def p0f_impersonate(pkt, osgenre=None, osdetails=None, signature=None,
                    extrahops=0, mtu=1500, uptime=None):
    """Modifies pkt so that p0f will think it has been sent by a
specific OS.  If osdetails is None, then we randomly pick up a
personality matching osgenre. If osgenre and signature are also None,
we use a local signature (using p0f_getlocalsigs). If signature is
specified (as a tuple), we use the signature.

For now, only TCP Syn packets are supported.
Some specifications of the p0f.fp file are not (yet) implemented."""
    pkt = pkt.copy()
    # pkt = pkt.__class__(raw(pkt))
    while pkt.haslayer(IP) and pkt.haslayer(TCP):
        pkt = pkt.getlayer(IP)
        if isinstance(pkt.payload, TCP):
            break
        pkt = pkt.payload

    if not isinstance(pkt, IP) or not isinstance(pkt.payload, TCP):
        raise TypeError("Not a TCP/IP packet")

    db = p0f_selectdb(pkt.payload.flags)
    if osgenre:
        pb = db.get_base()
        if pb is None:
            pb = []
        pb = [x for x in pb if x[6] == osgenre]
        if osdetails:
            pb = [x for x in pb if x[7] == osdetails]
    elif signature:
        pb = [signature]
    else:
        pb = p0f_getlocalsigs()[db]
    if db == p0fr_kdb:
        # 'K' quirk <=> RST+ACK
        if pkt.payload.flags & 0x4 == 0x4:
            pb = [x for x in pb if 'K' in x[5]]
        else:
            pb = [x for x in pb if 'K' not in x[5]]
    if not pb:
        raise Scapy_Exception("No match in the p0f database")
    pers = pb[random.randint(0, len(pb) - 1)]

    # options (we start with options because of MSS)
    # Take the options already set as "hints" to use in the new packet if we
    # can. MSS, WScale and Timestamp can all be wildcarded in a signature, so
    # we'll use the already-set values if they're valid integers.
    orig_opts = dict(pkt.payload.options)
    int_only = lambda val: val if isinstance(val, six.integer_types) else None
    mss_hint = int_only(orig_opts.get('MSS'))
    wscale_hint = int_only(orig_opts.get('WScale'))
    ts_hint = [int_only(o) for o in orig_opts.get('Timestamp', (None, None))]

    options = []
    if pers[4] != '.':
        for opt in pers[4].split(','):
            if opt[0] == 'M':
                # MSS might have a maximum size because of window size
                # specification
                if pers[0][0] == 'S':
                    maxmss = (2**16 - 1) // int(pers[0][1:])
                else:
                    maxmss = (2**16 - 1)
                # disregard hint if out of range
                if mss_hint and not 0 <= mss_hint <= maxmss:
                    mss_hint = None
                # If we have to randomly pick up a value, we cannot use
                # scapy RandXXX() functions, because the value has to be
                # set in case we need it for the window size value. That's
                # why we use random.randint()
                if opt[1:] == '*':
                    if mss_hint is not None:
                        options.append(('MSS', mss_hint))
                    else:
                        options.append(('MSS', random.randint(1, maxmss)))
                elif opt[1] == '%':
                    coef = int(opt[2:])
                    if mss_hint is not None and mss_hint % coef == 0:
                        options.append(('MSS', mss_hint))
                    else:
                        options.append((
                            'MSS', coef * random.randint(1, maxmss // coef)))
                else:
                    options.append(('MSS', int(opt[1:])))
            elif opt[0] == 'W':
                if wscale_hint and not 0 <= wscale_hint < 2**8:
                    wscale_hint = None
                if opt[1:] == '*':
                    if wscale_hint is not None:
                        options.append(('WScale', wscale_hint))
                    else:
                        options.append(('WScale', RandByte()))
                elif opt[1] == '%':
                    coef = int(opt[2:])
                    if wscale_hint is not None and wscale_hint % coef == 0:
                        options.append(('WScale', wscale_hint))
                    else:
                        options.append((
                            'WScale', coef * RandNum(min=1, max=(2**8 - 1) // coef)))  # noqa: E501
                else:
                    options.append(('WScale', int(opt[1:])))
            elif opt == 'T0':
                options.append(('Timestamp', (0, 0)))
            elif opt == 'T':
                # Determine first timestamp.
                if uptime is not None:
                    ts_a = uptime
                elif ts_hint[0] and 0 < ts_hint[0] < 2**32:
                    # Note: if first ts is 0, p0f registers it as "T0" not "T",
                    # hence we don't want to use the hint if it was 0.
                    ts_a = ts_hint[0]
                else:
                    ts_a = random.randint(120, 100 * 60 * 60 * 24 * 365)
                # Determine second timestamp.
                if 'T' not in pers[5]:
                    ts_b = 0
                elif ts_hint[1] and 0 < ts_hint[1] < 2**32:
                    ts_b = ts_hint[1]
                else:
                    # FIXME: RandInt() here does not work (bug (?) in
                    # TCPOptionsField.m2i often raises "OverflowError:
                    # long int too large to convert to int" in:
                    #    oval = struct.pack(ofmt, *oval)"
                    # Actually, this is enough to often raise the error:
                    #    struct.pack('I', RandInt())
                    ts_b = random.randint(1, 2**32 - 1)
                options.append(('Timestamp', (ts_a, ts_b)))
            elif opt == 'S':
                options.append(('SAckOK', ''))
            elif opt == 'N':
                options.append(('NOP', None))
            elif opt == 'E':
                options.append(('EOL', None))
            elif opt[0] == '?':
                if int(opt[1:]) in TCPOptions[0]:
                    optname = TCPOptions[0][int(opt[1:])][0]
                    optstruct = TCPOptions[0][int(opt[1:])][1]
                    options.append((optname,
                                    struct.unpack(optstruct,
                                                  RandString(struct.calcsize(optstruct))._fix())))  # noqa: E501
                else:
                    options.append((int(opt[1:]), ''))
            # FIXME: qqP not handled
            else:
                warning("unhandled TCP option " + opt)
            pkt.payload.options = options

    # window size
    if pers[0] == '*':
        pkt.payload.window = RandShort()
    elif pers[0].isdigit():
        pkt.payload.window = int(pers[0])
    elif pers[0][0] == '%':
        coef = int(pers[0][1:])
        pkt.payload.window = coef * RandNum(min=1, max=(2**16 - 1) // coef)
    elif pers[0][0] == 'T':
        pkt.payload.window = mtu * int(pers[0][1:])
    elif pers[0][0] == 'S':
        # needs MSS set
        mss = [x for x in options if x[0] == 'MSS']
        if not mss:
            raise Scapy_Exception("TCP window value requires MSS, and MSS option not set")  # noqa: E501
        pkt.payload.window = mss[0][1] * int(pers[0][1:])
    else:
        raise Scapy_Exception('Unhandled window size specification')

    # ttl
    pkt.ttl = pers[1] - extrahops
    # DF flag
    pkt.flags |= (2 * pers[2])
    # FIXME: ss (packet size) not handled (how ? may be with D quirk
    # if present)
    # Quirks
    if pers[5] != '.':
        for qq in pers[5]:
            # FIXME: not handled: P, I, X, !
            # T handled with the Timestamp option
            if qq == 'Z':
                pkt.id = 0
            elif qq == 'U':
                pkt.payload.urgptr = RandShort()
            elif qq == 'A':
                pkt.payload.ack = RandInt()
            elif qq == 'F':
                if db == p0fo_kdb:
                    pkt.payload.flags |= 0x20  # U
                else:
                    pkt.payload.flags |= random.choice([8, 32, 40])  # P/U/PU
            elif qq == 'D' and db != p0fo_kdb:
                pkt /= conf.raw_layer(load=RandString(random.randint(1, 10)))  # XXX p0fo.fp  # noqa: E501
            elif qq == 'Q':
                pkt.payload.seq = pkt.payload.ack
            # elif qq == '0': pkt.payload.seq = 0
        # if db == p0fr_kdb:
        # '0' quirk is actually not only for p0fr.fp (see
        # packet2p0f())
    if '0' in pers[5]:
        pkt.payload.seq = 0
    elif pkt.payload.seq == 0:
        pkt.payload.seq = RandInt()

    while pkt.underlayer:
        pkt = pkt.underlayer
    return pkt