dns: fix off-by-four in EDNS0 OWNER Option (#4955)

The lengths taken from
https://datatracker.ietf.org/doc/html/draft-cheshire-edns0-owner-option-01
are off by four because they include the length of the two-byte EDNS0
option code and the length of the two-byte EDNS0 option length but
according to https://datatracker.ietf.org/doc/html/rfc6891#section-6.1.2
the length should include the size of data only so all the lengths
should be decreased by 4 to parse the OWNER option containing the wakeup
MAC correctly.

Without this patch the wakeup MAC gets cut off:
```
>>> EDNS0OWN(b'\x00\x04\x00\x0e\x00\x9b\x11"3DUffUD3"\x11')
<EDNS0OWN  optcode=Owner optlen=14 v=0 s=155 primary_mac=11:22:33:44:55:66 |<Padding  load=b'fUD3"\x11' |>>
```
With this patch it's included as expected
```
<EDNS0OWN  optcode=Owner optlen=14 v=0 s=155 primary_mac=11:22:33:44:55:66 wakeup_mac=66:55:44:33:22:11 |>
```

Author: evverx

scapy/layers/dns.py

@@ -494,11 +494,11 @@ class EDNS0OWN(_EDNS0Dummy):
                    MACField("primary_mac", "00:00:00:00:00:00"),
                    ConditionalField(
                        MACField("wakeup_mac", "00:00:00:00:00:00"),
-                       lambda pkt: (pkt.optlen or 0) >= 18),
+                       lambda pkt: (pkt.optlen or 0) >= 14),
                    ConditionalField(
                        StrLenField("password", "",
-                                   length_from=lambda pkt: pkt.optlen - 18),
-                       lambda pkt: (pkt.optlen or 0) >= 22)]
+                                   length_from=lambda pkt: pkt.optlen - 14),
+                       lambda pkt: (pkt.optlen or 0) >= 18)]
 
     def post_build(self, pkt, pay):
         pkt += pay

test/scapy/layers/dns_edns0.uts

@@ -212,3 +212,32 @@ for opt_class in EDNS0OPT_DISPATCHER.values():
     p = DNSRROPT(raw(DNSRROPT(rdata=[EDNS0TLV(), opt_class(), opt_class()])))
     assert len(p.rdata) == 3
     assert all(Raw not in opt for opt in p.rdata)
+
+
++ EDNS0 - Owner
+
+= Dissection
+
+p = EDNS0OWN(b'\x00\x04\x00\x08\x00\x9b\x11"3DUf')
+assert p.optcode == 4
+assert p.optlen == 8
+assert p.v == 0
+assert p.s == 155
+assert p.primary_mac == '11:22:33:44:55:66'
+
+p = EDNS0OWN(b'\x00\x04\x00\x0e\x00\x9b\x11"3DUffUD3"\x11')
+assert p.optcode == 4
+assert p.optlen == 14
+assert p.v == 0
+assert p.s == 155
+assert p.primary_mac == '11:22:33:44:55:66'
+assert p.wakeup_mac == '66:55:44:33:22:11'
+
+p = EDNS0OWN(b'\x00\x04\x00\x12\x00\x9b\x11"3DUffUD3"\x11abcd')
+assert p.optcode == 4
+assert p.optlen == 18
+assert p.v == 0
+assert p.s == 155
+assert p.primary_mac == '11:22:33:44:55:66'
+assert p.wakeup_mac == '66:55:44:33:22:11'
+assert p.password == b'abcd'