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List:       openjdk-net-dev
Subject:    Re: RFR: 8290349: IP_DONTFRAGMENT doesn't set DF bit in IPv4 header [v4]
From:       Michael McMahon <michaelm () openjdk ! org>
Date:       2022-08-04 10:51:47
Message-ID: y97yHwPDOo1nqQUcOY85k7BasKnusWdpYmGxULWvSuY=.1e7e2481-72a5-4cab-a5ab-2d96b2828ec3 () github ! com
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On Wed, 3 Aug 2022 09:43:44 GMT, Daniel Jeliński <djelinski@openjdk.org> wrote:

> > This patch partially fixes the issue where IPv6 sockets were fragmenting outgoing \
> > IPv4 datagrams even when IP_DONTFRAGMENT flag was set. Specifically, it fixes the \
> > issue on Linux and Windows. As far as I could tell, the issue is unfixable on Mac \
> > OS X. 
> > All systems have a separate DONTFRAGMENT flag for IPv4 and IPv6. Each flag only \
> > affects packets from its address family; if we want to disable fragmentation of \
> > both IPv4 and IPv6 packets sent by an IPv6 socket, we need to set both IPv4 and \
> > IPv6 flags. This is similar to other already existing options like IP_TOS or \
> > IP_MULTICAST_*. 
> > On Mac OS X it's impossible to set an IPv4 socket option on an IPv6 socket; \
> > attempting to do so results in an error. This is a known issue with Mac OS X; on \
> > that system we return false from \
> > [Net#shouldSetBothIPv4AndIPv6Options](https://github.com/openjdk/jdk/blob/2342684f2cd91a2e5f43dd271e95836aa78e7d0a/src/java.base/unix/native/libnio/ch/Net.c#L159) \
> > to avoid setting IPv4 options. 
> > As far as I can tell, non-privileged users have no way to check if the DF flag \
> > was set or if the packet was fragmented. I implemented a test that attempted to \
> > send a large packet over a physical interface and expected SocketException / \
> > EMSGSIZE; the test frequently failed for unrelated reasons, so I decided against \
> > including it. Loopback interface has infinite MTU on some systems, so can not be \
> > used for this test. 
> > Testing performed (with IP_DONTFRAGMENT flag):
> > Windows 10:
> > With this patch, IPv4 packets sent from IPv6 socket have the DF flag; sending an \
> > IPv4 packet larger than the interface MTU results in EMSGSIZE and no packet is \
> > sent. Without this patch, the packet is fragmented and sent without DF flag. \
> > Sending IPv6 packets larger than the interface MTU usually results in EMSGSIZE. \
> > It may succeed if the destination address is non-routable. 
> > For other systems I could not capture packets, so I can only report the observed \
> > behavior of sending packets. Windows 2012 and 2016 (IP_MTU_DISCOVER not \
> > supported): With this patch, sending any packet exceeding MTU size fails with \
> > EMSGSIZE. Without this patch sending a large IPv6 packet succeeds. If a packet is \
> > sent to a non-routable address, send succeeds, no error is reported. 
> > Linux:
> > With this patch, sending any packet exceeding MTU size fails with EMSGSIZE. \
> > Without this patch, sending large IPv4 packets from IPv6 sockets succeeds. 
> > Mac OS X 12:
> > Sending an IPv6 packet exceeding MTU size fails with EMSGSIZE. Sending large IPv4 \
> > packets from IPv4 sockets also fails with EMSGSIZE. Sending large IPv4 packets \
> > from IPv6 sockets succeeds. The patch does not change the observed behavior.
> 
> Daniel Jeliński has updated the pull request incrementally with one additional \
> commit since the last revision: 
> Revert Net.c comment changes

The change is fine. The Net.c comment is not worth arguing over.

-------------

Marked as reviewed by michaelm (Reviewer).

PR: https://git.openjdk.org/jdk/pull/9575


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