netdev - Re: [Bugme-new] [Bug 8756] New: Route advmss copied to ALL routes when interface MTU changes

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Date:	Sat, 14 Jul 2007 21:48:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	netdev@...r.kernel.org
Cc:	"bugme-daemon@...nel-bugs.osdl.org" 
	<bugme-daemon@...nel-bugs.osdl.org>, bugzilla@...e.lp0.eu
Subject: Re: [Bugme-new] [Bug 8756] New: Route advmss copied to ALL routes
 when interface MTU changes

On Sat, 14 Jul 2007 15:34:31 -0700 (PDT) bugme-daemon@...zilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=8756

It is unclear which kernel version this applies to?

>            Summary: Route advmss copied to ALL routes when interface MTU
>                     changes
>            Product: Networking
>            Version: 2.5
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: IPV6
>         AssignedTo: yoshfuji@...ux-ipv6.org
>         ReportedBy: bugzilla@...e.lp0.eu
> 
> 
> Steps to reproduce:
> 1. Have multiple network devices and routes for them with standard (low, i.e.
> 1500) MTUs and default ADVMSS (e.g. 1440)
> 2. Change the MTU on one of them to something high (e.g. 7200)
> 3. The ADVMSS is incorrectly set for ALL routes based on the new MTU of the
> interface that changed.
> 
> Note: if addrconf is running then the ADVMSS will be reset on the next RA, so
> this will impede attempts to reproduce.
> 
> # ip -6 r
> fe80::/64 dev sit0  metric 256  expires 4481744sec mtu 1480 advmss 1440
> hoplimit 4294967295
> fe80::/64 dev ppp0  metric 256  expires 19071345sec mtu 1500 advmss 1440
> hoplimit 4294967295
> fe80::/64 dev eth0  metric 256  expires 19072430sec mtu 1500 advmss 1440
> hoplimit 4294967295
> default dev ppp0  metric 1024  expires 19071347sec mtu 1500 advmss 1440
> hoplimit 4294967295
> 
> # ifconfig eth0 mtu 7200
> 
> # ip -6 r
> fe80::/64 dev sit0  metric 256  expires 4481732sec mtu 1480 advmss 7140
> hoplimit 4294967295 <-- wrong
> fe80::/64 dev ppp0  metric 256  expires 19071334sec mtu 1500 advmss 7140
> hoplimit 4294967295 <-- wrong
> fe80::/64 dev eth0  metric 256  expires 19072419sec mtu 7200 advmss 7140
> hoplimit 4294967295 <-- correct
> 
> Explicitly setting the MTU on these other routes (although this is not possible
> without having to add another route due to bug 8755) automatically sets ADVMSS
> appropriately.
> 
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