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From: Portnoy, Gary (gportnoy
belenosinc.com)Date: Wed Jul 18 2001 - 13:47:09 CDT
There wouldn't be any harm in blocking all fragmented packets, unless your
users VPN in. I know that certain VPN protocols encapsulate the IP data,
creating packets larger than the Ethernet MTU of 1500. This causes the
packet to be fragmented. Just a word of advice: be careful. Sniff your
network to make sure that you don't normally generate or receive fragmented
packets...
-Gary-
-----Original Message-----
From: Jose Nazario [mailto:josebiocserver.BIOC.cwru.edu]
Sent: Wednesday, July 18, 2001 1:10 PM
To: Gamble
Cc: Russell Fulton; incidentssecurityfocus.com
Subject: Re: streams of fragments...
On Wed, 18 Jul 2001, Gamble wrote:
> This sounds like a DOS attack. By sending you many fragmented
> packets the attacker could consume a lot of the memory on your
> machine. You could counter this by blocking all IP fragments on your
> firewall, but that would also prevent legitimate activities.
a lot of sites block fragments to no great loss of theirs. in this day and
age it's usually not needed. i found this out some years ago helping a
friend with a Linux firewall on his PPP link. his ISP had a PPP MTU of
about 576, but his ethernet frames were set to an MTU 1500, and your
guessed it, he generated fragments. some sites were totally inaccessible
until he tuned down his MTU to under 576 on his internal ethernet LAN.
they're big names, but i wont post them here. *shrug* block fragments is
not that bad to do these days.
____________________________
jose nazario josecwru.edu
PGP: 89 B0 81 DA 5B FD 7E 00 99 C3 B2 CD 48 A0 07 80
PGP key ID 0xFD37F4E5 (pgp.mit.edu)
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