With IPv6 connection and RA the MTU value sometimes get reset to link default

[sz gy]

In the router the radvd.conf:

--- cut ---
interface eth0
{
        AdvSendAdvert on;

        # Disable Mobile IPv6 support
        AdvHomeAgentFlag off;
        AdvLinkMTU 1410;

        prefix 2001:470:212c::/64
        {
                AdvOnLink on;
                AdvAutonomous on;

                # Disable Mobile IPv6 support
                AdvRouterAddr off;
        };
};
--- cut ---

So the advertised MTU is 1410. The interfaces configured to dual stack. So there is an IPv4 link with MTU 1500.

If I read this in syslog:

--- cut ---
Feb 7 14:32:37 szimszon-kili NetworkManager[903]: <warn> (enp1s0): DHCPv6 request timed out.
Feb 7 14:32:37 szimszon-kili NetworkManager[903]: <info> (enp1s0): DHCPv6 state changed unknown -> timeout
Feb 7 14:32:37 szimszon-kili NetworkManager[903]: <info> (enp1s0): canceled DHCP transaction, DHCP client pid 26074
Feb 7 14:32:37 szimszon-kili NetworkManager[903]: <info> (enp1s0): DHCPv6 state changed timeout -> done
--- cut ---

The /proc/sys/net/ipv6/conf/enp1s0/mtu is: 1500 which is wrong.

After I read this in syslog:

--- cut ---
Feb 7 14:34:20 szimszon-kili dhclient: DHCPREQUEST of 192.168.55.70 on enp1s0 to 192.168.55.1 port 67 (xid=0x28751fdf)
Feb 7 14:34:20 szimszon-kili dhclient: DHCPACK of 192.168.55.70 from 192.168.55.1
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> address 192.168.55.70
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> plen 24 (255.255.255.0)
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> gateway 192.168.55.1
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> server identifier 192.168.55.1
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> lease time 600
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> nameserver '192.168.55.1'
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> domain name 'kopaszhegy'
Feb 7 14:34:20 szimszon-kili NetworkManager[903]: <info> (enp1s0): DHCPv4 state changed bound -> bound
Feb 7 14:34:20 szimszon-kili dbus[906]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service'
Feb 7 14:34:20 szimszon-kili dhclient: bound to 192.168.55.70 -- renewal in 293 seconds.
Feb 7 14:34:20 szimszon-kili systemd[1]: Starting Network Manager Script Dispatcher Service...
Feb 7 14:34:20 szimszon-kili dbus[906]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Feb 7 14:34:20 szimszon-kili systemd[1]: Started Network Manager Script Dispatcher Service.
Feb 7 14:34:20 szimszon-kili nm-dispatcher: Dispatching action 'dhcp4-change' for enp1s0
--- cut ---
 The /proc/sys/net/ipv6/conf/enp1s0/mtu is: 1410 which is good.

Because the IPv6 connection can't handle packet size bigger than 1420 if the interface has MTU 1500 than there are many connection problems.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: network-manager 1.0.4-0ubuntu5.2
ProcVersionSignature: Ubuntu 4.2.0-27.32-generic 4.2.8-ckt1
Uname: Linux 4.2.0-27-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Feb 7 17:25:09 2016
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
InstallationDate: Installed on 2015-11-10 (89 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
NetworkManager.state:
 [main]
 NetworkingEnabled=true
 WirelessEnabled=true
 WWANEnabled=true
 WimaxEnabled=true
SourcePackage: network-manager
UpgradeStatus: No upgrade log present (probably fresh install)
nmcli-nm: Error: command ['nmcli', '-f', 'all', 'nm'] failed with exit code 2: Error: Object 'nm' is unknown, try 'nmcli help'.