Raspbian: static IP for eth0 and dhcp for wlan0

Question

I run a Raspberry Pi at our university (Raspbian Stretch). I use it for a project where it has to rely on WiFi. I configured the connection using wpa_supplicant and it worked perfectly well. Now there's a problem with it, so I no longer get onto it using WiFi/ssh. I unplugged the SD card and modified /etc/network/interfaces file to configure a static network that allows me to acces the Raspi over Ethernet. Here's the file:

source-directory /etc/network/interfaces.d

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

auto eth0
allow-hotplug eth0
iface eth0 inet static
address 192.168.66.101
netmask 255.255.255.0
gateway 192.168.66.100

The Ethernet connection works perfectly, so I can acces my Raspi via ssh again. When I do /etc/init.d/neworking restart, it takes a while, but I don't get a WiFi connection. When i run sudo systemctl restart dhcpcd.service, it fails. systemctl status dhcpcd.service says:

Feb 23 09:33:01 pren7 systemd[1]: Starting dhcpcd on all interfaces...
Feb 23 09:33:01 pren7 dhcpcd[3065]: Not running dhcpcd because /etc/network/interfaces
Feb 23 09:33:01 pren7 dhcpcd[3065]: defines some interfaces that will use a
Feb 23 09:33:01 pren7 dhcpcd[3065]: DHCP client or static address
Feb 23 09:33:01 pren7 systemd[1]: dhcpcd.service: Control process exited, code=exited status=6
Feb 23 09:33:01 pren7 systemd[1]: Failed to start dhcpcd on all interfaces.
Feb 23 09:33:01 pren7 systemd[1]: dhcpcd.service: Unit entered failed state.
Feb 23 09:33:01 pren7 systemd[1]: dhcpcd.service: Failed with result 'exit-code'.

So can't I use DHCP for wlan0 when I configure eth0 using a static IP? Is there a workaround? I cannot just deactivate eth0, because if the wlan0 connection fails, I need to modify the config directly on the SD card again.

EDIT: Following your suggestion, I switched from classic networking to dhcpcd. Here's my new /etc/network/interfaces:

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto lo
iface lo inet loopback

iface eth0 inet manual

allow-hotplug wlan0
iface wlan0 inet dhcp
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

And here's my /etc/dhcpcd.conf:

interface eth0
static ip_address=192.168.66.101/24
static routers=192.168.66.100
static domain_name_servers=8.8.8.8

Answer 1

Now I figured it out, thanks for the link!

Raspi Setup

After copying image to SD card:

mount /dev/mmcblk0p1 /mnt # mount boot partition
touch /mnt/ssh # activates ssh
umount /mnt

mount /dev/mmcblk0p2 /mnt # mount root partition

Edit /mnt/etc/dhcpcd.conf:

# static ethernet interface for debugging
interface eth0
static ip_address=192.168.66.101/24
static routers=192.168.66.100
static domain_name_servers=8.8.8.8

Umount SD card:

umount /mnt

Remove SD card, plug it into the Raspberry.

On Arch Linux, set up the Ethernet as follows:

sudo ip link set enp0s31f6 up
sudo ip addr add 192.168.66.100/24 broadcast 192.168.66.255 dev enp0s31f6

Connect to Raspberry using ssh:

ssh pi@192.168.66.101 # default password: raspberry

Add WiFi credentials:

wpa_passphrase [essid] [passphrase] >> /etc/wpa_supplicant/wpa_supplicant.conf

Enable wpa_supplicant:

systemctl enable wpa_supplicant.service
systemctl start wpa_supplicant.service

Restart networking:

systemctl restart dhcpcd.service

Answer 2

You can continue to use obsolete Debian networking despite its limitations - it normally configures a single interface (your implementation is non-standard and it is anyone's guess what it would do).

Including dhcp in /etc/network/interface causes dhcpcd to disable itself.

However I suggest you use the recommended normal dhcpcd networking.

See How to set up networking/WiFi

Answer 3

Try to add line below to /etc/dhcpcd:

denyinterfaces wlan0