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On Mon, Feb 21, 2000 at 07:43:54AM -0800, LigerTeam wrote:
[...]
> In fact, TCP header is 6 kinds of
> tcp flag (SYN, ACK, PSH, RST, FIN, URG).
>=20
> problem is the flag value in TCP header
> approaches to 1byte variable of u_char type.
> ex)see tcp.h file
>=20
> The flag value Each one correspond to 1 bit,
> but it have unused 2 bit.
>=20
> |unused|unused|URG|ACK|PSH|RST|SYN|FIN|
>=20
> Understanding of the very problem is simple.
> Let's compare the two codes.
> ex)SYN Scan detecter program several code type
>=20
> i) if ( flag =3D=3D TH_SYN )
>=20
> ii) if ( flag & TH_SYN )
>=20
> (TH_SYN->SYN flag)
>=20
> The i) code is true, only when the syn
> flag bit is set at 1.
>=20
> So the flag value is 0x2,
> and |0|0|0|0|0|0|1|0| in bit.
>=20
> The next ii) code is true, only
> when SYN flag bit, the TH_SYN value
> in flags, is set at 1, and the other
> bit state is not influential.
>=20
> Eventually, we can easily know a very
> important thing.
>=20
> If hackers use the two higher bit(unused bit)
> one or all, to set at 1,
> ii) code type has false value,
> but i) code type last true value.
> and hackers avoid scan detecter
[...]=20
> Conclusion:
>=20
> When the flags variable in tcp header is adjusted
> totally with given value,
> higher two bit(unused bit) must be cleared
> and set at 0.
[...]
This is a known issue; it's in the category of "invalid TCP flags=20
scanning". In fact, the two unused bits in the TCP flags byte can=20
be used for TCP fingerprinting as the response to such TCP packets=20
is not specified in RFC 793 and therefore depends on the TCP/IP=20
implementation being used. In addition to TCP fingerprinting, TCP=20
packets with certain invalid (i.e. not covered by RFC 793) flag
combinations not including the SYN flag can be used to determine=20
which ports are open on the target machine.
This leads one to the conclusion that focussing on TCP packets with=20
the SYN flag set is completely insufficient for scan detection. Any=20
decent scan detector must, among other things, pay explicit=20
attention to those 2 unused bits in the TCP flags byte anyway.
--
Jochen Bauer
Security Team (RUS-CERT) =20
Computer Center of the University of Stuttgart =20
Germany
=20
************************************************************************=20
*Email: jtbat_private-stuttgart.de *
* jochen.bauerat_private-stuttgart.de *
* *
*PGP Public Key: *
*http://ca.uni-stuttgart.de:11371/pks/lookup?op=3Dindex&search=3D0xB5D92889*
************************************************************************=20
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