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DARWIN/MacOS patch to fix word alignment of netmask in sockaddr.sa_data and filter ARP entries #4895

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DARWIN/MacOS patch to fix word alignment of netmask in sockaddr.sa_data and filter ARP entries #4895

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a69e067
DARWIN/MacOS patch to fix:
jhogg Jan 5, 2026
20fc67a
Further review of DARWIN indicates the address structures (including …
jhogg Jan 6, 2026
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13 changes: 11 additions & 2 deletions scapy/arch/bpf/pfroute.py
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Original file line number Diff line number Diff line change
Expand Up @@ -406,7 +406,10 @@ class sockaddr(Packet):
XStrLenField(
"sa_data",
"",
length_from=lambda pkt: pkt.sa_len - 2 if pkt.sa_len >= 2 else 0,
# NOTE: Darwin right-justifies netmask on 4 byte word alignment
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@gpotter2 gpotter2 Jan 5, 2026

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Could you use a PadField instead? I will check, but most likely this padding isn't DARWIN-specific.

What's the padding. 8?

length_from=lambda pkt:
((pkt.sa_len+3)//4*4 - 2 if pkt.sa_len >= 2 else 0) if DARWIN else
(pkt.sa_len - 2 if pkt.sa_len >= 2 else 0),
),
lambda pkt: pkt.sa_family
not in [
Expand Down Expand Up @@ -1023,6 +1026,9 @@ def read_routes():
if DARWIN and flags.RTF_WASCLONED and msg.rtm_parentflags.RTF_PRCLONING:
# OSX needs filtering
continue
if DARWIN and flags.RTF_HOST and not flags.RTF_BROADCAST:
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@gpotter2 gpotter2 Jan 5, 2026

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Could you do a

if DARWIN:
    # OSX needs filtering
    if ....:
        continue
    # <...>
    if ....:
        continue

for clarity? Thanks.

# DARWIN includes the entire ARP table in the route response. Remove it.
continue
addrs = msg.rtm_addrs
net = 0
mask = 0xFFFFFFFF
Expand All @@ -1046,7 +1052,10 @@ def read_routes():
if nm.sa_family == socket.AF_INET:
mask = atol(nm.sin_addr)
elif nm.sa_family in [0x00, 0xFF]: # NetBSD
mask = struct.unpack("<I", nm.sa_data[:4].rjust(4, b"\x00"))[0]
if DARWIN:
mask = struct.unpack(">I", nm.sa_data[-4:].rjust(4, b"\x00"))[0]
else:
mask = struct.unpack("<I", nm.sa_data[:4].rjust(4, b"\x00"))[0]
else:
mask = int.from_bytes(nm.sa_data[:4], "big")
i += 1
Expand Down