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Make sure that a valid TCP over DNS message can be decoded #2408

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Jan 20, 2020
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Make sure that a valid TCP over DNS message can be decoded #2408

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eee80a0
Make sure that a valid TCP over DNS message can be decoded
guedou Jan 14, 2020
792e0dd
Warn that the DNS message is too small
guedou Jan 17, 2020
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17 changes: 17 additions & 0 deletions scapy/layers/dns.py
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Original file line number Diff line number Diff line change
Expand Up @@ -437,6 +437,23 @@ def compress(self):
"""Return the compressed DNS packet (using `dns_compress()`"""
return dns_compress(self)

def pre_dissect(self, s):
"""
Check that a valid DNS over TCP message can be decoded
"""
if isinstance(self.underlayer, TCP):
# Compute the length of the DNS packet
dns_len = 0
if len(s) >= 2:
dns_len = struct.unpack("!H", s[:2])[0]
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# Check if the length is valid
if dns_len < 14 or len(s) < dns_len:
message = "Malformed DNS message: invalid length!"
warning(message)
raise Scapy_Exception(message)
return s


# https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-4
dnstypes = {
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9 changes: 9 additions & 0 deletions test/regression.uts
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Expand Up @@ -7886,6 +7886,15 @@ p = DNS(raw(DNS(id=1,ra=1,an=DNSRR(rrname='secdev', type='TXT', rdata=["sweet",
assert p[DNS].an.rdata == [b"sweet", b"celestia"]
assert raw(p) == b'\x00\x01\x01\x80\x00\x00\x00\x01\x00\x00\x00\x00\x06secdev\x00\x00\x10\x00\x01\x00\x00\x00\x01\x00\x0f\x05sweet\x08celestia'

= DNS - Malformed DNS over TCP message

try:
p = IP(raw(IP()/TCP()/DNS(length=24, qdcount=1)))
assert False
except Scapy_Exception as e:
assert str(e) == "Malformed DNS message: invalid length!"


= Layer binding

* Test DestMACField & DestIPField
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