Install xtables and kernel headers:
$ sudo apt install dev-scripts linux-headers-`uname -r` xtables-addons-common xtables-addons-source xtables-addons-dkms
xtables-addons-dkms fails to build. Most likely due to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1014680.
Build the package:
$ cd xtables-addons-3.21
$ debuild -b -uc -us
In the parent directory you should now have 4 deb packages:
$ ls -1 *.deb
xtables-addons-common-dbgsym_3.21-1_arm64.deb
xtables-addons-common_3.21-1_arm64.deb
xtables-addons-dkms_3.21-1_all.deb
xtables-addons-source_3.21-1_all.deb
Install the xtables-addons-dkms and xtables-addons-common` packages:
$ sudo dpkg -i xtables-addons-dkms_3.21-1_all.deb xtables-addons-common_3.21-1_arm64.deb
This will build the xtables-addons kernel modules within /lib/modules/$(uname -r)/updates/dkms:
$ ls -1 /lib/modules/$(uname -r)/updates/dkms/xt_*.ko
/lib/modules/5.14.0-0.bpo.2-arm64/updates/dkms/xt_ACCOUNT.ko
/lib/modules/5.14.0-0.bpo.2-arm64/updates/dkms/xt_CHAOS.ko
[...]
Add the accounting config to /etc/shorewall/accounting:
ACCOUNT(int-ext,10.0.1.0/24) - eth1 eth0
ACCOUNT(int-ext,10.0.1.0/24) - eth0 eth1