syn flood, tcpdump readings
tomh at huppi.com
Thu Aug 7 18:00:56 UTC 2008
On 12:32 Thu 07 Aug , David DeSimone wrote:
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> Tom Huppi <tomh at huppi.com> wrote:
> > Anyway, I am getting what I believe to be syn floods
> > periodically. They dwarf my production traffic and sometimes
> > get close to producing as much bandwith as we are paying for. A
> > representative sample looks like so when viewed with tcpdump on
> > my outward interface ('em1'):
> > 21:36:53.870312 IP 126.96.36.199.x11 > 188.8.131.52.domain: S 27394048:27394048(0) win 16384
> > 21:36:53.870319 IP 184.108.40.206.x11 > 220.127.116.11.domain: S 1793916928:1793916928(0) win 16384
> Since you went to the trouble of obscuring the source IP, I presume that
> the source IP is your IP. So, these look like responses, i.e. outbound
> traffic, not inbound, since they are sourced from your IP. You can use
> tcpdump's -e flag to be sure who is sending and who is receiving.
I obscured my own IP range which is the 74.nnn.nnn. one and it
is a /24. Interestingly most of the IP's on my side are ones
where I have no host.
The reason why is that I figured that if I myself were a
semi-sophisticated cracker, I would look for targets of
opertunity on the various mailing lists where one could identify
both networks administered by newbie/part-time personel, and
often a fair amount about the configuration of said :)
The IP '18.104.22.168' is exactly as it appeared on my tcpdump.
It shows as a telcom company in India in this case...usually
it's some network company or another in China.
My network looks like so:
------------- em0 <---> internal range
Network Provider <----> em1 | pf firewall |
(Internap) ------------- bce1 <---> dmz range
I took the tcpdump output to indicate that Syn packets showing an Indian Origin were showing up addressed to (mainly non-existant) IP addresses within my /24 network.
I'll look at 'tcpdump -e'. Thanks for the hint!
> - --
> David DeSimone == Network Admin == fox at verio.net
> "I don't like spinach, and I'm glad I don't, because if I
> liked it I'd eat it, and I just hate it." -- Clarence Darrow
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