netdev - Re: [PATCH v3 net-next] net: ipv6: Add sysctl entry to disable MTU updates from RA

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Date:	Mon, 26 Jan 2015 18:27:09 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	David Miller <davem@...emloft.net>
Cc:	harouth@...eaurora.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next] net: ipv6: Add sysctl entry to disable MTU updates from RA

David Miller <davem@...emloft.net> writes:

> Under what circumstances would userland ignore a router advertized
> MTU, and are the RFCs ok with this?

RFC 4861 ( https://tools.ietf.org/html/rfc4861#page-54 ) says:

  "If the MTU option is present, hosts SHOULD copy the option's value
   into LinkMTU so long as the value is greater than or equal to the
   minimum link MTU [IPv6] and does not exceed the maximum LinkMTU value
   specified in the link-type-specific document (e.g., [IPv6-ETHER])."

So the RFC acknowledge that there may exist valid reasons in particular
circumstances to ignore the MTU option.  As others have stated: This
might be necessary for DoS prevention, working around bugs in other
equipment, etc.


Bjørn
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