<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<p>Hi Bill,</p>
<p>thanks for sharing the data. Indeed, I can't offer you a way to
backtrack the spoofed packets.</p>
<p>Anyway, I'm not sure what could you do legally as there is a very
high chance that these people are not in the USA and the CFAA
won't apply to them.<br>
</p>
<p>Here is what I would do if I was in your situation.</p>
<p>Since these packets are spoof and malformed, I would block all
SYN/ACK based on the length.</p>
<p>Depending on your hardware, it's very easy to inspect <u>only
the SYN/ACK by length</u> if you have modern hardware. On
linux/unix, it's also very straightforward. I'm not sure for
windows though.<br>
</p>
<p>Here is the details of the analysis:</p>
<p>Today, all the SYN and SYN/ACK includes a minimum of options like
MSS, WS, SACK, NOP (Only a spacer, sometimes 2) and extended TS.
There might be others, but let's use the basic one.</p>
<p>In your case, there are none. There is only MSS and the SYN
length is 44 bytes. These are spoof packets maybe generated by
either TCP-AMP like reported earlier.<br>
</p>
<p>I would try to block all SYN/ACK coming toward your network with
a length of 44 bytes or lower. But, this is weird because it
should be 54 bytes. Maybe there is some offloading of some sort in
your gear.<br>
</p>
<p>Now depending on your hardware, it could work or it could kill
your router as it will punt the cpu. I guess you have some modern
gear.<br>
</p>
<p>What I do when I am not sure about the length, I start to accept
and log at 60 bytes, then 58, 56, 54... 44 until I catch the
gremlins.</p>
<p>Once you found the sweet spot, you drop all SYN/ACK toward your
/23 lower than X bytes. It won't kill or block anything legitimate
if you do it properly. :)</p>
<p>What will happen is that you will not reply to these spoof
SYN/ACK with a RST and still allowing RST for legit SYN and
SYN/ACK. Akamai and your service providers will be happy and
should not penalize you.<br>
</p>
<p>I'm pretty sure that it will help you as it did for me in the
past.<br>
</p>
<p>Let me know if it's not clear and/or which part is foggy and I'll
try to give more details and better explanation.</p>
<p>Regards,<br>
</p>
<p>Jean St-Laurent<br>
</p>
<div class="moz-cite-prefix">On 2020-03-14 11:46, William Herrin
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAP-guGXuHmro4wOm0owTROwj+ibo_0yMTUn1d6ZEuWOmL1+D5Q@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">On Sat, Mar 14, 2020 at 4:02 AM Jean | ddostest.me via NANOG
<a class="moz-txt-link-rfc2396E" href="mailto:nanog@nanog.org"><nanog@nanog.org></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">can you post some forged packets please? You can send them offlist if
you prefer.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
Hi Jean,
Here are a couple examples (PDT this morning):
08:22:43.413250 IP (tos 0x0, ttl 55, id 10108, offset 0, flags [none],
proto ICMP (1), length 56)
45.89.93.26 > 199.33.225.218: ICMP host 45.89.93.26 unreachable -
admin prohibited filter, length 36
IP (tos 0x0, ttl 69, id 10108, offset 0, flags [DF], proto TCP
(6), length 40)
199.33.225.218.9851 > 45.89.93.26.443: [|tcp]
0x0000: 4500 0038 277c 0000 3701 28da 2d59 5d1a
0x0010: c721 e1da 030d 4b61 0000 0000 4500 0028
0x0020: 277c 4000 4506 dae4 c721 e1da 2d59 5d1a
0x0030: 267b 01bb a057 e903
08:25:47.787326 IP (tos 0x0, ttl 54, id 0, offset 0, flags [DF], proto
TCP (6), length 44)
104.87.78.95.80 > 199.33.225.143.8667: Flags [S.], cksum 0xc97a
(correct), seq 1216155085, ack 11765035, win 29200, options [mss
1156], length 0
0x0000: 4500 002c 0000 4000 3606 e564 6857 4e5f
0x0010: c721 e18f 0050 21db 487d 0dcd 00b3 852b
0x0020: 6012 7210 c97a 0000 0204 0484
I have observed no consistency in the remote IP addresses. I receive
no more than a few of each and they don't line up with particular
networks. Remote ports are heavily 80, 443, 22, 25, etc. but a
smattering of less common ports too. I'm not seeing any RSTs at all
nor any port-unreachables. Lots of syn/acks and a few time exceeded
and host unreachables. I don't know what to make of that.
On Sat, Mar 14, 2020 at 1:46 AM Andrew Smith
<a class="moz-txt-link-rfc2396E" href="mailto:andrew.william.smith@gmail.com"><andrew.william.smith@gmail.com></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Look inside the ICMP Unreachable backscatter at the truncated original packet that caused the unreachable message.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
Clever! I wouldn't have thought of that. Unfortunately as in the
example above, the TTLs in the packets encapsulated in ICMP are not
especially close to one of the common boundaries.
Regards,
Bill Herrin
--
William Herrin
<a class="moz-txt-link-abbreviated" href="mailto:bill@herrin.us">bill@herrin.us</a>
<a class="moz-txt-link-freetext" href="https://bill.herrin.us/">https://bill.herrin.us/</a>
</pre>
</blockquote>
</body>
</html>