IPv6 default route NOT being set netplan Ubuntu 18.04.2 LTS

Am unable to configure the default route using netplan on an Ubuntu 18.04.2 system. The route works when explicit ip -6 route add ... commands are issued.

Privacy extensions have been disabled: /etc/sysctl.d/10-ipv6-privacy.conf

net.ipv6.conf.all.use_tempaddr = 0
net.ipv6.conf.default.use_tempaddr = 0
net.ipv6.conf.ens192.use_tempaddr = 0

SLACC and RA have been disabled, IPv4/IPv6 forwarding are one (machine serves as an OpenVPN server)

/etc/sysctl.d/10-ipv6-router.conf

net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.all.proxy_ndp = 1

/etc/sysctl.conf

net.ipv4.ip_forward=1
...
net.ipv6.conf.all.forwarding=1

/etc/netplan/01-netcfg.yaml

network: version: 2 renderer: networkd ethernets: ens18: dhcp4: no addresses: - a.b.c.d/24 gateway4: a.b.c.e dhcp6: no accept-ra: no addresses: - "2001:0db8:0004:4a1a::dead:beef/64" gateway6: "2001:0db8:0004::0001" ens19: dhcp6: no accept-ra: no addresses: - "2001:0db8:0004:4a1a::dead:dead/64" gateway6: "2001:0db8:0004::0001"

This assigns the addresses to the interface as shown below:

$ ip -6 addr show
2: ens18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000 inet6 2001:0db8:0004:4a1a::dead:beef/64 scope global valid_lft forever preferred_lft forever inet6 fe80::abcd:efff:fea3:2a03/64 scope link valid_lft forever preferred_lft forever
3: ens19: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000 inet6 2001:0db8:0004:4a1a::dead:dead/64 scope global valid_lft forever preferred_lft forever inet6 fe80::defc:deff:fe98:7c74/64 scope link valid_lft forever preferred_lft forever

The routes using ip -6 route show

2001:0db8:0004:4a1a::/64 dev ens19 proto kernel metric 256 pref medium
2001:0db8:0004:4a1a::/64 dev ens18 proto kernel metric 256 pref medium
fe80::/64 dev ens19 proto kernel metric 256 pref medium
fe80::/64 dev ens18 proto kernel metric 256 pref medium

There is no default route configured and pinging external sites does NOT work:

$ ping -6 -c3 ipv6.google.com
connect: Network is unreachable

Running the following two commands:

$ sudo ip -6 route add 2001:0db8:0004::0001 dev ens18 metric 1
$ sudo ip -6 route add default via 2001:0db8:0004::0001 dev ens18 metric 1

Now running ip -6 route shows:

2001:0db8:4::1 dev ens18 metric 1 pref medium
2001:0db8:4:4a1a::/64 dev ens19 proto kernel metric 256 pref medium
2001:0db8:4:4a1a::/64 dev ens18 proto kernel metric 256 pref medium
fe80::/64 dev ens19 proto kernel metric 256 pref medium
fe80::/64 dev ens18 proto kernel metric 256 pref medium
default via 2001:0db8:4::1 dev ens18 metric 1 pref medium

A default route is now configured and pinging external sites works:

$ ping -6 -c3 ipv6.google.com
PING ipv6.google.com(ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e)) 56 data bytes
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=1 ttl=56 time=23.2 ms
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=2 ttl=56 time=23.2 ms
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=3 ttl=56 time=23.2 ms
--- ipv6.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 23.274/23.283/23.293/0.176 ms

So, what is wrong with the netplan configuration? Why are the default IPv6 routes not being configured?

Edited after making changes suggested by @slangasek:

 ethernets: ens18: ... dhcp6: no accept-ra: no addresses: - "2001:0db8:0004:4a1a::dead:beef/64" routes: - to: "::/0" via: "2001:0db8:0004::1" metric: 1 on-link: true

However, this does not work. I get messages like ens18: Could not set route: No route to host in the journal logs.

From the answer at , I added the additional route to the router using scope: link:

 routes: - to: "2001:0db8:0004::1/128" via: "2001:0db8:0004::1" metric: 1 scope: link - to: "::/0" via: "2001:0db8:0004::1" metric: 1 on-link: true

But, still not working!

3 Answers

Your configuration shows you have a local network of 2001:0db8:0004:4a1a::/64 but you are trying to configure a gateway of 2001:0db8:0004::0001 which is not part of that network. So networkd refuses to add a default route via a router that you to not have a route to.

Your manual configuration with ip route also reflects this: you are first adding a route for the router's address, declaring that it is locally connected to your Ethernet, and afterwards setting the default route.

So you must do the equivalent with netplan, which is to declare two entries in routes:, one that is scope: link and one that sets on-link: true:

 addresses: - "2001:0db8:0004:4a1a::dead:dead/64" routes: - to: "2001:0db8:0004::0001/128" scope: link - to: "::/0" via: "2001:0db8:0004::0001" on-link: true

network: version: 2 renderer: networkd ethernets: ens19: dhcp6: no accept-ra: no addresses: ["2001:0db8:0004:4a1a::dead:dead/64"] gateway6: "2001:0db8:0004::0001" routes: - to: "2001:0db8:0004::0001/128" scope: link

This works for me as of this writing on Ubuntu 18.04.4 LTS (using different netblocks but tested)

I just ran into this issue. The actual cause is written right in /etc/sysctl.conf

# Uncomment the next line to enable packet forwarding for IPv6
# Enabling this option disables Stateless Address Autoconfiguration
# based on Router Advertisements for this host
#net.ipv6.conf.all.forwarding=1

When IPv6 forwarding is disabled, the host will not listen to router advertisements, which is how the default route is usually determined. Therefore, you have to manually add the default route (and DNS servers and anything else you were getting out of the RA) or, set the accept_ra kernel option to 2, which overrides the behaviour.

See this blog article for some more info.