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IPSEC VPN on Ubuntu 15.04 with StrongSwan

Published: 20-12-2015 | Author: Remy van Elst | Text only version of this article

❗ This post is over eight years old. It may no longer be up to date. Opinions may have changed.

This is a guide on setting up an IPSEC VPN server on Ubuntu 15.04 using StrongSwan as the IPsec server and for authentication. It has a detailed explanation with every step. We choose the IPSEC protocol stack because of vulnerabilities found in pptpd VPNs and because it is supported on all recent operating systems by default.

Why a VPN?

More than ever, your freedom and privacy when online is under threat. Governments and ISPs want to control what you can and can't see while keeping a record of everything you do, and even the shady-looking guy lurking around your coffee shop or the airport gate can grab your bank details easier than you may think. A self hosted VPN lets you surf the web the way it was intended: anonymously and without oversight.

A VPN (virtual private network) creates a secure, encrypted tunnel through which all of your online data passes back and forth. Any application that requires an internet connection works with this self hosted VPN, including your web browser, email client, and instant messaging program, keeping everything you do online hidden from prying eyes while masking your physical location and giving you unfettered access to any website or web service no matter where you happen to live or travel to.

This tutorial is available for the following platforms:

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IPSEC encrypts your IP packets to provide encryption and authentication, so no one can decrypt or forge data between your clients and your server. It also provides a tunnel to send data to the server.

This VPN setup is called a road-warrior setup, because clients can connect from anywhere. Another much used VPN setup is called site-to-site, where two VPN servers connect two networks with one another. In a road warrior setup your local network isn't shared, but you do get access to the server's network.

To work trough this tutorial you should have:

  • 1 Ubuntu 15.04 server with at least 1 public IP address and root access
  • 1 (or more) clients running an OS that support IPsec IKEv2 vpns (Ubuntu, Mac OS, Windows 7+, Android 4+).
  • Ports 4500/UDP, 500/UDP, 51/UDP and 50/UDP opened in the firewall.

I do all the steps as the root user. You should do to, but only via sudo -i or su -.

No L2TP?

The previous tutorials all used L2TP to set up the VPN tunnel and use IPSEC only for the encryption. With the IKEv2 protocol and newer operating systems (like OS X 10.8+, Android 4+, iOS 6+ and Windows 7+) supporting IKEv2 we can also use IPSEC to set up the tunnel, before we used IPSEC to do that.

This VPN will therefore not work out of the box on older operating systems. See my other tutorials with L2TP on how to do that.


The tutorial consists out of the following steps:

  • Install packages
  • Generate certificates
  • Configure IPSEC
  • Configure Firewall

Android and Windows client configuration is covered at the end of the tutorial.

Install Strongswan

StrongSwan is a descendant of FreeS/WAN, just like Openswan or LibreSwan. Strongswan however is actively developed, whereas the other ones, except LibreSwan are less. StrongSwan is in default in the Ubuntu repositories. You can read more about Strongswan on wikipedia or their website.

apt-get install strongswan strongswan-plugin-af-alg strongswan-plugin-agent strongswan-plugin-certexpire strongswan-plugin-coupling strongswan-plugin-curl strongswan-plugin-dhcp strongswan-plugin-duplicheck strongswan-plugin-eap-aka strongswan-plugin-eap-aka-3gpp2 strongswan-plugin-eap-dynamic strongswan-plugin-eap-gtc strongswan-plugin-eap-mschapv2 strongswan-plugin-eap-peap strongswan-plugin-eap-radius strongswan-plugin-eap-tls strongswan-plugin-eap-ttls strongswan-plugin-error-notify strongswan-plugin-farp strongswan-plugin-fips-prf strongswan-plugin-gcrypt strongswan-plugin-gmp strongswan-plugin-ipseckey strongswan-plugin-kernel-libipsec strongswan-plugin-ldap strongswan-plugin-led strongswan-plugin-load-tester strongswan-plugin-lookip strongswan-plugin-ntru strongswan-plugin-pgp strongswan-plugin-pkcs11 strongswan-plugin-pubkey strongswan-plugin-radattr strongswan-plugin-sshkey strongswan-plugin-systime-fix strongswan-plugin-whitelist strongswan-plugin-xauth-eap strongswan-plugin-xauth-generic strongswan-plugin-xauth-noauth strongswan-plugin-xauth-pam strongswan-pt-tls-client 


The VPN server will identify itself with a certificate to the clients. The clients can use a certificate to authenticate themself, this tutorial however keeps it simple and sets up username and password authentication as well.

On Android with the StrongSwan Application you can just import the .p12 we are going to create later on. On Windows 7, we'll use EAP to configure a username and password for our client.

You might want to install haveged to speed up the key generation process:

apt-get install haveged
systemctl enable haveged
systemctl start haveged

Haveged provides a constant source of entropy and randomness.

Start by creating a self singed root CA private key:

cd /etc/ipsec.d/
mkdir private
mkdir cacerts
mkdir certs
mkdir p12
ipsec pki --gen --type rsa --size 4096 --outform der > private/strongswanKey.der
chmod 600 private/strongswanKey.der

Generate a self signed root CA certificate of that private key:

ipsec pki --self --ca --lifetime 3650 --in private/strongswanKey.der --type rsa --dn "C=NL, O=Example Company, CN=strongSwan Root CA" --outform der > cacerts/strongswanCert.der

You can view the certificate properties with the following command:

ipsec pki --print --in cacerts/strongswanCert.der

Example output:

cert:      X509
subject:  "C=NL, O=Example Company, CN=strongSwan Root CA"
issuer:   "C=NL, O=Example Company, CN=strongSwan Root CA"
validity:  not before Dec 20 08:12:27 2015, ok
           not after  Dec 17 08:12:27 2025, ok (expires in 3649 days)
serial:    1f:8e:0c:08:c4:a2:5b:1f
flags:     CA CRLSign self-signed 
authkeyId: d1:ad:f7:76:ad:10:02:7f:1d:04:e1:80:46:9d:b2:c7:fb:4d:d3:bb
subjkeyId: d1:ad:f7:76:ad:10:02:7f:1d:04:e1:80:46:9d:b2:c7:fb:4d:d3:bb
pubkey:    RSA 4096 bits
keyid:     88:ef:88:13:7f:da:5a:28:13:77:4b:4c:81:df:ee:db:fb:5c:69:54
subjkey:   d1:ad:f7:76:ad:10:02:7f:1d:04:e1:80:46:9d:b2:c7:fb:4d:d3:bb

Generate the VPN Host key. This is the keypair the VPN server host will use to authenticate itself to clients. First the private key:

ipsec pki --gen --type rsa --size 4096 --outform der > private/vpnHostKey.der
chmod 600 private/vpnHostKey.der

Generate the public key and use our earlier created root ca to sign the public key:

ipsec pki --pub --in private/vpnHostKey.der --type rsa | ipsec pki --issue --lifetime 730 --cacert cacerts/strongswanCert.der --cakey private/strongswanKey.der --dn "C=NL, O=Example Company," --san --san --san  --san @ --flag serverAuth --flag ikeIntermediate --outform der > certs/vpnHostCert.der

The domain name or IP address of your VPN server, which is later entered in the clients connection properties, MUST be contained either in the subject Distinguished Name (CN) and/or in a subject Alternative Name (--san). If this does not match the clients will fail to connect.

The built in Windows 7 VPN client needs the serverAuth extended key usage flag in your host certificate as shown above, or the client will refuse to connect. In addition, OS X 10.7.3 or older requires the ikeIntermediate flag, which we also add here.

We add the IP address twice, one with an @ in front so that it gets added as an subjectAltName of the DNSName type and one of the IPAddess type.

Let's view the certificate:

ipsec pki --print --in certs/vpnHostCert.der


cert:      X509
subject:  "C=NL, O=Example Company,"
issuer:   "C=NL, O=Example Company, CN=strongSwan Root CA"
validity:  not before Dec 20 08:15:22 2015, ok
           not after  Dec 19 08:15:22 2017, ok (expires in 729 days)
serial:    aa:31:ac:fd:4b:fa:41:5d
flags:     serverAuth iKEIntermediate 
authkeyId: d1:ad:f7:76:ad:10:02:7f:1d:04:e1:80:46:9d:b2:c7:fb:4d:d3:bb
subjkeyId: 27:c7:87:de:83:38:6c:f7:56:57:c2:b3:1f:05:11:ca:b9:2f:89:d4
pubkey:    RSA 4096 bits
keyid:     f8:03:95:ad:eb:a1:76:93:5f:8d:b8:77:5e:60:dc:ce:78:42:3b:dd
subjkey:   27:c7:87:de:83:38:6c:f7:56:57:c2:b3:1f:05:11:ca:b9:2f:89:d4

You can also use OpenSSL to see the contents, here is an excerpt:

openssl x509 -inform DER -in certs/vpnHostCert.der -noout -text


        Version: 3 (0x2)
        Serial Number: 12263773464207966557 (0xaa31acfd4bfa415d)
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=NL, O=Example Company, CN=strongSwan Root CA
            Not Before: Dec 20 07:15:22 2015 GMT
            Not After : Dec 19 07:15:22 2017 GMT
        Subject: C=NL, O=Example Company,
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (4096 bit)
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Authority Key Identifier: 

            X509v3 Subject Alternative Name: 
      ,, IP Address:, DNS:
            X509v3 Extended Key Usage: 
                TLS Web Server Authentication,
    Signature Algorithm: sha1WithRSAEncryption

The private key (/etc/ipsec.d/private/strongswanKey.der) of the CA should be moved somewhere safe, possibly to a special signing host without access to the Internet. Theft of this master signing key would completely compromise your public key infrastructure. Use it only to generate client certificates when needed.

Client certificate

Any client will require a personal certificate in order to use the VPN. The process is analogous to generating a host certificate, except that we identify a client certificate by the clients e-mail address rather than a hostname.

We create a keypair for the example user "John".

Private key:

ipsec pki --gen --type rsa --size 2048 --outform der > private/JohnKey.der
chmod 600 private/JohnKey.der

Public key, signed by our root ca we generated:

ipsec pki --pub --in private/JohnKey.der --type rsa | ipsec pki --issue --lifetime 730 --cacert cacerts/strongswanCert.der --cakey private/strongswanKey.der --dn "C=NL, O=Example Company," --san "" --san "" --san "john@" --outform der > certs/JohnCert.der

A VPN client needs a client certificate, its corresponding private key, and the signing CA certificate. The most convenient way is to put everything in a single signed PKCS#12 file and export it with a paraphrase.

Convert the required keys to PEM formt before converting to a .p12:

openssl rsa -inform DER -in private/JohnKey.der -out private/JohnKey.pem -outform PEM

openssl x509 -inform DER -in certs/JohnCert.der -out certs/JohnCert.pem -outform PEM

openssl x509 -inform DER -in cacerts/strongswanCert.der -out cacerts/strongswanCert.pem -outform PEM

Construct the .p12:

openssl pkcs12 -export -inkey private/JohnKey.pem -in certs/JohnCert.pem -name "John's VPN Certificate" -certfile cacerts/strongswanCert.pem -caname "strongSwan Root CA" -out p12/John.p12

Enter a passphrase twice, then you have a .p12. You can send John.p12 and its export paraphrase to the person who is going to install it onto the client. In some cases (iOS for example) you have to separately include the CA certificate cacerts/strongswanCert.pem.

Transport this John.p12 file and the password over seperate channels to a client.

If you need any more user certificates, repeat the above steps with other user data. You can also do this later on.

Revoking a certificate

If a certificate is lost or stolen, it must be revoked so nobody can use it to connect to your VPN server. Assuming the certificate from the previous step got stolen, we revoke it with:

cd /etc/ipsec.d/
ipsec pki --signcrl --reason key-compromise \
    --cacert cacerts/strongswanCert.der \
    --cakey private/strongswanKey.der \
    --cert certs/JohnCert.der \
    --outform der > crls/crl.der

Restart ipsec afterwards:

ipsec restart

This generates the new certificate revocation list (CRL) crls/crl.der. When someone tries to authenticate with the stolen certificate, he'll receive an authentication credentials error message, and your log file will contain something like:

04[CFG] using trusted certificate "C=NL, O=Example Company, CN=strongSwan Root CA"
04[CFG]   crl correctly signed by "C=NL, O=Example Company, CN=strongSwan Root CA"
04[CFG] certificate was revoked on Dec 20 14:51:24 UTC 2015, reason: key compromise

To add another revoked certificate to the same list, we need to copy the existing list into a temporary file:

cd /etc/ipsec.d/
cp crls/crl.der crl.der.tmp
ipsec pki --signcrl --reason key-compromise \
  --cacert cacerts/strongswanCert.der \
  --cakey private/strongswanKey.der \
  --cert certs/OtherStolenCert.der \
  --lastcrl crl.der.tmp \
  --outform der > crls/crl.der
rm crl.der.tmp

Restart ipsec afterwards:

ipsec restart

IPSEC Configuration

The main ipsec configuration file is located in /etc/strongswan.d/. We are going to edit it:

vim /etc/strongswan.d/VPN.conf

Place the following contents:

# ipsec.conf - strongSwan IPsec configuration file

config setup
    charondebug="ike 4, knl 4, cfg 4, net 4, esp 4, dmn 4,  mgr 4"

conn %default

conn IPSec-IKEv2

conn IPSec-IKEv2-EAP

conn CiscoIPSec

Remove the /etc/ipsec.conf file and create a symlink:

rm /etc/ipsec.conf
ln -s /etc/strongswan.d/VPN.conf /etc/ipsec.conf

This configuration has settings for three types of VPN services: IKEv2 + RSA certificate, IKEv2 + EAP and IKEv1 + Xauth, thus providing compatibility for a wide range of recent IPsec clients.

Apple added support for IKEv2 in iOS 8, but it needs to be configured using a custom configuration profile.

OS X does not support IKEv2 (not on 10.10 or lower).

For iOS 9+ and OS X 10.10+ you need to make sure the leftid= is the same as the CN in your certificate. You also need to enter that on the devices, otherwise you'll get a no matching peer config found log error.

Android 4+ and Windows 7+ support IKEv2.

Clients will get the Google DNS servers and an IP address in the range. We use a strong ciphersuite.

The leftcert=vpnHostCert.der expands to the path /etc/ipsec.d/certs/vpnHostCert.der.

VPN user accounts and secrets

The users are configured in the /etc/ipsec.secrets file.

vim /etc/ipsec.secrets

Example content:

: RSA vpnHostKey.der
: PSK 0sv+NkxY9LLZvwj4qCC2o/gGrWDF2d21jL

alice : EAP "YzCgnveYuL429fH" 
bob : EAP "E23pOjvW8z248iAp" 
hipster: XAUTH "xauth_ikev1_example_password"

In the example above the RSA private key file vpnHostKey.der stored in the /etc/openswan.d/private/ directory is not protected by symmetric encryption (a password).

The PSK for IKEv1 connections is also defined.

The format of the EAP MSCHAPv2 user credentials is:

[<domain>\]<username> : EAP "<plaintext password>" 

Add as many users as you like there. The first line allows all users with a valid certificate to use the VPN, the other lines allow users without a certificate to login with a username and password. The space between the username, the colon (:) and EAP needs to be there.

I strongly suggest adding NO users here, and setting NO PSK. Use certificates, they are much more secure.

Whenever you edit /etc/ipsec.secrets while strongSwan is running, you must reload the file:

ipsec rereadsecrets

If you need to generate password, OpenSSL can help you there:

 openssl rand -base64 24

Firewall & Packet Routing

Configure the iptables firewall to allow vpn traffic and to forward packets:

# for ISAKMP (handling of security associations)
iptables -A INPUT -p udp --dport 500 --j ACCEPT
# for NAT-T (handling of IPsec between natted devices)
iptables -A INPUT -p udp --dport 4500 --j ACCEPT
# for ESP payload (the encrypted data packets)
iptables -A INPUT -p esp -j ACCEPT
# for the routing of packets on the server
iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+

Replace %SERVERIP% with the external IP of your VPS. If your external interface is not named ethX (+ is a wildcard) then rename appropriately.

Execute the below commands to enable kernel IP packet forwarding and disable ICP redirects.

echo "net.ipv4.ip_forward = 1" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.accept_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.rp_filter = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.accept_source_route = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.icmp_ignore_bogus_error_responses = 1" |  tee -a /etc/sysctl.conf

Set these settings for other network interfaces:

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done

Apply them:

sysctl -p
Persistent settings via /etc/rc.local

To make sure this keeps working at boot you might want to add the following to /etc/rc.local:

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done
iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+
iptables -A INPUT -p udp --dport 500 --j ACCEPT
iptables -A INPUT -p udp --dport 4500 --j ACCEPT
iptables -A INPUT -p esp -j ACCEPT

Add it before the exit 0 line and replace %SERVERIP% with the external IP of your VPS.

Start the VPN

All the configuration on the server is now done. Enable the VPN at startup:

systemctl enable strongswan

And start it:

systemctl start strongswan

If you get a permission denied error, stroke the files with apparmor:

apparmor_parser -R /etc/apparmor.d/usr.lib.ipsec.charon
apparmor_parser -R /etc/apparmor.d/usr.lib.ipsec.stroke

Check the status of the service:

ipsec status


Security Associations (0 up, 0 connecting):

And a more elaborate status:

ipsec statusall


Status of IKE charon daemon (strongSwan 5.1.2, Linux 3.19.0-41-generic, x86_64):
  uptime: 94 seconds, since Dec 20 08:27:24 2015
  malloc: sbrk 1859584, mmap 266240, used 693200, free 1166384
  worker threads: 11 of 16 idle, 5/0/0/0 working, job queue: 0/0/0/0, scheduled: 0
  loaded plugins: charon test-vectors curl unbound ldap pkcs11 aes rc2 sha1 sha2 md4 md5 random nonce x509 revocation constraints pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp sshkey ipseckey pem openssl gcrypt af-alg fips-prf gmp xcbc cmac hmac ctr ccm gcm ntru attr kernel-netlink resolve socket-default farp stroke updown eap-identity eap-aka eap-aka-3gpp2 eap-gtc eap-mschapv2 eap-dynamic eap-radius eap-tls eap-ttls eap-peap xauth-generic xauth-eap xauth-noauth tnc-imc tnc-tnccs tnccs-20 tnccs-11 tnccs-dynamic dhcp whitelist lookip error-notify certexpire led duplicheck radattr addrblock
Listening IP addresses:
Security Associations (0 up, 0 connecting):

Client Configuration

See the Strongswan Wiki for guides on configuring Windows and OS X/iOS clients


Thanks to:

Tags: debian , ikev2 , ipsec , openswan , openvpn , pptp , strongswan , tutorials , ubuntu , vpn