systemd-hosts.d.service

[Unit]
Description=Keep your hosts records in logically separated files
After=local-fs.target

[Service]
Type=oneshot
Environment="HOSTS_FILE=/etc/hosts"
Environment="HOSTS_DIRECTORY=/etc/hosts.d"
Environment="HOSTS_HEADER=### Generated by systemd-hosts.d ###"

# Ensure hosts directory exist
ExecCondition=/bin/sh -c 'test -e "$HOSTS_DIRECTORY"'
# Ensure preexisting hosts file isn't going to be overwritten
ExecCondition=/bin/sh -c '\
    test ! -f "$HOSTS_FILE" || head -n1 -- "$HOSTS_FILE" | grep -q "$HOSTS_HEADER"\
'

# Assemble hosts file
ExecStart=/bin/sh -c '(\
    echo "$HOSTS_HEADER";\
    echo;\
    for conf_file in "$HOSTS_DIRECTORY"/*.conf; do\
        test -e "$conf_file" || continue;\
        echo "## From $conf_file ##";\
        while read -r line; do\
          printf "%%s\\n" "$line";\
        done <"$conf_file";\
        echo;\
    done;\
) >"$HOSTS_FILE"'

# Additionally, unset write permissions while at rest
ExecStartPre=-/usr/bin/env chmod ug+w -- "$HOSTS_FILE"
ExecStartPost=-/usr/bin/env chmod ugo=r -- "$HOSTS_FILE"

[Install]
RequiredBy=network-pre.target