netdev - Re: Fw: [Bug 55861] New: PMTU discovery no longer works in Linux 3.6+ with routers that do not send next hop MTU information

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Date:	Thu, 28 Mar 2013 09:59:04 +0800
From:	Li Wei <lw@...fujitsu.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
CC:	netdev@...r.kernel.org
Subject: Re: Fw: [Bug 55861] New: PMTU discovery no longer works in Linux
 3.6+ with routers that do not send next hop MTU information

It seems to be in icmp_unreach():

		case ICMP_FRAG_NEEDED:
			if (ipv4_config.no_pmtu_disc) {
				LIMIT_NETDEBUG(KERN_INFO pr_fmt("%pI4: fragmentation needed and DF set\n"),
					       &iph->daddr);
			} else {
				info = ntohs(icmph->un.frag.mtu);
				if (!info)
					goto out;

When MTU is zero, we skip the process in icmp_socket_deliver() which propagate
this error to transport protocols.

After some investigation it seems in transport protocols' err_handler which
finally called dst->update_pmtu(ip_rt_update_pmtu for IPv4) can deal with this
situation properly.

So I think we can simply kill the check of icmph->un.frag.mtu here.

On 03/27/2013 11:31 PM, Stephen Hemminger wrote:
> 
> 
> Begin forwarded message:
> 
> Date: Wed, 27 Mar 2013 08:25:40 -0700
> From: "bugzilla-daemon@...zilla.kernel.org" <bugzilla-daemon@...zilla.kernel.org>
> To: "stephen@...workplumber.org" <stephen@...workplumber.org>
> Subject: [Bug 55861] New: PMTU discovery no longer works in Linux 3.6+ with routers that do not send next hop MTU information
> 
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=55861
> 
>            Summary: PMTU discovery no longer works in Linux 3.6+ with
>                     routers that do not send next hop MTU information
>            Product: Networking
>            Version: 2.5
>     Kernel Version: 3.6 onwards
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: IPV4
>         AssignedTo: shemminger@...ux-foundation.org
>         ReportedBy: _@...b.eu
>         Regression: Yes
> 
> 
> After upgrading recently, I found that path MTU discovery no longer worked
> correctly for accessing some devices on the other side of an IPsec tunnel.
> 
> Bisection revealed the problems started with 3.6 and are still present in
> 3.9-rc4 (latest available at time of reporting).
> 
> Some investigation into code changes leads me to the belief that Linux lost
> support for handling ICMP destination unreachable fragmentation needed packets
> for which the next hop MTU field is zero. This is an expected condition when
> dealing with older routers, as RFC792 originally defined ICMP destination
> unreachable fragmentation needed without a next hop MTU field, and it was later
> added in bytes previously allocated as unused.
> 
> The particular router in my case generating such packets is a machine running
> OpenBSD 4.6.
> 
> A commit that appears to be of particular interest in this bug is
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=46517008e1168dc926cf2c47d529efc07eca85c0
> 
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