为了避免数据库服务器等内网应用资源暴露到公网中，打算利用VPN 技术实现链接到内网。
本文主要介绍CentOS 7 服务器上安装与配置OpenVPN服务器,以及如何编写客户端连接到新建立的OpenVPN服务器上所需的配置文件

OpenVPN的介绍

OpenVPN是一个开源的应用程序，它允许您通过公共互联网创建一个安全的专用网络。OpenVPN实现一个虚拟专用网（VPN）来创建一个安全连接。OpenVPN使用OpenSSL库提供加密，它提供了几种身份验证机制，如基于证书的、预共享密钥和用户名/密码身份验证。

openvpn 有两种模式

数据包（TUN模式）或数据帧（TAP模式）

• TUN模式：TUN模拟了网络层设备，第三层数据包如IP封包，底层数据隧道数据
• TAP模式等同于一个设备，第二操作层数据包如扩展数据帧，创建一个相对桥接接，复杂T

接口接口的好处可见，客户端优化VPN服务子网的IP（忽然忽隐忽现）物理上的区别，可以完全将客户端看做完全与VPN服务器相关的时间，而TUN接口下所有的客户端则出现一个独立的子网内，与VPN服务器相关的子网没有关系，这种使用比较好，和公司的网络区分开，完全是一个虚拟的网络

脚本内容

openvpn 一键安装脚本

#!/bin/bash
#
# https://github.com/Nyr/openvpn-install
#
# Copyright (c) 2013 Nyr. Released under the MIT License.


# Detect Debian users running the script with "sh" instead of bash
if readlink /proc/$$/exe | grep -q "dash"; then
    echo 'This installer needs to be run with "bash", not "sh".'
    exit
fi

# Discard stdin. Needed when running from an one-liner which includes a newline
read -N 999999 -t 0.001

# Detect OpenVZ 6
if [[ $(uname -r | cut -d "." -f 1) -eq 2 ]]; then
    echo "The system is running an old kernel, which is incompatible with this installer."
    exit
fi

# Detect OS
# $os_version variables aren't always in use, but are kept here for convenience
if grep -qs "ubuntu" /etc/os-release; then
    os="ubuntu"
    os_version=$(grep 'VERSION_ID' /etc/os-release | cut -d '"' -f 2 | tr -d '.')
    group_name="nogroup"
elif [[ -e /etc/debian_version ]]; then
    os="debian"
    os_version=$(grep -oE '[0-9]+' /etc/debian_version | head -1)
    group_name="nogroup"
elif [[ -e /etc/centos-release ]]; then
    os="centos"
    os_version=$(grep -oE '[0-9]+' /etc/centos-release | head -1)
    group_name="nobody"
elif [[ -e /etc/fedora-release ]]; then
    os="fedora"
    os_version=$(grep -oE '[0-9]+' /etc/fedora-release | head -1)
    group_name="nobody"
else
    echo "This installer seems to be running on an unsupported distribution.
Supported distributions are Ubuntu, Debian, CentOS, and Fedora."
    exit
fi

if [[ "$os" == "ubuntu" && "$os_version" -lt 1804 ]]; then
    echo "Ubuntu 18.04 or higher is required to use this installer.
This version of Ubuntu is too old and unsupported."
    exit
fi

if [[ "$os" == "debian" && "$os_version" -lt 9 ]]; then
    echo "Debian 9 or higher is required to use this installer.
This version of Debian is too old and unsupported."
    exit
fi

if [[ "$os" == "centos" && "$os_version" -lt 7 ]]; then
    echo "CentOS 7 or higher is required to use this installer.
This version of CentOS is too old and unsupported."
    exit
fi

# Detect environments where $PATH does not include the sbin directories
if ! grep -q sbin <<< "$PATH"; then
    echo '$PATH does not include sbin. Try using "su -" instead of "su".'
    exit
fi

if [[ "$EUID" -ne 0 ]]; then
    echo "This installer needs to be run with superuser privileges."
    exit
fi

if [[ ! -e /dev/net/tun ]] || ! ( exec 7<>/dev/net/tun ) 2>/dev/null; then
    echo "The system does not have the TUN device available.
TUN needs to be enabled before running this installer."
    exit
fi

new_client () {
    # Generates the custom client.ovpn
    {
    cat /etc/openvpn/server/client-common.txt
    echo "<ca>"
    cat /etc/openvpn/server/easy-rsa/pki/ca.crt
    echo "</ca>"
    echo "<cert>"
    sed -ne '/BEGIN CERTIFICATE/,$ p' /etc/openvpn/server/easy-rsa/pki/issued/"$client".crt
    echo "</cert>"
    echo "<key>"
    cat /etc/openvpn/server/easy-rsa/pki/private/"$client".key
    echo "</key>"
    echo "<tls-crypt>"
    sed -ne '/BEGIN OpenVPN Static key/,$ p' /etc/openvpn/server/tc.key
    echo "</tls-crypt>"
    } > ~/"$client".ovpn
}

if [[ ! -e /etc/openvpn/server/server.conf ]]; then
    clear
    echo 'Welcome to this OpenVPN road warrior installer!'
    # If system has a single IPv4, it is selected automatically. Else, ask the user
    if [[ $(ip -4 addr | grep inet | grep -vEc '127(\.[0-9]{1,3}){3}') -eq 1 ]]; then
        ip=$(ip -4 addr | grep inet | grep -vE '127(\.[0-9]{1,3}){3}' | cut -d '/' -f 1 | grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}')
    else
        number_of_ip=$(ip -4 addr | grep inet | grep -vEc '127(\.[0-9]{1,3}){3}')
        echo
        echo "Which IPv4 address should be used?"
        ip -4 addr | grep inet | grep -vE '127(\.[0-9]{1,3}){3}' | cut -d '/' -f 1 | grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}' | nl -s ') '
        read -p "IPv4 address [1]: " ip_number
        until [[ -z "$ip_number" || "$ip_number" =~ ^[0-9]+$ && "$ip_number" -le "$number_of_ip" ]]; do
            echo "$ip_number: invalid selection."
            read -p "IPv4 address [1]: " ip_number
        done
        [[ -z "$ip_number" ]] && ip_number="1"
        ip=$(ip -4 addr | grep inet | grep -vE '127(\.[0-9]{1,3}){3}' | cut -d '/' -f 1 | grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}' | sed -n "$ip_number"p)
    fi
    # If $ip is a private IP address, the server must be behind NAT
    if echo "$ip" | grep -qE '^(10\.|172\.1[6789]\.|172\.2[0-9]\.|172\.3[01]\.|192\.168)'; then
        echo
        echo "This server is behind NAT. What is the public IPv4 address or hostname?"
        # Get public IP and sanitize with grep
        get_public_ip=$(grep -m 1 -oE '^[0-9]{1,3}(\.[0-9]{1,3}){3}$' <<< "$(wget -T 10 -t 1 -4qO- "http://ip1.dynupdate.no-ip.com/" || curl -m 10 -4Ls "http://ip1.dynupdate.no-ip.com/")")
        read -p "Public IPv4 address / hostname [$get_public_ip]: " public_ip
        # If the checkip service is unavailable and user didn't provide input, ask again
        until [[ -n "$get_public_ip" || -n "$public_ip" ]]; do
            echo "Invalid input."
            read -p "Public IPv4 address / hostname: " public_ip
        done
        [[ -z "$public_ip" ]] && public_ip="$get_public_ip"
    fi
    # If system has a single IPv6, it is selected automatically
    if [[ $(ip -6 addr | grep -c 'inet6 [23]') -eq 1 ]]; then
        ip6=$(ip -6 addr | grep 'inet6 [23]' | cut -d '/' -f 1 | grep -oE '([0-9a-fA-F]{0,4}:){1,7}[0-9a-fA-F]{0,4}')
    fi
    # If system has multiple IPv6, ask the user to select one
    if [[ $(ip -6 addr | grep -c 'inet6 [23]') -gt 1 ]]; then
        number_of_ip6=$(ip -6 addr | grep -c 'inet6 [23]')
        echo
        echo "Which IPv6 address should be used?"
        ip -6 addr | grep 'inet6 [23]' | cut -d '/' -f 1 | grep -oE '([0-9a-fA-F]{0,4}:){1,7}[0-9a-fA-F]{0,4}' | nl -s ') '
        read -p "IPv6 address [1]: " ip6_number
        until [[ -z "$ip6_number" || "$ip6_number" =~ ^[0-9]+$ && "$ip6_number" -le "$number_of_ip6" ]]; do
            echo "$ip6_number: invalid selection."
            read -p "IPv6 address [1]: " ip6_number
        done
        [[ -z "$ip6_number" ]] && ip6_number="1"
        ip6=$(ip -6 addr | grep 'inet6 [23]' | cut -d '/' -f 1 | grep -oE '([0-9a-fA-F]{0,4}:){1,7}[0-9a-fA-F]{0,4}' | sed -n "$ip6_number"p)
    fi
    echo
    echo "Which protocol should OpenVPN use?"
    echo "   1) UDP (recommended)"
    echo "   2) TCP"
    read -p "Protocol [1]: " protocol
    until [[ -z "$protocol" || "$protocol" =~ ^[12]$ ]]; do
        echo "$protocol: invalid selection."
        read -p "Protocol [1]: " protocol
    done
    case "$protocol" in
        1|"") 
        protocol=udp
        ;;
        2) 
        protocol=tcp
        ;;
    esac
    echo
    echo "What port should OpenVPN listen to?"
    read -p "Port [1194]: " port
    until [[ -z "$port" || "$port" =~ ^[0-9]+$ && "$port" -le 65535 ]]; do
        echo "$port: invalid port."
        read -p "Port [1194]: " port
    done
    [[ -z "$port" ]] && port="1194"
    echo
    echo "Select a DNS server for the clients:"
    echo "   1) Current system resolvers"
    echo "   2) Google"
    echo "   3) 1.1.1.1"
    echo "   4) OpenDNS"
    echo "   5) Quad9"
    echo "   6) AdGuard"
    read -p "DNS server [1]: " dns
    until [[ -z "$dns" || "$dns" =~ ^[1-6]$ ]]; do
        echo "$dns: invalid selection."
        read -p "DNS server [1]: " dns
    done
    echo
    echo "Enter a name for the first client:"
    read -p "Name [client]: " unsanitized_client
    # Allow a limited set of characters to avoid conflicts
    client=$(sed 's/[^0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-]/_/g' <<< "$unsanitized_client")
    [[ -z "$client" ]] && client="client"
    echo
    echo "OpenVPN installation is ready to begin."
    # Install a firewall in the rare case where one is not already available
    if ! systemctl is-active --quiet firewalld.service && ! hash iptables 2>/dev/null; then
        if [[ "$os" == "centos" || "$os" == "fedora" ]]; then
            firewall="firewalld"
            # We don't want to silently enable firewalld, so we give a subtle warning
            # If the user continues, firewalld will be installed and enabled during setup
            echo "firewalld, which is required to manage routing tables, will also be installed."
        elif [[ "$os" == "debian" || "$os" == "ubuntu" ]]; then
            # iptables is way less invasive than firewalld so no warning is given
            firewall="iptables"
        fi
    fi
    read -n1 -r -p "Press any key to continue..."
    # If running inside a container, disable LimitNPROC to prevent conflicts
    if systemd-detect-virt -cq; then
        mkdir /etc/systemd/system/openvpn-server@server.service.d/ 2>/dev/null
        echo "[Service]
LimitNPROC=infinity" > /etc/systemd/system/openvpn-server@server.service.d/disable-limitnproc.conf
    fi
    if [[ "$os" = "debian" || "$os" = "ubuntu" ]]; then
        apt-get update
        apt-get install -y openvpn openssl ca-certificates $firewall
    elif [[ "$os" = "centos" ]]; then
        yum install -y epel-release
        yum install -y openvpn openssl ca-certificates tar $firewall
    else
        # Else, OS must be Fedora
        dnf install -y openvpn openssl ca-certificates tar $firewall
    fi
    # If firewalld was just installed, enable it
    if [[ "$firewall" == "firewalld" ]]; then
        systemctl enable --now firewalld.service
    fi
    # Get easy-rsa
    easy_rsa_url='https://github.com/OpenVPN/easy-rsa/releases/download/v3.0.8/EasyRSA-3.0.8.tgz'
    mkdir -p /etc/openvpn/server/easy-rsa/
    { wget -qO- "$easy_rsa_url" 2>/dev/null || curl -sL "$easy_rsa_url" ; } | tar xz -C /etc/openvpn/server/easy-rsa/ --strip-components 1
    chown -R root:root /etc/openvpn/server/easy-rsa/
    cd /etc/openvpn/server/easy-rsa/
    # Create the PKI, set up the CA and the server and client certificates
    ./easyrsa init-pki
    ./easyrsa --batch build-ca nopass
    EASYRSA_CERT_EXPIRE=3650 ./easyrsa build-server-full server nopass
    EASYRSA_CERT_EXPIRE=3650 ./easyrsa build-client-full "$client" nopass
    EASYRSA_CRL_DAYS=3650 ./easyrsa gen-crl
    # Move the stuff we need
    cp pki/ca.crt pki/private/ca.key pki/issued/server.crt pki/private/server.key pki/crl.pem /etc/openvpn/server
    # CRL is read with each client connection, while OpenVPN is dropped to nobody
    chown nobody:"$group_name" /etc/openvpn/server/crl.pem
    # Without +x in the directory, OpenVPN can't run a stat() on the CRL file
    chmod o+x /etc/openvpn/server/
    # Generate key for tls-crypt
    openvpn --genkey --secret /etc/openvpn/server/tc.key
    # Create the DH parameters file using the predefined ffdhe2048 group
    echo '-----BEGIN DH PARAMETERS-----
MIIBCAKCAQEA//////////+t+FRYortKmq/cViAnPTzx2LnFg84tNpWp4TZBFGQz
+8yTnc4kmz75fS/jY2MMddj2gbICrsRhetPfHtXV/WVhJDP1H18GbtCFY2VVPe0a
87VXE15/V8k1mE8McODmi3fipona8+/och3xWKE2rec1MKzKT0g6eXq8CrGCsyT7
YdEIqUuyyOP7uWrat2DX9GgdT0Kj3jlN9K5W7edjcrsZCwenyO4KbXCeAvzhzffi
7MA0BM0oNC9hkXL+nOmFg/+OTxIy7vKBg8P+OxtMb61zO7X8vC7CIAXFjvGDfRaD
ssbzSibBsu/6iGtCOGEoXJf//////////wIBAg==
-----END DH PARAMETERS-----' > /etc/openvpn/server/dh.pem
    # Generate server.conf
    echo "local $ip
port $port
proto $protocol
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh.pem
auth SHA512
tls-crypt tc.key
topology subnet
server 10.8.0.0 255.255.255.0" > /etc/openvpn/server/server.conf
    # IPv6
    if [[ -z "$ip6" ]]; then
        echo 'push "redirect-gateway def1 bypass-dhcp"' >> /etc/openvpn/server/server.conf
    else
        echo 'server-ipv6 fddd:1194:1194:1194::/64' >> /etc/openvpn/server/server.conf
        echo 'push "redirect-gateway def1 ipv6 bypass-dhcp"' >> /etc/openvpn/server/server.conf
    fi
    echo 'ifconfig-pool-persist ipp.txt' >> /etc/openvpn/server/server.conf
    # DNS
    case "$dns" in
        1|"")
            # Locate the proper resolv.conf
            # Needed for systems running systemd-resolved
            if grep -q '^nameserver 127.0.0.53' "/etc/resolv.conf"; then
                resolv_conf="/run/systemd/resolve/resolv.conf"
            else
                resolv_conf="/etc/resolv.conf"
            fi
            # Obtain the resolvers from resolv.conf and use them for OpenVPN
            grep -v '^#\|^;' "$resolv_conf" | grep '^nameserver' | grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}' | while read line; do
                echo "push \"dhcp-option DNS $line\"" >> /etc/openvpn/server/server.conf
            done
        ;;
        2)
            echo 'push "dhcp-option DNS 8.8.8.8"' >> /etc/openvpn/server/server.conf
            echo 'push "dhcp-option DNS 8.8.4.4"' >> /etc/openvpn/server/server.conf
        ;;
        3)
            echo 'push "dhcp-option DNS 1.1.1.1"' >> /etc/openvpn/server/server.conf
            echo 'push "dhcp-option DNS 1.0.0.1"' >> /etc/openvpn/server/server.conf
        ;;
        4)
            echo 'push "dhcp-option DNS 208.67.222.222"' >> /etc/openvpn/server/server.conf
            echo 'push "dhcp-option DNS 208.67.220.220"' >> /etc/openvpn/server/server.conf
        ;;
        5)
            echo 'push "dhcp-option DNS 9.9.9.9"' >> /etc/openvpn/server/server.conf
            echo 'push "dhcp-option DNS 149.112.112.112"' >> /etc/openvpn/server/server.conf
        ;;
        6)
            echo 'push "dhcp-option DNS 94.140.14.14"' >> /etc/openvpn/server/server.conf
            echo 'push "dhcp-option DNS 94.140.15.15"' >> /etc/openvpn/server/server.conf
        ;;
    esac
    echo "keepalive 10 120
cipher AES-256-CBC
user nobody
group $group_name
persist-key
persist-tun
verb 3
crl-verify crl.pem" >> /etc/openvpn/server/server.conf
    if [[ "$protocol" = "udp" ]]; then
        echo "explicit-exit-notify" >> /etc/openvpn/server/server.conf
    fi
    # Enable net.ipv4.ip_forward for the system
    echo 'net.ipv4.ip_forward=1' > /etc/sysctl.d/99-openvpn-forward.conf
    # Enable without waiting for a reboot or service restart
    echo 1 > /proc/sys/net/ipv4/ip_forward
    if [[ -n "$ip6" ]]; then
        # Enable net.ipv6.conf.all.forwarding for the system
        echo "net.ipv6.conf.all.forwarding=1" >> /etc/sysctl.d/99-openvpn-forward.conf
        # Enable without waiting for a reboot or service restart
        echo 1 > /proc/sys/net/ipv6/conf/all/forwarding
    fi
    if systemctl is-active --quiet firewalld.service; then
        # Using both permanent and not permanent rules to avoid a firewalld
        # reload.
        # We don't use --add-service=openvpn because that would only work with
        # the default port and protocol.
        firewall-cmd --add-port="$port"/"$protocol"
        firewall-cmd --zone=trusted --add-source=10.8.0.0/24
        firewall-cmd --permanent --add-port="$port"/"$protocol"
        firewall-cmd --permanent --zone=trusted --add-source=10.8.0.0/24
        # Set NAT for the VPN subnet
        firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to "$ip"
        firewall-cmd --permanent --direct --add-rule ipv4 nat POSTROUTING 0 -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to "$ip"
        if [[ -n "$ip6" ]]; then
            firewall-cmd --zone=trusted --add-source=fddd:1194:1194:1194::/64
            firewall-cmd --permanent --zone=trusted --add-source=fddd:1194:1194:1194::/64
            firewall-cmd --direct --add-rule ipv6 nat POSTROUTING 0 -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to "$ip6"
            firewall-cmd --permanent --direct --add-rule ipv6 nat POSTROUTING 0 -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to "$ip6"
        fi
    else
        # Create a service to set up persistent iptables rules
        iptables_path=$(command -v iptables)
        ip6tables_path=$(command -v ip6tables)
        # nf_tables is not available as standard in OVZ kernels. So use iptables-legacy
        # if we are in OVZ, with a nf_tables backend and iptables-legacy is available.
        if [[ $(systemd-detect-virt) == "openvz" ]] && readlink -f "$(command -v iptables)" | grep -q "nft" && hash iptables-legacy 2>/dev/null; then
            iptables_path=$(command -v iptables-legacy)
            ip6tables_path=$(command -v ip6tables-legacy)
        fi
        echo "[Unit]
Before=network.target
[Service]
Type=oneshot
ExecStart=$iptables_path -t nat -A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to $ip
ExecStart=$iptables_path -I INPUT -p $protocol --dport $port -j ACCEPT
ExecStart=$iptables_path -I FORWARD -s 10.8.0.0/24 -j ACCEPT
ExecStart=$iptables_path -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
ExecStop=$iptables_path -t nat -D POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to $ip
ExecStop=$iptables_path -D INPUT -p $protocol --dport $port -j ACCEPT
ExecStop=$iptables_path -D FORWARD -s 10.8.0.0/24 -j ACCEPT
ExecStop=$iptables_path -D FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT" > /etc/systemd/system/openvpn-iptables.service
        if [[ -n "$ip6" ]]; then
            echo "ExecStart=$ip6tables_path -t nat -A POSTROUTING -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to $ip6
ExecStart=$ip6tables_path -I FORWARD -s fddd:1194:1194:1194::/64 -j ACCEPT
ExecStart=$ip6tables_path -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
ExecStop=$ip6tables_path -t nat -D POSTROUTING -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to $ip6
ExecStop=$ip6tables_path -D FORWARD -s fddd:1194:1194:1194::/64 -j ACCEPT
ExecStop=$ip6tables_path -D FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT" >> /etc/systemd/system/openvpn-iptables.service
        fi
        echo "RemainAfterExit=yes
[Install]
WantedBy=multi-user.target" >> /etc/systemd/system/openvpn-iptables.service
        systemctl enable --now openvpn-iptables.service
    fi
    # If SELinux is enabled and a custom port was selected, we need this
    if sestatus 2>/dev/null | grep "Current mode" | grep -q "enforcing" && [[ "$port" != 1194 ]]; then
        # Install semanage if not already present
        if ! hash semanage 2>/dev/null; then
            if [[ "$os_version" -eq 7 ]]; then
                # Centos 7
                yum install -y policycoreutils-python
            else
                # CentOS 8 or Fedora
                dnf install -y policycoreutils-python-utils
            fi
        fi
        semanage port -a -t openvpn_port_t -p "$protocol" "$port"
    fi
    # If the server is behind NAT, use the correct IP address
    [[ -n "$public_ip" ]] && ip="$public_ip"
    # client-common.txt is created so we have a template to add further users later
    echo "client
dev tun
proto $protocol
remote $ip $port
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
auth SHA512
cipher AES-256-CBC
ignore-unknown-option block-outside-dns
block-outside-dns
verb 3" > /etc/openvpn/server/client-common.txt
    # Enable and start the OpenVPN service
    systemctl enable --now openvpn-server@server.service
    # Generates the custom client.ovpn
    new_client
    echo
    echo "Finished!"
    echo
    echo "The client configuration is available in:" ~/"$client.ovpn"
    echo "New clients can be added by running this script again."
else
    clear
    echo "OpenVPN is already installed."
    echo
    echo "Select an option:"
    echo "   1) Add a new client"
    echo "   2) Revoke an existing client"
    echo "   3) Remove OpenVPN"
    echo "   4) Exit"
    read -p "Option: " option
    until [[ "$option" =~ ^[1-4]$ ]]; do
        echo "$option: invalid selection."
        read -p "Option: " option
    done
    case "$option" in
        1)
            echo
            echo "Provide a name for the client:"
            read -p "Name: " unsanitized_client
            client=$(sed 's/[^0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-]/_/g' <<< "$unsanitized_client")
            while [[ -z "$client" || -e /etc/openvpn/server/easy-rsa/pki/issued/"$client".crt ]]; do
                echo "$client: invalid name."
                read -p "Name: " unsanitized_client
                client=$(sed 's/[^0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-]/_/g' <<< "$unsanitized_client")
            done
            cd /etc/openvpn/server/easy-rsa/
            EASYRSA_CERT_EXPIRE=3650 ./easyrsa build-client-full "$client" nopass
            # Generates the custom client.ovpn
            new_client
            echo
            echo "$client added. Configuration available in:" ~/"$client.ovpn"
            exit
        ;;
        2)
            # This option could be documented a bit better and maybe even be simplified
            # ...but what can I say, I want some sleep too
            number_of_clients=$(tail -n +2 /etc/openvpn/server/easy-rsa/pki/index.txt | grep -c "^V")
            if [[ "$number_of_clients" = 0 ]]; then
                echo
                echo "There are no existing clients!"
                exit
            fi
            echo
            echo "Select the client to revoke:"
            tail -n +2 /etc/openvpn/server/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | nl -s ') '
            read -p "Client: " client_number
            until [[ "$client_number" =~ ^[0-9]+$ && "$client_number" -le "$number_of_clients" ]]; do
                echo "$client_number: invalid selection."
                read -p "Client: " client_number
            done
            client=$(tail -n +2 /etc/openvpn/server/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | sed -n "$client_number"p)
            echo
            read -p "Confirm $client revocation? [y/N]: " revoke
            until [[ "$revoke" =~ ^[yYnN]*$ ]]; do
                echo "$revoke: invalid selection."
                read -p "Confirm $client revocation? [y/N]: " revoke
            done
            if [[ "$revoke" =~ ^[yY]$ ]]; then
                cd /etc/openvpn/server/easy-rsa/
                ./easyrsa --batch revoke "$client"
                EASYRSA_CRL_DAYS=3650 ./easyrsa gen-crl
                rm -f /etc/openvpn/server/crl.pem
                cp /etc/openvpn/server/easy-rsa/pki/crl.pem /etc/openvpn/server/crl.pem
                # CRL is read with each client connection, when OpenVPN is dropped to nobody
                chown nobody:"$group_name" /etc/openvpn/server/crl.pem
                echo
                echo "$client revoked!"
            else
                echo
                echo "$client revocation aborted!"
            fi
            exit
        ;;
        3)
            echo
            read -p "Confirm OpenVPN removal? [y/N]: " remove
            until [[ "$remove" =~ ^[yYnN]*$ ]]; do
                echo "$remove: invalid selection."
                read -p "Confirm OpenVPN removal? [y/N]: " remove
            done
            if [[ "$remove" =~ ^[yY]$ ]]; then
                port=$(grep '^port ' /etc/openvpn/server/server.conf | cut -d " " -f 2)
                protocol=$(grep '^proto ' /etc/openvpn/server/server.conf | cut -d " " -f 2)
                if systemctl is-active --quiet firewalld.service; then
                    ip=$(firewall-cmd --direct --get-rules ipv4 nat POSTROUTING | grep '\-s 10.8.0.0/24 '"'"'!'"'"' -d 10.8.0.0/24' | grep -oE '[^ ]+$')
                    # Using both permanent and not permanent rules to avoid a firewalld reload.
                    firewall-cmd --remove-port="$port"/"$protocol"
                    firewall-cmd --zone=trusted --remove-source=10.8.0.0/24
                    firewall-cmd --permanent --remove-port="$port"/"$protocol"
                    firewall-cmd --permanent --zone=trusted --remove-source=10.8.0.0/24
                    firewall-cmd --direct --remove-rule ipv4 nat POSTROUTING 0 -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to "$ip"
                    firewall-cmd --permanent --direct --remove-rule ipv4 nat POSTROUTING 0 -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to "$ip"
                    if grep -qs "server-ipv6" /etc/openvpn/server/server.conf; then
                        ip6=$(firewall-cmd --direct --get-rules ipv6 nat POSTROUTING | grep '\-s fddd:1194:1194:1194::/64 '"'"'!'"'"' -d fddd:1194:1194:1194::/64' | grep -oE '[^ ]+$')
                        firewall-cmd --zone=trusted --remove-source=fddd:1194:1194:1194::/64
                        firewall-cmd --permanent --zone=trusted --remove-source=fddd:1194:1194:1194::/64
                        firewall-cmd --direct --remove-rule ipv6 nat POSTROUTING 0 -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to "$ip6"
                        firewall-cmd --permanent --direct --remove-rule ipv6 nat POSTROUTING 0 -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to "$ip6"
                    fi
                else
                    systemctl disable --now openvpn-iptables.service
                    rm -f /etc/systemd/system/openvpn-iptables.service
                fi
                if sestatus 2>/dev/null | grep "Current mode" | grep -q "enforcing" && [[ "$port" != 1194 ]]; then
                    semanage port -d -t openvpn_port_t -p "$protocol" "$port"
                fi
                systemctl disable --now openvpn-server@server.service
                rm -rf /etc/openvpn/server
                rm -f /etc/systemd/system/openvpn-server@server.service.d/disable-limitnproc.conf
                rm -f /etc/sysctl.d/99-openvpn-forward.conf
                if [[ "$os" = "debian" || "$os" = "ubuntu" ]]; then
                    apt-get remove --purge -y openvpn
                else
                    # Else, OS must be CentOS or Fedora
                    yum remove -y openvpn
                fi
                echo
                echo "OpenVPN removed!"
            else
                echo
                echo "OpenVPN removal aborted!"
            fi
            exit
        ;;
        4)
            exit
        ;;
    esac
fi