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Scapy packet, field, and layer patterns

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Use this when adding or modifying Scapy protocol layers, fields, payload binding logic, or UTScapy tests.

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Scapy packet, field, and layer patterns

Use this skill when working on protocol implementation in Scapy core/layers, especially around Packet, Field, fields_desc, layer binding, and regression tests via UTScapy.

Core model

  • A layer is a Packet subclass with a fields_desc list.
  • Field values flow through human/internal/machine conversions (h2i, i2h, i2m, m2i, any2i).
  • Building and dissecting are centralized in Packet (do_build, post_build, do_dissect, guess_payload_class, extract_padding).

Packet and field usage patterns

1) Define layers as Packet subclasses with fields_desc

  • Prefer explicit defaults in fields_desc.
  • Use field types that encode semantics (e.g. EnumField, FlagsField, PacketListField) instead of generic numeric/string fields when possible.
  • For optional/variant fields, use ConditionalField and MultipleTypeField.

Examples:

  • scapy/layers/inet.py (IP, TCP, UDP, ICMP)
  • scapy/layers/l2.py (Ether, ARP, GRE, Dot1Q)

2) Compute deferred values in post_build

  • Keep None defaults for values that must be computed from final bytes (checksums, lengths, header offsets).
  • Implement updates in post_build(self, p, pay) after payload is available.

Common examples:

  • IP.post_build: IHL/len/checksum
  • TCP.post_build: data offset + checksum
  • UDP.post_build: len + checksum
  • GRE.post_build: conditional checksum

3) Split payload/padding with extract_padding when length is explicit

  • If a layer encodes payload length, override extract_padding.
  • Return (payload, padding) correctly to keep dissection aligned.

Examples:

  • IP.extract_padding
  • UDP.extract_padding
  • Dot3.extract_padding

4) Payload dispatch and layer binding (guess_payload_class, bind_*)

  • Packet.guess_payload_class(payload) selects the next layer during dissection.
    • Default behavior iterates payload_guess entries and matches field constraints.
    • payload_guess is populated by bind_bottom_up() (and therefore by bind_layers()).
    • If no match applies, fallback is Packet.default_payload_class() (conf.raw_layer).
  • Override guess_payload_class only when dispatch depends on dynamic logic that cannot be expressed as simple field equality.
  • Base-class dispatch pattern: a base layer class can decode into a concrete sibling/subclass before normal dissection.
    • Commonly implemented with dispatch_hook(cls, _pkt, ...).
    • Example: Ether.dispatch_hook can return Dot3, and Dot3.dispatch_hook can return Ether.
  • Binding APIs:
    • bind_bottom_up(lower, upper, ...): dissection-time binding only.
    • bind_top_down(lower, upper, ...): build-time default field propagation only.
    • bind_layers(lower, upper, ...): convenience helper that applies both directions.

Examples:

  • scapy/packet.py: guess_payload_class, bind_bottom_up, bind_top_down, bind_layers
  • scapy/layers/l2.py: Ether.dispatch_hook / Dot3.dispatch_hook
  • scapy/layers/inet.py: protocol-specific guess_payload_class overrides

5) Request/response matching and stream reassembly hooks

  • Implement hashret() when a protocol needs stable request/response correlation keys.
    • Base Packet.hashret() delegates to payload; many layers override it (e.g. IP/TCP/UDP) to include flow context.
  • Implement answers(other) to express protocol-level "is response to" logic.
    • Base behavior matches same class then delegates to payload.
    • Typical checks include type/code pairs, id/seq tuples, and src/dst or sport/dport relationships.
  • For stream-aware sniffing:
    • IPSession performs on-the-fly IPv4 defragmentation.
    • TCPSession reconstructs TCP byte streams and can call layer tcp_reassemble(data, metadata, session) when implemented.
    • tcp_reassemble should return a packet when enough bytes are present, else None and keep state in metadata / session.

References:

  • scapy/packet.py (hashret, answers)
  • scapy/layers/inet.py (IP/TCP/UDP/ICMP overrides)
  • scapy/sessions.py (IPSession, TCPSession, streamcls)

6) Complete scapy.fields field-type catalog (and intended use)

This list covers field types defined in scapy/fields.py that are used when authoring packet layouts.

  • Base and wrappers (core composition/control)
    • Field: base class for conversion and build/dissect behavior.
    • Emph: display emphasis wrapper.
    • MayEnd: allows legal early stop while dissecting optional tail fields.
    • ActionField: triggers side-effect callbacks during value assignment/build.
    • ConditionalField: include/exclude field based on runtime predicate.
    • MultipleTypeField: pick one of several field definitions from packet context.
    • PadField, ReversePadField: alignment/padding wrappers.
    • TrailerField, FCSField: model trailer fields appended after main payload.
  • Address/network identity fields
    • DestField, MACField, LEMACField: MAC/address-like values.
    • IPField, SourceIPField: IPv4 values (including context-derived source behavior).
    • IP6Field, SourceIP6Field, DestIP6Field: IPv6 values.
    • IPPrefixField, IP6PrefixField: address + prefix-length encoding.
    • OUIField: 24-bit organization identifiers.
    • UUIDField, UUIDEnumField: UUID values (optionally enumerated).
  • Integer scalar fields
    • Byte-sized: ByteField, XByteField, OByteField, SignedByteField, YesNoByteField
    • 3-byte and variable-width: ThreeBytesField, X3BytesField, LEThreeBytesField, XLE3BytesField, NBytesField, XNBytesField
    • 16-bit: ShortField, SignedShortField, LEShortField, LESignedShortField, XShortField, XLEShortField
    • 32-bit: IntField, SignedIntField, LEIntField, LESignedIntField, XIntField, XLEIntField
    • 64-bit: LongField, SignedLongField, LELongField, LESignedLongField, XLongField, XLELongField
  • Floating/scaled numeric fields
    • IEEEFloatField, IEEEDoubleField: IEEE-754 float/double.
    • BCDFloatField: BCD-encoded decimal values.
    • FixedPointField: fixed-point values stored in bitfields.
    • ScalingField, BitScalingField: physical-unit scaling (offset/resolution) wrappers.
  • String/bytes fields
    • Generic strings: StrField, StrFieldUtf16
    • Enumerated strings: StrEnumField
    • Fixed-length strings: StrFixedLenField, StrFixedLenFieldUtf16, StrFixedLenEnumField, NetBIOSNameField
    • Length-coupled strings: StrLenField, StrLenFieldUtf16, StrLenEnumField, BoundStrLenField
    • Hex-rendered byte strings: XStrField, XStrLenField, XStrFixedLenField, XLEStrLenField
    • Terminated strings: StrNullField, StrNullFieldUtf16, StrStopField
  • Packet/container and length-coupling fields
    • Nested packets: PacketField, PacketLenField, PacketListField
    • Generic repeated values: FieldListField
    • Length/count references: FieldLenField, LEFieldLenField, LenField
  • Bit and varint-like fields
    • BitField, BitLenField, BitFieldLenField, XBitField
    • BitExtendedField, LSBExtendedField, MSBExtendedField
  • Enum-capable fields
    • Generic and char: EnumField, CharEnumField
    • Bit enums: BitEnumField, BitLenEnumField
    • Width/endianness-specific enums: ShortEnumField, LEShortEnumField, LongEnumField, LELongEnumField, ByteEnumField, XByteEnumField, IntEnumField, SignedIntEnumField, LEIntEnumField, XLEIntEnumField, XShortEnumField, LE3BytesEnumField, XLE3BytesEnumField
    • Contextual enums: MultiEnumField, BitMultiEnumField
    • Enum-key variants: ByteEnumKeysField, ShortEnumKeysField, IntEnumKeysField
  • Flags fields
    • FlagsField: bitmask rendered as named flags.
    • MultiFlagsField: multi-set flag representation across grouped masks.
  • Time fields
    • UTCTimeField: timestamps with human-readable UTC conversion support.
    • SecondsIntField: integer-seconds timestamp fields.
  • Raw override
    • RawVal (from scapy.packet) is used with fields when you intentionally bypass normal conversion.

Examples:

  • scapy/layers/inet.py: IP.options (PacketListField), ICMP conditional/enum usage
  • scapy/layers/l2.py: ARP MultipleTypeField usage, GRE conditional/checksum fields

7) Additional protocol implementation patterns (from Scapy implementation slides)

  • Layer navigation while debugging/building packets
    • pkt[Layer] selects a specific layer instance.
    • .underlayer accesses the previous (encapsulating) layer.
    • .payload accesses the next layer.
    • Useful to validate binding/dispatch behavior in quick REPL checks.
  • Field-family naming shortcuts
    • X* variants: hex-oriented display.
    • LE* variants: little-endian integer encoding.
    • Signed* variants: signed integer representation.
    • *Enum* variants: symbolic names via mapping tables.
  • PacketField / PacketListField dynamic decoding
    • PacketField decodes one embedded packet object.
    • PacketListField decodes repeated embedded packets; dispatcher may be a class or a function returning a class.
    • Common with TLV/IE lists where element type depends on runtime bytes.
  • Post-dissection mutation hooks
    • post_dissect(self, s) can adjust decoded state after full layer parsing (e.g., decrypting payload-dependent content).
    • Use only when mutation is required after field extraction.
  • Explicit payload vs. extra-bytes control
    • extract_padding can intentionally discard/redirect trailing bytes when they are not protocol padding. This is important when parsing variable-length lists of embedded packets where trailing bytes should not be interpreted as padding.
  • Custom field implementation workflow
    • Model field states explicitly:
      • internal (i): Scapy runtime value
      • machine (m): serialized bytes representation
      • human (h): display representation
    • Override conversion/encoding points as needed:
      • i2h, i2m, m2i for state conversion
      • addfield / getfield for custom serialization and parsing behavior

References:

  • scapy/contrib/mpls.py (guess_payload_class dispatch pattern)
  • scapy/layers/inet6.py (IPv46.dispatch_hook)
  • scapy/contrib/automotive/doip.py (tcp_reassemble)
  • scapy/layers/dot11.py (Dot11WEP.post_dissect)
  • scapy/layers/tftp.py (TFTP_Option.extract_padding)

UTScapy integration patterns

Use UTScapy for regression coverage of layer behavior.

Campaign structure

  • Campaign syntax in test files:
    • % campaign
    • + test set
    • = unit test
    • ~ keywords
    • * comments
  • The last Python expression in a unit test determines pass/fail truthiness.

References:

  • doc/scapy/development.rst (Testing with UTScapy)
  • scapy/tools/UTscapy.py (parse_campaign_file, campaign execution/filtering)

Useful UTScapy CLI patterns

  • Run one or more campaigns: -t
  • Include/exclude keyword groups: -k / -K
  • Select tests by number: -n
  • Load .utsc JSON config: -c
  • Output formats: -f text|ansi|HTML|LaTeX|xUnit|live
  • Generate docs from campaign comments/tests: -R
  • Non-root mode keyword filtering: -N

Reference:

  • scapy/tools/UTscapy.py (usage(), main())

Typical test workflow

  1. Add/modify protocol layer fields and binding logic.
  2. Add/adjust UTScapy tests in test/ configs/campaign files with meaningful keywords.
  3. Run through existing wrappers (./test/run_tests or tox environments using scapy.tools.UTscapy).
  4. Keep tests focused on:
    • build/dissect roundtrips
    • computed fields (len/checksum/options)
    • payload dispatch and edge-case fallback

Quick references

  • Core packet behavior: scapy/packet.py
  • Field internals: scapy/fields.py
  • Common layer patterns: scapy/layers/inet.py, scapy/layers/l2.py
  • Test runner internals: scapy/tools/UTscapy.py
  • Design guidance: doc/scapy/build_dissect.rst