Conditional SLAAC with bird – Only advertise default route if one is present on the router

When people want to deploy a Linux box as a 1st hop router in an IPv6 network RAdvd usually is the first choice for sending out Router Advertisments. The configuration is rather straight forward and on most distributions extensive example configuration files exist. Once configured RAdvd will silently do it’s job and happilly send out periodic RAs and answer any Router Solicitations, even thought the router itself may not have any network connectivity (anymore).

While designing the Freifunk Hochstift Backbone a while back I was looking for a way to only send out RAs with a default route if the router actually has one itself to prevent clients from getting broken IPv6 connectivity (and people complaining).

I found that the Bird Internet Routing Daemon – which already was in use for OSPF/BGP – also provides a ravd protocol which does what I wanted using a trigger. The following configuration will keep sending out RAs but will zero the router lifetime, which will allow clients to configure an IP address from this prefix but not use this router as default gateway, which is OK for my particular setup.

protocol radv {
    # ONLY advertise prefix, IF default route is available
    import all;
    export all;
    trigger ::/0;

    interface "vlan0815" {
        default lifetime 600 sensitive yes;

        prefix 2001:db8:23:42::/64 {
        preferred lifetime 3600;

This is the default behaviour of the radv trigger but it can be fine tuned to your requirements:

trigger prefix

RAdv protocol could be configured to change its behavior based on availability of routes. When this option is used, the protocol waits in suppressed state until a trigger route (for the specified network) is exported to the protocol, the protocol also returns to suppressed state if the trigger route disappears. Note that route export depends on specified export filter, as usual. This option could be used, e.g., for handling failover in multihoming scenarios.

During suppressed state, router advertisements are generated, but with some fields zeroed. Exact behavior depends on which fields are zeroed, this can be configured by sensitive option for appropriate fields. By default, just default lifetime (also called router lifetime) is zeroed, which means hosts cannot use the router as a default router. preferred lifetime and valid lifetime could also be configured as sensitive for a prefix, which would cause autoconfigured IPs to be deprecated or even removed.

The radv protocol has a long list of knobs and switches for fine tuning, see the bird 1.6 and 2.0 documentation for further details.

OpenVPN and VRFs

Using VRFs on Linux enables a whole new set of network setups.

As described in the previous post about VRFs on Linux, VRFs allow to isolate different interfaces at layer 3. In the Freifunk Hochstift network we chose to consider the main or default VRF as the internal network and move any internet facing interfaces into an external VRF. Chosing this concept allows, to safely contain traffic within the internal network and only at  designated border routers leak eligible traffic into the internet.

In an distributed environment like the Freifunk Hochstift network, it is inevitable to connect different islands using VPN tunnels over the Internet. This could be done by the means of GRE tunnels as shown in the previous article about the border routers, or by means of encrypted VPNs like OpenVPN, IPsec or Wireguard. As the old infrastructure quite heavily relied on OpenVPN tunnels, and they worked quite well, the new setup should keep this building block in place.

Continue reading OpenVPN and VRFs

(Vlan-aware) Bridges on Linux

Lately I’ve had some conversations about how Linux sucks at bridging tagged VLANs into VMs, which just isn’t true anymore.

With recent Kernels Linux bridges have become vlan-aware and now allow configuring any bridge port like a port of any decent network switch with respect to 802.1q VLANS. A port can present a VLAN as untagged traffic as well as a number of VLANs in tagged mode. As can be expected, SVIs can be configured as vlan interfaces of a bridge, too.

The old brctl utility has been integrated within the iproute suite as part of of ip link. The commands map as follows:

brctl addbr br0
ip link add br0 type bridge
    [ forward_delay FORWARD_DELAY ]
    [ vlan_filtering VLAN_FILTERING ]
    [ vlan_default_pvid VLAN_D_PVID ]
    [ nf_call_iptables NF_CALL_IPT ]
    [ nf_call_ip6tables NF_CALL_IP6TABLES ]
    [ nf_call_arptables NF_CALL_ARPTABLES ]

brctl addif br0 eth0
ip link set eth0 master br0

Continue reading (Vlan-aware) Bridges on Linux

Seriously predictable interface names – An introduction to systemd .link files

Predictable interface names are a new thing. The most common argument made is that they are not really predictable though, depending on the point of view. How about making interface names predictable and meaningful in the same time?

Most admins will probably think of udev right now, which previously was heavily used to achieve exactly that. In times of systemd the new hotness are .link files which provide similar capabilities and allow even more options to be set for interfaces.

Continue reading Seriously predictable interface names – An introduction to systemd .link files

FrOSCon 13 Network Track – Videos and Slides

A month ago the 13th Free and Open Source Software Conference (FrOSCon) took place in St. Augustin, Germany.  At this years event I organized a two day Network Track designed for a broad audience of Linux folks, system administrators and developers to answer questions about networking topics they were afraid to ask or didn’t realize they wanted or had to know! As the lines between system engineering and network engineering keep on blurring it’s getting more and more important to broaden the focus in both worlds, keywords being things like SDN, IP-Fabric, Segment Routing etc. here.

The track started with a lecture about networking basics and over both days advanced to technically more sophisticated topics following a red line.

On Saturday the focus was on Layer 2 and Layer 3 fundamentals (Ethernet switching and routing), dynamic routing protocols as well as the Linux packet-filter. Sundays track started gently with VLANs, Bonding and Bridging and advanced to more sophisticated topics like policy-based routing, VRFs, Open vSwitch, Segment Routing and Software Defined Networking. The track concluded with an overview about Best Current Operational Practices and a Q&A sessions.

All these talks are – thanks to the nice folks at CCC-VOC – available on Video at (german audio) as are the slides (english):

Day 1

Day 2

  • Adv. topics in Layer2/3 – Light and dark magic with the Linux network stack  (Video) (Slides)
  • Segment Routing  (Video) (Slides)
  • Open vSwitch – The switch within your machine  (Video) (Slides)
  • Best Current Opertional Practices – Dos, Don’ts and lessons learned
    (Video) (Slides)
  • Building your own SDN – The penguin orchestrates the network  (Video) (Slides)
  • Grand Q&A  (Video)

Anycasted services with Debian, bird, anycast-healthchecker and Cisco Nexus 7000

A while ago we started getting alerts, that one of our Kerberos KDCs had problem with the Kerberos database replication. A little digging revealed, that the problems are caused by load spikes on the KDC which were the result of a burst of legitimate queries fired by some systems we didn’t have much control over. Additionally we found that the MIT Kerberos implementation queries all KDCs provided in the configuration file in sequential order, so the first KDC get’s nearly all queries. While thinking about load balancing solutions, quickly anycast came to mind, so we decided to set it up. Anycast leverages the Equal Cost Multipath Routing (ECMP)  capability of common routers to distribute traffic to multiple next-hops for the same destination.

The solution consists of three corner stones:

  1. anycast-healtchecker as a means to check service availability
  2. bird as a BGP speaker on the KDCs and route reflectors
  3. Data center routers (Cisco Nexus 7010) speaking BGP to the route reflectors

The topology is as follows:

Topology KDCs

Continue reading Anycasted services with Debian, bird, anycast-healthchecker and Cisco Nexus 7000

Beware of the details (and VLANs)

A friend today challenged me with this problem on an Ubuntu box:

auto eth2
iface eth2 inet static
    netmask 31

auto eth2.210
iface eth2.210 inet static
    netmask 31

root@box:~# ifup eth2.210
Cannot find device "eth2.210"
Failed to bring up eth2.210.

The first thing coming to mind is “package vlan missing?”, which it was. After installing it, it got more interesting:

root@box:~# ifup eth2.210
Set name-type for VLAN subsystem. Should be visible in /proc/net/vlan/config
ifup: recursion detected for interface eth2 in parent-lock phase
ERROR: trying to add VLAN #210 to IF -:eth2:-  error: File exists
Cannot find device "eth2.210"
Failed to bring up eth2.210

A little poking around showed, that there’s no interface eth2.210 present in the system, but

root@box:~# cat /proc/net/vlan/config 
VLAN Dev name | VLAN ID
rename10       | 210  | eth2

root@box:~# ip l
10: rename10@eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether DE:AD:BE:EF:23:42 brd ff:ff:ff:ff:ff:ff

Deleting the renameNN interface an running ifup again, just created a renameNN+1 interface with kernel log entries like

[7366672.699018] rename14: renamed from eth2.210

I suspected some bugs in /etc/network/if-{,pre-}up.d/ but this even happend, when manually running

ip link add link eth2 name eth2.210 type vlan id 210


ip link add link eth2 name vlan210 type vlan id 210


Continue reading Beware of the details (and VLANs)

Building your own Software Defined Network with Linux and Open Source Tools

Nearly two years ago, I started thinking about a next generation design for the Freifunk Hochstift backbone infrastructure, motivated by the limits and design choices made before (we were young and didn’t know better.. or didn’t listen..).

This post is the starting point of a series of posts about building a software defined wireless ISP network with Linux, a fistful of Open Source tools and low cost hardware.

Continue reading Building your own Software Defined Network with Linux and Open Source Tools

FFRL Routingdays – Learn to build the Internet

Over a year ago we (the Freifunk Rheinland Backbone Operators) organized the FFRL Routindays, a two day event where we gave a hands-on trainig on “How to build the Internet” or any internal network. As of periodic quesions about where to find the slides and videos here’s a summary of all media available of this event.

The event has entirely been in german language, as are all slides and lab guides. This is provided in the hope, that it still might be of some use even for non-german speakers.

The building blocks were

  1. Networking / IP basics
  2. Static routing + bird hands-on Lab
  3. OSPF
  4. eBGP
  5. eBGP -> OSPF redistribution
  6. iBGP
  7. Policy-based Routing

The complete slidedeck is available here. Thanks to the CCC VideoOperation Center, there are recordings of the event avilable on, too.