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IPv6 basics: verify ULA, RA/SLAAC, PD and DNS

Five address/mechanism terms

Section titled “Five address/mechanism terms”
Name Example/role Common mistake
link-local fe80::/10 every NIC has one; not public
ULA inside fd00::/8 not directly routable on the internet
GUA 2000::/3 public address requires upstream PD/allocation
RA router advertises prefix/gateway/DNS no RA → no SLAAC
SLAAC client generates an address from RA prefix not DHCPv6
DHCPv6 server assigns address/prefix/DNS different from IPv4 DHCP
PD upstream delegates a prefix to the router no PD → no LAN GUA

Current state

Section titled “Current state”
Router shellCheck addresses, routes and UCI
Terminal window
ip -6 addr show
ip -6 route
uci show network.lan
uci show dhcp.lan
uci show network.globals
logread -e odhcpd | tail -30

This lab: br-lan had ULA fd2f:385b:7d0a::1/60 and link-local; no default IPv6 route and no GUA.

ExpectedKnow whether you have only link-local, ULA, or GUA.

LAN: ULA + RA/SLAAC

Section titled “LAN: ULA + RA/SLAAC”
Router shellConfirm LAN prefix and RA service
Terminal window
uci show network.globals.ula_prefix
uci show network.lan.ip6assign
uci show dhcp.lan.ra
uci show dhcp.lan.ra_slaac
uci show dhcp.lan.dhcpv6
ps | grep [o]dhcpd

This lab:

network.globals.ula_prefix='fd2f:385b:7d0a::/48'
network.lan.ip6assign='60'
dhcp.lan.ra='server'
dhcp.lan.ra_slaac='1'
dhcp.lan.dhcpv6='server'

ip6assign='60' takes a /60 for LAN out of the ULA /48; clients usually get /64.

ExpectedLAN has a ULA prefix and odhcpd sends RA.

ComputerLet a client get an address from RA

Linux example (accept_ra=2 only in this netns; normal clients usually keep defaults):

Terminal window
sysctl -w net.ipv6.conf.eth0.accept_ra=2
ip -6 addr show eth0
ip -6 route
ping -6 -c 2 <router-ULA>

This lab netns client got:

fd2f:385b:7d0a:0:5088:71ff:fed4:32d2/64  scope global dynamic
fe80::5088:71ff:fed4:32d2/64             scope link

and pinged fd2f:385b:7d0a::1. The client only had fd2f:385b:7d0a::/64, no default IPv6 route (no upstream).

ExpectedThe client gets a ULA and can ping the router IPv6.

WAN: GUA and PD

Section titled “WAN: GUA and PD”
Router shellCheck whether WAN6/upstream provides a prefix
Terminal window
uci show network.wan6 2>/dev/null
ubus call network.interface.wan6 status 2>/dev/null
ip -6 route
ip -6 addr show eth1
logread | grep -Ei 'odhcp6c|dhcpv6|pd' | tail -30

Typical main router:

Terminal window
uci set network.wan6=interface
uci set network.wan6.device='eth1'
uci set network.wan6.proto='dhcpv6'
uci commit network
/etc/init.d/network reload
  • Upstream must support IPv6/PD; some ONTs/ISPs give only /64 or nothing;
  • Without PD, do not invent a “public prefix” on LAN;
  • This lab had no upstream IPv6, so GUA/PD were not verified; only ULA/RA/SLAAC were.

ExpectedA GUA and default route, or a clear upstream “no IPv6”.

DNS and firewall

Section titled “DNS and firewall”
Router shellTest AAAA and IPv6 DNS separately
Terminal window
nslookup -type=AAAA openwrt.org 127.0.0.1
nslookup -type=AAAA openwrt.org <router-ULA>
cat /tmp/resolv.conf.d/resolv.conf.auto
  • AAAA can resolve even when DNS uses IPv4;
  • client DNS may come from RA/DHCPv6 or manual config;
  • Do not switch all DNS to IPv6 for “IPv6 works”; keep IPv4/IPv6 fallback.

Firewall:

Terminal window
uci show firewall
nft list ruleset 2>/dev/null | head -80
  • Allow ICMPv6/ND/RA (otherwise IPv6 breaks);
  • Allow LAN→WAN IPv6 forward;
  • Do not disable the IPv6 firewall “to test connectivity”; that exposes the LAN.

ExpectedKnow whether it is “no AAAA” or “DNS did not use IPv6”.

Common failures

Section titled “Common failures”

Only fe80 addresses

Section titled “Only fe80 addresses”
  • LAN has no ULA prefix/ip6assign;
  • odhcpd is not running / RA blocked;
  • client accept_ra=0;
  • switch/AP blocks multicast/ICMPv6.

ULA but no GUA

Section titled “ULA but no GUA”
  • upstream has no IPv6/PD;
  • wan6 not configured/prefix not received;
  • ISP gives only /64, cannot split for LAN;
  • relay/ND proxy scenarios are different; do not force it.

Address but no internet

Section titled “Address but no internet”
  • no default IPv6 route;
  • firewall forward/ICMPv6;
  • DNS only on IPv4;
  • upstream route/prefix invalid.

IPv4 slows/breaks after enabling IPv6

Section titled “IPv4 slows/breaks after enabling IPv6”
  • DNS preference/dual-stack fallback;
  • client privacy addresses/RA parameters;
  • Keep IPv4 working; add IPv6 one step at a time.

E2 lab run (2026-10-20)

Section titled “E2 lab run (2026-10-20)”
  • Environment: OpenWrt 25.12.6 x86 VM 200; LAN 192.168.1.2/24 + ULA fd2f:385b:7d0a::1/60.
  • ULA prefix fd2f:385b:7d0a::/48; ip6assign=60; ra=server, ra_slaac=1, dhcpv6=server; odhcpd running.
  • Client: SLAAC got fd2f:385b:7d0a:0:5088:71ff:fed4:32d2/64; pinged the router ULA.
  • No upstream IPv6/GUA/PD; WAN6, real ISP, DHCPv6 client and IPv6 firewall/NAT untested.
  • Recipe: tests/recipes/ipv6-25.12.md.

Sources and review

Section titled “Sources and review”
  • This is E2 partial: LAN ULA/RA/SLAAC were tested; WAN GUA/PD and a real ISP were not.
  • No independent reviewer; conclusions are limited to this VM/netns environment.
  • Do not turn “LAN has a ULA” into “IPv6 internet works”; ULA is internal only.

Sources

Router.Homes

Independent OpenWrt notes. Every page states the firmware, device, package manager and way back.

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