Override dhcpcd.conf from SD card

Question

I attempted to set up a static ip in dhcpcd.conf, but it appears I made a mistake somewhere, as the Pi does not appear on the network anymore. I have no monitors or keyboards, and ssh was my only method of accessing it. Is there any way to override the file and get it connected again through the sd card?

Answer 1

Take the SD card out of the pi and put it in whatever other device you have

Edit the cmdline.txt file in the boot partition, it'll look something like

console=serial0,115200 console=tty1 root=PARTUUID=xxxxxxxx-02 rootfstype=ext4 fsck.repair=yes rootwait quiet 

add to the end of that line

systemd.run=/boot/firstrun.sh systemd.run_success_action=reboot systemd.unit=kernel-command-line.target

Create a file in the boot partition called firstrun.sh with the following:

#!/bin/bash
set +e
cat > /etc/dhcpcd.conf <<'DHCPCDEOF'
# A sample configuration for dhcpcd.
# See dhcpcd.conf(5) for details.
Allow users of this group to interact with dhcpcd via the control socket.
#controlgroup wheel
Inform the DHCP server of our hostname for DDNS.
hostname
Use the hardware address of the interface for the Client ID.
clientid
or
Use the same DUID + IAID as set in DHCPv6 for DHCPv4 ClientID as per RFC4361.
Some non-RFC compliant DHCP servers do not reply with this set.
In this case, comment out duid and enable clientid above.
#duid
Persist interface configuration when dhcpcd exits.
persistent
Rapid commit support.
Safe to enable by default because it requires the equivalent option set
on the server to actually work.
option rapid_commit
A list of options to request from the DHCP server.
option domain_name_servers, domain_name, domain_search, host_name
option classless_static_routes
Respect the network MTU. This is applied to DHCP routes.
option interface_mtu
Most distributions have NTP support.
#option ntp_servers
A ServerID is required by RFC2131.
require dhcp_server_identifier
Generate SLAAC address using the Hardware Address of the interface
#slaac hwaddr
OR generate Stable Private IPv6 Addresses based from the DUID
slaac private
Example static IP configuration:
#interface eth0
#static ip_address=192.168.0.10/24
#static ip6_address=fd51:42f8:caae:d92e::ff/64
#static routers=192.168.0.1
#static domain_name_servers=192.168.0.1 8.8.8.8 fd51:42f8:caae:d92e::1
It is possible to fall back to a static IP if DHCP fails:
define static profile
#profile static_eth0
#static ip_address=192.168.1.23/24
#static routers=192.168.1.1
#static domain_name_servers=192.168.1.1
fallback to static profile on eth0
#interface eth0
#fallback static_eth0
DHCPCDEOF
rm -f /boot/firstrun.sh
sed -i 's| systemd.run.*||g' /boot/cmdline.txt
exit 0

Stick the card back in the pi and boot

Note: this will set the content of /etc/dhcpcd.conf to the default state, exactly as it was when you first flashed the OS

Answer 2

You have made 2 fundamental errors; changing a system file without backup and setting a static IP address.

There are many ways to fix this;

The easiest is to mount the SD Card on a Linux computer.

You can connect to another computer with an Ethernet cable and ssh using ssh pi@raspberrypi.local

Rebooting to a root shell can repair most problems on the Pi, but needs keyboard & screen or serial console.

1. Append init=/bin/sh at the end of cmdline.txt and reboot.
2. After booting you will be at the prompt in a root shell.
3. Your root file system is mounted as readonly now, so remount it as read/write mount -n -o remount,rw /
4. Fix your problem
5. Remove init=/bin/sh and reboot.