April 15, 2020

T&C – How To configure Raspbian to prefer eth0 over wlan0

The idea is reduce energy, radio emission, heat emission: make a handles scripts that leave switched-on only one interface with a minimal usage of network script.

The simplest way is just to create a cron script that enable or disable interface in order to leave eth0 up, or wifi if eth0 is down.

What do we need to perform that? it is necessary to know if the interface have a valid ip then disable the other accordly:

Get the IP from interface

The main script /sbin/ifconfig provide all the information:

pi@reflu:~ $ /sbin/ifconfig eth0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.178.34  netmask 255.255.255.0  broadcast 192.168.178.255
        inet6 fe80::375d:dd72:2185:6c17  prefixlen 64  scopeid 0x20<link>
        ether b8:27:eb:b9:90:a5  txqueuelen 1000  (Ethernet)
        RX packets 10089732  bytes 15115613319 (14.0 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 5222734  bytes 356142008 (339.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

but here we have too many information, so need to filter it 😉

pi@reflu:~ $ /sbin/ifconfig eth0 | grep 'netmask' | awk '{ print $2 }'
192.168.178.34

do we need really the IP? maybe no, maybe just need to known if the eth0 have the IP

/sbin/ifconfig eth0 | grep -q 'netmask'
# 0 is preset, 1 is not present
echo $?

Enable/Disable interface

This command is easy… just use iwconfig wlan0 txpower off disable the Wifi and for the enabling iwconfig wlan0 txpower auto but take attention about the last command: it’s come in error! I don’t have any idea about the reason it, the facto before this command the Wifi restart to work… it’s the troll of raspberry

The Final Scripts

Install the script

The final steps, just run every minutes… the cron help on it: add the command in root crontab with the command

sudo crontab -e -u root

and the line is just

* * * * /bin/bash -c ". /root/.bashrc; /usr/local/sbin/disable-wifi.sh > /var/log/disable-wifi.log 2>&1"

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