StrongSwan Ipsec VPN for Remote Users with Certificate Based Authentication

This is a working strongswan ipsec config that can be used for a roadwarrior setup for remote users utilizing certificate based authentication instead of id/pw.

This is a pure IPSEC with ESP setup, not L2tp.
This is not 2 factor, it is cert only.

To get started:

sudo apt-get install strongswan

You need is a CA that is capable of registering AltNames in a cert. OpenSSL can do this easily. I used this guide to setup the basic openssl CA.

Once the CA is ready and you have generated your ca cert and ca private key, you next need to create a cert for the ipsec host and a cert for the end user.

For the Server:
Since I need the Alt Names in the certs, make a copy of /etc/ssl/openssl.cnf to be used for the Server.

cp /etc/ssl/openssl.cnf /etc/ssl/openssl-for-server.cnf
# Extension copying option: use with caution. copy_extensions = copy [ v3_req ] # Extensions to add to a certificate request basicConstraints = CA:FALSE keyUsage = nonRepudiation, digitalSignature, keyEncipherment subjectAltName=@alt_names [alt_names] DNS.1 = DNS.2 =

Now, using Openssl, create the request for the server, fill in the details of the req as needed, then sign the request.

openssl req -new -nodes -out ipsechost-req.pem -keyout private/ipsechost-key.pem -config /etc/ssl/openssl-for-server.cnf
openssl ca -config /etc/ssl/openssl-for-server.cnf -out ipsechost-cert.pem -in ipsechost-req.pem

Copy the certs to the correct locations for strongswan to use.

cp cacert.pem /etc/ipsec.d/cacerts
cp ipsechost-cert.pem /etc/ipsec.d/certs
cp private/ipsechost-key.pem /etc/ipsec.d/private/

Stongswan is configured using the /etc/ipsec.conf and /etc/ipsec.secrets files.
This is a very simple config that will work for providing access to remote users:
Edit /etc/ipsec.conf

# /etc/ipsec.conf - strongSwan IPsec configuration file
config setup

conn %default

conn common
        left=IP_OF_IPSEC_HOST          # Ip of the host
        leftcert=ipsechost-cert.pem    # the cert we just created and copied  # the Alt name in the Cert we just created
        leftsubnet=      # The internal subnet the remote user wants to access
        right=%any                     # Connections can come from anywhere
        rightsourceip=   # Use this pool of IPs to assign to these inbound connections

conn ikev2

Edit the /etc/ipsec.secrets file

: RSA ipsechost-key.pem

Restart/Reload IPsec.

ipsec restart

I like to watch logs just to be sure there are no errors:

tail -f /var/log/syslog /var/log/auth.log

Next we create a client cert. We need another copy of the openssl config file for user requests since the Alt Name changes from DNS to Email.

cp /etc/ssl/openssl-for-server.cnf /etc/ssl/openssl-for-users.cnf
[ v3_req ] # Extensions to add to a certificate request basicConstraints = CA:FALSE keyUsage = nonRepudiation, digitalSignature, keyEncipherment subjectAltName=email:copy #[alt_names] #DNS.1 = #DNS.2 =

Create the request, fill in the details as needed for the user especially the email address, and Sign the request. The email address specified in the request prompts will be used in the cert for the Alt name and in the config for the user’s side of the tunnel.

openssl req -new -nodes -out user1-req.pem -keyout private/user1-key.pem -config /etc/ssl/openssl.cnf 
openssl ca -config /etc/ssl/openssl-for-users.cnf -out user1-cert.pem -in user1-req.pem 

You need to copy the user1-cert.pem, user1-key.pem, and the cacert.pem to the user’s machine. We will need these file to complete the VPN.

On the User’s Side:

sudo apt-get install strongswan

Copy the files into the proper directories
user1-cert.pem to /etc/ipsec.d/certs
user1-key.pem to /etc/ipsec.d/private
cacert.pem to /etc/ipsec.d/cacerts

Edit the client side ipsec.conf. This is a working config:

conn %default

conn roadwarrior
     leftsourceip=%config                # This will take an IP from the ip pool on server
     leftcert=ipsecuser1-cert.pem        # The user cert we copied in      # This is the email included in the Alt Name in the user cert
     leftfirewall=yes   # The location of the host, FQDN or IP # the Altname used by the ipsec host
     rightsubnet=          # the subnet on the servers side you want to access. 

Edit the ipsec.secrets file

: RSA ipsecuser1-key.pem

On the client, issue an “ipsec restart” and it should attempt to build the tunnel with that you are done.

Use “ipsec statusall” to get details on the tunnels. From the server, a healthy tunnel looks like this:

Security Associations (1 up, 0 connecting):
       ikev2[11]: ESTABLISHED 3 minutes ago, HOST_IP[]...REMOTE_IP[]
       ikev2[11]: IKEv2 SPIs: 49c4512b56436e5b_i 6276554588ce1803_r*, public key reauthentication in 50 minutes
       ikev2[11]: IKE proposal: AES_CBC_128/HMAC_SHA1_96/PRF_HMAC_SHA1/MODP_2048
       ikev2{9}:  INSTALLED, TUNNEL, ESP in UDP SPIs: cd1e015f_i cd3cb1c1_o
       ikev2{9}:  AES_CBC_128/HMAC_SHA1_96, 0 bytes_i, 0 bytes_o, rekeying in 12 minutes

Use “ipsec listall” for details on the host’s certs and configs. Here we want to be sure Alt Names are good, and the CA and certs are loaded correctly.

List of X.509 End Entity Certificates:
  subject:  "C=US, ST=NY, O=OpenPeak, OU=IT,,"
  issuer:   "C=US, ST=NY, L=NY, O=mydomain, OU=IT, CN=ipsecserver-ca,"
  serial:    10:03
  validity:  not before Mar 18 20:44:25 2015, ok
             not after  Nov 24 20:44:25 2028, ok 
  pubkey:    RSA 2048 bits, has private key
  keyid:     10:15:77:ae:2e:a4:e8:3f:cc:1f:6d:a9:d9:80:bd:9f:41:fb:63:e5
  subjkey:   3f:0c:bf:01:2f:c7:16:be:d4:83:5c:76:81:56:a9:f1:3a:84:b4:5f
  authkey:   b7:61:d7:32:19:65:c3:10:1a:43:23:27:bc:46:29:e5:ff:df:03:1c

List of X.509 CA Certificates:

  subject:  "C=US, ST=NY, L=NY, O=mydomain, OU=IT, CN=ipsecserver-ca,"
  issuer:   "C=US, ST=NY, L=NY, O=mydomain, OU=IT, CN=ipsecserver-ca,"
  serial:    db:e9:16:e0:44:a3:57:83
  validity:  not before Mar 18 15:49:45 2015, ok
             not after  Mar 15 15:49:45 2025, ok 
  pubkey:    RSA 2048 bits
  keyid:     18:47:07:92:b8:3d:a0:bb:88:bf:27:2b:4d:0b:a7:79:8b:c1:1b:ba
  subjkey:   b7:61:d7:32:19:65:c3:10:1a:43:23:27:bc:46:29:e5:ff:df:03:1c
  authkey:   b7:61:d7:32:19:65:c3:10:1a:43:23:27:bc:46:29:e5:ff:df:03:1c

Note that if you want to enable 2 factor with this, change the openssl request for the Clients to omit the -nodes option. This will prompt you for a password during the certificate creation that must be entered every time the client wants to connect.

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2 Responses to StrongSwan Ipsec VPN for Remote Users with Certificate Based Authentication

  1. James Emadoye says:

    I run a dedicated server running window server 2008r2. I want to establish a site to site vpn connection to a site with the following parameters.

    VPN Tunnel Endpoint IP address:196.6.103.x
    IKE Encryption (DES, 3DES,AES):AES 256
    Authentication Method (MD5, SHA): SHA
    Security Association Lifetime:28800
    IPSEC Encryption (DES, 3DES,AES):AES 256
    Authentication Method (MD5, SHA):SHA
    Diffle-Helman Group (1,2): 2
    Security Association Lifetime (Sec): 1800
    Perfect Forward Secrecy (Yes, No): Yes
    PFS Diffie-Helman Group (1,2,5):5
    Host Ip: 196.6.103.x

    Please can someone please come to my aid. Which is the best option for me to connect to the site with the above parameters from my window server.

    I await you reply.

  2. Deepak Prasad says:

    Thanks for the writeup – it helped me configuring my IPSec tunnel

    I now want to extend the configuration to achieve HA for strongswan. How can i do that? Can some one pls provide some insight


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