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Re: linux-ipsec: cornered: MTU and fragmentation bugs
- To: email@example.com (John Gilmore)
- Subject: Re: linux-ipsec: cornered: MTU and fragmentation bugs
- From: firstname.lastname@example.org (Alan Cox)
- Date: Wed, 24 Mar 1999 22:33:32 +0000 (GMT)
- 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
- In-Reply-To: <199903242110.NAA21766@toad.com> from "John Gilmore" at Mar 24, 99 01:10:02 pm
- Sender: firstname.lastname@example.org
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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)))
> 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.
> 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.
> 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.