and Ubuntu.

[liwenwu-amazon]

@mogren I just unit tested the net.Interfaces() call on ubuntu t3.large instance. It is working as expected.
Here is the test program

cat test.go
package main

import (
	"fmt"
	"net"
)

func main() {

	interfaces, _ := net.Interfaces()

	for _, intf := range interfaces {
		fmt.Printf("interface %v, mac %v\n", intf.Name, intf.HardwareAddr)
	}
}

Here is the test output

root@ip-172-31-0-186:/home/ubuntu# ./test
interface lo, mac 
interface ens5, mac 0a:5f:92:e3:59:86

Here is the output of ifconfig

root@ip-172-31-0-186:/home/ubuntu# ifconfig
ens5: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9001
        inet 172.31.0.186  netmask 255.255.240.0  broadcast 172.31.15.255
        inet6 fe80::85f:92ff:fee3:5986  prefixlen 64  scopeid 0x20<link>
        ether 0a:5f:92:e3:59:86  txqueuelen 1000  (Ethernet)
        RX packets 6072  bytes 7297970 (7.2 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1699  bytes 199849 (199.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 191  bytes 14956 (14.9 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 191  bytes 14956 (14.9 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

[liwenwu-amazon]

@ewbankkit @phyllisstein, can you verify if this PR fixed your CNI in your environment? thanks

[phyllisstein]

@liwenwu-amazon This is awesome, thanks so much for taking up the issue! I just migrated from Kops with Ubuntu back to EKS with Amazon's Kube AMI, but I'll try rebuilding my Kops cluster again with this change in the next couple of days.

[ewbankkit]

Yes, I should be able to verify in the next couple of days too.

[mogren]

Thanks for testing this!

The change will fix this issue, but it does feel a bit strange that we need to use the MAC address to figure out the primary interface name.

[ewbankkit]

@liwenwu-amazon I have verified that building from the latest master source and running the CNI plugin on CentOS Linux release 7.5.1804 is now successful.

[phyllisstein]

Confirmed that v1.2.1 does the trick with Ubuntu 18.04 as well. Thanks again!