Add support for HSRPv1 Advertise packets

[amlamarra]

Scapy was not properly recognizing HSRPv1 Advertisement (opcode 3) packets. The fields are different in these packets.

It doesn't help that the Advertise packets are not detailed in the RFC (2281).

This MR adds support for such packets.

Before:

>>> pkts = rdpcap("hsrp.pcap")
>>> pkts.show()
0000 Ether / IP / UDP 10.28.165.252:1985 > 224.0.0.2:1985 / HSRP / Padding
0001 Ether / IP / UDP 10.28.169.252:1985 > 224.0.0.2:1985 / Raw / Padding

After:

>>> pkts = rdpcap("hsrp.pcap")
>>> pkts.show()
0000 Ether / IP / UDP 10.28.165.252:1985 > 224.0.0.2:1985 / HSRP / Padding
0001 Ether / IP / UDP 10.28.169.252:1985 > 224.0.0.2:1985 / HSRP / Padding
>>> pkts[1].show()
###[ Ethernet ]###
  dst       = 01:00:5e:00:00:02
  src       = 00:12:7f:ba:1f:02
  type      = IPv4
###[ IP ]###
     version   = 4
     ihl       = 5
     tos       = 0xc0
     len       = 44
     id        = 0
     flags     =
     frag      = 0
     ttl       = 1
     proto     = udp
     chksum    = 0x24e7
     src       = 10.28.169.252
     dst       = 224.0.0.2
     \options   \
###[ UDP ]###
        sport     = 1985
        dport     = 1985
        len       = 24
        chksum    = 0xeb0d
###[ HSRP ]###
           version   = 0
           opcode    = Advertise
           adv_type  = HSRP interface state
           adv_length= 14
           adv_state = Passive
           adv_reserved1= 0
           adv_active_grps= 0
           adv_passive_grps= 1
           adv_reserved2 = 1862270976
###[ Padding ]###
              load      = b'\x00\x00'

[p-l-]

Rather than having many ConditionalFields, maybe you may try having two distinct layers (say, HSRP and HSRPAdvertise) and use a dispatch_hook() method?

[amlamarra]

Thanks, I had never used dispatch_hook() before.

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.27%. Comparing base (5009233) to head (e2ed46d).
⚠️ Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/hsrp.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4972      +/-   ##
==========================================
- Coverage   80.31%   80.27%   -0.04%     
==========================================
  Files         381      381              
  Lines       93630    93734     +104     
==========================================
+ Hits        75202    75249      +47     
- Misses      18428    18485      +57     

Files with missing lines	Coverage Δ
scapy/layers/hsrp.py	91.66% <94.73%> (+1.66%)	⬆️

... and 15 files with indirect coverage changes

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[gpotter2]

Thanks for the PR !