Mailinglist Archive: opensuse-security (192 mails)

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Re: [suse-security] IPChains
  • From: Gerhard Sittig <Gerhard.Sittig@xxxxxxx>
  • Date: Sat, 27 May 2000 11:09:32 +0200
  • Message-id: <20000527110932.B2305@xxxxxxxxxxxxx>
On Sat, May 27, 2000 at 01:57 +0200, Roman Drahtmueller wrote:
> Gerhard,
> >
> > And to make it security relevant, again (that's what we're
> > here for after all): ping can act as a tunnel transporting
> > data from and to the outside *if* you have a relay station
> > inside your LAN. That's why admins sometimes decide to block
> > pings and traceroutes and no user should feel any real loss
> > about it.
> This is right, but it's also a good advice _not_ to filter ICMP
> packets coming through the firewall into the internal network
> or at least to the hosts that can take advantage of ICMPs
> (notably mailservers or such). Without these control messages
> long timeouts or inefficient bandwidth usage is the result.

That's why I wrote something along the lines of "decide to block
pings and traceroutes" since I remember the discussions bubbling
up time after time on blocking ICMP completely either breaks
whole TCP stacks or important functionality thereof. The most
popular feature suffering from missing ICMP seems to be MTU
discovery, other problems include the timeouts you talk about.

But looking at all the ICMP packet types one should at least
block the redirect ones. And besides "dest unreach", "param
prob", "source quench" and "time exceeded" everything else seems
luxurious to pass through. The "unreach" could be filtered even
more for its subtypes. And *if* you have to enable echo reqs and
replies, you better block icmp to the network and broadcast
addresses (remember smurf, tfn and the other DoSes?). To further
protect against attacks, one would wish for a feature like
FreeBSD's icmp bandwidth limiting -- is there something similar
for Linux?

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Gerhard Sittig true | mail -s "get gpg key" Gerhard.Sittig@xxxxxxx
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