This would also apply to Intrusion Detection Systems fingerprinting when the administrator choose to tear down connections with a tcp rst packet. For example Snort 1.8.4 flexresp code, ttl value is always 64. Of course you can also take this thought further and fingerprint on other things in the tcp rst packet and you should be able to tell exactly which IDS running in the other end. Thanks, Jonas Eriksson jeat_private On Sun, 31 Mar 2002, Meder Kydyraliev wrote: > Hi fellow bugtraqers, > > recently playing with raw sockets and PF (OpenBSD 3.0) i noticed > that when you have return-rst rule for some tcp packet, ttl field > in ip header of rst packet, that is sent by PF, equals 128, while > default ttl for OpenBSD 3.0 is 64, so we can actually see what > tcp ports are blocked by pf and which are open, but closed(nothing > on them). > So then i grabed ipfilter(3.4.25) source and saw that for Solaris > (SunOS5) ipfilter's default ttl for rst packets is 60, while when > the port is open(by ipfilter) but nothing listens on it ttl is > reflected from the packet (ttl reflection could also be used for OS > figerprinting, some OSs (Sol7) reflect ttl and some use their default > ttl) the rst is being sent to; and for linux ipfilter's ttl is 127 > while default is 255. > > So if we know the distance to our target we could: > > - use the information for packetfilter fingerprinting, possibly OS > fingerprinting; > - find out firewall acls, and what ports are actually blocked by > firewall and what ports are open but nothing listens on them(so that > we modify exploit to bind a shell on an open port, for example); > > Regards, > Meder Kydyraliev > > PS: it is fixed in OpenBSD -CURRENT, thanks to Daniel Hartmeier. > PPS: didn't have linux, so couldn't check that with iptables/ipchains > > > here is snort output for OpenBSD(PF) with return-rst for port 5555: > > 03/31-17:26:02.282644 xxx.xxx.xxx.xxx:61230 -> xxx.xxx.xxx.xxx:5555 > TCP TTL:255 TOS:0x0 ID:24383 IpLen:20 DgmLen:44 DF > ******S* Seq: 0x9379CC65 Ack: 0x0 Win: 0x2238 TcpLen: 24 > TCP Options (1) => MSS: 1460 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > 03/31-17:26:02.282793 xxx.xxx.xxx.xxx:5555 -> xxx.xxx.xxx.xxx:61230 > TCP TTL:128 TOS:0x0 ID:48505 IpLen:20 DgmLen:40 > ^^^^^^^ > ***A*R** Seq: 0x0 Ack: 0x9379CC66 Win: 0x0 TcpLen: 20 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > > > here is snort output for OpenBSD with an open(by PF) but closed(nothing > listening on it) port: > > 03/31-17:26:33.326327 xxx.xxx.xxx.xxx:61257 -> xxx.xxx.xxx.xxx:52000 > TCP TTL:255 TOS:0x0 ID:24384 IpLen:20 DgmLen:44 DF > ******S* Seq: 0x39DCC231 Ack: 0x0 Win: 0x2238 TcpLen: 24 > TCP Options (1) => MSS: 1460 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > 03/31-17:26:33.326481 xxx.xxx.xxx.xxx:52000 -> xxx.xxx.xxx.xxx:61257 > TCP TTL:64 TOS:0x0 ID:57309 IpLen:20 DgmLen:40 DF > ^^^^^^ > ***A*R** Seq: 0x0 Ack: 0x39DCC232 Win: 0x0 TcpLen: 20 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > > and now Solaris 7 with ipfilter(3.4.25) with return-rst rule: > > 03/31-17:30:16.997579 xxx.xxx.xxx.xxx:12879 -> xxx.xxx.xxx.xxx:1521 > TCP TTL:64 TOS:0x10 ID:7967 IpLen:20 DgmLen:64 DF > ******S* Seq: 0x6A3518CE Ack: 0x0 Win: 0x4000 TcpLen: 44 > TCP Options (9) => MSS: 1460 NOP NOP SackOK NOP WS: 0 NOP NOP > TCP Options => TS: 1615975242 0 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > 03/31-17:30:16.997785 xxx.xxx.xxx.xxx:1521 -> xxx.xxx.xxx.xxx:12879 > TCP TTL:60 TOS:0x10 ID:15731 IpLen:20 DgmLen:40 DF > ^^^^^^ > ***A*R** Seq: 0x0 Ack: 0x6A3518CF Win: 0x0 TcpLen: 20 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > and just a port w/o daemon on it: > > 03/31-17:30:57.379170 xxx.xxx.xxx.xxx:29599 -> xxx.xxx.xxx.xxx:42000 > TCP TTL:64 TOS:0x10 ID:25418 IpLen:20 DgmLen:64 DF > ******S* Seq: 0xF016BF9 Ack: 0x0 Win: 0x4000 TcpLen: 44 > TCP Options (9) => MSS: 1460 NOP NOP SackOK NOP WS: 0 NOP NOP > TCP Options => TS: 1615975323 0 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ > > 03/31-17:30:57.379449 xxx.xxx.xxx.xxx:42000 -> xxx.xxx.xxx.xxx:29599 > TCP TTL:64 TOS:0x10 ID:15732 IpLen:20 DgmLen:40 DF > ^^^^^^ > ***A*R** Seq: 0x0 Ack: 0xF016BFA Win: 0x0 TcpLen: 20 > > =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ >
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