Tag Archives: multiple

Rename interfaces on Linux

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I just reinstalled a Debian stable on a laptop but messed with the interfaces so that an external USB WiFi card appeared as wlan0 while the main card appeared as wlan1. In case you wondered you can rename or reset interface names in /etc/udev/rules.d/70-persistent-net.rules. That’s on systemd though.
I wonder how we can change that on sysvinit? Nobody cares, probably, but I do.

According to what I read there, it is not consistent. Interfaces are named in the order in which they appear during the boot process. However it is possible to use ifrename from the wireless tools package. Why this tool that should work for all type of interface is part of the wireless tools package is beyond my comprehension. But hey whatever, Linux, and it just works.

If you are curious and want to know how ifrename actually does rename an interface, according to the code it uses a SIOCSIFNAME ioctl on a socket file descriptor. There it passes a struct ifreq in which you can provide a new name for the interface. Just man netdevice(7) for more info.

Source based routing

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My two home servers are down for the moment. This also means that our two IPv6 SixXS tunnels are down which costs us 100 ISK per week. Argh! I need to get these up and running as soon as possible. Fortunately we have another VPS on Linux that can save us. So we just have to enable the two tunnels there and make sure that we can ping to/from both interfaces.

Setting up the two tunnels is easy. Use one configuration file per tunnel. Ensure that you change the parameters “tunnel_id” to the tunnel associated to this configuration file, one “pidfile” and “ipv6_interface” for each tunnel and “defaultroute” to false because we already have a default IPv6 route. Now you can start/stop each tunnel with:

aiccu start /etc/aiccu/tunnel0.conf
aiccu start /etc/aiccu/tunnel1.conf

aiccu stop  /etc/aiccu/tunnel0.conf
aiccu stop  /etc/aiccu/tunnel1.conf

Don’t forget to hack /etc/init.d/aiccu to start/stop both tunnel on each reboot. OK! So now ifconfig list the two interfaces, up and running sixxs0 and sixxs1. This is great but wait… Nobody outside can ping these interfaces. The tunnel must ping to be considered active by SixXS so we better get this running.

For now we have these three interfaces and IPs (not the actual names/IPs):

  1. net0 (2001::1) default
  2. sixxs0 (2a02::1)
  3. sixxs1 (2a02::2)

By default, all our IPv6 traffic goes through net0. However and unsurprisingly our ISP filters the traffic at the output of net0. So we cannot use this interface to answer the echo-requests. Actually, what we want is that traffic originating 2a02::1 goes through sixxs0 and from 2a02::2 goes through sixxs1. That is, one default route based on the source address.

Linux has long had support for multiple routing tables (CONFIG_IP_MULTIPLE_TABLES). Basically what we will do here:

  • Create two routing tables for each tunnel interface (sixxs0, sixxs1).
  • Each table will have a default route through its interface.
  • Lookup into one of the two tables according to the source IP.

You can find some relevant documentation in Linux Advanced Routing & Traffic Control, Chapter 4.

We first list the actual rules:

# ip rule list
0:  from all lookup local
32766:  from all lookup main
32767:  from all lookup default

We can see that we have three routing tables. One for the local addresses, the normal routing table (what you get with ip -6 route) and the fallback default table.
Let’s first check the local routing table (we are just curious):

# ip -6 route list table local
local ::1 via :: dev lo  proto none  metric 0  mtu 16436 advmss 16376 hoplimit 0
local 2a02::1 via :: dev lo  proto none  metric 0  mtu 16436 advmss 16376 hoplimit 0
local 2001::1 via :: dev lo  proto none  metric 0  mtu 16436 rtt 6ms rttvar 7ms cwnd 10 advmss 16376 hoplimit 0
local 2a02::2 via :: dev lo  proto none  metric 0  mtu 16436 advmss 16376 hoplimit 0
local fe80::1 via :: dev lo  proto none  metric 0  mtu 16436 advmss 16376 hoplimit 0
local fe80::2 via :: dev lo  proto none  metric 0  mtu 16436 advmss 16376 hoplimit 0
ff00::/8 dev net0  metric 256  mtu 1500 advmss 1440 hoplimit 0
ff00::/8 dev sixxs1  metric 256  mtu 1280 advmss 1220 hoplimit 0
ff00::/8 dev sixxs0  metric 256  mtu 1280 advmss 1220 hoplimit 0

So now you know what’s going on when you ping one of your local interfaces. But back to our point. We name our two new routing tables in /etc/iproute2/rt_tables:

# SixXS tables
200 sixxs0
201 sixxs1

Now we add the default route in each of these two tables:

ip -6 route add default dev sixxs0 table sixxs0
ip -6 route add default dev sixxs1 table sixxs1

And finally we use two rules to map the source address to the correct routing table:

# ip -6 rule add from 2a02::1 table sixxs0
# ip -6 rule add from 2a02::2 table sixxs1
# ip -6 rule list
0:  from all lookup local
16383:  from 2a02::1 lookup sixxs0
16383:  from 2a02::2 lookup sixxs1
32766:  from all lookup main
32767:  from all lookup default

It should be OK but let’s check that. We can ping from the sixxs interfaces:

ping6 -c1 -I 2a02::1 www.kame.net
ping6 -c1 -I 2a02::2 www.kame.net

We also check that we can ping our interfaces from another host:

ping6 -c1 2a02::1
ping6 -c1 2a02::2

Everything works, that’s great! Finally we just hack /etc/init.d/aiccu to configure the routing tables on each reboot. Note that you need to sleep a bit when you issue the aiccu start because the daemon needs a bit of time to enable the tunnels. Also note that you must be careful when you test your script (quoting the SixXS FAQ):

“If a client connects more than 4 times in 60 seconds (1 minute) the client will not be allowed to connect again for the next 5 minutes. In case this threshold is exceeded more than once in 24 hours a client will be automatically blocked for a week.”

As you can guess, I have been blocked. Oops!

Get rid of that Non-Breaking space

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The non-breaking space is a variant of the space character which as the name suggests prevents automatic line breaking when using a space character. Another common use of it is to avoid collapsing of white-spaces in formats such as TeX or HTML. There are also some others specific typographic uses but nevermind.

From my point of view this character is a real pain, a annoying remain of a former era when the only editor you had was plain-text and which is now just good at invisibly polluting your work so that no compiler, interpreter or any other ASCII oriented tool would accept it anymore. For ASCII is not dead, it’s 7-bit, it’s common, it’s the neutral zone of all character set and No-Break Space shall not interfere therefore it must be eradicated from the very surface of earth or at least from my own keyboards.
There are several options to do that under Xorg. The first one would be to use the setxkbmap command to specify the nbsp:none option for your keyboard.
setxkbmap -option "nbsp:none"

However this won’t work as you might expect it especially when you have multiple keyboards with different layouts. So another solution would be to specify XkbOption directly within the Xorg configuration file, one for each input device. But this won’t work neither if you use a Bépo layout as this option will simply disable the use of underscore (AltGr Space). So the final solution is based on xmodmap to modify the list of keysyms assigned to the space keycode (0x41). You may just add the following line to ~/.xmodmap and ensure that the file is loaded when your session starts with xmodmap ~/.xmodmap.

keycode 65 = space space space space underscore underscore space space