Using pf to mitigate DDoS - discussion at openbsd-misc

[dhatz]

There was a discussion about DDoS mitigation recently at openbsd-misc, which I thought some people here might find interesting.

It seems that from a variety of attacks, a big SYN flood is the most harmful.

From: Roger S.
Subject: How to deal with DDoS ?
Newsgroups: gmane.os.openbsd.misc
Date: 2012-02-20 16:57:05 GMT (1 week, 6 days, 3 hours and 16 minutes ago)
Greetings misc@

I am facing regular and consequent DDoS, and I would like to know how
the OpenBSD community deal with these. Hints and inputs welcome.

The obvious first : my input pipes are not filled, there is plenty of
bandwith available for my regular users. As OpenBSD is not enough (in
my setup, I am sure there is a solution) to mitigate such attacks we
use a proprietary product, but this solution has some undesirable
side-effects and is not a viable long term solution for us.

Methodology is more or less always the same :

• massive UDP flood          :  2 Gbps / 150 Kpps -> dropped
  directly on the router, not a problem
• moderate ICMP flood        :  10 Mbps /  12 Kpps
• moderate IP fragments flood : 380 Mbps /  57 Kpps
• moderate TCP RST flood      :  10 Mbps /  30 Kpps
• massive TCP SYN flood      : 640 Mbps /  2 Mpps -> yup, that hurts

So, UDP never ever reaches my OpenBSD box. The SYN are made with a
very vicious method : each used IP send exactly one SYN, but there are
millions of them (traffic probably spoofed, but can not use uRPF as we
have asymmetric traffic and routes). I tried to set limit states with
1M entries, and it was quickly filled (tried 5M but the box collapses
way before that). So in the end, the state table collapses and no
traffic can pass, even for regular users with already established
connections.

I ran some experiments in a lab trying to reproduce this, with a box
roughly identical to what I have in production (but much weaker, of
course). The box collapses at 600 Kpps SYN (100% interrupts), but
handles everything very gently (less than 50% interrupts and no packet
loss) if the first rule evaluated is block drop in quick from !
<whitelisted_users>. So it seems that my bottleneck is PF here, not
the hardware. A consequence of this saturation : both my main firewall
and my backup claims MASTER ownership of the CARP (split brain
syndrome). CARP works just fine when I add the block rule, though.

Some configuration details :

• OS  : OpenBSD 5.0/amd64 box, using GENERIC.MP
• CPU : Intel X3460 CPU (4 cores, 2.80GHz)
• RAM : 4GB
• NIC : 2x Intel 82576 (2 ports each)

Each network card has the following setup : one port to the LAN, one
port to the WAN. Each pair (LAN1/LAN2 and WAN1/WAN2) is trunked using
LACP. Already bumped net.inet.ip.ifq.maxlen, as all NICs are
supported. My benchmarks did highlight two interesting things : amd64
has better performance than i386 (roughly 5-10% less interrupts, with
same rules and traffic), but the difference between GENERIC and
GENERIC.MP is insignificant.

My current idea is to hack a daemon to track established connections
(extracting them la netstat), and inject my block rule in an anchor ( la relayd) when needed (watching some stats from pf, with its ioctl
interface). Pros: regular users the firewall saw before the attack can
still use the service. Cons: no new users are allowed until the
removal of the rule, obviously. Better than nothing, but I welcome any
other hints :)

One other solution may be to add boxes. I tried a carpnodes cluster,
but at 600 Kpps I got a "split brain" with both nodes claiming MASTER
for each carpnode. Maybe if I configure ALTQ it could help this ? As I
have more boxes, I could deal with the performance impact of ALTQ.

I am willing to test any patch/suggestion you may have, of course.
Even just hints about kernel code, as I am currently messing with PF
code myself. I did compile a profiled kernel, I must now check the
results but that will be another story.

To finish, here is the typical load on the box (errors are from
various DDoS, not related to normal use) :

Status: Enabled for 77 days 02:17:58            Debug: err

Interface Stats for trunk1            IPv4            IPv6
  Bytes In                  8885330383273                0
  Bytes Out                72449316050298            20224
  Packets In
    Passed                    48738702875                0
    Blocked                    10152865611                0
  Packets Out
    Passed                    67293792876              281
    Blocked                    4557637133                0

State Table                          Total            Rate
  current entries                    37135
  searches                    130771929548        19632.2/s
  inserts                      4718030394          708.3/s
  removals                      4717993259          708.3/s
Source Tracking Table
  current entries                    7455
  searches                      4951426366          743.3/s
  inserts                        623672861          93.6/s
  removals                      623665406          93.6/s
Counters
  match                        5600111978          840.7/s
  bad-offset                            0            0.0/s
  fragment                        3591379            0.5/s
  short                            2500133            0.4/s
  normalize                          10968            0.0/s
  memory                            71750            0.0/s
  bad-timestamp                          0            0.0/s
  congestion                      3863476            0.6/s
  ip-option                              0            0.0/s
  proto-cksum                            0            0.0/s
  state-mismatch                  3722058            0.6/s
  state-insert                          0            0.0/s
  state-limit                            0            0.0/s
  src-limit                      234360390          35.2/s
  synproxy                    13817759263        2074.4/s
Limit Counters
  max states per rule                    0            0.0/s
  max-src-states                    89727            0.0/s
  max-src-nodes                          0            0.0/s
  max-src-conn                          0            0.0/s
  max-src-conn-rate                      0            0.0/s
  overload table insertion              0            0.0/s
  overload flush states                  0            0.0/s</whitelisted_users>

From: Roger S.
Subject: Re: How to deal with DDoS ?
Newsgroups: gmane.os.openbsd.misc
Date: 2012-02-22 08:39:51 GMT (1 week, 4 days, 11 hours and 31 minutes ago)
On Tue, Feb 21, 2012 at 9:51 PM, Joachim Schipper
<joachim <at="">joachimschipper.nl> wrote:

Just the most obvious idea, since you mention that this sort-of-works if
you put "block drop in quick from !<whitelisted_users>": does it handle
this load if you turn off pf, or only include one or two trivial rules?

Did not try to turn off pf (I need it anyway), and my pf.conf is very
simple and already optimized following the good book of pf and some
undeadly posts.

It certainly suggests that you may be well-served by optimizing your
pf.conf… (also, you've probably found the "synproxy" directive? If
not, try that too.)

I already use synproxy, the problem is that I get so much SYN that
pf/state table collapses.

Also, state tracking is apparently faster than stateless pf for normal
firewalls. I'd double-check if this is still true in your case, though;
if nothing else, stateless pf makes a CARP'ed setup easier.

I am not sure to understand here. I want to use synproxy to protect my
backend servers, so I need state stracking.

I'm pretty sure you can muck with the rules without dropping existing
connections. (pf essentially does "does this packet match a known state?
If not, look at pf.conf".) This is almost certainly easier than your
proposed daemon.

Sure thing, the daemon is only a workaround to provide degraded but
working service when under attack.

A final, rather hackish, idea that probably does need a bit of
programming: greylisting for SYNs. Legitimate users will send you a
second SYN, so you could do something like (this has not even been
syntax-checked!)
  block drop log in quick from ! <syn_seen>no state flags S/SA

I like the idea. This may need some programming indeed, but it seems
even better than my idea. Thanks, I'll take a look at this.

and then add every logged IP to syn_seen. Obviously, this will slow down
access to the service for legitimate users, which may or may not be
acceptable.

We are speaking of a slower but working service, or no service at all.
I prefer the first alternative :)</syn_seen></whitelisted_users></joachim>

[cmb]

A flood of any type of traffic you're passing is bad news for every firewall. The lowest performance limit on anything any firewall does is new connections per second, and you'll hit that pretty quickly under a decent sized flood, or if the box is fast enough to handle that, you'll hit the state table limit quickly regardless of how high it is. Traffic you're blocking has little impact though. That's generically true of every firewall, they're almost always the most susceptible thing on any network to DDoS attacks (if you have adequate bandwidth to where you aren't knocked offline entirely and at the mercy of your provider).