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fix _packet_subnet calculation logic #4943

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fix _packet_subnet calculation logic #4943

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72d7f9d
fix _packet_subnet calculation logic
wenxuan70 Mar 13, 2026
923b03d
fix(dns): default source_plen to /24 for IPv4 and /48 for IPv6 when N…
wenxuan70 Mar 20, 2026
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29 changes: 15 additions & 14 deletions scapy/layers/dns.py
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Original file line number Diff line number Diff line change
Expand Up @@ -558,34 +558,35 @@ def m2i(self, pkt, x):
x = x[: operator.floordiv(self.af_length, 8)]
return inet_ntop(self.af_familly, x)

def _pack_subnet(self, subnet):
# type: (bytes) -> bytes
def _pack_subnet(self, subnet, pkt):
# type: (bytes, Packet) -> bytes
packed_subnet = inet_pton(self.af_familly, plain_str(subnet))
for i in list(range(operator.floordiv(self.af_length, 8)))[::-1]:
if packed_subnet[i] != 0:
i += 1
break
return packed_subnet[:i]
sz = operator.floordiv(self.length_from(pkt) + 7, 8)
return packed_subnet[:sz]

def i2m(self, pkt, x):
# type: (Optional[Packet], Optional[Union[str, Net]]) -> bytes
if x is None:
return self.af_default
try:
return self._pack_subnet(x)
return self._pack_subnet(x, pkt)
except (OSError, socket.error):
pkt.family = 2
return ClientSubnetv6("", "")._pack_subnet(x)
return ClientSubnetv6("", "", lambda p: p.source_plen or 32). \
_pack_subnet(x, pkt)

def i2len(self, pkt, x):
# type: (Packet, Any) -> int
if x is None:
return 1
try:
return len(self._pack_subnet(x))
return len(self._pack_subnet(x, pkt))
except (OSError, socket.error):
pkt.family = 2
return len(ClientSubnetv6("", "")._pack_subnet(x))
return len(
ClientSubnetv6(
"", "", lambda p: p.source_plen or 32
)._pack_subnet(x, pkt))


class ClientSubnetv6(ClientSubnetv4):
Expand All @@ -607,13 +608,13 @@ class EDNS0ClientSubnet(_EDNS0Dummy):
ByteField("scope_plen", 0),
MultipleTypeField(
[(ClientSubnetv4("address", "192.168.0.0",
length_from=lambda p: p.source_plen),
length_from=lambda p: p.source_plen or 16),
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@wenxuan70 wenxuan70 Mar 20, 2026

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The default value of 16 here is to maintain backward compatibility: when scope_plen is None, the original _pack_subnet method computes 16 bits from the default address 192.168.0.0.

lambda pkt: pkt.family == 1),
(ClientSubnetv6("address", "2001:db8::",
length_from=lambda p: p.source_plen),
length_from=lambda p: p.source_plen or 32),
lambda pkt: pkt.family == 2)],
ClientSubnetv4("address", "192.168.0.0",
length_from=lambda p: p.source_plen))]
length_from=lambda p: p.source_plen or 32))]


class EDNS0COOKIE(_EDNS0Dummy):
Expand Down
3 changes: 3 additions & 0 deletions test/scapy/layers/dns.uts
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Original file line number Diff line number Diff line change
Expand Up @@ -520,3 +520,6 @@ for i in pkt:
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::'

b = EDNS0ClientSubnet(source_plen=23, address='101.132.0.0')
assert bytes(b) == b'\x00\x08\x00\x07\x00\x01\x17\x00e\x84\x00'
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