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Explanation of the 'spank' attack
-- a new breed stream/raped
------------------------------------------------

By: lst (yardley@uiuc.edu)

This is a tad different than the previous release.  Stream/Raped mearly 
flooded the host with ack's (or no flags) and came from random ips with 
random sequence numbers and/or ack numbers.  The difference now is that 
this not only does the previous stuff, but also directly attacks from and 
to multicast addresses as well.  Just as before, rate limiting should be 
done to counteract its effect (the same idea as ICMP_BANDLIM).  The 
multicast handling should also be checked to verify that it is behaving 
properly.

The attacker specifies the port[s] that they want to send the attack to, 
depending on what ports are selected, you will have different net 
results.  If the port is an open port, then you will possibly have a longer 
kernel path to follow before the drop.  Therefore, a smart attacker will 
hit open ports, but havoc can also come about from random ports due to 
states and processing.

In the best case scenario, you will experience only the lag of the flood 
and the lag of the processing (currently) and then be fine when the 
attacker stops,  In the worst case, you lockup, kill the network, and 
possibly have to reboot.  Once you patch it, you deal with a lot less 
processing time (the drops are handled without the RST flag when 
appropriate--bandlim type idea).  In other words, you go to the drop 
routine instead of dropwithrst silencing your response, which decreases 
your processing time, the hit on your network, and the effect of the flood 
(once a threshold is reached, all those bad packets are silently dropped 
and the attack has less of a net effect).

The filters that were presented at the beginning of this email will block 
all multicast packets that come out (and in) the tcp stack I have been
getting mailed a lot about this.  Here is why I said the previous 
statement.  Receiving a packet with no flags is considered an illegal
packet (obviously) and is often dumped, however, as we have seen in 
the past, illegal packets often wreak havoc and often go untested.

There is very little that "raped.c" or "stream.c" actually showed as 
problems in the TCP/IP stacks.  The true problem lies more in the effects 
of the response (caused by the attack).  This is the same concept as the 
SYN floods of yesteryear, and the same type of thing will be done to handle 
it.  The main difference is that it will be on a simpler note because there 
isn't much need for a "cookie" based system.  One should just throttle the 
response of the reset packets which in turn will help stop the storm that 
you generate and in general, harden the tcp/ip stack to behave the way it 
is supposed to.

The main effect of this attack is that you are shooting back RST+ACK's at 
all the spoofed hosts.  Obviously, a lot of these hosts will not exist and 
you will get ICMP unreaches (as an example) bounced back at you.  There are 
other possibilities as well, but unreach would be the most common 
(redirects might be common as well although i did not spend the time to 
analyze that).  The ones that don't respond back may send you some packets 
back as well (depending on if the port was valid or not and what their 
firewall rules are).  This type of attack is complicated by the multicasts, 
and the effect is amplified as well.  All in all, it becomes very nasty 
very quick.  Basically, this causes a nice little storm of packets, in the 
ideal case.

Note that I said ideal case in the previous paragraph.  This is not always 
the observed behavior.  It all depends on what is on the subnet, what type 
of packets are recieved, what rules and filters you have setup, and even 
the duration of the flood.  It has been pointed out several times that the 
machine will go back to normal once the attack is stopped, which is exactly 
why something like ICMP_BANDLIM will work.

I have also been asked a lot about what this "bug" affects.  I have seen it 
have effects on *BSD, Linux, Solaris, and Win* as far as OS's go.  It has 
also seemed to affect some hubs, switches, routers, or gateways since 
entire subnets have "disappeared" briefly after the attack.  The multicast 
attack seems to be more deadly to teh network than the previous attack and 
its affects get amplified and even carried over to the rest of the network 
(bypassing secluded network bounds).  I don't have more specifics on the 
systems affected because of the difficulty in testing it (and keeping the 
network up) since I do not have local access to the networks that I tested 
on, and remote access gets real ugly real fast.

Another possibility that has been suggested as to why some machines die is 
that the machine's route table is being blown up by the spoofed 
packets.  Each spoofed packet has a different source address which means 
that a temporary route table entry is being created for each one.  These 
entries take time to timeout.  Use 'vmstat -m' and check the 'routetbl' 
field while the attack is going on.

Route table entries can be controlled somewhat under freebsd with:

[root@solid]::[~] sysctl -a | fgrep .rt
net.inet.ip.rtexpire: 3600
net.inet.ip.rtminexpire: 10
net.inet.ip.rtmaxcache: 128

You can do the following, to help if the route table is at least part of 
the problem:

sysctl -w net.inet.ip.rtexpire=2
sysctl -w net.inet.ip.rtminexpire=2

Things that will help:

1. Drop all multicast packets (ingress and egress) that are addressed to 
   the tcp stack because multicasts are not valid for tcp.
2. Extend bandwidth limiting to include RST's, ACK's and anything else 
   that you feel could affect the stability of the machine.
3. Don't look for listening sockets if the packet is not a syn

I hope that this helps, or explains a little more at least.

---------------------------------------------------
Temporary remedy
---------------------------------------------------

If you use ipfilter, this MAY help you, but the issue is quite a bit 
different than the previous issue.

-- start rule set --
block in quick proto tcp from any to any head 100
block in quick proto tcp from 224.0.0.0/28 to any group 100
pass in quick proto tcp from any to any flags S keep state group 100
pass out proto tcp from any to any flags S keep state
pass in all
-- end rule set --

optionally, a rule like the following could be inserted to handle outgoing 
packets (if they send from the firewall somehow) but you have bigger 
problems than the attack if that is the case.

-- start additional rule --
block out proto tcp from any to 224.0.0.0/28
-- end additional rule --

That will help you "stop" the attack (actually it will just help minimize 
the affects), although it will still use some CPU though

Note: If you use IPFW, there is no immediate way to solve this problem due 
to the fact that it is a stateless firewall.  If you are getting attacked, 
then temporarily use ipfilter (or any other state based firewall) to stop 
it.  Otherwise, wait for vendor patches or read more about the explanation 
for other possible workarounds.

FreeBSD "unofficial patch" by Don Lewis: 
http://solid.ncsa.uiuc.edu/~liquid/patch/don_lewis_tcp.diff

-----------------------
Conclusion
-----------------------

This bug was found in testing.  It seems a bit more lethal than the 
previous and should be addressed as such.  Patches should be available now, 
but I do not follow all the platforms.

--------------------
References
--------------------

This was done independantly, although some of the analysis and reverse 
engineering of concept was done by other people.  As a result, I would like 
to give credit where credit is due.  The following people contributed in 
some way or another:

Brett Glass <brett@lariat.org>
Alfred Perlstein <bright@wintelcom.net>
Warner Losh <imp@village.org>
Darren Reed <avalon@coombs.anu.edu.au>
Don Lewis <Don.Lewis@tsc.tdk.com>

Also, I would like to send shouts out to w00w00 (http://www.w00w00.org)

-------------------
Attached
-------------------
These programs are for the sake of full disclosure, don't abuse 
them.  Spank was written with libnet, so you will need to obtain that as
well.  You can find that at http://www.packetfactory.net/libnet

For an "unofficial" patch:
http://www.w00w00.org/files/spank/don_lewis_tcp.diff

For spank.c:
http://www.w00w00.org/files/spank/spank.c