Allow loopback network interface monitoring.

[ossama-othman]

Add a new mptcpd_nm_monitor_loopback() function that allows the user to enable monitoring of loopback network interfaces. Monitoring of loopback network interfaces is meant primarily for testing purposes. Mptcpd will retain the previous behavior of only monitoring non-loopback network interfaces by default.

Fixes #208.

[ossama-othman]

Expected test-network-monitor results (non-loopback IP addresses may vary):

Loopback Network Interface Monitoring Disabled

test-network-monitor.c:check_interface() 
interface
  family: 0
  type:   1
  index:  2
  flags:  0x00011043
  name:   enp3s0
test-network-monitor.c:check_interface()   addrs:
test-network-monitor.c:dump_addr()     192.168.1.48
...
PASS test-network-monitor (exit status: 0)

Loopback Network Interface Monitoring Enabled

test-network-monitor.c:check_interface() 
interface
  family: 0
  type:   772
  index:  1
  flags:  0x00010049
  name:   lo
test-network-monitor.c:check_interface()   addrs:
test-network-monitor.c:dump_addr()     127.0.0.1
test-network-monitor.c:dump_addr()     ::1
test-network-monitor.c:check_interface() 
interface
  family: 0
  type:   1
  index:  2
  flags:  0x00011043
  name:   enp3s0
test-network-monitor.c:check_interface()   addrs:
test-network-monitor.c:dump_addr()     192.168.1.48
...
PASS test-network-monitor (exit status: 0)

Notice the loopback interface and address dump.

[coveralls]

Pull Request Test Coverage Report for Build 1860521727

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 11 of 12 (91.67%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 53.734%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/network_monitor.c	11	12	91.67%

Files with Coverage Reduction	New Missed Lines	%
lib/network_monitor.c	8	43.91%

Totals
Change from base Build 1849124342:	0.5%
Covered Lines:	1072
Relevant Lines:	1995

---

💛 - Coveralls

[mjmartineau]

A couple of questions inline

[mjmartineau]

I'm not set up to test this directly, but looks good to me.

[ossama-othman]

Verified with non-loopback interface down:

$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> ...
2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> ...

$ sudo ip link set enp3s0 down
$ make check
...

$ sudo ip link set enp3s0 up

$ cat tests/test-network-monitor.log 
test-network-monitor.c:check_interface() 
interface
  family: 0
  type:   772
  index:  1
  flags:  0x00010049
  name:   lo
test-network-monitor.c:check_interface()   addrs:
test-network-monitor.c:dump_addr()     127.0.0.1
test-network-monitor.c:dump_addr()     ::1
PASS test-network-monitor (exit status: 0)