Running and troubleshooting an OpenVPN server

wlanboy

Content Contributer
After helping some people (here and on irc) to setup their OpenVPN server I would like to start a collection of useful tips regarding the operation of an OpenVPN server.

First the basic steps:

  • Click on the "enable TUN/TAP device" within for server panel and restart the vps. [for OpenVZ]
  • After that a

    cat /dev/net/tun

    [SIZE=small]should return:[/SIZE]
    Code:
    cat: /dev/net/tun: File descriptor in bad state
  • Installing openvpn
    Code:
    apt-get install openvpn
  • If /dev/net/tun is missing
    Easy way:
    Code:
    openvpn --mktun --dev tun0
    Hard way (yup there are still images out there which do not have a /dev/net):


    mkdir -p /dev/net
    mknod /dev/net/tun c 10 200
    chmod 600 /dev/net/tun

  • Setup configuration of openvpn
    Code:
    cp -r /usr/share/doc/openvpn/ /etc/ 
    cd /etc/openvpn/examples/easy-rsa/2.0
    nano vars
    Setup the last exports to save you some typing:


    export KEY_SIZE=2048

    export KEY_COUNTRY=""
    export KEY_PROVINCE=""
    export KEY_CITY=""
    export KEY_ORG=""
    export KEY_EMAIL=""
    export KEY_CN=changeme
    export KEY_NAME=changeme
    export KEY_OU=changeme

    Key size should be at least 2048 bit!
    Of course no empty values ...
     
  • If they key tools reporting that they cannot find the openssl lib...


    nano whichopensslcnf

    Change following line (the double [[:digit:]] is wrong):


    elif $OPENSSL version | grep -E "1\.0\.[[:digit:]]" > /dev/null; then

  • Start generating the keys:


    ./clean-all [to clean all generated stuff - if you want to start right from the beginning]
    ./build-ca [certificate authority of all your certs]
    ./build-key-server servername [cert for server]
    ./build-key clientname [certs per client]
    ./build-dh [dh parameter for key-exchange algorithm]

  • Copy keys to openvpn server


    mkdir /etc/openvpn/keys
    cd keys
    cp ca.crt ca.key dh1024.pem server.crt server.key /etc/openvpn/keys
    [depends on your naming of the build-* calls, if you called your server mybox it is e.g.
    mybox.crt and mybox.key]
    chmod 600 /etc/openvpn/keys/*

  • Creating the server config


    cd /etc/openvpn/examples/sample-config-files/
    gunzip server.conf.gz
    cp server.conf /etc/openvpn/
    cd /etc/openvpn/
    nano /etc/openvpn/server.conf

  • Alterting the config


    port 1149 #[port you want to use]
    dev tun #[device type you want to use]
    #relative paths of the keys:
    ca keys/ca.crt
    cert keys/servch.crt
    key keys/servch.key
    dh keys/dh1024.pem
    server 10.10.10.0 255.255.255.0 #[network of openvpn server - server itself will get 10.10.10.1]
    push "redirect-gateway def1 bypass-dhcp" #[if you want to surf through the vpn]
    client-to-client #[if each openvpn client should see the other openvpn clients (gaming)]
    comp-lzo #[compression is good]
    max-clients 3 #[number of concurrent users]
    user nobody
    group nogroup #[downgrade rights of openvpn for security]
    persist-key
    persist-tun #[nobdy cannot alter devices - so persist there settings]
    status openvpn-status.log
    log-append openvpn.log #[logging is good for checking problems]

  • Track the usage of your openvpn server
    OpenVPN server does have some nice hookups that you can use to modify its workflow. One nice feature are following hooks (added to the server config):


    script-security 3 system
    client-connect /etc/openvpn/client-login.sh
    client-disconnect /etc/openvpn/client-logout.sh

    So the scripts are called whenever a client connects or disconnects!
     


    nano /etc/openvpn/client-login.sh

    Code:
    #!/bin/bash
    #Send an email when a client connects with today's time and date
    NOW="$(date +"%H:%M:%S - %Y-%m-%d")"
    
    sendmail [email protected] <<EOF
    FROM: [email protected]
    TO: [email protected]
    SUBJECT: OpenVPN - LOGIN: $common_name - $NOW
    At $NOW, $common_name logged out of the OpenVPN server red.
    
    IP: $trusted_ip
    PORT: $trusted_port
    MTU: $tun_mtu
    .
    EOF
    
    exit 0
    Code:
    nano client-logout.sh
    Code:
    #!/bin/bash
    #Send an email when a client connects with today's time and date
    NOW="$(date +"%H:%M:%S - %Y-%m-%d")"
    
    sendmail [email protected] <<EOF
    FROM: [email protected]
    TO: [email protected]
    SUBJECT: OpenVPN - LOGOUT: $common_name - $NOW
    At $NOW, $common_name logged out of the OpenVPN server red.
    
    IP: $trusted_ip
    PORT: $trusted_port
    MTU: $tun_mtu
    
    Sent: $bytes_sent byte
    Resc: $bytes_received byte
    .
    EOF
    
    exit 0
    Yup I really like this global vars. So this script can be altered to put this information into a csv file or into a database.
    You get all information you need - and the used bandwith too.


    chmod +x /etc/openvpn/*.ssh

    And we are done.
     
  • Next hook is the login mode
    Need a password instead of certs? Or an additional password to the certs?
     


    auth-user-pass-verify /etc/openvpn/auth.sh via-file

    This just has to return "0" or "1" for good and bad auth:
     


    nano /etc/openvpn/auth.sh

    Code:
    #!/bin/sh
    #Simplest way
    ALLOWED_USER1="vpsboard"
    ALLOWED_PASS1="supersecure"
    
    if [ "$username" == "$ALLOWED_USER1" ] && [ "$password" == "$ALLOWED_PASS1" ]
     then exit 0
    fi
    
    exit 1
    Want no client certs? Add this to the server.conf


    client-cert-not-required
    username-as-common-name

  • Networking
    Well nothing new if you read my iptables tutorial, but maybe you did not:


    device="venet0" #// OpenVZ
    yourexternalid="1.1.1.1" #// Change this value!
    yourvpnsubnet="10.10.10.0/24" #// Change this value!
    yourvpnport="1149" #//Change this value!

    # allow OPENVPN and enable ip forwarding
    iptables -A INPUT -i tun0 -j ACCEPT
    iptables -A FORWARD -i tun0 -j ACCEPT

    iptables -A FORWARD -o $device -i tun0 -j ACCEPT
    iptables -A FORWARD -o tun0 -i $device -m state --state RELATED,ESTABLISHED -j ACCEPT
    iptables -A INPUT -i $device -p udp --dport $yourvpnport -m state --state NEW -j ACCEPT
    iptables -t nat -A POSTROUTING -s $yourvpnsubnet -j SNAT --to $yourexternalid

    Depens on your choice if you are using a startup script or persist these iptables rules.
     
  • Revoking certs


    . ./vars
    ./revoke-full clientname


So this is everything I am duing to setup a OpenVPN server.

Looking forward to your input and your addons.
 
Last edited by a moderator:

drmike

100% Tier-1 Gogent
I *really* need to start monkeying with OpenVPN.   Long avoided to complexity and many steps like documented above.

For the paranoids, is there a way/recommendation on how to increase the crypto security (different encryption uses, longer key length, etc.)?
 

wlanboy

Content Contributer
For the paranoids, is there a way/recommendation on how to increase the crypto security (different encryption uses, longer key length, etc.)?
Four points:

  • More bits for the keys

    nano vars

    Change this line too (e.g. to 2048):


    export KEY_SIZE=1024

  • Switching dh to 2048bit
    Code:
    openssl dhparam -out /etc/openvpn/dh2048.pem 2048
    server.conf:


    dh dh2048.pem

  • Activating AES-256-CBC in server.conf
    Code:
    cipher AES-256-CBC
  • Adding tls-auth
    Code:
    openvpn --genkey --secret /etc/openvpn/ta.key
    and server.conf
    Code:
    tls-auth ta.key 0
    and on client side:


    tls-auth ta.key 1



Regardings tls:

The tls-auth directive adds an additional HMAC signature to all SSL/TLS handshake packets for integrity verification. Any UDP packet not bearing the correct HMAC signature can be dropped without further processing. The tls-auth HMAC signature provides an additional level of security above and beyond that provided by SSL/TLS.
 

johnlth93

New Member
I *really* need to start monkeying with OpenVPN.   Long avoided to complexity and many steps like documented above.

For the paranoids, is there a way/recommendation on how to increase the crypto security (different encryption uses, longer key length, etc.)?
If you browse through the openvpn man page you would see those parameters that mentioned by wlanboy on hardening the encryption.

You could also customer --auth and --cipher if that's what you wanted.

Anyway @wlan

never really tried "openvpn --mktun --dev tun0" because usually there are all setup along with the template and some fancy button on solusvm but I did used the old fashion command back then


mkdir -p
/dev/net
mknod /dev/net/tun
c 10 200
chmod 600
/dev/net/tun
Good job on the guide though, hope people take their time to read instead of asking the same question again and again :) 
 

peterw

New Member
Good job.


script-security 3 system
client-connect /etc/openvpn/client-login.sh
client-disconnect /etc/openvpn/client-logout.sh

And thank you for this scripts!
 

rupe

New Member
Minor typo (excess typing :) ) - you've got a double entry in 'save you some typing' code: 

Code:
export KEY_EMAIL
 

wlanboy

Content Contributer
Minor typo (excess typing :) ) - you've got a double entry in 'save you some typing' code: 
Thank you for the hint - I modified the post.

Just checked the default config file and it does have the double entry too.
 

peterw

New Member
You don't need a client config. Openvpn runs fine on the console

Code:
openvpn --remote SERVER_IP --dev tun1 --ifconfig 10.10.10.2 10.10.10.1 --ca ~/keys/ca.crt --cert ~/keys/client.crt --key ~/keys/client.key --reneg-sec 60 --verb 5
 

tdc-adm

New Member
For multi-clients/one-server model: each client certificate requires a unique Common Name. If you set a same Common Name for your clients then only one client can login, other clients will be kicked out automatically.
 

wlanboy

Content Contributer
If you set a same Common Name for your clients then only one client can login, other clients will be kicked out automatically.
There is a parameter for that too (if you want to):

Code:
# 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
 

fixidixi

Active Member
You don't need a client config. Openvpn runs fine on the console


openvpn --remote SERVER_IP --dev tun1 --ifconfig 10.10.10.2 10.10.10.1 --ca ~/keys/ca.crt --cert ~/keys/client.crt --key ~/keys/client.key --reneg-sec 60 --verb 5
yea and god save you from reboots.. ..or if you are running openvpn between vpses for instance.. the only reason i'd use params as you've written would be testing.. well even then i'd hit up a tail -f  logfile and modify config, restart service instead..
 
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