Fix ClientSubnetv4 Calculates Address Length Incorrectly issue

Author: wenxuan70

scapy/layers/dns.py

@@ -546,7 +546,7 @@ class ClientSubnetv4(StrLenField):
 
     def getfield(self, pkt, s):
         # type: (Packet, bytes) -> Tuple[bytes, I]
-        sz = operator.floordiv(self.length_from(pkt), 8)
+        sz = operator.floordiv(self.length_from(pkt) + 7, 8)
         sz = min(sz, operator.floordiv(self.af_length, 8))
         return s[sz:], self.m2i(pkt, s[:sz])
 

test/scapy/layers/dns.uts

@@ -516,3 +516,7 @@ assert p == eval(p.command())
 pkt = DNSQR(qname=["domain1.com", "domain2.com"], qtype="A")
 for i in pkt:
     assert i.qname in [b"domain1.com.", b"domain2.com."]
+
+b = b'\x00\x08\x00\x0c\x00\x02=\x00+\xaf\xa3\xc4\xed\xeeW\xb8'
+p = EDNS0ClientSubnet(b)
+assert p.source_plen == 61 and p.address == '2baf:a3c4:edee:57b8::'