Comment 3 for bug 1838045

ii dkms 2.2.0.3-2ubuntu11.7

# dkms build -m xtables-addons -v 2.12 -k 4.15.0-56-generic --kernelsourcedir="/root/unpack/usr/src/linux-headers-4.15.0-56-generic"
Error! Your kernel headers for kernel 4.15.0-56-generic cannot be found.
Please install the linux-headers-4.15.0-56-generic package,
or use the --kernelsourcedir option to tell DKMS where it's located

ii dkms 2.2.0.3-2ubuntu11.8

# dkms build -m xtables-addons -v 2.12 -k 4.15.0-56-generic --kernelsourcedir="/root/unpack/usr/src/linux-headers-4.15.0-56-generic"

Preparing kernel 4.15.0-56-generic for module build:
(This is not compiling a kernel, just preparing kernel symbols)
Storing current .config to be restored when complete
...
DKMS: build completed.

# dkms status
xtables-addons, 2.12, 4.15.0-56-generic, x86_64: built
xtables-addons, 2.12, 4.4.0-157-generic, x86_64: installed