Fyodor writes in his paper Idle scanning and related IPID games: Almost four years ago, security researcher Antirez posted an innovative new TCP port scanning technique. Idlescan, as it has become known, allows for completely blind port scanning. Attackers can actually scan a target without sending a single packet to the target from their own IP address! Instead, a clever side-channel attack allows for the scan to be bounced off a dumb "zombie" host. Intrusion detection system (IDS) reports will finger the innocent zombie as the attacker. Besides being extraordinarily stealthy, this scan type permits mapping out IP-based trust relationships between machines.
The following is my original posting to the Bugtraq mailing list. It was sent in Decenber 1998! A lot of time ago.
The english is almost unreadable, but for sure who was in the field perfectly understood the idea.
I have uncovered a new tcp port scan method.
Instead all others it allows you to scan using spoofed
packets, so scanned hosts can't see your real address.
In order to perform this i use three well known tcp/ip
implementation peculiarities of most OS:
(1) * hosts reply SYN|ACK to SYN if tcp target port is open,
reply RST|ACK if tcp target port is closed.
(2) * You can know the number of packets that hosts are sending
using id ip header field. See my previous posting 'about the ip
header' in this ml.
(3) * hosts reply RST to SYN|ACK, reply nothing to RST.
host A - evil host, the attacker.
host B - silent host.
host C - victim host.
A is your host.
B is a particular host: It must not send any packets while
you are scanning C. There are a lot of 'zero traffic' hosts
in internet, especially in the night :)
C is the victim, it must be vulnerable to SYN scan.
I've called this scan method 'dumb host scan' in honour of host
How it works:
Host A monitors number of outgoing packets from B using id iphdr.
You can do this simply using hping:
#hping B -r
HPING B (eth0 xxx.yyy.zzz.jjj): no flags are set, 40 data bytes
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=0 ttl=64 id=41660 win=0 time=1.2 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=1 ttl=64 id=+1 win=0 time=75 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=2 ttl=64 id=+1 win=0 time=91 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=3 ttl=64 id=+1 win=0 time=90 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=4 ttl=64 id=+1 win=0 time=91 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=5 ttl=64 id=+1 win=0 time=87 ms
As you can see, id increases are always 1. So this host have the
characteristics that host B should to own.
Now host A sends SYN to port X of C spoofing from B.
(using hping => 0.67 is very easy, http://www.kyuzz.org/antirez)
if port X of C is open, host C will send SYN|ACK to B (yes,
host C don't know that the real sender is A). In this
case host B replies to SYN|ACK with a RST.
If we send to host C a few of SYN it will reply to B with a few
of SYN|ACK, so B will reply to C a few of RST... so
we'll see that host B is sending packets!
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=17 ttl=64 id=+1 win=0 time=96 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=18 ttl=64 id=+1 win=0 time=80 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=19 ttl=64 id=+2 win=0 time=83 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=20 ttl=64 id=+3 win=0 time=94 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=21 ttl=64 id=+1 win=0 time=92 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=22 ttl=64 id=+2 win=0 time=82 ms
The port is open!
Instead, if port X of C is closed sending to C a few
of SYN spoofed from B, it will reply with RST to B, and
B will not reply (see 3). So we'll see that host B is not sending
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=52 ttl=64 id=+1 win=0 time=85 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=53 ttl=64 id=+1 win=0 time=83 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=54 ttl=64 id=+1 win=0 time=93 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=55 ttl=64 id=+1 win=0 time=74 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=56 ttl=64 id=+1 win=0 time=95 ms
60 bytes from xxx.yyy.zzz.jjj: flags=RA seq=57 ttl=64 id=+1 win=0 time=81 ms
The port is closed.
All this can appear complicated to perform, but using two sessions
of hping on Linux virtual consoles or under X makes it more simple.
First session listen host B: hping B -r
Second session send spoofed SYN: hping C -a B -S
Sorry if my english is not so clear.
However this posting is not adequate to describe exaustively
this scan method, so i'll write a paper on this topic, specially
about how to implement this in a port scanner (i.e. nmap), and
about players characteristics and OS used.
happy new year,
resumindo mais facilmente, o idle scan funciona assim, voce primeiro encontra
um host desprotegido,com alguma porta aberta/nao filtrada, depois usa esse host
para scanear otras maquinas, o nmap faz iso p/ voce com seguinte syntaxe
nmap -P0 -sI 126.96.36.199:80 -vv 200.203.251.asterisco -oN teste
essa tecnica de scan alem de ser totalmente stealth, ela pode servir para enganar
firewall e IDS, veja o exemplo acima, eu econtrei um host, que serve como zumbi,
p/ atravez dele mesmo scanear sua propria rede, oque e bem provavel que eu consiga,
pois geralmente computadores da mesma rede tem relacao de confianca,
aceitando pedidos de conexoes uns dos outros. nao vou dar mais exemplos de comandos
pq acho que ja e sufuciente para bons entendedores.