rad

Synopsis #

IPv6 addressing on OpenBSD is typically handled using Stateless Address Autoconfiguration (SLAAC) rather than DHCPv6. The rad(8) daemon provides Router Advertisements (RA) that clients such as slaacd(8) use to self-configure IPv6 addresses, routes, and optional DNS resolver information without requiring a central DHCPv6 server.

Older OpenBSD releases used rtadvd(8) for this role. rtadvd(8) was replaced by rad(8) in OpenBSD 6.4, so new OpenBSD systems should use rad(8) and /etc/rad.conf.

In contrast with IPv4, where DHCP is commonly used for address configuration, OpenBSD’s base system intentionally omits DHCPv6 server functionality. A lightweight DHCPv6 server is available via ports (dhcp6s) but is seldom needed in practice.

DHCP and Autoconfiguration Components in OpenBSD #

The following table summarizes the major DHCP and IPv6 autoconfiguration components available on OpenBSD, covering both IPv4 and IPv6.

For most IPv6 environments on OpenBSD, rad(8) and slaacd(8) are sufficient to provide robust autoconfiguration without requiring additional software.

Configuration #

The configuration file for rad(8) is /etc/rad.conf. A minimal configuration advertises on the selected interface:

interface em0

To advertise IPv6 prefixes on interface em0, enable and start the daemon as follows:

# rcctl enable rad
# rcctl start rad

This instructs rad(8) to send Router Advertisements on em0, which are then processed by client machines, typically running slaacd(8).

Customizing Advertisement Behavior #

A more explicit /etc/rad.conf can set the advertised prefix and DNS information:

interface em0 {
    prefix 2001:db8:1::/64
    dns {
        nameserver 2001:db8:1::1
        search example.org
    }
}

This announces the LAN prefix and provides Recursive DNS Server (RDNSS) information to clients that support it.

Example: LAN Router with Static IPv6 Prefix #

Given a statically assigned IPv6 prefix (for example, 2001:db8:1::/64), the router must:

1. Assign an address to its LAN interface:

   # ifconfig em0 inet6 2001:db8:1::1 prefixlen 64 alias
   

2. Create /etc/rad.conf with an interface em0 block.

3. Ensure that packet forwarding is enabled:

   # sysctl net.inet6.ip6.forwarding=1
   

4. Persist forwarding in /etc/sysctl.conf:

   net.inet6.ip6.forwarding=1
   

5. Enable and start rad(8):

   # rcctl enable rad
   # rcctl start rad
   

Notes on DHCPv6 and Mixed Environments #

OpenBSD’s base system does not include a DHCPv6 server. The dhcp6s daemon is available via the ports collection, but it is generally used only when stateful IPv6 configuration is required, such as central IP management or lease tracking.

Most OpenBSD systems will operate using:

• rad(8) to announce prefixes, routes, and optional DNS resolver information
• slaacd(8) on clients to auto-configure

This configuration provides full IPv6 connectivity without requiring DHCPv6 and aligns with common IPv6 deployment practice.

Debugging and Verification #

Check that rad(8) is enabled and running:

# rcctl check rad

To verify that router advertisements are being sent:

# tcpdump -n -i em0 icmp6

Look for Router Advertisement ICMPv6 messages.

To verify that a client interface accepted the RA and configured an address:

# ifconfig em0

Expected output should include an inet6 autoconfigured address, typically starting with the advertised prefix.

Additional Considerations #

Ensure that the system firewall (pf(4)) allows IPv6 traffic, including ICMPv6 Router Advertisements:

Example /etc/pf.conf fragment:

pass inet6 proto ipv6-icmp from :: to ff02::1

This permits outgoing multicast advertisements necessary for SLAAC.