There also seems to be another bug for ipv6 only; the sysctl setting of mtu happens before the ip link set dev up; which means, at up time, the interface gets the default mtu which clobbers the ipv6 mtu.
/sbin/sysctl -q -e -w net.ipv6.conf.ens8.autoconf=0 /sbin/sysctl -q -e -w net.ipv6.conf.ens8.mtu=1480 /bin/ip link set dev ens8 up /bin/ip -6 addr add fd0d:ffe0:5771::1:0:0/64 dev ens8 I'm testing this post-up hook which I think covers all of the cases. # cat /etc/network/if-up.d/mtuipv6 #!/bin/sh [ "$IFACE" != "lo" ] || exit 0 # Trigger only if MTU configured [ -n "$IF_MTU" ] || exit 0 # Target ipv6 MTU settings only [ "$ADDRFAM" = "inet6" ] || exit 0 # We need to check the underlying interface MTU and # raise it if the IPV6 mtu is larger CUR_DEV_MTU=$(/bin/cat /sys/class/net/$IFACE/mtu) if [ $CUR_DEV_MTU -lt $IF_MTU ]; then /bin/ip link set $IFACE mtu $IF_MTU fi /sbin/sysctl -q -e -w net.ipv6.conf.$IFACE.mtu=$IF_MTU Thoughts? -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1609367 Title: ifupdown does not set ipv6-only large mtu To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/1609367/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs