Hello,
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, Adam. auto lo iface lo inet loopback auto eth0 iface eth0 inet static #address 192.168.50.216 address 192.168.50.181 netmask 255.255.255.0 gateway 192.168.50.254 dns-domain domain.lan dns-nameservers 192.168.50.201 192.168.50.202 iface eth0 inet6 static #address 2001:eee:4444:150::216 address 2001:eee:4444:150::181 netmask 64 gateway 2001:eee:4444:150::1 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