practical dynamic network setup for laptops

I really wanted to describe my current network setup I use on my laptop. So here is this post.

My objectives were: keep the configuration simple and easy to use for the user.

What I have

My laptop automatically connects to wifi (for configured SSIDs) and obtains an IP address. If I connect the ethernet cable it gets an IP there as well and the same goes for USB (tethering). Each interface gets the default route and they are inserted with different metrics. The wired connection wins. If the preferred interface gets disconnected (e.g cable pulled out) it automatically starts to use the other available interface.

# ip route show

default via 192.168.44.129 dev usb0  metric 267 
default via 141.70.74.1 dev wlan0  metric 303 
141.70.74.0/21 dev wlan0  proto kernel  scope link  src 141.70.74.145  metric 303 
141.70.74.145 via 127.0.0.1 dev lo  metric 303 
192.168.44.0/24 dev usb0  proto kernel  scope link  src 192.168.44.18  metric 267 
192.168.44.18 via 127.0.0.1 dev lo  metric 267

Components

Setup

dhcpd5

This is the dhcp client daemon that tries to get IP addresses on interfaces that are operational mode up. When a link comes up which is: usually after cable insertion for wired ethernet or successful AP association for wireless.

This tool also takes care to add the default routes with the mentioned metrics attached.

To get rid the legacy isc dhclient I did (there may be cleaner ways):

dpkg-divert --divert /sbin/dhclient.orig --rename /sbin/dhclient
ln -s /bin/true /sbin/dhclient

I created a systemd unit file (the default init script is also fine): /etc/systemd/system/dhcpcd.service

[Unit]
Description=dhcpcd5 - IPv4 DHCP client with IPv4LL support
Before=runlevel2.target runlevel3.target runlevel4.target runlevel5.target shutdown.target
After=local-fs.target
Conflicts=shutdown.target

[Service]
ExecStart=/sbin/dhcpcd5 -B -L -d

[Install]
WantedBy=multi-user.target

wpa_supplicant and /etc/network/interfaces

iface wlan0 inet dhcp
  wpa-conf /etc/network/wpa_supplicant.conf

wpa_supplicant.conf looks like this for me:

ctrl_interface=/var/run/wpa_supplicant
update_config=1

network={
    ssid="eduroam"
    scan_ssid=1
    key_mgmt=WPA-EAP
    group=CCMP TKIP
    eap=PEAP
    identity="someuser@login.ppke.hu"
    password="arealpasswordgoeshere"
    ca_path="/etc/ssl/certs"
    subject_match="/C=HU/ST=Budapest/L=Budapest/O=Pazmany Peter Katolikus Egyetem/CN=tutela.itk.ppke.hu"
    phase1="peapver=0"
    phase2="MSCHAPV2"
}

network={
    ssid="Butterfly13"
    scan_ssid=1
    key_mgmt=NONE
    priority=4
    disabled=1
}

network={
    ssid="homewifi"
    scan_ssid=1
    psk="thepskgoeshere"
    key_mgmt=WPA-PSK
    priority=4
}

I use wpa_cli to control the wifi configuration or if I need to add ad-hoc changes (e.g for debconf). It is also easy to select networks to prefer or just disable all except for one.

The networks needs to be up:

ifup eth0
ifup wlan0