Everybody running a server on a Raspberry with an open internet connection should protect against unauthorized access. There are various ways to protect. An additional protection is to restrict access to the Raspberry to specific IP ranges. The easiest way to do this is by using geoip and iptables and allow access from IPs from your country only. Actually this makes sense only if the server is used by you only and is no open server for everybody (owncloud, seafile, ...).

Execute following steps in order to install geoip on Raspbian Buster

Buster now uses nsf instead of iptables and requires a different format of the geoip files.

1) install xtables-addon

sudo apt install xtables-addons-common libnet-cidr-lite-perl libtext-csv-xs-perl libgeoip2-perl

2) Enable xt_geoip

sudo modprobe xt_geoip

echo "xt_geoip" | sudo tee -a /etc/modules-load.d/modules.conf

3) Download geoip files

mkdir /tmp/geoip

cd /tmp/geoip

/usr/lib/xtables-addons/xt_geoip_dl

4) Build geoip database now

mkdir -P /usr/share/xt_geoip
cd GeoLite2-Country-CSV_20190709
sudo /usr/lib/xtables-addons/xt_geoip_build -D /usr/share/xt_geoip

5) Add iptables rules to accept IPs from US and Germany

Example:

iptables -A INPUT -m geoip --src-cc DE,US -m conntrack --ctstate NEW -j ACCEPT

6) Create following script to update your geoip database on a regular base

#!/bin/bash
geotmpdir=$(mktemp -d)
OLDPWD="${PWD}"
cd "${geotmpdir}"
/usr/lib/xtables-addons/xt_geoip_dl
dir="$(ls)"
cd $dir
/usr/lib/xtables-addons/xt_geoip_build -D /usr/share/xt_geoip 
cd "${OLDPWD}"
rm -r "${geotmpdir}"

Execute following steps in order to install geoip on Raspbian Stretch

1) Install the xtables-addons

sudo apt-get install raspberrypi-kernel-headers

wget http://downloads.sourceforge.net/project/xtables-addons/Xtables-addons/xtables-addons-2.14.tar.xz
tar xf xtables-addons-2.14.tar.xz
cd xtables-addons-2.14
./configure
make
make install

or

Kudos to @Basti

You can also use DKMS to build this module. Place source to /usr/src/xtables-addons-2.14 for example and create a dkms.conf in there. I have used the file shipped with xtables-addons-dkms_2.12-0.1_all.deb and edit the PACKAGE_VERSION="2.14" and
DEST_MODULE_LOCATION[0]="/extra". More infos about dkms (https://wiki.ubuntuusers.de/DKMS/).

2) Create a file /usr/local/bin/installGeoIP.sh and insert following code

#!/bin/bash
set -euo pipefail

set +e
if ! dpkg -l xtables-addons-common >/dev/null ; then
        apt install xtables-addons-common
fi
if ! dpkg -l libtext-csv-xs-perl >/dev/null ; then
        apt install libtext-csv-xs-perl
fi
set -e

if [ ! -d /usr/share/xt_geoip ]; then
        mkdir /usr/share/xt_geoip
fi

geotmpdir=$(mktemp -d)
csv_files="${geotmpdir}/GeoIPCountryWhois.csv ${geotmpdir}/GeoIPv6.csv"
OLDPWD="${PWD}"
cd "${geotmpdir}"
/usr/lib/xtables-addons/xt_geoip_dl
/usr/lib/xtables-addons/xt_geoip_build -D /usr/share/xt_geoip ${csv_files}
cd "${OLDPWD}"
rm -r "${geotmpdir}"
exit 0

3) Make this file executable and invoke it

chmod +x /usr/local/bin/installGeoIP.sh

installGeoIP.sh

4) Add iptables rules to accept IPs from US and Germany

Example:

iptables -A INPUT -m geoip --src-cc DE,US -m conntrack --ctstate NEW -j ACCEPT

Issues

If you get iptables: No chain/target/match by that name. error messages test whether the xtables_addons are installed correctly

modprobe -c | grep x_tab

should display a long list of modules.

modprobe xt_geoip

Should succeed.

depmod -a

may also help to fix the issue.

References

Maxmind geoip

Linoxide: Block IP from countries using Geoip

Netfilter: geoip howto

Xtables-addons (source code)

Blocklist ipsets

How to install kernel headers

Linxu headers rpi from mhieenka
Solved: iptables & geoip

rpi-source wiki

Alternative: ipset usage (German)

Reddit: Firewall with geoIP capability on Debian 10

Sourceforge: XTables-addons

nft-blacklist

 asds