catarrh/simtrace-analyser
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Surface GSMTAP BAD_FCS desync flag end-to-end

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sigrok-iso7816-stream sets GSMTAP_FLAG_BAD_FCS (0x01) in the GSMTAP
header `res` byte on desynced TPDUs (mis-framed exchanges kept for
forensics). Propagate that flag through the whole pipeline so such
packets are explicitly surfaced instead of decoding as garbage:

- gsmtap.py: GSMTAP_FLAG_BAD_FCS const; GsmtapReceiver.read_packet()
  now returns (sub_type, data, flags) from the res byte.
- server/capture.py: GsmtapListener forwards the flags to the DB.
- pcap.py: _parse_gsmtap_hdr/_scan_gsmtap/extract_gsmtap and
  parse_pcap/parse_pcapng now yield a (ts, type, payload, flags)
  tuple, so the flag survives a PCAP/PCAPNG round-trip.
- server/server.py: PCAP import stores the flags.
- server/decode.py: gsmtap_flag_names() maps bit 0x01 to
  'bad FCS (desynced)'; merged into decoded errors for tpdu/atr/pps
  and rst/vcc, so the frontend ⚠ badge + Errors row flag the entry.

User-visible: desynced TPDUs show a ⚠ "bad FCS (desynced)" marker
in the trace and detail views. Tests: +4 (decode BAD_FCS markers,
PCAP flag round-trip); existing tests updated for the new tuple
arities. Version 1.22.0, SW cache simtrace-analyser-v32.
This commit is contained in:
catarrh committed 2026-08-26 18:08:25 +03:00
1 parent 7eaf7e9b53
commit 66094a0114
13 files changed
+90 -35
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simtrace2_pysniff/__init__.py
+1 -1
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@@ -6,7 +6,7 @@ from .gsmtap import GsmtapSender, GsmtapReceiver
from .pcap import PcapWriter
from .dump import FileDumper, format_message
__version__ = '1.21.0'
__version__ = '1.22.0'
__all__ = [
'SniffSession',
'DeviceDisconnected',
simtrace2_pysniff/gsmtap.py
+11 -5
View File
@@ -19,6 +19,10 @@ GSMTAP_SIM_PPS_RSP = 0x03
GSMTAP_SIM_RST_EVENT = 0x10
GSMTAP_SIM_VCC_EVENT = 0x11
# Per-packet flags carried in the GSMTAP header `res` byte (libosmocore
# gsmtap.h). sigrok-iso7816-stream sets BAD_FCS on a desynced TPDU.
GSMTAP_FLAG_BAD_FCS = 0x01
GSMTAP_UDP_PORT = 4729
_GSMTAP_HDR_FMT = '!BBBBHBBIBBBB' # 16 bytes, big-endian
@@ -79,27 +83,29 @@ class GsmtapReceiver:
def read_packet(self):
"""Block until a GSMTAP-SIM packet arrives.
Returns ``(sub_type, data)`` on success or ``(None, None)``
on timeout. *sub_type* is ``GSMTAP_SIM_ATR`` or ``GSMTAP_SIM_APDU``.
Returns ``(sub_type, data, flags)`` on success or ``(None, None, 0)``
on timeout. *sub_type* is ``GSMTAP_SIM_ATR``/``GSMTAP_SIM_APDU`` or
a custom sub-type; *flags* are the per-packet bits from the GSMTAP
header ``res`` byte (see ``GSMTAP_FLAG_*``).
Raises ``ValueError`` for non-SIM or unknown GSMTAP packets.
"""
try:
packet, addr = self._sock.recvfrom(65536)
except (socket.timeout, OSError):
return None, None
return None, None, 0
if len(packet) < _GSMTAP_HDR_SIZE:
raise ValueError(f'GSMTAP packet too short: {len(packet)} bytes')
(version, hdr_len, pkt_type, _timeslot, _arfcn, _noise, _signal,
_frame, sub_type, _antenna, sub_slot, _res) = \
_frame, sub_type, _antenna, sub_slot, res) = \
struct.unpack(_GSMTAP_HDR_FMT, packet[:_GSMTAP_HDR_SIZE])
if pkt_type != GSMTAP_TYPE_SIM:
raise ValueError(f'Not a GSMTAP SIM packet (type={pkt_type})')
data = packet[_GSMTAP_HDR_SIZE:]
return sub_type, data
return sub_type, data, res
def close(self):
self._sock.close()
simtrace2_pysniff/pcap.py
+12 -12
View File
@@ -224,22 +224,22 @@ def _frame_to_udp_payload(frame, linktype):
def _parse_gsmtap_hdr(data):
"""Parse a GSMTAP header; return (sub_type, payload) or None if not SIM."""
"""Parse a GSMTAP header; return (sub_type, payload, flags) or None."""
if len(data) < _GSMTAP_HDR_SIZE:
return None
(version, hdr_len, pkt_type, _ts, _arfcn, _noise, _signal,
_frame, sub_type, _ant, _slot, _res) = struct.unpack(
_frame, sub_type, _ant, _slot, res) = struct.unpack(
_GSMTAP_HDR_FMT, data[:_GSMTAP_HDR_SIZE])
if version != 0x02 or pkt_type != GSMTAP_TYPE_SIM:
return None
hdr_size = hdr_len * 4
if hdr_size < _GSMTAP_HDR_SIZE or len(data) < hdr_size:
return None
return sub_type, data[hdr_size:]
return sub_type, data[hdr_size:], res
def _scan_gsmtap(frame):
"""Fallback: scan a frame for a GSMTAP-SIM header, return (sub_type, payload)."""
"""Fallback: scan a frame for a GSMTAP-SIM header, return (sub_type, payload, flags)."""
for off in range(0, len(frame) - _GSMTAP_HDR_SIZE + 1):
parsed = _parse_gsmtap_hdr(frame[off:])
if parsed is not None:
@@ -250,8 +250,8 @@ def _scan_gsmtap(frame):
def extract_gsmtap(frame, linktype):
"""Extract a GSMTAP-SIM message from a link-layer frame.
Returns ``(sub_type, payload)`` or ``None`` if the frame carries no
GSMTAP-SIM data.
Returns ``(sub_type, payload, flags)`` or ``None`` if the frame carries
no GSMTAP-SIM data.
"""
udp = _frame_to_udp_payload(frame, linktype)
if udp:
@@ -304,10 +304,10 @@ def parse_pcap(data):
parsed = extract_gsmtap(frame, linktype)
if parsed is None:
continue
sub_type, payload = parsed
sub_type, payload, flags = parsed
msg_type = _gsmtap_to_msg(sub_type, payload)
if msg_type is not None:
yield ts, msg_type, payload
yield ts, msg_type, payload, flags
def parse_pcapng(data):
@@ -354,10 +354,10 @@ def parse_pcapng(data):
ts = _scale_ts(ts_hi, ts_lo, tsresols.get(iface))
parsed = extract_gsmtap(frame, linktypes.get(iface))
if parsed is not None:
sub_type, payload = parsed
sub_type, payload, flags = parsed
msg_type = _gsmtap_to_msg(sub_type, payload)
if msg_type is not None:
yield ts, msg_type, payload
yield ts, msg_type, payload, flags
elif block_type == _PNG_SPB and len(body) >= 4:
cap_len, = struct.unpack(endian + 'I', body[:4])
if cap_len <= len(body) - 4:
@@ -365,10 +365,10 @@ def parse_pcapng(data):
# SPB has no timestamp and no interface; use file-order ts.
parsed = _scan_gsmtap(frame)
if parsed is not None:
sub_type, payload = parsed
sub_type, payload, flags = parsed
msg_type = _gsmtap_to_msg(sub_type, payload)
if msg_type is not None:
yield float(spb_ts), msg_type, payload
yield float(spb_ts), msg_type, payload, flags
spb_ts += 1
simtrace2_pysniff/server/capture.py
+2 -2
View File
@@ -34,10 +34,10 @@ class GsmtapListener:
def iter_messages(self):
while self._running:
sub_type, data = self._receiver.read_packet()
sub_type, data, flags = self._receiver.read_packet()
if sub_type is None:
continue
yield gsmtap_msg_type(sub_type), data, 0
yield gsmtap_msg_type(sub_type), data, flags
class DirectSniffer:
simtrace2_pysniff/server/decode.py
+21 -4
View File
@@ -4030,6 +4030,19 @@ def decode_data_flags(flags):
return names or None
def gsmtap_flag_names(flags):
"""Decode the GSMTAP header ``res`` per-packet flag bits into a list of
human-readable names (or None).
GSMTAP_FLAG_BAD_FCS is set by the sigrok-iso7816-stream decoder on a
desynced TPDU (mis-framed exchange / implausible status word).
"""
names = []
if flags & 0x01:
names.append('bad FCS (desynced)')
return names or None
def decode_fidi(raw_data):
"""Decode a SIMtrace2 sniff_fidi payload (1-byte Fi/Di)."""
if len(raw_data) < 1:
@@ -4274,27 +4287,31 @@ def decode_sniff_msg(raw_data, msg_type, flags=0, prev=None):
if msg_type == 'tpdu' and raw_data:
result = decode_message(raw_data, prev=prev)
if result is not None:
errs = decode_data_flags(flags)
errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
if errs:
result['errors'] = errs
return result
if msg_type == 'change':
return decode_change(flags)
if msg_type in ('rst', 'vcc'):
return decode_line_event(raw_data, msg_type)
result = decode_line_event(raw_data, msg_type)
errs = gsmtap_flag_names(flags)
if errs:
result['errors'] = errs
return result
if msg_type == 'fidi' and raw_data:
return decode_fidi(raw_data)
if msg_type == 'atr':
result = _decode_atr(raw_data)
if result is not None:
errs = decode_data_flags(flags)
errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
if errs:
result['errors'] = errs
return result
if msg_type == 'pps':
result = _decode_pps(raw_data)
if result is not None:
errs = decode_data_flags(flags)
errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
if errs:
result['errors'] = errs
return result
simtrace2_pysniff/server/server.py
+2 -2
View File
@@ -331,9 +331,9 @@ class RequestHandler(BaseHTTPRequestHandler):
first_ts = packets[0][0]
last_ts = packets[-1][0]
rows = []
for ts, msg_type, payload in packets:
for ts, msg_type, payload, flags in packets:
elapsed = round(ts - first_ts, 6) if ts is not None else 0.0
rows.append((max(0.0, elapsed), msg_type, payload, 0))
rows.append((max(0.0, elapsed), msg_type, payload, flags))
self.db.insert_messages(session_id, rows)
# Use the packet timestamps as the session window when they are real
# (classic pcap / pcapng EPB). pcapng SPB-only files have synthetic