IPv6 on L2TP softwire

This page documents how to configure IPv6 over a L2TP softwire, which is a method used by some ISP to provide IPv6 connectivity.

Softwire is the new fancy term for network tunnels, aka encapsulation. Reasonably accurate definitions about softwires are given in RFC 4925, and RFC 5571 describes an implementation using L2TPv2.

Softwires are used as basic blocks to transport newer protocols (typically IPv6) over an older network (typically, the IPv4 core network of an ISP).

SFR, in France, is known to use softwires to provide IPv6 to its residential customers. See some documentation (in French).

This howto is derived from an experience with SFR, in France (FTTH residential access). It might applies to other ISPs as well, but you'll need to adapt IP addresses, PPP login and passwords, and so on.

The high-level description of the tunneling is the following:

  1. a L2TP tunnel is created, encapsulated in UDP packets over IPv4
  2. a PPP session is established inside the tunnel
  3. IPv6CP (see RFC 5072) is used to negotiate link-local IPv6 addresses
  4. an IPv6 prefix is obtained thanks to DHCPv6

In the case of SFR, steps 1 and 2 require an authentication. Fortunately, the L2TP password is hardcoded. The PPP password is not, but it's sent as cleartext, so a simple sniffing is enough to recover it.

You need to install xl2tpd, which will handle the L2TP tunnel and PPP session.

# /etc/config/network
config interface 6pe
        option proto l2tp
        option server <LNS address>
        option username '<PPP username>'
        option password '<PPP password>'
        option keepalive '6'
        option ipv6 '1'
config interface 'wan6'
        option ifname '@6pe'
        option proto 'dhcpv6'

If you need authentication at the L2TP level (before PPP):

# /etc/xl2tpd/xl2tp-secrets
* * my_l2tp_password

At this point, rebooting or simply running ifup wan6 should give you a fully working IPv6 setup. To debug, look at the logs (logread) and the interfaces status (ifstatus 6pe and ifstatus wan6).

Note that SFR's CPE, the Neufbox, is running a modified version of OpenWrt. Since they publish their firmware (I used the NB6-MAIN-R3.3.4 firmware), it's possible to look at their config files (and hardcoded passwords), which greatly simplifies the task.

# /etc/x2ltpd/x2ltpd.conf
port = 1701
auth file = /etc/xl2tpd/xl2tp-secrets
access control = no
[lac 6pe]
lns = ; address of the LNS (L2TP Network Server)
ppp debug = yes
hostname = XX.XX.XX.XX ; your public IP address
hidden bit = no
; ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
require authentication = no
refuse authentication = no
refuse chap = no
flow bit = yes
length bit = yes
# /etc/xl2tpd/xl2tp-secrets
*	*	6pe

Starting the L2TP tunnel

You need to start xl2tpd, and connect the profile we defined:

/etc/init.d/xl2tpd start
echo "c 6pe" > /var/run/xl2tpd/l2tp-control

There doesn't seem to be an easy way to start a profile automatically at startup. Quick & dirty:

# /etc/rc.d/S60xl2tpd
(sleep 10 && echo "c 6pe" > /var/run/xl2tpd/l2tp-control) &


  • look at the logs (logread)
  • try to activate some xl2tpd debug options
  • use tcpdump to see what's going on with the LNS

Last, you need to set PPP options for IPv6 negotiation.

# /etc/ppp/options.xl2tpd
# From the official firmware
ipv6 ,
child-timeout 20
lcp-echo-failure 3
lcp-echo-interval 20
name <your PPP login>

For SFR, the PPP login seems to be dhcp/XX.XX.XX.XX@YYYYYYYYYYYY, where XX.XX.XX.XX is your public IP address, and YYYYYYYYYYYY is the MAC address of the WAN interface of the official box, without the colons.

You then need to define the PPP password in /etc/ppp/chap-secrets:

dhcp/XX.XX.XX.XX@YYYYYYYYYYYY * <PPP password>

For SFR, the password is not obvious. It's sent in cleartext, thus recoverable by sniffing the WAN port of the official box.

Once the PPP session is established inside the L2TP tunnel, a new interface ppp0 should appear.

The only remaining step is to request an IPv6 prefix to the ISP, by using for instance the wide-dhcp6c client.

OpenWrt integration

Note that this is specific to Attitude Adjustment, as IPv6 support is expected to changed a lot in the upcoming Barrier Breaker release.

Interface declaration

We need to tell OpenWrt about the new interface:

# /etc/config/network
config interface wan6
        option ifname   ppp0
        option proto    none

If, at some point, you don't get a default route for IPv6, you could try to add the route yourself, where the gateway is the link-local address of the router at the other end of the softwire:

# /etc/config/network
config route6                           
        option interface wan6          
        option target '::/0'            
        option gateway 'fe80::XXXX:XXff:feXX:XXXX'
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  • Last modified: 2021/11/15 13:05
  • by vgaetera