Initial commit: simtrace2-pysniff v0.1.0

Python-based SIMtrace2 sniffer — drop-in replacement for simtrace2-sniff.
Single dependency (pyusb), auto-reconnect, GSMTAP/PCAP output, library API.

Modules:
  protocol.py — SIMtrace2 USB sniffing protocol parser
  device.py   — USB discovery + SniffSession with exponential backoff reconnect
  gsmtap.py   — GSMTAP UDP sender for Wireshark
  pcap.py     — PCAP file writer (LINKTYPE_GSMTAP)
  dump.py     — hex dump formatters + file output
  __main__.py — CLI entry point

Startup script sniff-start.sh handles udev permission checks and shows
install instructions. udev rule uses TAG+=uaccess (systemd-logind ACL).

simtrace2_pysniff/gsmtap.py

@@ -0,0 +1,61 @@
+"""GSMTAP UDP sender for Wireshark integration.
+
+GSMTAP header format (big-endian, 16 bytes). Based on libosmocore's gsmtap.h.
+"""
+
+import struct
+import socket
+
+GSMTAP_VERSION = 0x02
+GSMTAP_HDR_LEN = 4  # in 32-bit words (16 bytes)
+GSMTAP_TYPE_SIM = 0x07
+
+GSMTAP_SIM_ATR = 0x01
+GSMTAP_SIM_APDU = 0x02
+
+GSMTAP_UDP_PORT = 4729
+
+_GSMTAP_HDR_FMT = '!BBBBHBBIBBBB'  # 16 bytes, big-endian
+_GSMTAP_HDR_SIZE = struct.calcsize(_GSMTAP_HDR_FMT)
+
+
+def build_gsmtap_packet(sub_type, data, slot_nr=0):
+    """Build a GSMTAP packet as bytes.
+
+    Returns *gsmtap_hdr* + *data* as a single bytes object.
+    """
+    hdr = struct.pack(
+        _GSMTAP_HDR_FMT,
+        GSMTAP_VERSION,    # version
+        GSMTAP_HDR_LEN,    # hdr_len (in 32-bit words)
+        GSMTAP_TYPE_SIM,   # type
+        0,                 # timeslot
+        0,                 # arfcn
+        0,                 # noise_db
+        0,                 # signal_db
+        0,                 # frame_number
+        sub_type,          # sub_type
+        0,                 # antenna_nr
+        slot_nr,           # sub_slot
+        0,                 # res
+    )
+    return hdr + data
+
+
+class GsmtapSender:
+    """Send ATR/APDU data as GSMTAP UDP packets."""
+
+    def __init__(self, host='127.0.0.1', port=GSMTAP_UDP_PORT):
+        self._addr = (host, port)
+        self._sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+
+    def send_atr(self, data, slot_nr=0):
+        packet = build_gsmtap_packet(GSMTAP_SIM_ATR, data, slot_nr)
+        self._sock.sendto(packet, self._addr)
+
+    def send_apdu(self, data, slot_nr=0):
+        packet = build_gsmtap_packet(GSMTAP_SIM_APDU, data, slot_nr)
+        self._sock.sendto(packet, self._addr)
+
+    def close(self):
+        self._sock.close()