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
