Give a LAN SLAAC addresses with DNS from stateless DHCPv6

Cisco IOSWorking

Hosts on R1's LAN build their own addresses with SLAAC and learn the DNS server and search domain from R1 acting as a stateless DHCPv6 server. No per-host leases are kept.

Item	Value
LAN prefix	2001:db8:acad:10::/64
R1 Gi0/0/0	2001:db8:acad:10::1/64, link-local fe80::1
DNS server	2001:db8:acad:100::53
Search domain	corp.example.com

Tasks

Make sure R1 routes IPv6 and advertises the LAN prefix for SLAAC.
Create a DHCPv6 pool that carries only the DNS server and domain name.
Set the O flag in R1's RAs so hosts ask for those options.
Attach the pool to the LAN interface.
Verify on R1 and on both hosts.

Solution

1Routing and the LAN prefixR1

R1 on Cisco IOS

ipv6 unicast-routing
!
interface GigabitEthernet0/0/0
 ipv6 address fe80::1 link-local
 ipv6 address 2001:db8:acad:10::1/64
 no shutdown

The /64 is advertised with A=1 by default, so SLAAC works as soon as routing is on.

2Stateless DHCPv6 poolR1
R1 on Cisco IOS
```
ipv6 dhcp pool LAN10-INFO
 dns-server 2001:db8:acad:100::53
 domain-name corp.example.com
```
No address prefix line. That is what makes the pool stateless.
3Set the O flag and attach the poolR1
R1 on Cisco IOS
```
interface GigabitEthernet0/0/0
 ipv6 nd other-config-flag
 ipv6 dhcp server LAN10-INFO
```
other-config-flag sets O=1. M stays 0 and A stays 1, so hosts keep using SLAAC for addresses and send a DHCPv6 Information-Request for the options only.

Verify

R1 output

R1# show ipv6 interface GigabitEthernet0/0/0 | include Hosts use
Hosts use stateless autoconfig for addresses.
Hosts use DHCP to obtain other configuration.

SLAAC for addresses, DHCPv6 for the rest. Exactly the M=0, O=1 combination.

R1 output

R1# show ipv6 dhcp pool
DHCPv6 pool: LAN10-INFO
  DNS server: 2001:DB8:ACAD:100::53
  Domain name: corp.example.com
  Active clients: 0

The options are in the pool. Zero active clients is correct for a stateless pool, which keeps no bindings.

R1 output

R1# show ipv6 dhcp interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is in server mode
  Using pool: LAN10-INFO
  Preference value: 0
  Hint from client: ignored
  Rapid-Commit: disabled

The interface serves the right pool.

R1 output

R1# debug ipv6 dhcp
IPv6 DHCP: Received INFORMATION-REQUEST from FE80::8D2C:4E1F:3B6A:91D0 on GigabitEthernet0/0/0
IPv6 DHCP: Sending REPLY to FE80::8D2C:4E1F:3B6A:91D0 on GigabitEthernet0/0/0

The host asks with Information-Request, not Solicit, which proves it is doing stateless DHCPv6. Turn the debug off with undebug all.

PC1 output

PC1# ipconfig /all
Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . : corp.example.com
   IPv6 Address. . . . . . . . . . . : 2001:db8:acad:10:8d2c:4e1f:3b6a:91d0(Preferred)
   Temporary IPv6 Address. . . . . . : 2001:db8:acad:10:5d1:9a3e:c27f:4b18(Preferred)
   Link-local IPv6 Address . . . . . : fe80::8d2c:4e1f:3b6a:91d0%12(Preferred)
   Default Gateway . . . . . . . . . : fe80::1%12
   DNS Servers . . . . . . . . . . . : 2001:db8:acad:100::53

Two SLAAC addresses (stable and temporary, both random on Windows), the gateway from the RA, and the DNS server and suffix from DHCPv6. Output trimmed to the IPv6 lines.

PC2 output

PC2# resolvectl dns eth0
Link 2 (eth0): 2001:db8:acad:100::53

The Linux host learned the same DNS server.

Mistakes people make

Forgetting ipv6 nd other-config-flag. The pool is fine but hosts never ask for it.
Forgetting ipv6 dhcp server on the interface. Hosts ask and nobody answers.
Expecting DHCPv6 to provide the default gateway. It never does; the RA does.
Testing only with Android phones. They ignore DHCPv6 completely, so they get addresses but not these options.
A host firewall or ACL blocking UDP 546 and 547, the DHCPv6 client and server ports.

On other platforms

radvd on a Linux router. RDNSS and DNSSL put DNS straight into the RA, which also reaches Android.

Linux

interface eth1 {
    AdvSendAdvert on;
    AdvOtherConfigFlag on;
    prefix 2001:db8:acad:10::/64 {
        AdvOnLink on;
        AdvAutonomous on;
    };
    RDNSS 2001:db8:acad:100::53 {
    };
    DNSSL corp.example.com {
    };
};

Setting the O flag in Junos RAs. The DHCPv6 server itself is configured separately under system services or access.

Junos

set protocols router-advertisement interface ge-0/0/0.0 other-stateful-configuration
set protocols router-advertisement interface ge-0/0/0.0 prefix 2001:db8:acad:10::/64

Say it in the interview

"Addresses come from SLAAC, so the prefix keeps A=1 and M stays off. I set the O flag so hosts send an Information-Request, and I give R1 a pool with only DNS and domain name, no address prefix. I verify with show ipv6 interface for the flags, the pool showing zero clients because it is stateless, and the host showing the DNS server. For Android I would add RDNSS to the RA, because it ignores DHCPv6."

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