DHCP and read-only root filesystem

dhcpetcnetworkingreadonly

When doing ifup wlan0 on a system with / mounted as read-only (embedded computer), I get this error:

Failed to connect to non-global ctrl_ifname: wlan0  error: Read-only file system
Internet Systems Consortium DHCP Client 4.3.1
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All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

can't create /var/lib/dhcp/dhclient.wlan0.leases: Read-only file system
Listening on LPF/wlan0/80:1f:02:d3:42:b8
Sending on   LPF/wlan0/80:1f:02:d3:42:b8
Sending on   Socket/fallback
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 13
...
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 5
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

On the other hand, when doing ifup wlan0 with / mounted as read-write, no problem, an IP is succesfully attributed.

How to make DHCP work on a read-only root filesystem?

# /etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-ssid "<myssid>"
wpa-psk "<mypasswd>"

Best Answer

Create a tmpfs partition in RAM, and then symlink /var/lib/dhcp to a directory in it.

Edit: I'll assume that you can edit your root filesystem before changing it to read-only. So:

  1. Set up a tmpfs mount point with mkdir /mnt/ramdisk && mkdir /mnt/ramdisk/var-lib-dhcp
  2. Add the tmpfs to your fstab with tmpfs /mnt/ramdisk/var-lib-dhcp tmpfs size=10M 0 0
  3. Add a symbolic link from /var/lib/dhcp to /mnt/ramdisk/var-lib-dhcp
  4. Test while file system is read/write
  5. Make file system read-only, test again
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