IPv6 configuration

See also: Routing example: IPv6, IPv4/IPv6 transitioning, IPv6 extras

The default firmware provides full IPv6 support with a DHCPv6 client (odhcp6c), an RA & DHCPv6 Server (odhcpd) and a IPv6 firewall (ip6tables).
Also, the default installation of the web interface includes the package luci-proto-ipv6, required to configure IPv6 from the luci web interface.

If you are making a custom build please note that the packages stated above must be installed to provide the corresponding IPv6 functionality.

Our aim is to follow RFC 7084 where possible.
Please notify us if you find any standard violations.

The following requirements of RFC 7084 are currently known not to be met:

• RFC 7084 WAA-5 (SHOULD-requirement): The NTP-Server is requested and received but currently not processed or used.

• Prefix handling:
  • Management of prefixes, addresses and routes from upstream connections and local ULA-prefixes
  • Management of prefix unreachable-routes, prefix deprecation (RFC 7084) and prefix classes
  • Distribution of prefixes onto downstream interfaces (including size, ID and class hints)
  • Source-based policy routing to correctly handle multiple uplink interfaces, ingress policy filtering (RFC 7084)

The following sections describe the configuration of IPv6 connections to your ISP or an upstream router. Please note that most tunneling mechanisms like 6in4, 6rd and 6to4 may not work behind a NAT-router. Multiple IPv6 addresses can be assigned with aliases.

• Automatic bootstrap from SLAAC, stateless DHCPv6, stateful DHCPv6, DHCPv6-PD and any combination
• Handling of preferred and valid address and prefix lifetimes
• Duplicate address (DAD) and Link-MTU detection
• DHCPv6 Extensions: Reconfigure, Information-Refresh, SOL_MAX_RT=3600
• DHCPv6 Extensions: RDNSS, DNS Search Domain, NTP, SIP, ds-lite, prefix exclusion (experimental)

For an uplink with native IPv6-connectivity you can use the following example configuration. It will work both for uplinks supporting DHCPv6 with Prefix Delegation and those that don't support DHCPv6-PD or DHCPv6 at all (SLAAC-only).

# cat /etc/config/network
config interface wan
        option ipv6 1 # only required for PPP-based protocols
        ...
 
config interface wan6
        option device   eth1 # use same device as in wan-section or "@wan"
        option proto    dhcpv6
 
config interface lan
        option proto    static
        option ip6assign 60
        ...

See below for advanced configuration options of protocol dhcpv6.

PPP-based protocols - for example pppoe and pppoa - require that option ipv6 is specified in the parent config interface wan section. See WAN interface protocols. option ipv6 can take the value:

• 0: disable IPv6 on the interface
• 1: enable IPCP6 negotiation on the interface, but nothing else. If successful, the parent interface will be assigned a link-local address (prefix fe80::/10). All other IPv6 configuration is made in the wan6 interface which must be configured manually, as described below.
• auto: (default) enable IPv6 on the interface. Spawn a virtual interface wan_6 (note the underscore) and start DHCPv6 client odhcp6c to manage prefix assignment. Ensure the lan interface has option ip6assign 64 (or a larger prefix size) set to redistribute the received prefix downstream.

Further configuration options, if required, can be given in the config interface wan6 section.

Note: In order to successfully receive DHCPv6 advertisement unicast messages from the dhcp6s to OpenWrt dhcp6c, you will need to have firewall rule for the WAN zone (already allowed in default):

# cat /etc/config/firewall
....
config rule
        option target 'ACCEPT'
        option src 'wan'
        option proto 'udp'
        option dest_port '546'
        option name 'Allow DHCPv6 replies'
        option family 'ipv6'
        option src_port '547'

These are available options in uci configuration of client ipv6 interface (using the “dhcpv6” protocol).

Note: To automatically configure ds-lite from dhcpv6, you need to create an interface with option auto 0 and put its name as the 'iface_dslite' parameter. In addition, you also need to add its name to a suitable firewall zone in /etc/config/firewall.

Static configuration of the IPv6 uplink is supported as well. The following example demonstrates this.

# cat /etc/config/network
config interface wan
        option device   eth1
        option proto    static
        option ip6addr  2001:db80::2/64   # Own address
        option ip6gw    2001:db80::1      # Gateway address
        option ip6prefix 2001:db80:1::/48 # Prefix addresses for distribution to downstream interfaces
        option dns      2001:db80::1      # DNS server
 
config interface lan
        option proto    static
        option ip6assign 60
        ...

For advanced configuration options see below for the usable options in a IPv6 “static” protocol:

• Server support for Router Advertisement, DHCPv6 (stateless and stateful) and DHCPv6-PD
• Automatic detection of announced prefixes, delegated prefixes, default routes and MTU
• Change detection for prefixes and routes triggering resending of RAs and DHCPv6-Reconfigure
• Detection of client hostnames and export as augmented hosts-file
• Support for RA & DHCPv6-relaying and NDP-proxying to e.g. support uplinks without prefix delegation

OpenWrt provides a flexible local prefix delegation mechanism.

It can be tuned for each downstream-interface individually with 3 parameters which are all optional:

• ip6assign: Prefix size used for assigned prefix to the interface (e.g. 64 will assign /64-prefixes)
• ip6hint: Subprefix ID to be used if available (e.g. 1234 with an ip6assign of 64 will assign prefixes of the form ...:1234::/64 or given LAN ports, LAN & LAN2, and a prefix delegation of /56, use ip6hint of 00 and 80 which would give prefixes of LAN ...:xx00::/64 and LAN2 ...:xx80::/64)
• ip6class: Filter for prefix classes to accept on this interface (e.g. wan6 - only assign prefix from the respective interface, local - only assign the ULA-prefix)

ip6assign and / or ip6hint settings might be ignored if the desired subprefix cannot be assigned. In this case, the system will first try to assign a prefix with the same length but different subprefix-ID. If this fails as well, the prefix length is reduced until the assignment can be satisfied. If ip6hint is not set, an arbitrary ID will be chosen. Setting the ip6assign parameter to a value < 64 will allow the DHCPv6-server to hand out all but the first /64 via DHCPv6-Prefix Delegation to downstream routers on the interface. If ip6hint is not suitable for the given ip6assign, it will be rounded down to the nearest possible value.

If ip6class is not set, then all prefix classes are accepted on this interface. Specify one or multiple interface names such as wan6 to accept only prefix from the respective interface, or specify local accept only the ULA-prefix when using IPv6 NAT or NPT. This can be used to select upstream interfaces from which subprefixes are assigned. For prefixes received from dynamic-configuration methods like DHCPv6, it is possible that the prefix-class is not equal to the source-interface but e.g. augmented with an ISP-provided numeric prefix class-value.

# cat /etc/config/network
config globals globals
        option ula_prefix fd00:db80::/48
 
config interface wan6
        option proto static
        option ip6prefix 2001:db80::/56
        ...
 
config interface lan
        option proto static
        option ip6assign 60
        option ip6hint 10
        ...
 
config interface guest
        option proto static
        option ip6assign 64
        option ip6hint abcd
        list ip6class wan6
        ...

The results of that configuration would be:

• The lan interface will be assigned the prefixes 2001:db80:0:10::/60 and fd00:db80:0:10::/60.
• The DHCPv6-server can offer both prefixes except 2001:db80:0:10::/64 and fd00:db80:0:10::/64 to downstream routers on lan via DHCPv6-PD.
• The guest interface will only get assinged the prefix 2001:db80:0:abcd::/64 due to the class filter.

For multiple interfaces, the prefixes are assigned based on firstly the assignment length (smallest first) then on weight and finally alphabetical order of interface names. e.g. if wlan0 and eth1 have ip6assign 61 and eth2 has ip6assign 62, the prefixes are assigned to eth1 then wlan0 (alphabetic) and then eth2 (longest prefix). Note that if there are not enough prefixes, the last interfaces get no prefix - which would happen to eth2 if the overall prefix length was 60 in this example.

If the router can ping6 the internet, but lan machines get “Destination unreachable: Unknown code 5” or “Source address failed ingress/egress policy” then the ip6assign option is missing on your lan interface.

OpenWrt features a versatile RA & DHCPv6 server and relay. Per default, SLAAC and both stateless and stateful DHCPv6 are enabled on an interface. If there are any prefixes of size /64 or shorter present then addresses will be handed out from each prefix. If all addresses on an interface have prefixes shorter than /64, then DHCPv6 Prefix Delegation is enabled for downstream routers. If a default route is present, the router advertises itself as default router on the interface.

The system is also able to detect when there is no prefix available from an upstream interface and can switch into relaying mode automatically to extend the upstream interface configuration onto its downstream interfaces. This is useful for putting the target router behind another IPv6 router which doesn't offer prefixes via DHCPv6-PD.

Example configuration section for SLAAC + DHCPv6 server mode. This is suitable also for a typical 6in4 tunnel configuration, where you specify the fixed LAN prefix in the tunnel interface config. Make sure to disable NDP-Proxy by removing the ndp option if any.

# cat /etc/config/dhcp
config dhcp lan
    option dhcpv6 server
    option ra server
    option ra_flags 'managed-config other-config'
    ...

Example configuration section for SLAAC alone. Make sure to deactivate RA flags, otherwise clients expect the presence of a DHCPv6 and consequently may fail to activate the network connection. Note that disabling DHCPv6 makes some clients (e.g. Android devices) prefer IPv4 over IPv6.

# cat /etc/config/dhcp
config dhcp lan
    option dhcpv6 disabled
    option ra server
    list ra_flags 'none'
    ...

Example configuration section for relaying

# cat /etc/config/dhcp
config dhcp lan
    option dhcpv6 relay
    option ra relay
    option ndp relay
    ...
 
config dhcp wan6
    option dhcpv6 relay
    option ra relay
    option ndp relay
    option master 1
    option interface wan6

OpenWrt uses a source-address and source-interface based policy-routing system. This is required to correctly handle different uplink interfaces. Each delegated prefix is added with an unreachable route to avoid IPv6-routing loops.

To determine the current status of routes you can consult the information provided by ifstatus.

Example (ifstatus wan6):

...
        "ipv6-address": [
                {
                        "address": "2001:db80::a00:27ff:fe67:cd9c",
                        "mask": 64,
                        "preferred": 1681,
                        "valid": 7081
                }
        ],
        "ipv6-prefix": [
                {
                        "address": "2001:db80:0:100::",
                        "mask": 56,
                        "preferred": 86282,
                        "valid": 86282,
                        "class": "wan6",
                        "assigned": {
                                "lan": {
                                        "address": "2001:db80:0:110::",
                                        "mask": 60
                                }
                        }
                }
        ],
        "route": [
                {
                        "target": "2001:db80::",
                        "mask": 48,
                        "nexthop": "fe80::800:27ff:fe00:0",
                        "metric": 1024,
                        "valid": 7081
                },
                {
                        "target": "::",
                        "mask": 0,
                        "nexthop": "fe80::800:27ff:fe00:0",
                        "metric": 1024,
                        "valid": 7081
                }
        ],
...

Interpretation:

• On the interface 2 routes are provided: 2001:db80::/48 and a default-route via the router fe80::800:27ff:fe00:0.
• These routes can only be used by locally generated traffic and traffic with a suitable source-address, that is either one of the local addresses or an address out of the delegated prefix.

IPv6 ULA prefix can serve the following purposes:

• Predictable static IPv6 suffix allocation with DHCPv6.
• Predictable site-to-site connectivity with dynamic or missing GUA prefix.
• IPv6 routing for LAN clients behind NAT66 with missing GUA prefix.

If IPv6 GUA is not available, a workaround is generally required to make applications prefer IPv6 over IPv4.