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RE: attacks

> -----Original Message-----
> From: sim ton [mailto:firewall38@lycos.com] 
> Sent: Tuesday, June 04, 2002 11:09 AM
> To: debian
> Subject: attacks

A couple of opinions here; I'm no expert on any of this, so take
these with a grain of salt:

> * to protect against ping of DEATH :
> $IPTABLES --append FORWARD -p icmp --icmp-type echo-request 
> --match limit --limit 1/s --jump ACCEPT

That's how it's shown in the documentation for iptables, so I
hope it works.  ;)  You've got the options spelled out in full
(--append instead of -A, --jump instead of -j) but otherwise it
looks okay to me.

You probably want to add some route verification too:

  for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
      echo 1 > $f

> * to protect against UDP flooding :
> i don't know yet, i heard about udp flood with chargen(19) 
> and echo(7), must i forbid these ports ??

These (along with discard[9]) are diagnostic ports that I would
venture to say, nobody uses anymore; for example, ICMP is used
now instead of echo.  In fact, they've been removed from NT due
to the DoS aspects.  I would make sure they are turned off in
your inetd/xinetd, because I *have* seen them pop up as enabled
in a default installation.

If you want to know how they could be exploited, have a look at:


That having been said, see the last comment at bottom...

> * to protect against tiny fragments and frangment overlapping 
> nothing yet... the only thing i know is that i can't forbid 
> incoming fragment packet... 

I guess you could have -f in a rule.  However, there may be some
implications when using NAT, since all the packets get reassembled
first anyway.  But my understanding of this is limited.  To quote
the docs:

  "If you are doing connection tracking or NAT, then all
   fragments will get merged back together before they reach
   the packet filtering code, so you need never worry about

> last thing i heard an attack on port 0 with UDP
> can i forbid this port ? what is port 0 ?
> is true ?

I'm not really sure what port 0 is used for; one site referred
to ICMP Click as a possible exploit, but IANA just shows it as
simply "reserved".  I hope someone else can clarify what it's

Anyway, what I would do is block TCP & UDP 0-19.  This tosses
"port 0", as well as tcpmux, compressnet, rje, echo, discard,
systat, daytime, netstat, qotd, msp, and chargen all at once:

  $IPTABLES -A INPUT -i eth0 -p tcp --dport 0:19 -j DROP
  $IPTABLES -A INPUT -i eth0 -p udp --dport 0:19 -j DROP

I specified the interface, just in case netstat somehow may
get blocked on the internal machines or the localhost... this
may not be necessary and you can experiment accordingly.


Jeff Bonner

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