ipv4.py's parse() grabs the IP headers and passes the rest of its input on to
some next layer (which may be TCP, UDP, ICMP, or just raw data, depending).
Sounds good, but… there are times when the rest of its input isn't actually all
IP payload. Specifically, when the ethernet frame has been padded to meet the
minimum frame size. Then when tcp.py parses itself, for example, it thinks the
padding is TCP payload, which is really annoying.
Since IP knows how much of the rest of the data it's actually encapsulating, it
seems like the right thing to do is just to not pass the ethernet frame padding
forward and throw it away instead. I can't think of any reason you'd want to
do otherwise. Here's a patch:
--- /home/user/nox3/src/nox/lib/packet/ipv4.py 2010-03-31 23:49:59.000000000
-0700
+++ ipv4.py 2010-04-13 19:29:50.000000000 -0700
@@ -137,19 +137,22 @@
# packet
self.parsed = True
+ length = self.iplen
+ if length > dlen: length = dlen # Clamp to what we've got
+
if self.protocol == ipv4.UDP_PROTOCOL:
- self.next = udp(arr=self.arr[self.hl*4:], prev=self)
+ self.next = udp(arr=self.arr[self.hl*4:length], prev=self)
elif self.protocol == ipv4.TCP_PROTOCOL:
- self.next = tcp(arr=self.arr[self.hl*4:], prev=self)
+ self.next = tcp(arr=self.arr[self.hl*4:length], prev=self)
elif self.protocol == ipv4.ICMP_PROTOCOL:
- self.next = icmp(arr=self.arr[self.hl*4:], prev=self)
+ self.next = icmp(arr=self.arr[self.hl*4:length], prev=self)
elif dlen < self.iplen:
self.msg('(ip parse) warning IP packet data shorter than IP len:
%u < %u' % (dlen, self.iplen))
else:
- self.next = self.arr[self.hl*4:].tostring()
+ self.next = self.arr[self.hl*4:length].tostring()
if isinstance(self.next, packet_base) and not self.next.parsed:
- self.next = self.arr[self.hl*4:].tostring()
+ self.next = self.arr[self.hl*4:length].tostring()
def checksum(self):
data = struct.pack('!BBHHHBBHII', (self.v << 4) + self.hl, self.tos, \
-- Murphy
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