RE: streams of fragments...

[Rich Ostergard <rostergard () radiocentral com>]

You could get around this by setting the MTU to 1480, thus making it
divisible by 8.  This also has the added bonus of reducing the network
overhead by 98.7%  All this info is in that fragmentation paper posted Dug
Song, I highly recommend reading it.

-----Original Message-----
From: Portnoy, Gary [mailto:gportnoy () belenosinc com]
Sent: Wednesday, July 18, 2001 11:47 AM
To: 'Jose Nazario'; Gamble
Cc: Russell Fulton; incidents () securityfocus com
Subject: RE: streams of fragments...


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:jose () biocserver BIOC cwru edu]
Sent: Wednesday, July 18, 2001 1:10 PM
To: Gamble
Cc: Russell Fulton; incidents () securityfocus 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                                                 jose () cwru 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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