fix: Fake broadcast address for 127.x.x.x

[iacore]

Closes #2749

Thanks @zoff99 for the idea

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This change is 

[zoff99]

@iacore does it work in parallel with normal LAN discovery? meaning will it use lo: and eth0: at the same time?
if you have some time, can you add a test case that it works?
here: https://github.com/TokTok/c-toxcore/blob/master/auto_tests/lan_discovery_test.c

[iacore]

@iacore does it work in parallel with normal LAN discovery? meaning will it use lo: and eth0: at the same time?

the code says yes, so i guess yes.

if you have some time, can you add a test case that it works? here: https://github.com/TokTok/c-toxcore/blob/master/auto_tests/lan_discovery_test.c

i have no idea how to do that. sorry.

[zoff99]

it works pretty well. nicely done. what would it do on localhost in ipv6 ?

[zoff99]

@iphydf @Green-Sky this PR works for me. are there any security implications of this? or other bad things in your view?

[iacore]

it works pretty well. nicely done. what would it do on localhost in ipv6 ?

No, and I'm afraid that the whole broadcast logic doesn't take ipv6 into account. See https://github.com/TokTok/c-toxcore/pull/2760/files#diff-907bc1ea87830b3899d91912c9b7c2eb62d713dadf887012b60bc79aa9bc4b01R186

[zoff99]

i added a custom test in my branch to check that its actually working:
https://github.com/zoff99/c-toxcore/blob/7c891986872bada402fb940b005d31fbc2628872/custom_tests/0005_tox_local_lan_only_127001_test.c#L601-L615

[Green-Sky]

it works pretty well. nicely done. what would it do on localhost in ipv6 ?

No, and I'm afraid that the whole broadcast logic doesn't take ipv6 into account. See https://github.com/TokTok/c-toxcore/pull/2760/files#diff-907bc1ea87830b3899d91912c9b7c2eb62d713dadf887012b60bc79aa9bc4b01R186

ipv4 should be available everywhere, at least for localhost.
Also the ioctl interface we use here only supports ipv4...

@Green-Sky this PR works for me. are there any security implications of this? or other bad things in your view?

will have to check, code looks ok beside some styling nitpicks.