On Nov 24, 2008, at 23:12, Pieter de Boer wrote:
Hi Eirik,
Perform the nmap scan and look at the tcpdump output to see how your
firewall and/or server react.
nmap command:
nmap -PN -sT --scanflags SYNFIN -p<port> anduin.net
where <port> was either 80 (open) or 8585 (closed).
tcpdump command on firewall (which NATs to internal IPs):
tcpdump -i <interface> -p -vvv host alge.anart.no and \(port 80 or
port 8585\)
where <interface> was the publicly facing interface on the firewall.
Results for port 80:
IP (tos 0x0, ttl 59, id 12785, offset 0, flags [DF], proto: TCP
(6), length: 64) alge.anart.no.40283 > 213.225.74.230.http: S,
cksum 0xa720 (correct), 3300467486:3300467486(0) win 16384 <mss
1460,nop,nop,sackOK,nop,wscale 0,nop,nop,timestamp 2747936488 0>
IP (tos 0x0, ttl 63, id 10914, offset 0, flags [DF], proto: TCP
(6), length: 60) 213.225.74.230.http > alge.anart.no.40283: S,
cksum 0x8ef5 (correct), 347647336:347647336(0) ack 3300467487 win
65535 <mss 1460,nop,wscale 3,sackOK,timestamp 2946365534 2747936488>
Results for port 8585:
IP (tos 0x0, ttl 59, id 44156, offset 0, flags [DF], proto: TCP
(6), length: 64) alge.anart.no.1839 > 213.225.74.230.8585: S, cksum
0xf765 (correct), 1324215952:1324215952(0) win 16384 <mss
1460,nop,nop,sackOK,nop,wscale 0,nop,nop,timestamp 4070158112 0>
IP (tos 0x0, ttl 63, id 34488, offset 0, flags [DF], proto: TCP
(6), length: 40) 213.225.74.230.8585 > alge.anart.no.1839: R, cksum
0x52ef (correct), 0:0(0) ack 1324215953 win 0
I can't tell what's going on here, except I wouldn't have expected
a reply at all to the second one at least, and maybe not even the
first. However, I don't have enough experience to tell if nmap is
doing the "right thing" here at all.
First of all, this is not a scan with both the SYN and FIN flags
set. This can be seen from the tcpdump output only showing the 'S'
flag. You're using -sT, which makes nmap use connect(), and thus the
regular SYN, SYN/ACK, ACK 3-way-handshake. For a SYN/FIN scan,
you'll need root access. I tested this locally without supplying
further TCP scan options to nmap. Could you retest and make sure you
see 'SF' as flags in tcpdump?
I don't. With
nmap --scanflags SYNFIN -p<port> <host>
as root, I got, from what I can tell, exactly the same. May be this is
filtered on the way out, so I need to find an unhampered box to try
from? I could simply try crossing vlans through the firewall, I guess.
Secondly, it would be useful if you'd explain the following: is your
firewall NATting port 8585 also, or is traffic sent to that port
handled by the TCP/IP stack of the firewall itself? Furthermore, it
appears the firewall is not actually filtering traffic to port 8585..
This particular machine is behind 1:1 NATing. I usually do NAT+fwrules
for needed ports only, but even in those cases I get the (false?) syn
+fin alerts from (in this case) securityspace.com.
The strictest firewall configuration would be to have everything
filtered except the ports you actually use. Those ports are either
NATted to the back-end system or handled by the firewall itself (in
case you want that functionality). From a security perspective,
simply dropping incoming traffic is better than sending back RST's.
In pf this is the default.
That is correct, however in this case I do 1:1 and no pf on the target
host (it is in a DMZ). I ran the scan on this system out of curiosity
only, however as stated above this problem is far from unique to this
particular system.
Thanks for your input, i'll keep trying to reproduce this..
/Eirik
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