The Linux IPv6 Router Advertisement Daemon (radvd)

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Background

radvd (the router advertisement daemon) sends IPv6 advertisements and responds to node IPv6 requests. Here's the description from the man page.

radvd is the router advertisement daemon for IPv6. It listens to router solicitations and sends router advertisements as described in "Neighbor Discovery for IP Version 6 (IPv6)" (RFC 4861). With these advertisements hosts can automatically configure their addresses and some other parameters. They also can choose a default router based on these advertisements.

Installation and Setup

Setting radvd is suspiciously easy compared to other linux services. First you install it.

sudo apt install radvd

Installing it will also start the service so we can use systemctl to check how it's doing.

systemctl status radvd

systemctl status radvd
● radvd.service - LSB: Router Advertising Daemon
   Loaded: loaded (/etc/init.d/radvd; bad; vendor preset: enabled)
   Active: active (exited) since Fri 2018-07-20 12:39:19 PDT; 36s ago
     Docs: man:systemd-sysv-generator(8)

Jul 20 12:39:18 asgaard systemd[1]: Starting LSB: Router Advertising Daemon...
Jul 20 12:39:19 asgaard radvd[8532]: Starting radvd:
Jul 20 12:39:19 asgaard radvd[8532]: * /etc/radvd.conf does not exist or is empty.
Jul 20 12:39:19 asgaard radvd[8532]: * See /usr/share/doc/radvd/README.Debian
Jul 20 12:39:19 asgaard radvd[8532]: * radvd will *not* be started.
Jul 20 12:39:19 asgaard systemd[1]: Started LSB: Router Advertising Daemon.

You can see in the output that there's a line with /etc/radvd.conf does not exist or is empty.. To fix this we're going to create the configuration file.

The Configuration File

Now create a configuration file (named radvd.conf). Here's an example.

interface eth0
{
MinRtrAdvInterval 3;
MaxRtrAdvInterval 4;
AdvSendAdvert on;
AdvManagedFlag on;
prefix 2001:db7::/64
{ AdvValidLifetime 14300; AdvPreferredLifetime 14200; }
;
};

Note that the interface name depends on your system. Most linux systems don't use the eth0 convention anymore. On my desktop the ethernet inteface is called enp1s0 and on my raspberry pi it's enxb827eb6c9129 so make sure you check what your interface name is using ifconfig.

Now move (or copy) the config file into /etc/

sudo mv radvd.conf /etc/

And then restart radvd and check its state.

sudo service radvd restart
systemctl status radvd

● radvd.service - LSB: Router Advertising Daemon
   Loaded: loaded (/etc/init.d/radvd; bad; vendor preset: enabled)
   Active: active (exited) since Fri 2018-07-20 12:41:36 PDT; 2s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 10008 ExecStop=/etc/init.d/radvd stop (code=exited, status=0/SUCCESS)
  Process: 10017 ExecStart=/etc/init.d/radvd start (code=exited, status=0/SUCCESS)

Jul 20 12:41:36 asgaard systemd[1]: Starting LSB: Router Advertising Daemon...
Jul 20 12:41:36 asgaard radvd[10017]: Starting radvd:
Jul 20 12:41:36 asgaard radvd[10017]: * IPv6 forwarding seems to be disabled.
Jul 20 12:41:36 asgaard radvd[10017]: * See /usr/share/doc/radvd/README.Debian
Jul 20 12:41:36 asgaard radvd[10017]: * radvd will *not* be started.
Jul 20 12:41:36 asgaard systemd[1]: Started LSB: Router Advertising Daemon.

So the warning about the configuration file went away, now we have to fix the IPv6 forwarding seems to be disabled. error.

Enable IPv6 Forwarding

For now you can enable it at the command line.

sudo sysctl -w net.ipv6.conf.all.forwarding=1

And restart and check radvd again.

sudo service radvd restart
systemctl status radvd

● radvd.service - LSB: Router Advertising Daemon
   Loaded: loaded (/etc/init.d/radvd; bad; vendor preset: enabled)
   Active: active (running) since Fri 2018-07-20 12:45:02 PDT; 20s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 12255 ExecStop=/etc/init.d/radvd stop (code=exited, status=0/SUCCESS)
  Process: 12264 ExecStart=/etc/init.d/radvd start (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/radvd.service
           ├─12275 /usr/sbin/radvd -u radvd -p /var/run/radvd/radvd.pid
           └─12276 /usr/sbin/radvd -u radvd -p /var/run/radvd/radvd.pid

Jul 20 12:45:01 asgaard systemd[1]: Starting LSB: Router Advertising Daemon...
Jul 20 12:45:02 asgaard radvd[12274]: version 2.11 started
Jul 20 12:45:02 asgaard radvd[12264]: Starting radvd: radvd.
Jul 20 12:45:02 asgaard systemd[1]: Started LSB: Router Advertising Daemon.

So it looks like our daemon is working. If you restart the server, though, the IPv6 port forwarding won't be enabled so open up /etc/sysctl.conf and un-comment out the following line.

net.ipv6.conf.all.forwarding=1

Now the next time you power on the computer it should be enabled.

Checking

To check if it's working you can put another device on the LAN and check ifconfig. In our radvd.conf file we defined the prefix for the address:

prefix 2001:db7::/64

So in the ifconfig output we should see an entry for an IPv6 address that starts with 2001:db7::. In fact I ended up with two.

inet6 2001:db7::b8b4:6adf:6267:5571  prefixlen 64  scopeid 0x0<global>
inet6 2001:db7::a80c:1a05:eaac:c1e8  prefixlen 64  scopeid 0x0<global>

Sources