I have a serious issue with IPv6 after upgrading several hosts from Jessie to Stretch.
Default gateway cannot be set using gateway kword in /etc/network/interfaces. After some research over Internet, I figured out this is probably related to router advertisement so I added a list of pre-up command to disable everything.
systemctl status networking was then okay, but after some time (days ?) the IPv6 gateway is gone, again....
Thanks in advance,
iface lo inet loopback
iface eth0 inet static
dns-nameservers 192.168.50.201 192.168.50.202
iface eth0 inet6 static
pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/autoconf
pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra
pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_defrtr
pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_pinfo
pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_rtr_pref