"dummy" network adapters do not work with GetInterfaceIP

[karl-tpio]

A picture console log is worth 1000 words:

TL;DR: interfaces of type "dummy" (like a loopback, but usually not given an address in the 127.x space) are not supported(?) by the GetInterfaceIP template function

0. Grab the binary to play around:

root@ip-172-25-26-131:~/go/bin# go get -u github.com/hashicorp/go-sockaddr/cmd/sockaddr

1. List all the network interfaces on the host:

root@ip-172-25-26-131:~/go/bin# ip -o link show | awk -F': ' '{print $2}'
lo
ens5
dummy0
docker0

2. demonstrate the tool works:

root@ip-172-25-26-131:~/go/bin# ./sockaddr eval -d 'GetInterfaceIP "ens5"'
[0] in: "{{GetInterfaceIP \"ens5\"}}"
[0] out: "172.25.26.131"

root@ip-172-25-26-131:~/go/bin# ifconfig ens5
ens5: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9001
        inet 172.25.26.131  netmask 255.255.240.0  broadcast 172.25.31.255
        inet6 fe80::e9:d4ff:fe2c:7eb4  prefixlen 64  scopeid 0x20<link>
        inet6 2600:1f1c:61e:1701:d208:ba80:cf7c:9e9e  prefixlen 128  scopeid 0x0<global>
        ether 02:e9:d4:2c:7e:b4  txqueuelen 1000  (Ethernet)
        RX packets 159045  bytes 89658689 (89.6 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 83925  bytes 13061624 (13.0 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

3. test the tool with the intended interface:

root@ip-172-25-26-131:~/go/bin# ./sockaddr eval -d 'GetInterfaceIP "dummy0"'
[0] in: "{{GetInterfaceIP \"dummy0\"}}"
[0] out: ""

a Nil result was not expected.

4. Show the expected result:

root@ip-172-25-26-131:~/go/bin# ifconfig dummy0
dummy0: flags=195<UP,BROADCAST,RUNNING,NOARP>  mtu 1500
        inet 169.254.1.1  netmask 255.255.255.255  broadcast 0.0.0.0
        inet6 fe80::1cc3:f6ff:fe14:c441  prefixlen 64  scopeid 0x20<link>
        ether 1e:c3:f6:14:c4:41  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 11  bytes 770 (770.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

5. show how the dummy interface came into existence:

root@ip-172-25-26-131:~/go/bin# cat /etc/systemd/network/dummy0.netdev
# Creates a "dummy" network interface
# we'll configure this interface with a link-local address
# See: https://www.freedesktop.org/software/systemd/man/systemd.netdev.html
##

[NetDev]
Name=dummy0
Kind=dummy

Additional info that may be helpful:

root@ip-172-25-26-131:~/go/bin# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.2 LTS
Release:	18.04
Codename:	bionic

For context, this issue was discovered while troubleshooting this specific configuration of consul:
hashicorp/consul#5371

[karl-tpio]

After a casual conversation w/ a colleague, i think i have a "why" for the bug and possibly a solution, too.

I'm not a golang person, but i think i can read well enough to figure out what's going on.

0. Gets all interfaces
1. Filters on the name match
2. Filters on forwardable flag: https://github.com/hashicorp/go-sockaddr/blob/master/ifaddrs.go#L688

Here's where the bug happens. Because the dummy interface has an IP of 169.254.1.1, which is in the 169.254.0.0/16 range that MustIPv4Addr("169.254.0.0/16") filters out here: https://github.com/hashicorp/go-sockaddr/blob/master/rfc.go#L902

Basically: It appears that the GetInterfaceIP is not well named. That is, the GetInterfaceIP function should have been named GetForwardableInterfaceIP or similar. I can understand why adjusting the existing function name may not be ideal and would suggest an alternative:

• keep the GetInterfaceIP function, but add documentation indicated the limitations w/r/t the filtered address ranges

• create a new GetInterfaceIPRegardlessOfInterrfaceFlags function which will omit the check against ForwardingBlacklist.

• update the documentation describing the limits in GetInterfaceIP to indicate that there's an alternative function, GetInterfaceIPRegardlessOfInterrfaceFlags that may be better suited to the users purposes.