NAT6 and IPv6 masquerading

This page describes how to set up NAT6 (also known as NAT66) masquerading on your OpenWrt router. Most users will not need or want to do this in IPv6, but there are some more specific use cases for NAT with IPv6. These might include:

  • IPv6 multihoming without BGP
  • Performing stateless 1:1 NAT for migration purposes
  • Your ISP uses a dynamic prefix and you need stable addressing
  • Creating a subnet for when the network doesn't support subnetting
  • Being provided a smaller prefix than a /64 or worse, none at all or a ULA address

Note: If you are provided a /64 prefix, you may not need to use NAT6 and instead should try relay mode.

This guide relies on OpenWrt default settings and assumes you already have a working IPv6 connection on your OpenWrt router.

Install the necessary packages.

# Install packages
opkg update
opkg install kmod-ipt-nat6

If you are handing out only local addresses, i.e. not part of delegated prefix from your upstream, change the first letter of the IPv6 ULA prefix.

# Configure network
uci set network.globals.ula_prefix="$(uci get network.globals.ula_prefix | sed -e "s/^./d/")"
uci commit network
/etc/init.d/network restart

Set the DHCPv6 server to always announce default router.

# Configure odhcpd
uci set dhcp.lan.ra_default="1"
uci commit dhcp
/etc/init.d/odhcpd restart

Enable the new masq6 option in your firewall on your upstream zone.

# Configure firewall
uci set $(uci show firewall | sed -n -e "/\.name='wan'$/s//.masq6='1'/p" | sed -n -e "1p")
uci commit firewall

Since masquerading is enabled, disable the redundant firewall rule “Allow-ICMPv6-Forward”.

# Configure firewall
uci set $(uci show firewall | sed -n -e "/\.name='Allow-ICMPv6-Forward'$/s//.enabled='0'/p" | sed -n -e "1p")
uci commit firewall

Save the NAT6 firewall script.

# Configure firewall
cat << "EOF" > /etc/firewall.nat6
# NAT6 + masquerading firewall script
# trevorj <>
# You can configure in /etc/config/firewall per zone:
# * IPv4 masquerading
#     option masq 1
# * IPv6 masquerading
#     option masq6 1
# * IPv6 privacy extensions
#     option masq6_privacy 1
set -e -o pipefail
. /lib/
. /lib/functions/
. /usr/share/libubox/
log() {
    logger -t nat6 -s "${@}"
get_ula_prefix() {
    uci get network.globals.ula_prefix
validate_ula_prefix() {
    local ula_prefix="${1}"
    if [ $(echo "${ula_prefix}" | grep -c -E -e "^([0-9a-fA-F]{4}):([0-9a-fA-F]{0,4}):") -ne 1 ] ; then
        log "Fatal error: IPv6 ULA ula_prefix=\"${ula_prefix}\" seems invalid. Please verify that a ula_prefix is set and valid."
        return 1
ip6t() {
    ip6tables "${@}"
ip6t_add() {
    if ! ip6t -C "${@}" &> /dev/null; then
        ip6t -I "${@}"
nat6_init() {
    iptables-save -t nat \
    | sed -e "/\s[DS]NAT\s/d;/\sMASQUERADE$/d;/\s--match-set\s\S*/s//\06/" \
    | ip6tables-restore -T nat
masq6_network() {
    # ${config} contains the ID of the current section
    local network_name="${1}"
    local device
    network_get_device device "${network_name}" || return 0
    local done_net_dev
    for done_net_dev in ${DONE_NETWORK_DEVICES}; do
        if [ "${done_net_dev}" = "${device}" ]; then
            log "Already configured device=\"${device}\", so leaving as is."
            return 0
    log "Found device=\"${device}\" for network_name=\"${network_name}\"."
    if [ "${zone_masq6_privacy}" -eq 1 ]; then
        log "Enabling IPv6 temporary addresses for device=\"${device}\"."
        log "Accepting router advertisements on ${device} even if forwarding is enabled (required for temporary addresses)"
        echo 2 > "/proc/sys/net/ipv6/conf/${device}/accept_ra" \
        || log "Error: Failed to change router advertisements accept policy on ${device} (required for temporary addresses)"
        log "Using temporary addresses for outgoing connections on interface ${device}"
        echo 2 > "/proc/sys/net/ipv6/conf/${device}/use_tempaddr" \
        || log "Error: Failed to enable temporary addresses for outgoing connections on interface ${device}"
    append DONE_NETWORK_DEVICES "${device}"
handle_zone() {
    # ${config} contains the ID of the current section
    local config="${1}"
    local zone_name
    config_get zone_name "${config}" name
    # Enable masquerading via NAT6
    local zone_masq6
    config_get_bool zone_masq6 "${config}" masq6 0
    log "Firewall config=\"${config}\" zone=\"${zone_name}\" zone_masq6=\"${zone_masq6}\"."
    if [ "${zone_masq6}" -eq 0 ]; then
        return 0
    # IPv6 privacy extensions: Use temporary addrs for outgoing connections?
    local zone_masq6_privacy
    config_get_bool zone_masq6_privacy "${config}" masq6_privacy 1
    log "Found firewall zone_name=\"${zone_name}\" with zone_masq6=\"${zone_masq6}\" zone_masq6_privacy=\"${zone_masq6_privacy}\"."
    log "Setting up masquerading nat6 for zone_name=\"${zone_name}\" with zone_masq6_privacy=\"${zone_masq6_privacy}\""
    local ula_prefix="$(get_ula_prefix)"
    validate_ula_prefix "${ula_prefix}" || return 1
    local postrouting_chain="zone_${zone_name}_postrouting"
    log "Ensuring ip6tables chain=\"${postrouting_chain}\" contains our MASQUERADE."
    ip6t_add "${postrouting_chain}" -t nat \
        -m comment --comment "!fw3" -j MASQUERADE
    local input_chain="zone_${zone_name}_input"
    log "Ensuring ip6tables chain=\"${input_chain}\" contains our permissive DNAT rule."
    ip6t_add "${input_chain}" -t filter -m conntrack --ctstate DNAT \
        -m comment --comment "!fw3: Accept port forwards" -j ACCEPT
    local forward_chain="zone_${zone_name}_forward"
    log "Ensuring ip6tables chain=\"${forward_chain}\" contains our permissive DNAT rule."
    ip6t_add "${forward_chain}" -t filter -m conntrack --ctstate DNAT \
        -m comment --comment "!fw3: Accept port forwards" -j ACCEPT
    config_list_foreach "${config}" network masq6_network
    log "Done setting up nat6 for zone=\"${zone_name}\" on devices: ${DONE_NETWORK_DEVICES}"
main() {
    config_load firewall
    config_foreach handle_zone zone
main "${@}"
cat << "EOF" >> /etc/sysupgrade.conf

Include the NAT6 firewall script in the configuration.

# Configure firewall
uci -q delete firewall.nat6
uci set firewall.nat6="include"
uci set firewall.nat6.path="/etc/firewall.nat6"
uci set firewall.nat6.reload="1"
uci commit firewall
/etc/init.d/firewall restart

The solution provided here can be considered more robust and portable. Especially the init script comes with benefits such as configurability, sanity checks and error handling, as well as logging (which might help debugging, if something is not working as expected).

Configuration is done per firewall zone, just like standard masquerading.

This provides two configurables in UCI's firewall zone section:

# Configure firewall
uci set firewall.@zone[1].masq6="1"
uci set firewall.@zone[1].masq6_privacy="1"
uci commit firewall
/etc/init.d/firewall restart

If masq6_privacy is set to 1 (default is 0 aka off), then outgoing IPv6 connections will use temporary addresses that change dynamically. This supposedly makes it harder to track users and surf behavior.

If you prefer to use static addresses (at least as long your ISP assigned prefix doesn't change), leave it at the default of 0.

The script logs informational and error messages to the system log. The messages are tagged with nat6. You can view the log messages with the command:

logread -e nat6

If all was configured successfully, the output should look similar to this:

2017-02-04T18:03:44-08:00 notice nat6[]: Found firewall zone_name="wan" with zone_masq6="1" zone_masq6_privacy="1".
2017-02-04T18:03:44-08:00 notice nat6[]: Setting up masquerading nat6 for zone_name="wan" with zone_masq6_privacy="1"
2017-02-04T18:03:44-08:00 notice nat6[]: Ensuring ip6tables chain="zone_wan_postrouting" contains our MASQUERADE.
2017-02-04T18:03:44-08:00 notice nat6[]: Found device="eth1" for network_name="wan".
2017-02-04T18:03:44-08:00 notice nat6[]: Enabling IPv6 temporary addresses for device="eth1".
2017-02-04T18:03:44-08:00 notice nat6[]: Accepting router advertisements on eth1 even if forwarding is enabled (required for temporary addresses)
2017-02-04T18:03:44-08:00 notice nat6[]: Using temporary addresses for outgoing connections on interface eth1
2017-02-04T18:03:44-08:00 notice nat6[]: Already configured device="eth1", so leaving as is.
2017-02-04T18:03:44-08:00 notice nat6[]: Done setting up nat6 for zone="wan" on devices: eth1

Failures that occur during initialization (upon each firewall reload), will be logged and marked as such. Pay attention to any noted as error or fatal error.

You're also welcome to run the script from a shell to see it progress; it is just a shell script after all.

Make sure DHCPv6 uses the following settings (on an unmodified OpenWrt installation these should by the default):

  • “Router Advertisement-Service” and “DHCPv6-Service” are set to server mode*
  • “DHCPv6-Mode” is stateless + stateful
  • “NDP-Proxy” is disabled

You can check this by running the following command:

# uci show dhcp.lan

If the output is different, you are not using the defaults and you should set these options to the ones shown above. If there is an additional line starting with dhcp.lan.ndp, the NDP-Proxy is enabled and should be disabled. Setups with “DHCPv6-Service” disabled have been reported working as well by some users. However, if “DHCPv6-Service” is disabled, some clients (e.g. Android devices) will prefer IPv4 over IPv6. Therefore, enabling the “DHCPv6-Service” server mode is recommended.

The default ULA (Unique local address) prefix represents an address that is not globally routed on the internet by design. A lot of clients will prefer IPv4 over a ULA IPv6 address if there is no global IPv6 address assigned, so you may need to change your existing ULA prefix to indicate a global address to ensure traffic goes over IPv6 by default when possible.

When changing the ULA prefix, it doesn't necessarily have to start with d, but to avoid conflicts, you should use a prefix that is not being used yet. The letters are unassigned and therefore safe choices.

Setting ula_prefix to auto will auto-generate a new valid ULA prefix.

Using your ISP assigned prefix as ULA should also work. However, unless you have a static IPv6 prefix assigned by your ISP, this is not recommended, since it can cause address conflicts once the prefix changes. If you have a static prefix that you can delegate across your LAN, then you won't need to change your ULA prefix.

Provide IPv6 raw table support, this can help with connection tracking.

# Install packages
opkg update
opkg install kmod-ipt-raw6
/etc/init.d/firewall restart

Install the necessary packages and force the default gateway announcement. Enable NAT6 with IPv6 masquerading, cloning the IPv4 setup.

# Configure firewall
cat << "EOF" > /etc/firewall.nat6
iptables-save -t nat \
| sed -e "/\s[DS]NAT\s/d;/\s--match-set\s\S*/s//\06/" \
| ip6tables-restore -T nat
cat << "EOF" >> /etc/sysupgrade.conf
uci -q delete firewall.nat6
uci set firewall.nat6="include"
uci set firewall.nat6.path="/etc/firewall.nat6"
uci set firewall.nat6.reload="1"
uci commit firewall
/etc/init.d/firewall restart
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  • Last modified: 2021/03/17 04:25
  • by vgaetera