Cannot describe an IPv6-only SLAAC interface in yaml

[Daniel Axtens]

Say I have an interface I want to configure as IPv6-only, and I want the interface to be configured with SLAAC.

I tried to describe this interface like this:

network:
    version: 2
    ethernets:
        ens7:
            match:
                macaddress: 52:54:00:e7:a7:97

I don't want to set dhcp6, as that is described as being for stateful configuration, and I just want stateless auto-configuration via router advertisements.

However, this file doesn't work. If I reboot, the interface stays down:

$ ip a show ens7
3: ens7: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 52:54:00:e7:a7:97 brd ff:ff:ff:ff:ff:ff

I think this is due to the way src/networkd.c works:

static void
write_network_file(net_definition* def, const char* rootdir, const char* path)
{
    GString* s = NULL;

    /* do we need to write a .network file? */
    if (!def->dhcp4 && !def->dhcp6 && !def->bridge && !def->bond &&
        !def->ip4_addresses && !def->ip6_addresses && !def->gateway4 && !def->gateway6 &&
        !def->ip4_nameservers && !def->ip6_nameservers && !def->has_vlans)
        return;

This interface has none of those, so no network file is generated, so the interface stays down.

I'm not sure how you could actually get netplan to generate a network file without changing the code.

(FWIW, this is done in /e/n/i as 'iface ens7 inet6 auto')

[Daniel Axtens (daxtens)]

Ok, so further investigation shows that you can achieve the desired effect by specifying dhcp6: true - this will kick off autoconfiguration even if DHCP is not involved at all (e.g. SLAAC+RDNSS).

There are 2 things I'd like to do for this:
 - clarify the docs
 - (potentially) add an auto6 option that is an alias for dhcp6, just to make things more clear.

[Daniel Axtens (daxtens)]

This is now be documented based on https://github.com/CanonicalLtd/netplan/pull/10/files#diff-4e40b87174cfd3efbd425ab9f3bcec15, marking as fix committed.

[Daniel Axtens (daxtens)]

The fix is in Bionic, marking as Fix Released.