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Fix DNSRRTSIG length fields not auto-computing #5045

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Fix DNSRRTSIG length fields not auto-computing #5045

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4 changes: 2 additions & 2 deletions scapy/layers/dns.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1138,11 +1138,11 @@ class DNSRRTSIG(_DNSRRdummy):
DNSStrField("algo_name", "hmac-sha1"),
TimeSignedField("time_signed", 0),
ShortField("fudge", 0),
FieldLenField("mac_len", 20, fmt="!H", length_of="mac_data"), # noqa: E501
FieldLenField("mac_len", None, fmt="!H", length_of="mac_data"), # noqa: E501
StrLenField("mac_data", "", length_from=lambda pkt: pkt.mac_len), # noqa: E501
ShortField("original_id", 0),
ShortField("error", 0),
FieldLenField("other_len", 0, fmt="!H", length_of="other_data"), # noqa: E501
FieldLenField("other_len", None, fmt="!H", length_of="other_data"), # noqa: E501
StrLenField("other_data", "", length_from=lambda pkt: pkt.other_len) # noqa: E501
]

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13 changes: 11 additions & 2 deletions test/scapy/layers/dns_dnssec.uts
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Expand Up @@ -132,11 +132,20 @@ t.type == 16 and t.rdlen == 312 and t.rdata[0][:5] == b"Scapy" and t.rdata[1][-1

= DNSRRTSIG basic instantiation
t = DNSRRTSIG()
raw(t) == b"\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x14\x00\x00\x00\x00\x00\x00"
raw(t) == b"\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"

= DNSRRTSIG(), check parameters
t = DNSRRTSIG(rrname="SAMPLE-ALG.EXAMPLE.", time_signed=853804800, fudge=300)
raw(t) == b"\nSAMPLE-ALG\x07EXAMPLE\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x002\xe4\x07\x00\x01,\x00\x14\x00\x00\x00\x00\x00\x00"
raw(t) == b"\nSAMPLE-ALG\x07EXAMPLE\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x002\xe4\x07\x00\x01,\x00\x00\x00\x00\x00\x00\x00\x00"

= DNSRRTSIG mac_len/other_len auto-computed, round-trips
# Regression: mac_len/other_len defaulted to a hardcoded 20/0 instead of None,
# so FieldLenField never auto-computed and a non-empty MAC did not round-trip
# (the length prefix stayed 20, truncating the MAC to 20 bytes on re-dissect).
t = DNSRRTSIG(mac_data=b"M" * 32, other_data=b"O" * 6)
raw_t = raw(t)
p = DNSRRTSIG(raw_t)
p.mac_len == 32 and p.other_len == 6 and p.mac_data == b"M" * 32 and p.other_data == b"O" * 6 and raw(p) == raw_t

= TimeField methods

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