Wireshark-bugs: [Wireshark-bugs] [Bug 11634] ISIS Hello trigger Error/Malformed Packet length 16
Comment # 2
on bug 11634
from Jim Young
My analysis is the problem is ultimately with the following from
dissect_hello_checksum_clv():
<snip>
> checksum = tvb_get_ntohs(tvb, offset);
>
> /* the check_and_get_checksum() function needs to know how big
> * the packet is. we can either pass through the pdu-len through several layers
> * of dissectors and wrappers or extract the PDU length field from the PDU specific header
> * which is offseted 17 bytes in IIHs (relative to the beginning of the IS-IS packet) */
> pdu_length = tvb_get_ntohs(tvb, 17);
<snip>
In these two packets from the attached trace the actual pdu_length is at offset
9, not 17. The value of the ISIS HELLO's PDU Length was previously extracted
as 173 from offset 9, not offset 17. If we subtract 8 from the value extracted
from offset 9 (the real PDU length) we end up with a cooked pdu_length value
that passes the three tests at the top of check_and_get_checksum().
The quick fix to satisfy the attached trace is to replace the one line above in
dissect_hello_checksum_clv() with
pdu_length = tvb_get_ntohs(tvb, 9) - 8;
But I'm not certain this is truly the proper fix with investigating the details
of the protocol. I'm out of time to proceed until later this evening.
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