On Wed, 20 Jul 2022 17:19:50 GMT, Daniel Jeliński <djelin...@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.

src/jdk.net/macosx/native/libextnet/MacOSXSocketOptions.c line 207:

> 205:         }
> 206:     }
> 207:     return JNI_TRUE;

Hello Daniel, if I'm reading this diff correctly, then we have inverted the 
check on `fd`. So I suspect this final `return` now needs to be changed to 
`return JNI_FALSE` to account for `fd` being `-1`?

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

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

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