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Re: backtracking forged packets?

From: "Jean | ddostest.me via NANOG" <nanog () nanog org>
Date: Sun, 15 Mar 2020 12:36:03 -0400
I believe that Oculus blocked the RST and not the SYN/ACK.

It sounds the same but, it's not.

I see 2 options here:
1. Continue to be DDoS and abuse. The result is maybe they will move on, but I doubt.
2. Try to block the malformed SYN/ACK and it will probably solve your issue. You have nothing to lose to try as you can easily fallback.
You can let us know what are the results and if I was wrong, I will publicly state that I was wrong.
But, make sure you do it properly. _Do not block the RST_, you need to block the malformed SYN/ACK before they hit your open reflectors. 

Jean


On 2020-03-15 12:05, Amir Herzberg wrote:
Bill: I agree with Damian that you should try to ensure responding with RST to SYN/ACK; in fact, attackers are sometimes (often?) _looking_ for networks that do not send RST in response to unsolicited SYN/ACK, to spoof their addresses in syn-flooding and other attacks (eg., syn-ack) against victim servers.
Not sending RST could even result in you receiving ICMP unreachable - esp. indicating filtering as you received - since server admins may have installed a filter against your prefix (to deal with such abuse). So, I wonder, it is possible that your network/FW/provider already filter the RST responses so they don't reach the (victim) servers?
BTW, I'm covering these issues in my DoS lecture as part of the Net-Sec course (see URL below). The foils are available (although not yet latest version, will upload `soon' :) ), the text of the net-sec (2nd) part - not so much, it may take me quite a while to make this (2nd) part useable. 
--
Amir Herzberg

Comcast professor of Security Innovations, University of Connecticut

Homepage: https://sites.google.com/site/amirherzberg/home
Foundations of Cyber-Security (part I: applied crypto, part II: network-security): 
https://www.researchgate.net/project/Foundations-of-Cyber-Security
On Sat, Mar 14, 2020 at 7:51 PM Damian Menscher via NANOG <nanog () nanog org <mailto:nanog () nanog org>> wrote: 

    I don't recommend filtering the SYN-ACK packets.  That's what
    Octolus did, and the result was leaving half-open SYN_RECV
    connections on all the nodes used for reflection.  That has two
    downsides:

      - the reflectors will retry the SYN-ACK (several times), which
    increases your PPS load (amplifying the attack)
      - the providers may notice the large number of SYN_RECV
    connections from your network and put you on a blacklist

    I don't want to leave you with the impression that it's
    hopeless... these attacks aren't impossible to stop --- it just
    requires convincing the transit providers to care.

    Damian

    On Sat, Mar 14, 2020 at 1:31 PM Jean | ddostest.me
    <http://ddostest.me> via NANOG <nanog () nanog org
    <mailto:nanog () nanog org>> wrote:

        Hi Bill,

        thanks for sharing the data. Indeed, I can't offer you a way
        to backtrack the spoofed packets.

        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.

        Here is what I would do if I was in your situation.

        Since these packets are spoof and malformed, I would block all
        SYN/ACK based on the length.

        Depending on your hardware, it's very easy to inspect _only
        the SYN/ACK by length_ if you have modern hardware. On
        linux/unix, it's also very straightforward. I'm not sure for
        windows though.

        Here is the details of the analysis:

        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.

        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.

        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.

        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.

        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.

        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. :)

        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.

        I'm pretty sure that it will help you as it did for me in the
        past.

        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.

        Regards,

        Jean St-Laurent

        On 2020-03-14 11:46, William Herrin wrote:

        On Sat, Mar 14, 2020 at 4:02 AM Jean |ddostest.me  <http://ddostest.me>  via NANOG
        <nanog () nanog org>  <mailto:nanog () nanog org>  wrote:

        can you post some forged packets please? You can send them offlist if
        you prefer.

        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  <http://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
        <andrew.william.smith () gmail com>  <mailto:andrew.william.smith () gmail com>  wrote:

        Look inside the ICMP Unreachable backscatter at the truncated original packet that caused the unreachable 
message.

        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
        bill () herrin us  <mailto:bill () herrin us>
        https://bill.herrin.us/
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