How To Install OpenVPN on Debian with IPTABLES

Posted on January 14th, 2011 by admin

Installing OpenVPN on Debian with IPTables

I must admit there seem to be so many misleading / poorly demonstrated OpenVPN tutorials on the internet.

In this tutorial I will show you how setup OpenVPN from the ground up, networking and also configuring IPTables properly.

It might sound daunting to a newer user of *nix, but I have taken an informative approach.

Firstly we must check whether your VPS or dedi has a TUN / TAP interface:

   modprobe tun
   mkdir /dev/net
   mknod /dev/net/tun c 10 200

If it outputs the following (or something similar) you are good to go!

   mkdir: cannot create directory `/dev/net’: File exists
   mknod: `/dev/net/tun’: File exists

If you are on a VPS, you might need to ask your VPS provider to enable the TUN / TAP interface. (In the case the above fails)

We will now install OpenVPN:

   apt-get install openvpn

Now by default OpenVPN configuration files are situated in:

   # /etc/openvpn

We will create a new directory within our openvpn config (/etc/openvpn/keys)

   cd /etc/openvpn
   mkdir keys

So we will change directory to the EASYRSA examples.

   cd /usr/share/doc/openvpn/examples/easy-rsa/2.0

We will now run the following commands:

   . ./vars
   ./clean-all
   ./build-ca

This will clean any previous keys and create RSA private key.

If you get an permission errors, chances are you need to make the file executable:

e.g. chmod +x clean-all

The text step is to setup the server configuration:

   ./build-key-server vpnserver

(Where “vpnserver” is your server name)

You will now be asked a series of questions regarding server information.

Don’t worry, the server won’t test any information you put in, so you can put anything you want.

Commit the new entry. Done!

Now we will create the client(s) configuration files:

   ./build-key client1

Again we will be asked several question, commit and save.

Obviously if you want a second client you would enter:

   ./build-key client2

etc.

   openssl x509 -in file.crt -noout -text

Now when you have finished adding your clients, you will need run:

   ./build-dh

Upon completion the configurations files for the server and client(s) will be output to the keys directory (the directory will be automatically created)

The next step is to copy the keys folder to our open vpn configuration folder, like so:

   cp /usr/share/doc/openvpn/examples/easy-rsa/2.0/keys /etc/openvpn/keys

Now we will create our server config for openvpn:

   cd /etc/openvpn/keys
   nano vpnserver.conf

We will copy the following in to the vpnserver.conf:

   # Sample OpenVPN Server Config

   # Which local IP address should OpenVPN
   # listen on? (optional)
   local 127.0.0.1

   # Which TCP/UDP port should OpenVPN listen on?
   # If you want to run multiple OpenVPN instances
   # on the same machine, use a different port
   # number for each one. You will need to
   # open up this port on your firewall.
   port 1194

   # TCP or UDP server?
   ;proto tcp
   proto udp

   # “dev tun” will create a routed IP tunnel,
   # “dev tap” will create an ethernet tunnel.
   # Use “dev tap0″ if you are ethernet bridging
   # and have precreated a tap0 virtual interface
   # and bridged it with your ethernet interface.
   # If you want to control access policies
   # over the VPN, you must create firewall
   # rules for the the TUN/TAP interface.
   # On non-Windows systems, you can give
   # an explicit unit number, such as tun0.
   # On Windows, use “dev-node” for this.
   # On most systems, the VPN will not function
   # unless you partially or fully disable
   # the firewall for the TUN/TAP interface.
   ;dev tap
   dev tun

   # Windows needs the TAP-Win32 adapter name
   # from the Network Connections panel if you
   # have more than one. On XP SP2 or higher,
   # you may need to selectively disable the
   # Windows firewall for the TAP adapter.
   # Non-Windows systems usually don’t need this.
   ;dev-node MyTap

   # SSL/TLS root certificate (ca), certificate
   # (cert), and private key (key). Each client
   # and the server must have their own cert and
   # key file. The server and all clients will
   # use the same ca file.
   #
   # See the “easy-rsa” directory for a series
   # of scripts for generating RSA certificates
   # and private keys. Remember to use
   # a unique Common Name for the server
   # and each of the client certificates.
   #
   # Any X509 key management system can be used.
   # OpenVPN can also use a PKCS #12 formatted key file
   # (see “pkcs12″ directive in man page).
   ca /etc/openvpn/keys/ca.crt
   cert /etc/openvpn/keys/vpnservernew.crt
   key /etc/openvpn/keys/vpnserver.key # This file should be kept secret

   # Diffie hellman parameters.
   # Generate your own with:
   # openssl dhparam -out dh1024.pem 1024
   # Substitute 2048 for 1024 if you are using
   # 2048 bit keys.
   dh /etc/openvpn/keys/dh1024.pem

   # Configure server mode and supply a VPN subnet
   # for OpenVPN to draw client addresses from.
   # The server will take 10.8.0.1 for itself,
   # the rest will be made available to clients.
   # Each client will be able to reach the server
   # on 10.8.0.1. Comment this line out if you are
   # ethernet bridging. See the man page for more info.
   server 10.8.0.0 255.255.255.0

   # Maintain a record of client <-> virtual IP address
   # associations in this file. If OpenVPN goes down or
   # is restarted, reconnecting clients can be assigned
   # the same virtual IP address from the pool that was
   # previously assigned.
   ifconfig-pool-persist ipp.txt

   # Configure server mode for ethernet bridging.
   # You must first use your OS’s bridging capability
   # to bridge the TAP interface with the ethernet
   # NIC interface. Then you must manually set the
   # IP/netmask on the bridge interface, here we
   # assume 10.8.0.4/255.255.255.0. Finally we
   # must set aside an IP range in this subnet
   # (start=10.8.0.50 end=10.8.0.100) to allocate
   # to connecting clients. Leave this line commented
   # out unless you are ethernet bridging.
   ;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100

   # Configure server mode for ethernet bridging
   # using a DHCP-proxy, where clients talk
   # to the OpenVPN server-side DHCP server
   # to receive their IP address allocation
   # and DNS server addresses. You must first use
   # your OS’s bridging capability to bridge the TAP
   # interface with the ethernet NIC interface.
   # Note: this mode only works on clients (such as
   # Windows), where the client-side TAP adapter is
   # bound to a DHCP client.
   ;server-bridge

   # Push routes to the client to allow it
   # to reach other private subnets behind
   # the server. Remember that these
   # private subnets will also need
   # to know to route the OpenVPN client
   # address pool (10.8.0.0/255.255.255.0)
   # back to the OpenVPN server.
   ;push “route 192.168.10.0 255.255.255.0″
   ;push “route 192.168.20.0 255.255.255.0″

   # To assign specific IP addresses to specific
   # clients or if a connecting client has a private
   # subnet behind it that should also have VPN access,
   # use the subdirectory “ccd” for client-specific
   # configuration files (see man page for more info).

   # EXAMPLE: Suppose the client
   # having the certificate common name “Thelonious”
   # also has a small subnet behind his connecting
   # machine, such as 192.168.40.128/255.255.255.248.
   # First, uncomment out these lines:
   ;client-config-dir ccd
   ;route 192.168.40.128 255.255.255.248
   # Then create a file ccd/Thelonious with this line:
   # iroute 192.168.40.128 255.255.255.248
   # This will allow Thelonious’ private subnet to
   # access the VPN. This example will only work
   # if you are routing, not bridging, i.e. you are
   # using “dev tun” and “server” directives.

   # EXAMPLE: Suppose you want to give
   # Thelonious a fixed VPN IP address of 10.9.0.1.
   # First uncomment out these lines:
   ;client-config-dir ccd
   ;route 10.9.0.0 255.255.255.252
   # Then add this line to ccd/Thelonious:
   # ifconfig-push 10.9.0.1 10.9.0.2

   # Suppose that you want to enable different
   # firewall access policies for different groups
   # of clients. There are two methods:
   # (1) Run multiple OpenVPN daemons, one for each
   # group, and firewall the TUN/TAP interface
   # for each group/daemon appropriately.
   # (2) (Advanced) Create a script to dynamically
   # modify the firewall in response to access
   # from different clients. See man
   # page for more info on learn-address script.
   ;learn-address ./script

   # If enabled, this directive will configure
   # all clients to redirect their default
   # network gateway through the VPN, causing
   # all IP traffic such as web browsing and
   # and DNS lookups to go through the VPN
   # (The OpenVPN server machine may need to NAT
   # or bridge the TUN/TAP interface to the internet
   # in order for this to work properly).

   push “redirect-gateway def1 bypass-dhcp”

   # Certain Windows-specific network settings
   # can be pushed to clients, such as DNS
   # or WINS server addresses. CAVEAT:
   # http://openvpn.net/faq.html#dhcpcaveats
   # The addresses below refer to the public
   # DNS servers provided by opendns.com.
   push “dhcp-option DNS 208.67.222.222″
   push “dhcp-option DNS 208.67.220.220″

   # Uncomment this directive to allow different
   # clients to be able to “see” each other.
   # By default, clients will only see the server.
   # To force clients to only see the server, you
   # will also need to appropriately firewall the
   # server’s TUN/TAP interface.
   ;client-to-client

   # Uncomment this directive if multiple clients
   # might connect with the same certificate/key
   # files or common names. This is recommended
   # only for testing purposes. For production use,
   # each client should have its own certificate/key
   # pair.
   #
   # IF YOU HAVE NOT GENERATED INDIVIDUAL
   # CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
   # EACH HAVING ITS OWN UNIQUE “COMMON NAME”,
   # UNCOMMENT THIS LINE OUT.
   ;duplicate-cn

   # The keepalive directive causes ping-like
   # messages to be sent back and forth over
   # the link so that each side knows when
   # the other side has gone down.
   # Ping every 10 seconds, assume that remote
   # peer is down if no ping received during
   # a 120 second time period.
   keepalive 10 120

   # For extra security beyond that provided
   # by SSL/TLS, create an “HMAC firewall”
   # to help block DoS attacks and UDP port flooding.
   #
   # Generate with:
   # openvpn –genkey –secret ta.key
   #
   # The server and each client must have
   # a copy of this key.
   # The second parameter should be ’0′
   # on the server and ’1′ on the clients.
   ;tls-auth ta.key 0 # This file is secret

   # Select a cryptographic cipher.
   # This config item must be copied to
   # the client config file as well.
   ;cipher BF-CBC # Blowfish (default)
   ;cipher AES-128-CBC # AES
   ;cipher DES-EDE3-CBC # Triple-DES

   # Enable compression on the VPN link.
   # If you enable it here, you must also
   # enable it in the client config file.
   comp-lzo

   # The maximum number of concurrently connected
   # clients we want to allow.
   ;max-clients 100

   # It’s a good idea to reduce the OpenVPN
   # daemon’s privileges after initialization.
   #
   # You can uncomment this out on
   # non-Windows systems.
   ;user nobody
   ;group nogroup

   # The persist options will try to avoid
   # accessing certain resources on restart
   # that may no longer be accessible because
   # of the privilege downgrade.
   persist-key
   persist-tun

   # Output a short status file showing
   # current connections, truncated
   # and rewritten every minute.
   status openvpn-status.log

   # By default, log messages will go to the syslog (or
   # on Windows, if running as a service, they will go to
   # the “\Program Files\OpenVPN\log” directory).
   # Use log or log-append to override this default.
   # “log” will truncate the log file on OpenVPN startup,
   # while “log-append” will append to it. Use one
   # or the other (but not both).
   ;log openvpn.log
   ;log-append openvpn.log

   # Set the appropriate level of log
   # file verbosity.
   #
   # 0 is silent, except for fatal errors
   # 4 is reasonable for general usage
   # 5 and 6 can help to debug connection problems
   # 9 is extremely verbose
   verb 3

   # Silence repeating messages. At most 20
   # sequential messages of the same message
   # category will be output to the log.
   ;mute 20

Now lets concentrate on the following directives:

   # Leave for local interface or change to public IP etc.
   local 127.0.0.1

   # Leave as default (unless you are a bit to security consious like me :P)
   port 1194

   # If you have followed this tutorial you shouldn’t have to modify anything here, but best to double check the location of the files
   ca /etc/openvpn/keys/ca.crt
   cert /etc/openvpn/keys/vpnservernew.crt
   key /etc/openvpn/keys/vpnserver.key # This file should be kept secret

   # Modify the subnet to your suiting
   server 10.8.0.0 255.255.255.0

   # You can modify the DNS servers here, although I have included OpenDNS in the config.
   push “dhcp-option DNS 208.67.222.222″
   push “dhcp-option DNS 208.67.220.220″

   # This will tell all network traffic from the client’s IP to go through the VPN (which in most cases is what you want)
   push “redirect-gateway def1 bypass-dhcp”

Now lets restart the openvpn service:

   /etc/init.d/openvpn restart

—————————–

You may encounter an error if your server is using IPV6 when sending outbound connection via the VPN. Make sure IPV4 forwarding is enabled!

You can check if it is by:

   cat /proc/sys/net/ipv4/ip_forward

   0 = Off / 1 = On

If it’s turned off add the following to sysctl.conf (if it’s on skip this stage):

   net.ipv4.ip_forward = 1

   nano /etc/sysctl.conf

And now we make the change permenent:

   sysctl -p /etc/sysctl.conf

And restart networking:

   /etc/init.d/procps.sh restart

—————————–

Now we need to setup the client end. You will need to copy the following configuration files to the users openvpn config folder:

   ca.crt
   client1.crt
   client1.key

We will also be required to create a config file for the client:

In this example, my assumption is the user will be using a Windows system

Create a new file called vpnserver.conf and add the following:

   # VPN Client Config File

   # Specify that we are a client and that we
   # will be pulling certain config file directives
   # from the server.
   client

   # Use the same setting as you are using on
   # the server.
   # On most systems, the VPN will not function
   # unless you partially or fully disable
   # the firewall for the TUN/TAP interface.
   ;dev tap
   dev tun

   # Windows needs the TAP-Win32 adapter name
   # from the Network Connections panel
   # if you have more than one. On XP SP2,
   # you may need to disable the firewall
   # for the TAP adapter.
   ;dev-node MyTap

   # Are we connecting to a TCP or
   # UDP server? Use the same setting as
   # on the server.
   ;proto tcp
   proto udp

   # The hostname / IP and port of the server.
   # You can have multiple remote entries
   # to load balance between the servers.
   remote XX.XX.XX.XX 1194
   ;remote my-server-2 1194

   # Choose a random host from the remote
   # list for load-balancing. Otherwise
   # try hosts in the order specified.
   ;remote-random

   # Keep trying indefinitely to resolve the
   # host name of the OpenVPN server. Very useful
   # on machines which are not permanently connected
   # to the internet such as laptops.
   resolv-retry infinite

   # Most clients don’t need to bind to
   # a specific local port number.
   nobind

   # Downgrade privileges after initialization (non-Windows only)
   ;user nobody
   ;group nobody

   # Try to preserve some state across restarts.
   persist-key
   persist-tun

   # If you are connecting through an
   # HTTP proxy to reach the actual OpenVPN
   # server, put the proxy server/IP and
   # port number here. See the man page
   # if your proxy server requires
   # authentication.
   ;http-proxy-retry # retry on connection failures
   ;http-proxy [proxy server] [proxy port #]

   # Wireless networks often produce a lot
   # of duplicate packets. Set this flag
   # to silence duplicate packet warnings.
   ;mute-replay-warnings

   # SSL/TLS parms.
   # See the server config file for more
   # description. It’s best to use
   # a separate .crt/.key file pair
   # for each client. A single ca
   # file can be used for all clients.
   ca ca.crt
   cert client1.crt
   key client1.key

   # Verify server certificate by checking
   # that the certicate has the nsCertType
   # field set to “server”. This is an
   # important precaution to protect against
   # a potential attack discussed here:
   # http://openvpn.net/howto.html#mitm
   #
   # To use this feature, you will need to generate
   # your server certificates with the nsCertType
   # field set to “server”. The build-key-server
   # script in the easy-rsa folder will do this.
   ns-cert-type server

   # If a tls-auth key is used on the server
   # then every client must also have the key.
   ;tls-auth ta.key 1

   # Select a cryptographic cipher.
   # If the cipher option is used on the server
   # then you must also specify it here.
   ;cipher x

   # Enable compression on the VPN link.
   # Don’t enable this unless it is also
   # enabled in the server config file.
   comp-lzo

   # Set log file verbosity.
   verb 3

   # Silence repeating messages
   ;mute 20

   # Max routes
   max-routes 300

   # The directives below allow us to exclude different IPs from OpenVPN I.E. Any connections made to the hosts
   # are made directly via the internet, not going through OpenVPN’s tunnel.
   route XX.XX.XX.XX 255.255.255.255 net_gateway
   route XX.XX.XX.XX 255.255.255.255 net_gateway

There is only a few directives that need to be modified:

   # Change this to your remote server
   remote XX.XX.XX.XX 1194
   ;remote my-server-2 1194

   # Make sure these filenames corrospond to the file you have placed in the client config!
   ca ca.crt
   cert client1.crt
   key client1.key

   # If for example you use usenet or another source that consumes large proportions of bandwidth you might
   # want to consider creating a “route” so that any connections to XX.XX.XX.XX will be excluded from the tunnel.
   # Just replace the IP with XX.XX.XX.XX
   route XX.XX.XX.XX 255.255.255.255 net_gateway
   route XX.XX.XX.XX 255.255.255.255 net_gateway

IPTABLES

You may well find that the TUN / TAP interface has been automatically firewalled, although in our case this is good! We will now set some IPTABLE rules so that we can allow clients traffic to use our primary network adapter to access the internet.

Create a new file named firewall.log

   cd /root
   nano firewall.log

and add the following:

IP Tables Config

Define conditions for allowing inbound / outbound connections

mangle

PREROUTING ACCEPT [34:2484]

INPUT ACCEPT [34:2484]

FORWARD ACCEPT [0:0]

OUTPUT ACCEPT [37:7472]

POSTROUTING ACCEPT [37:7472]

COMMIT

Define blocked ports

filter

INPUT ACCEPT [34:2484]

FORWARD ACCEPT [0:0]

OUTPUT ACCEPT [37:7472]

-A FORWARD -s ! XX.XX.XX.XX/32 -i tun0 -p tcp -m tcp --dport 1:21 -j REJECT --reject-with icmp-port-unreachable -A FORWARD -s ! XX.XX.XX.XX/32 -i tun0 -p tcp -m tcp --dport 23:79 -j REJECT --reject-with icmp-port-unreachable -A FORWARD -s ! XX.XX.XX.XX/32 -i tun0 -p tcp -m tcp --dport 81:442 -j REJECT --reject-with icmp-port-unreachable -A FORWARD -s ! XX.XX.XX.XX/32 -i tun0 -p tcp -m tcp --dport 444:65535 -j REJECT --reject-with icmp-port-unreachable COMMIT

Define conditions to forward clients network access

nat

PREROUTING ACCEPT [0:0]

POSTROUTING ACCEPT [0:0]

OUTPUT ACCEPT [0:0]

-A POSTROUTING -s 10.8.0.0/24 -j SNAT --to-source XX.XX.XX.XX COMMIT

Replace any occurances of XX.XX.XX.XX to your servers IP address.

In this example it routes all requests from OpenVPN’s ip range to the primary networking adapter.

It also defines a port ruleset:

Accept Port 22 (SSH), 80 (HTTP), 443 (HTTPS) Block ANY OTHER PORT

Note: In the example I am using the TUN0 interface, make sure this is correct (you might have multiple TUN interfaces – unlikely)

Modify as needed.

Now we will apply these rules:

   iptables-restore < firewall.log

IMPORTANT: As we have used IP forwarding, we must enable this on the system, by default Debian Lenny doesn’t have it turned on. So we do the following:

   nano /proc/sys/net/ipv4/ip_forward

Change the default value (0) to 1 to enable IP forwarding.

Now we will initiate the the openvpn server by pointing it to our server .conf file we created earlier:

   openvpn –config /etc/openvpn/keys/vpnserver.conf

You should see something similar to the following:

OpenVPN Server Startup

Now we will connect from the client-side. Open up OpenVPN (needs administrative privilages) and connect to the appropriate configuration.

You should now see a status box, and hopefully within a few seconds – minute will be connected.

Hint: If you encounter any errors on the client-side right hand click on OpenVPN and review the log output.

IMPORTANT: As IPTABLES rules will be lost after reboot, if you reboot your system often I would strongly recommend you create a start-up script to apply the rules.

If you have any questions, please feel free to leave a comment and I will try my best to assist.

VPN on demand

script to start/stop

/usr/local/sbin/customvpn.sh

 #!/bin/bash

 abc=`curl -s -k -L https://servername.domain/vpn-home.php ` | grep '^[Y|N]'`
 ps -fax | grep client.conf | grep openvpn | grep -v grep
 ovp=$?
 
 if [[ "$abc" == 'YES' && $ovp != 0 ]]; then /etc/init.d/openvpn start; fi
 if [[ "$abc" == 'NO' && $ovp == 0 ]]; then /etc/init.d/openvpn stop; fi

Systemd

 #!/bin/bash
 
 abc=`curl -s -k -L https://servername.domain/vpn-home.php | grep '^[Y|N]'`
 ps -fax | grep client.conf | grep openvpn | grep -v grep
 ovp=$?
 
 if [[ "$abc" == 'YES' && $ovp != 0 ]]; then systemctl start openvpn@client.service; fi
 if [[ "$abc" == 'NO' && $ovp == 0 ]]; then systemctl stop openvpn@client.service; fi

CronJob

/etc/cron.d/openvpn

# Example of job definition:
#   .---------------- minute (0 - 59)
#   |  .------------- hour (0 - 23)
#   |  |  .---------- day of month (1 - 31)
#   |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
#   |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
#   |  |  |  |  |
#   *  *  *  *  * user-name command to be executed

   */5 *  *  *  * root      test -e /usr/local/sbin/customvpn.sh && /usr/local/sbin/customvpn.sh

OpenVPN

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