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Re: linux-ipsec: cornered: MTU and fragmentation bugs
- To: email@example.com (Alan Cox)
- Subject: Re: linux-ipsec: cornered: MTU and fragmentation bugs
- From: Richard Guy Briggs <firstname.lastname@example.org>
- Date: Thu, 25 Mar 1999 15:28:57 -0500 (EST)
- Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
- In-Reply-To: <m10PwDM-0007U1C@the-village.bc.nu> from "Alan Cox" at Mar 24, 99 10:33:32 pm
- Sender: email@example.com
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> > I'm surprised that the Linux kernel (2.0) is not sending ICMP
> > "fragmentation required and DF set" responses. I hope this is fixed
> I am not aware of any circumstances where Linux does not generate
> the appropriate messages. I think people would have noticed before
> two years ago if it was doing this wrong.
> if (skb->len+encap > dev2->mtu && (iph->frag_off & htons(IP_DF)))
Where is this code from? I assume ip_forward()? If so, I found the
ICMP message call that I think you intended to reference, but I don't
quite know why you quoted the ip_send() above...
Can it always be assumed that ip_fragment is called after checking DF?
ip_fragment() is not exclusively called from ip_forward().
The place John Denker speaks of is in ip_fragment. If a packet is
sent to ip_fragment() (such as at the end of ip_output()) without
checking DF, then the packet gets dropped silently.
> > 4) Heretofore linux has not generated these frag-needed messages. I
> > consider this a weakness in linux. I have a patch for this, as mentioned
> > in previous notes.
> No patch ever seen.
In case you didn't get it:
*** ip_fragment.c.old Tue Mar 23 17:22:41 1999
- --- ip_fragment.c Tue Mar 23 17:24:34 1999
*** 685,692 ****
- --- 685,693 ----
if (iph->frag_off & htons(IP_DF))
NETDEBUG(printk("ip_queue_xmit: frag needed\n"));
, dev); /* jsd */
> > 5) What's worse, there are some firewalls (the Firewall-One brand in
> > particular, and quite likely others) that in their usual configuration do
> > not pass these ICMP frag-needed datagrams. I consider this a weakness in
> > the firewalls. This is a pain in the neck to fix.
> It isnt the products, its the morons who install them. They have basically
> made the internet a 1500 mtu fixed size network. If you attempt to reason
> with the people who paid stupid amounts of money to have a 'highly certified
> security consultant' install it they know they paid $30K so even if they are
> wrong they'd get fired by their boss if they admitted it.
> It is perfectly possible to correctly install these products.
How easy will it be to fix this critical mass?
> > Solution #1 (ideal): Fix linux and fix the firewalls so that ICMP
> > frag-needed messages are returned to servers who depend on them. This
> > results in maximum efficiency.
> I've no evidence from the past two years to believe Linux does this wrongly.
That doesn't mean it doesn't exist... I'm still trying to verify
slainte mhath, RGB
Richard Guy Briggs -- PGP key available Auto-Free Ottawa! Canada
rgb at conscoop dot ottawa dot on dot ca <http://www.flora.org/afo/>
Please send all spam to firstname.lastname@example.org Marillion:<http://www.marillion.co.uk>
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