Fallback to IPv6 default routes for network interface detection

[guedou]

Fixes #4312 and #4304.

To maintainers: shoud I add unit tests?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.52%. Comparing base (ac3d5bb) to head (478cb0c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4321      +/-   ##
==========================================
- Coverage   82.20%   81.52%   -0.68%     
==========================================
  Files         353      353              
  Lines       83529    83655     +126     
==========================================
- Hits        68662    68202     -460     
- Misses      14867    15453     +586     

Files	Coverage Δ
scapy/interfaces.py	96.80% <100.00%> (+0.20%)	⬆️

... and 22 files with indirect coverage changes

[gpotter2]

I would argue this needs tests considering it's already a regression.

Also you need to rebase ;p

[gpotter2]

This makes me kinda wonder why we deprecated conf.iface6.

[guedou]

I always found conf.iface6 confusing. It makes sense to use it to control how IPv6 packets are built, but fe80::/64 and multiple interfaces break its usefulness. However, for sending with high-level functions or SuperSocket, it adds complexity and non-standard behavior. When iface= is not specified by the user, shall Scapy use conf.iface6, conf.iface or both ? It gets even more complicated if we consider that sr*() can receive a list of IP and IPv6 packets that must be sent to different interfaces.

[oskar456]

Hey, I only noticed this after I opened my own PR #4380.
According to my tests, it is not going to work because conf.route6 is not populated by the time get_working_if() is called. This can be fixed by calling conf.ifaces.reload() at the end of route6.py.
Also I think it would be better to consider only default or generic enough routes. When someone has for instance an IPv6-only machine with a VPN that pushes route say 10.0.0.0/8, then the interface with IPv6 default route should be preferred over the VPN interface with /8 IPv4 route.