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unix 2017-05-09 12-56-50

IPv6 with Debian

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Debian seems to be ready for IPv6 (although there are still some problems). In less than a day, I put a few of my machines online and joined the Internet of the future. In the following, I'd like to share with you how I did it.

Throughout this document, I use IPv6 addresses out of the 2001:db8::/32 subnet, which is reserved for documentation purposes. You will need to use your own prefix(es) when running the command. Similarly, I use as example IPv4 address of the PoP endpoint, which you will need to alter.

Configuring the packet filter

The first thing I did was to configure the packet filter on each host appropriately. Unfortunately, this is harder than it should be, because — to quote one of the netfilter developers — "when ip6tables was conceived, someone had a big bad brainfart": rather than adding IPv6 rules to your existing iptables ruleset, you have to create a new ruleset, duplicate all chains, networks, hosts, and individual rules, and maintain the two in parallel. Even though there are efforts of unification on the way, I speculate it'll take another couple of years until PF_INET6 will be fused into PF_INET and one will be able to do sensible cross-address-family packet filtering with Linux. Since I've recently started to look (again) at pyroman, maybe the most logical way forward would be to extend it to write both, IPv4 and IPv6 rulesets from its knowledge about the hosts and networks you configured.

Anyway, we want to get stuff working now! Thus, let's configure ourselves a packet filter. (Almost) all IPv6-related filtering must be configured via ip6tables (read on further down about IPv6 in IPv4 tunneling, the reason I said "almost"). The following is a simple default ruleset to start with, which I put into /etc/network/ip6tables to load with ip6tables-restore:

:in-new - [0:0]

### INPUT chain

# allow all loopback traffic
-A INPUT -i lo -j ACCEPT

# RT0 processing is disabled since
#-A INPUT -m rt --rt-type 0 -j REJECT

# allow all ICMP traffic
-A INPUT -p icmpv6 -j ACCEPT

# packets belonging to an establish connection or related to one can pass
# packets that are out-of-sequence are silently dropped
-A INPUT -m state --state INVALID -j DROP
# new connections unknown to the kernel are handled in a separate chain
-A INPUT -m state --state NEW -j in-new

# pass SYN packets for SSH
-A in-new -p tcp -m tcp --dport 22 --syn -j ACCEPT

# log and reject everything else
-A INPUT -m limit --limit 3/min --limit-burst 10 -j LOG --log-prefix "[INPUT6]: "

### OUTPUT chain

# RT0 processing is disabled since
#-A OUTPUT -m rt --rt-type 0 -j REJECT

# allow outgoing traffic, explicitly (despite chain policy)

### FORWARD chain

# RT0 processing is disabled since
#-A FORWARD -m rt --rt-type 0 -j REJECT

# disallow forwarded traffic, explicitly (despite chain policy)


Note that this recipe is pretty much unusable on pre-2.6.20 kernels due to their broken implementation of stateful connection tracking.

The ruleset should be fairly obvious, but you might wonder about my use of REJECT and allowing all ICMP — after all, you've heard for the past 30 years that ICMP is a "bad hacker protocol", and Internet security is no domain for being nice to people, so to prevent any information disclosure, you should DROP connections, not let people know that they're simply not allowed.

Well, to hell with all that! I don't see a single reason or attack vector that is foiled by DROP or disallowing ICMP. In fact, it's just security by obscurity, and mighty inconvenient at the same time. ICMP is also much more important with IPv6 than with IPv4 (it replaces ARP, for instance), and it's actually useful to be able to ping hosts, or get back informational messages on why something failed. Finally, rejecting traffic rather than dropping it doesn't suggest to a hacker that something's hidden here.

Then there is RFC 4890, which almost made me puke. This document is part of the reason why I say: let's fix problems in the kernel, rather than shielding them with unreadable and unmanageable rulesets!

Setting system parameters

The /proc/sys/net/ipv6 filesystem exports a number of parameters that you might want to set. The Linux IPv6 HOWTO explains all available parameters, so let me just show you the ones I set in /etc/sysctl.d/ipv6.conf and load with a call to sysctl -p:

net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.default.accept_ra = 0
net.ipv6.conf.default.accept_ra_defrtr = 0
net.ipv6.conf.default.accept_ra_rtr_pref = 0
net.ipv6.conf.default.accept_ra_pinfo = 0
net.ipv6.conf.default.accept_source_route = 0
net.ipv6.conf.default.accept_redirects = 0
net.ipv6.conf.default.forwarding = 0
net.ipv6.conf.all.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.all.accept_ra_defrtr = 0
net.ipv6.conf.all.accept_ra_rtr_pref = 0
net.ipv6.conf.all.accept_ra_pinfo = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.all.forwarding = 0

Obviously, for gateways, you want to enable forwarding, and on machines where you want to use autoconfiguration, those accept_ra* entries ought to be 1 accordingly.

If you're curious, the default values (the first seven lines) set the defaults for new interfaces, while the all values override the values for all interfaces. I am not sure whether this is a permanent overwrite, or just overwrites all existing interfaces, which is why I simply set the parameters for both.

Unfortunately, this does not seem to work. It seems as if writing to all does not affect existing interfaces, as documented in this thread on the linux-kernel mailing list.

At this moment, I suggest that you add pre-up headers to the iface stanzas of all interfaces to be sure:

pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/autoconf
pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/accept_ra
pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/accept_ra_defrtr
pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/accept_ra_pinfo
pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/accept_ra_rtr_pref

Getting connected

If you already have an IPv6 address, you're basically ready to go, but may want to read further down on how to connect your local network to the IPv6 Internet as well. If you are searching for a provider, have a look at the list of providers with native IPv6 connectivity over at

If you are reading up to here, I assume you are connected to the 'Net with IPv4. There are two ways for you to move towards IPv6: 6to4 or by way of a tunnel provider. A Kiwi website explains how to setting up 6to4 connectivity, and thus I will concentrate only on the tunnel approach. Even though everyone can set up 6to4 in a breeze without any accounts or waiting, there are a number of security considerations, it's pretty ugly to debug (due in part to asymmetric routing), and makes your life unnecessarily difficult when all you have is a dynamic IP that changes from time to time. If you are stuck behind a NAT gateway, you cannot use 6to4 either. Thus, I prefer the tunnel approach.

With the tunnel approach, IPv6 packets are wrapped up in IPv4 packets on your host, and sent to the IPv4 address of your tunnel provider, who has native IPv6 connectivity. The tunnel provider unwraps your packet and shoves the contained IPv6 packet onto the backbone. The IPv6 address you used as source address is routed to the tunnel provider, so any replies arrive at their machines, where they're again wrapped into IPv4 packets and sent to your (publicly-accessible) IPv4 address. Those IPv4 packets specify payload type 41 ("ipv6"), which is why we need -p ipv6 -j ACCEPT rules in the IPv4 iptables ruleset (see below).

There are a few tunnel providers out there. I chose SixXS and have not regretted my choice. I shall thus assume that you do the same: sign up for an account right now, so that you have it by the time you finished reading this document! SixXS works on a credit system: tunnels and subnets cost credits, which you can accumulate by maintaining your tunnels properly. This ensures that everyone can play around, but to do more advanced stuff, you need to first display competence with the basic concepts.

Tunnel types

Your first step with SixXS will be to request a tunnel. SixXS offers three types of tunnels:

static tunnels, for those with static IPs,
heartbeat tunnels, for those with dynamic IPs, and
AYIYA tunnels, for those behind NAT gateways.
Each of these tunnels have advantages and disadvantages, as everything does: the first two types of tunnels use IP protocol 41 packets to encapsulate the IPv6 packets. As such, there are security considerations involving the impersonation by spoofing, and all upstream firewalls must let protocol 41 pass. AYIYA addresses these problems by using signed packets, but that solution comes with extra computation overhead and smaller MTUs.

I suggest to use the first type of tunnel that fits your situation. Debian's aiccu package can take care of heartbeat and AYIYA tunnels for you, and it can even set up static ones.

During registration, you will also need to choose a "PoP", which stands for "Point of Presence". If your country only has a single PoP, that's the one you will end up using (unless you have a good reason for another one), but if there are more options, I strongly suggest that you go through the list of PoPs and select the one with the best roundtrip time and lowest latency from your location! Note that you must answer ping requests (ICMP echo-request) from the PoP you chose, or else the tunnel will not be created.

Other tunnel brokers include HE, which give you up to four tunnels and subnets within minutes, but they only support proto-41 tunnels ("static tunnels"), which do not work everywhere.

Setting up the tunnel

Once your tunnel request gets approved, you'll get a /64 prefix, in which you only use two addresses: the PoP will configure the :1 address and you need to configure your host to use the :2 address on the tunnel interface. You'll also be told the IPv4 address of your PoP "endpoint".

Joey Hess taught me that aiccu can set up the interface for you, using the data it queries from the SixXS registration (TIC) server. I tried it, and it works. However, I prefer the pure ifupdown approach, as it makes things explicit and allows me to use the hooks for stuff like loading the packet filter. So in my /etc/network/interfaces, you can find (remember that these are fake addresses and you need to use your own prefixes and PoP endpoint address):

auto sixxs
iface sixxs inet6 v4tunnel
address 2001:db8:ff00:3b::2
netmask 64
gateway 2001:db8:ff00:3b::1
ttl 64
pre-up ip6tables-restore < /etc/network/ip6tables
up ip link set mtu 1480 dev $IFACE
up invoke-rc.d aiccu start
down invoke-rc.d aiccu stop

Make sure to read about MTU values of the tunnel and adjust the 1480 value in the above to your tunnel settings and ISP connectivity.

The last two lines are only needed for heartbeat tunnels and assume that you have configured aiccu appropriately.

Also, when using /etc/network/interfaces, make sure to set noconfigure true in /etc/aiccu.conf, or else aiccu will bring up a duplicate/additional interface. If your version of aiccu does not honour this configuration option, you can set ipv6_interface sixxs instead: if you tell it to configure the same interface as specified in /etc/network/interfaces, it will actually execute all the same commands (which will fail), but won't report any errors.

For AYIYA tunnels, the following can be used, if you prefer to be able to bring up the interface with the usual ifup command, rather than having it appear when the daemon starts.

auto sixxs
iface sixxs inet manual
pre-up invoke-rc.d aiccu start
up sleep 1
up ip link set mtu 1280 dev $IFACE
post-down invoke-rc.d aiccu stop

It is also a good idea to prevent aiccu from starting at boot when using this method:

update-rc.d aiccu disable

Allowing proto-41 traffic

Unfortunately, these methods are likely to fail, because your regular IP packet filter (iptables, without the 6) doesn't let those encapsulating IPv4 packets pass, unless you tell it to; you probably want to do this early on in the chain, and also limit it to the tunnel peer, so in my case, I use the following rule:

iptables -I INPUT -p ipv6 -s -j ACCEPT

For AYIYA, you need to open port 5072, either for UDP, TCP, or SCTP, depending on how you configured it. Also have a look at this FAQ entry on what a firewall needs to pass. If it still doesn't work, you have an upstream packet filter that needs some of those holes poked. Good luck.

In most situations, the FORWARD chain does not get such a rule, since the tunnel terminates at the gateway, which routes to a native IPv6 network, or another tunnel. Allowing tunnels through a gateway is almost always a bad thing, as it would allow undetected and untraceable traffic from compromised boxes in the local network. The OUTPUT chain also does not need such a rule, if you have configured stateful filtering properly.

Now bring up the interface and verify the connection:

# ifup sixxs
# ping6 -nc1 2001:db8:ff00:3b::1
PING 2001:db8:ff00:3b::1(2001:db8:ff00:3b::1) 56 data bytes
64 bytes from 2001:db8:ff00:3b::1: icmp_seq=1 ttl=64 time=74.0 ms
# ping6 -nc1
PING 56 data bytes
64 bytes from 2001:a60:9002:1::184:1: icmp_seq=1 ttl=55 time=91.5 ms

Welcome to the Internet of the future!

Setting up an IPv6-capable gateway

Your IPv6 connection works, but it's limited to a single address, and you do not get to specify the reverse DNS PTR record for it. Since the concept of NAT is mostly absent from IPv6 (thanks! thanks! thanks!), you will not be able to connect any other hosts to the IPv6 network. If your local network has a few hosts behind a gateway, you will need to request a subnet from SixXS and configure your gateway (which has the tunnel connection) appropriately. Don't worry, this is not really very difficult.

First, request a subnet for your tunnel from your PoP via your SixXS homepage. Once approved, you will get a /48 prefix for your own use: 2^80 — 1.2 heptillion addresses which are yours to assign to every dust particle in your office or home, if you so desire.

The way I set it up is to add the first of these addresses to your internal interface on the gateway, by adding the following two lines to the interface's stanza in /etc/network/interfaces; you will need the iproute package installed (which you should be using for everything network-related anyway; remember to substitute your own prefixes):

up ip -6 addr add 2001:db8:ff12::1/64 dev $IFACE
down ip -6 addr del 2001:db8:ff12::1/64 dev $IFACE

Instead of bringing the interface down and up, just run ip -6 addr add <your prefix> dev eth0. Note the use of the /64 prefix instead of the /48 that got assigned, leaving only 20 pentillion addresses. Oh no! The reason for this is buried in the specs: basically, /48 is a site prefix, but individual networks should not be larger than /64, which is the prefix length of links in the IPv6 domain.

The /64 prefix is only one of 65536 different /64 prefixes you can use from your /48 prefix. Since it's unlikely that you'll use them all, it's a good idea to route unused ones to an unreachable destination, such as the loopback interface, which conveniently causes packets to any addresses outside the used /64 networks to be answered with ICMP destination network unreachable. You could route them to the special unreachable target instead, which would cause host unreachable messages, but the following is more explicit (again, remember to use your own prefix in place of the one used in this document):

up ip -6 route add 2001:db8:ff12::/48 dev lo
down ip -6 route del 2001:db8:ff12::/48 dev lo

Enabling forwarding

Now is also a good time to enable IPv6 forwarding, e.g. like so:

# echo net.ipv6.conf.all.forwarding = 1 >> /etc/sysctl.d/ipv6_forwarding.conf
# sysctl -p

Obviously, you will also need to change the policy on the ip6tables FORWARD chain. For now, let's just set it to accept all traffic between the local network behind eth0 and the Internet behind eth1. You should later create a proper ruleset, though! (remember to use your own prefix)

# ip6tables -I FORWARD -i eth0 -o eth1 -s 2001:db8:ff12::/64 -j ACCEPT
# ip6tables -I FORWARD -i eth1 -o eth0 -d 2001:db8:ff12::/64 -j ACCEPT

Advertising addresses on your local network

The final step is to spread the love to your local network. Refrain from selecting addresses from your subnet and assigning them to the local hosts, or wondering how to configure the DHCP server, because IPv6 does it differently: your gateway will advertise its routes (which includes a default route) to your network, and each host will pick an address based on its MAC address (unless it already has an EUI-64 address assigned. This all happens automagically, at least with current Debian and Windows machines.

On the gateway, you need to install radvd and simply tell it which prefix to use on which interface. My /etc/radvd.conf looks like this, and I won't explain it (remember to use your own prefix, not the one used in this document):

interface eth0
AdvSendAdvert on;
prefix 2001:db8:ff12::/64

Note again how we advertise a /64 network rather than the /48 we "own". You cannot advertise smaller networks if you want automatic configuration to work, and you should not use networks larger than /64 in any case. If 2^64 addresses are not enough for you, I trust you'll be able to figure out how to advertise another of your 65536 /64 prefixes in the /48 subnet to appropriate hosts.

Restart radvd and run over to another host to witness how it automagically gets connected to the IPv6 network by scanning /var/log/kern.log and watching the output of ip -6 addr and ip -6 route. Try ping6ing from there! Find the dancing turtle! It should all work.

If you don't like the automagic aspect of this, look into stateful configuration, using DHCPv6, as provided by dibbler-server and wide-dhcpv6-server.

Choosing source addresses

Even though you have obtained a /48 network and configured the gateway appropriately, connections originating from the gateway itself will have the tunnel endpoint IP as source address. This is because Linux picks the address of the outgoing interface when it sends packets, and we only slapped the address from our own subnet onto the LAN interface. However, if the firewall answers to 2001:db8:ff12::1 (or whatever address you gave it from your prefix space), it ought to use that address for sending, too.

Unfortunately, there's no guaranteed way to achieve this, due to RFC 3484, which is explained nicely on this page.

The best solution is to add 2001:db8:ff12::1/128 as last address to the external interface, which will cause Linux to use it in most cases. When it doesn't, you ought to just ignore it (remember to use your own prefixes instead):

iface sixxs inet6 v4tunnel
up ip -6 addr add 2001:db8:ff12::1/128 dev $IFACE
down ip -6 addr del 2001:db8:ff12::1/128 dev $IFACE

Here is a thread on the issue on the SixXS forum.

Resolving names

Take note of the IPv6 address of each host. There's a way to determine it given the host's MAC address, but this is easier (ipv6calc is also useful). You might want to let your local DNS server know by adding AAAA records in parallel to the existing A records, and possibly even adding PTR entries.

If you're serious about IPv6, you can tell SixXS to delegate reverse lookups for the IPv6 addresses to your DNS servers, but you ought to refrain from polluting the DNS namespace. The dig tool and its +trace option are awesome to debug and analyse reverse delegations:

dig +trace -x 2001:41e0:ff1a::1

Note that bind9-host provides an improved host tool, which fetches all kinds of information about names, not just the one single information configured as default:

% host has address has IPv6 address 2001:41e0:ff1a::1 mail is handled by 99 mail is handled by 10

% host 2001:41e0:ff1a::1
domain name pointer

Oh, and if you're really that curious about how IPv6 addresses are computed from MAC addresses, read RFC 2464. Basically, given a prefix 2001:db8:ff1a:: and a MAC address aa:bb:cc:dd:ee:ff, the resulting IPv6 address is obtained by:

inserting ff:fe into the middle of the MAC address to yield aa:bb:cc:ff:fe:dd:ee:ff;
removing the odd colons to yield aabb:ccff:fedd:eeff, the EUI-64;
flipping the second lowest bit of the first octet to yield a8bb:ccff:fedd:eeff; this marks the EUI-64 globally unique.
concatenating the prefix and this result to get 2001:db8:ff1a::a8bb:ccff:fedd:eeff.
Other (easier) ways to determine the local component of an IPv6 address are:

look at the output of ip -6 addr and find the address starting with fe80:: on the interface you want to connect to the IPv6 network; following the two colons is the interface's EUI-64.
use ipv6calc or sipcalc.
If you find your (Windows) IPv6 addresses changing all the time, you might be faced by "privacy features".

Remaining issues

Even though my IPv6 connectivity works, I have two remaining issues.

Mapping names to laptops

Laptops generally have two interfaces, one with a cable, and the other wireless. Both of these interfaces will have separate MAC addresses, and by extension, the laptop will have different IPv6 addresses depending on how it is connected to the local network.

I want to be able to connect to laptops without knowing the medium they use to connect to the network. Unfortunately, there seems to be no feasible way. The solutions I see are:

override the MAC address of one interface with that of the other, which is going to cause bgi problems in the case when the laptop (accidentally) gets connected to the same network twice;
add both IPv6 addresses as AAAA records to the laptop's DNS name, which will cause random delays when connecting as the resolver may return the currently inactive address first;
set up mobile IPv6, e.g. by following this Mobile IPv6 how-to, which would allow accessing the laptop uniformly, no matter where in the world it is. Unfortunately, Debian's support for Mobile IPv6 is severly lacking at time of writing. Also, Yves-Alexis Perez notes that this how-to is horribly outdated and promised to tend to it Real Soon Now™.
The second solution works for me for now, but I am interested in the third.

In response to this document, Andreas Henriksson has suggested the replace the stateless configuration (radvd) with stateful configuration, using DHCPv6. I have yet to investigate this option.

Jeroen Massar suggests to unite cable and wireless into a bridged interface, which seems like a very good idea.

Kernel versions and stateful connection tracking

Even though the default kernel of Debian etch, our current stable release, speaks IPv6, I cannot recommend it for deployment, as the 2.6.18 kernel does not support proper stateful connection tracking for IPv6, and thus makes it impossible to firewall hosts in a sensible manner (I always add local packet filters to all my hosts, and if only to guard against the situation when a user installs a malicious programme to listen on a high port). Of course, it is possible to configure a packet filter statelessly in an acceptable manner once you know the use case, so do with this information what you wish; I prefer to stay general for now.

A remedy exists, however: Debian "etch" 4.0r4 adds the so-called etch-and-a-half kernels, e.g. linux-image-2.6.24-etchnhalf.1-686. The 2.6.24 kernel seems to support stateful connection tracking for IPv6. Alternatively, you could use the 2.6.24 linux kernel packages on

Xen and IPv6

One drawback of switching to 2.6.24 is that you cannot run a dom0 on that machine any longer, so by practical extension, you cannot connect it to the IPv6 network with a packet filter in place. Supposedly, running 2.6.24 instances on a 2.6.18 dom0 is reported to work, however.

Debian "lenny" 5.0 fixes this: it comes with a patched 2.6.26 kernel and can be used for both, as dom0 as well as regular IPv6 citizen. / 2023-12-10_22-52-44 UTC.