How to advertise the IPv6 address of the DNS server with DHCP-Option 6

Installing and Using OpenWrt
1

I have an OpenWrt router running 23.05.5 within an x86 appliance. I have AdGuard Home running within an LXC container in Proxmox on a different appliance.

In the OpenWrt router, using DHCP-Options within Interfaces --> LAN --> DCHP Server --> Advanced Settings, I have set the IPv4 address of AdGuard Home server, using Option 6 (as documented).

All the client devices are getting the custom DNS IPv4 address of the AdGuard Home server running in Proxmox, and AdBlocking is working fine.

I am concern that client devices using IPv6 might be skipping AdGuard Home, because if I check any of the iOS devices, in the DNS server section in Settings --> Wifi --> DNS, I can see two DNS servers, the IPv4 configured with Option 6 in OpenWrt, and I can also see the IPv6 DNS server from my ISP (fe80::), I have confirmed that by checking the DNS 1 value in LuCI --> Status --> Overview --> IPv6 Upstream, that it is actually the IPv6 ISP DNS. I haven't modified that in the WAN interface to use custom DNS, and that's by design, I am OK with the router itself using the ISP DNS. I only need the Client devices using AdGuard Home.

What is the correct way to advertise the IPv6 DNS address (I can get it from the LXC container running ip a from the console) of the AdGuard Home server in the OpenWrt DHCP server, I have tried using Option 6 (6,fe80::......) but didn't work. Any feedback will be appreciated, thank you

2

Fe80 are Link local addresses so they are only valid on the link layer. Do you see an ULA, too? Starting with FC or FD?

3

I have ULA disabled, long story short, My ISP (T-Mobile Home Internet) doesn’t provide IPv6 Prefix Delegation, therefore my IPv6 options are limited, as a matter of fact the only way to get IPv6 addresses to the client devices is by configuring the OpenWrt router in Relay mode (NDP-Proxy), and I had to disable ULA for the Apple devices (iOS, macOS, etc.) to be able to get an IPv6 address.

4

Ok. But you can advertise any DNS GUA via slaac and or dhcpv6.

1 Like
5

Edit the LAN interface here:

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6

A v6 address cannot be advertised within a DHCPv4 packet. However, OpenWrt UCI has a flexible list dns within a config dhcp. Depending on the type of the address specified, the netifd script will direct the configuration to the proper server.

Note that DHCP option numbers are different for DHCPv6. It is not clear if an equivalent to dhcp_option exists for v6.

7

DNSMasq has that it works like this:

dhcp-option=option6:dns-server,[2606:4700:4700::1001]

By default DNSMasq should advertise its own address (=the routers address) as DNS server, I am certain that is the case for IPv4 and I think that also works for IPv6, provided RA is enabled.

8

For DHCPv6 in dnsmasq, it would be option 23.

# dnsmasq --help dhcp6
Known DHCPv6 options:
 21 sip-server-domain
 22 sip-server
 23 dns-server
 24 domain-search
 27 nis-server
 28 nis+-server
 29 nis-domain
 30 nis+-domain
 31 sntp-server
 32 information-refresh-time
 41 posix-timezone
 42 tzdb-timezone
 56 ntp-server
 59 bootfile-url
 60 bootfile-param
1 Like
9

You are probably right, I know for sure that option6:dns-server works as that is what I am using :slight_smile: