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WIP ICMP Extension Header bugfixes #4451

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WIP ICMP Extension Header bugfixes #4451

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29 changes: 16 additions & 13 deletions scapy/layers/inet.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1003,28 +1003,19 @@ def __init__(self):

def getfield(self, pkt, s):
# RFC4884 section 5.2 says if the ICMP packet length
# is >144 then ICMP extensions start at byte 137.
# is >144 then ICMP extensions start at least at byte 137.
if len(pkt.original) < 144:
return s, None
offset = 136 + len(s) - len(pkt.original)
if pkt.length == 0:
return s, None
offset = pkt.length * 4 # original padded datagram len
data = s[offset:]
# Validate checksum
if checksum(data) == data[3:5]:
return s, None # failed
# Dissect
return s[:offset], ICMPExtension_Header(data)

def addfield(self, pkt, s, val):
if val is None:
return s
data = bytes(val)
# Calc how much padding we need, not how much we deserve
pad = 136 - len(pkt.payload) - len(s)
if pad < 0:
warning("ICMPExtension_Header is after the 136th octet of ICMP.")
return data
return super(_ICMPExtensionField, self).addfield(pkt, s, b"\x00" * pad + data)


class _ICMPExtensionPadField(TrailerField):
def __init__(self):
Expand Down Expand Up @@ -1201,6 +1192,18 @@ class ICMP(Packet):
post_dissection = _ICMP_extpad_post_dissection

def post_build(self, p, pay):
if self.ext is not None and self.extpad in [None, b""]:
padding_index = pay.rindex(bytes(self.ext))
payload_len = len(pay[:padding_index])
padding_len = (4 - payload_len % 4) % 4
if payload_len + padding_len < 128:
padding_len = 128 - payload_len
padding = b"\x00" * padding_len
pay = pay[:padding_index] + padding + pay[padding_index:]
if self.ext is not None and self.length in [None, 0]:
ext_index = pay.rindex(bytes(self.ext))
length = len(pay[:ext_index]) // 4
p = p[:5] + chb(length) + p[6:]
p += pay
if self.chksum is None:
ck = checksum(p)
Expand Down