Support for Scope Identifiers in IP addresses for multicast/link-local support

[gpotter2]

This PR does a few changes to how Scapy handles IP addresses, in order to properly support link-layer / multicast scopes on L3. In particular:

• add implicit multicast routes in Linux when the interface supports multicast (note: on Windows, those routes are explicitly provided by the OS so no change was required)
• add support for a RFC6874-like scope identifier. One can now do:

pkt1 = IPv6(dst="ff02::fb%eth0")
pkt2 = IPv6(dst="ff02::fb%eth1")
assert pkt1.src != pkt2.src
# etc.
pkt3 = IP(dst="224.0.0.1%eth0")

The interface is then taken into account when calling route(). This enables support for multicast addresses using L3-functions (send, sr, etc.) as it also automatically choses the correct source addresses.

• The above behavior is allowed through the introduction of a hidden ScopedIP (I'm open to some other name) class/function that returns a str that has a secret scope attribute (in order to remember the interface)
• Deprecate the iface attribute of L3 functions. It was already mostly doing nothing, as L3sockets already implement chosing the proper interface when sending. This was confusing.
• remove the second attribute of SourceIPField and SourceIP6Field. The code path was generally a duplicate since those packets already implement a route() function.

Demo (see doc)

conf.checkIPaddr = False
sr(IPv6(dst="ff02::2%eth0")/ICMPv6EchoRequest(), multi=True)
sr(IPv6(dst="ff02::2%eth1")/ICMPv6EchoRequest(), multi=True)

Other changes

• extend Add DNS-SD command #4481 to support scoped identifiers

[codecov]

Codecov Report

Attention: Patch coverage is 81.56028% with 26 lines in your changes missing coverage. Please review.

Project coverage is 81.65%. Comparing base (18b3d6c) to head (3b50178).
Report is 11 commits behind head on master.

Files	Patch %	Lines
scapy/sendrecv.py	61.53%	10 Missing ⚠️
scapy/base_classes.py	82.92%	7 Missing ⚠️
scapy/fields.py	88.46%	3 Missing ⚠️
scapy/layers/dns.py	40.00%	3 Missing ⚠️
scapy/route.py	77.77%	2 Missing ⚠️
scapy/arch/linux/rtnetlink.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4461      +/-   ##
==========================================
+ Coverage   81.42%   81.65%   +0.22%     
==========================================
  Files         355      355              
  Lines       84815    84880      +65     
==========================================
+ Hits        69062    69307     +245     
+ Misses      15753    15573     -180     

Files	Coverage Δ
scapy/layers/hsrp.py	90.00% <ø> (ø)
scapy/layers/inet.py	71.84% <100.00%> (+0.05%)	⬆️
scapy/layers/inet6.py	88.54% <100.00%> (+0.07%)	⬆️
scapy/layers/l2.py	84.95% <100.00%> (+0.48%)	⬆️
scapy/main.py	70.37% <ø> (ø)
scapy/route6.py	88.39% <100.00%> (+2.20%)	⬆️
scapy/arch/linux/rtnetlink.py	93.39% <92.85%> (+0.40%)	⬆️
scapy/route.py	90.37% <77.77%> (+0.14%)	⬆️
scapy/fields.py	92.75% <88.46%> (+0.13%)	⬆️
scapy/layers/dns.py	84.20% <40.00%> (+0.20%)	⬆️
... and 2 more

... and 17 files with indirect coverage changes

[gpotter2]

/packit build

[gpotter2]

I'm gonna go ahead and merge this. It worked fine on the BSDs and should avoids issues like #4506