Category Archives: Linux General

Notes which apply to all flavours of Linux systems

Iperf – multicast IPv4

Using Iperf version 2.0.9 on Debian (Stretch) it’s possible to test networks using a multicast connection.

On the receiving end execute the command:
iperf -s -u -B -i 1

On the sending end execute the command:
iperf -c -u -T 3 -t 10 -i 1 -b 100M

This will run a bandwidth test using UDP traffic at 100Mbps.

Note, if your system is multi-homed you must make sure your multicast traffic is routed out of the correct interface. For example:
ip route add dev eth0

Bash – Internal Field Separator (IFS)

By default in Bash, the Internal Field Separator (IFS) is a ‘ ‘ character.

This is used to determine how bash splits text input into different fields. For example:

for file in *.jpg; do echo $file; done

will output each jpg filename on a separate line, unless there are spaces used in the filename.

For a more complete solution, change the IFS to the new line character and list each filename on a new line:

for file in `ls -1 *.jpg;` do echo "$file"; done

Of course the above doesn’t really achieve much, but it might be used as part of a simple script to resize images:

for file in `ls -1 *.jpg`; do convert "$file" -resize 200 "thumb-$file"; done

Debian 9 (Stretch) – Systemd network interface names

One of the effects of introducing Systemd into Debian is that the traditional naming convention for network interfaces has changed. No more is your Ethernet interface called eth0, instead it has a name based on its MAC address.

It’s still possible to use the old naming convention by creating a Systemd configuration file for each interface under /etc/systemd/network/. Each filename should start with a number lower than 99. The default configuration file starts with ’99’, the new config should run before that one.

The system config files can be found in /lib/systemd/network/, and are overlaid on top of the files in /etc/systemd/network/

For example, create the file: /etc/systemd/network/ which contains:


For more info about the options available for the Match and Link sections, see the man page.

Update your initramfs which is used during boot-up to configure your system.
update-initramfs -u

After the next reboot, the Ethernet interface with MAC address 01:02:03:04:05:06 will be called eth0.

USB Ethernet devices

Annoyingly the configuration of USB Ethernet devices is interfered with by udev, so that the above Systemd config file will not have any effect. To prevent this, either delete or comment out the contents of the file /lib/udev/rules.d/73-usb-net-by-mac.rules and restart udev.

The USB Ethernet device will now be named based on the Systemd configuration file above.

WiFi Cards

I had some difficulty using the MACAddress Match option with my WiFi card, so I instead matched on the basis of card type:
$ cat /etc/systemd/network/


ISC DHCP Server – Option 43 (Vendor specific attribute)

DHCP (RFC 2132) allows for vendor specific data to be distributed to clients.

Important notes:
– clients must request Option 43 in their Parameter Request List (Option 55).
– the Vendor Class Identifier (Option 60) sent by the client in the DHCP Request, must match ‘VendorName’ which is used in the ISC DHCP Server configuration below.

Example server configuration section from dhcpd.conf:

    option space VendorName;
    option VendorName.serviceName code 1 = text;
    option local-encapsulation code 43 = encapsulate VendorName;
    option VendorName.serviceName "data";

– where VendorName matches the value of the client’s Vendor Class Identifier (Option 60).
– ‘serviceName’ is used only as an internal reference within the DHCP server’s configuration, and must be different for each ‘code’ value.

Up to 256 codes may be used for each Vendor specific configuration. All fields will be returned to the client which matches the Vendor Identifier.

The DHCP server will return the Vendor attributes in the DHCP Response. The data returned is encoded in binary in the following format for each code:
Byte 0: code
Byte 1: length
Byte 2: data

Byte length+2: final data byte
… then follows the next field, starting with code, length, data‚Ķ

Restarting Enlightenment (e17) from the command line

The scenario is that your Enlightenment (e17) desktop has locked up, and your keyboard and mouse appear to be non-responsive. However you do not want to reboot as you will lose all of your open windows.

It is possible to restart the Enlightenment engine without affecting the windows being currently displayed.

Either switch to a TTY (by pressing Ctrl+Alt+F1) or login remotely over ssh, and then execute:
env DISPLAY=:0 enlightenment_remote -restart

Your desktop should have now recovered its pre-locked up state.
(on most setups you can switch back to the desktop from your TTY by pressing Alt+F7)