Class StrFixedLenField

scapy/fields.py:1860–1905

class StrFixedLenField(StrField):
    __slots__ = ["length_from"]

    def __init__(
            self,
            name,  # type: str
            default,  # type: Optional[bytes]
            length=None,  # type: Optional[int]
            length_from=None,  # type: Optional[Callable[[Packet], int]]
    ):
        # type: (...) -> None
        super(StrFixedLenField, self).__init__(name, default)
        self.length_from = length_from or (lambda x: 0)
        if length is not None:
            self.sz = length
            self.length_from = lambda x, length=length: length  # type: ignore

    def i2repr(self,
               pkt,  # type: Optional[Packet]
               v,  # type: bytes
               ):
        # type: (...) -> str
        if isinstance(v, bytes) and not conf.debug_strfixedlenfield:
            v = v.rstrip(b"\0")
        return super(StrFixedLenField, self).i2repr(pkt, v)

    def getfield(self, pkt, s):
        # type: (Packet, bytes) -> Tuple[bytes, bytes]
        len_pkt = self.length_from(pkt)
        if len_pkt == 0:
            return s, b""
        return s[len_pkt:], self.m2i(pkt, s[:len_pkt])

    def addfield(self, pkt, s, val):
        # type: (Packet, bytes, Optional[bytes]) -> bytes
        len_pkt = self.length_from(pkt)
        if len_pkt is None:
            return s + self.i2m(pkt, val)
        return s + struct.pack("%is" % len_pkt, self.i2m(pkt, val))

    def randval(self):
        # type: () -> RandBin
        try:
            return RandBin(self.length_from(None))  # type: ignore
        except Exception:
            return RandBin(RandNum(0, 200))


class StrFixedLenFieldUtf16(StrFixedLenField, StrFieldUtf16):