IPSEC VPN on Ubuntu 15.10 with StrongSwan
Published: 20-12-2015 | Author: Remy van Elst | Text only version of this article
❗ This post is over five years old. It may no longer be up to date. Opinions may have changed.
Table of Contents
This is a guide on setting up an IPSEC VPN server on Ubuntu 15.10 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:
Consider sponsoring me on Github. It means the world to me if you show your appreciation and you'll help pay the server costs.
You can also sponsor me by getting a Digital Ocean VPS. With this referral link you'll get $100 credit for 60 days.
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 15.10 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
A few of the previous tutorials 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.
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 should use a certificate to authenticate themself. Previous tutorials also configured usernames and password and pre-shared keys, this tutorial does not. Certificates are easier to use, can be revoked and are less hassle than managing usernames and passwords.
On Android with the StrongSwan Application you can just import the
.p12 we are
going to create later on. OS X and iOS from 10.10 and 9 upwards also support
this authentication method.
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
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, CN=vpn.example.org" --san vpn.example.com --san vpn.example.net --san 188.8.131.52 --san @184.108.40.206 --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
subjectAltName of the
DNSName type and one of the
Let's view the certificate:
ipsec pki --print --in certs/vpnHostCert.der
cert: X509 subject: "C=NL, O=Example Company, CN=vpn.example.org" 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 altNames: vpn.example.com, vpn.example.net, 220.127.116.11, 18.104.22.168 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
Certificate: Data: Version: 3 (0x2) Serial Number: 12263773464207966557 (0xaa31acfd4bfa415d) Signature Algorithm: sha1WithRSAEncryption Issuer: C=NL, O=Example Company, CN=strongSwan Root CA Validity Not Before: Dec 20 07:15:22 2015 GMT Not After : Dec 19 07:15:22 2017 GMT Subject: C=NL, O=Example Company, CN=vpn.example.org Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (4096 bit) [...] Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Authority Key Identifier: keyid:D1:AD:F7:76:AD:10:02:7F:1D:04:E1:80:46:9D:B2:C7:FB:4D:D3:BB X509v3 Subject Alternative Name: DNS:vpn.example.com, DNS:vpn.example.net, IP Address:22.214.171.124, DNS:126.96.36.199 X509v3 Extended Key Usage: TLS Web Server Authentication, 188.8.131.52.184.108.40.206.2 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
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".
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, CNemail@example.com" --san "firstname.lastname@example.org" --san "email@example.com" --san "firstname.lastname@example.org" --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
John.p12 file and the password over seperate channels to a
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:
This generates the new certificate revocation list (CRL)
someone tries to authenticate with the stolen certificate, he'll receive an
authentication credentials error message, and your log file will contain
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 configuration file is located in
/etc/strongswan.d/. We are
going to edit it:
Place the following contents:
# ipsec.conf - strongSwan IPsec configuration file config setup charondebug="ike 2, knl 2, cfg 2, net 2, esp 2, dmn 2, mgr 2" conn %default keyexchange=ikev2 ike=aes128-sha1-modp1024,aes128-sha1-modp1536,aes128-sha1-modp2048,aes128-sha256-ecp256,aes128-sha256-modp1024,aes128-sha256-modp1536,aes128-sha256-modp2048,aes256-aes128-sha256-sha1-modp2048-modp4096-modp1024,aes256-sha1-modp1024,aes256-sha256-modp1024,aes256-sha256-modp1536,aes256-sha256-modp2048,aes256-sha256-modp4096,aes256-sha384-ecp384,aes256-sha384-modp1024,aes256-sha384-modp1536,aes256-sha384-modp2048,aes256-sha384-modp4096,aes256gcm16-aes256gcm12-aes128gcm16-aes128gcm12-sha256-sha1-modp2048-modp4096-modp1024,3des-sha1-modp1024! esp=aes128-aes256-sha1-sha256-modp2048-modp4096-modp1024,aes128-sha1,aes128-sha1-modp1024,aes128-sha1-modp1536,aes128-sha1-modp2048,aes128-sha256,aes128-sha256-ecp256,aes128-sha256-modp1024,aes128-sha256-modp1536,aes128-sha256-modp2048,aes128gcm12-aes128gcm16-aes256gcm12-aes256gcm16-modp2048-modp4096-modp1024,aes128gcm16,aes128gcm16-ecp256,aes256-sha1,aes256-sha256,aes256-sha256-modp1024,aes256-sha256-modp1536,aes256-sha256-modp2048,aes256-sha256-modp4096,aes256-sha384,aes256-sha384-ecp384,aes256-sha384-modp1024,aes256-sha384-modp1536,aes256-sha384-modp2048,aes256-sha384-modp4096,aes256gcm16,aes256gcm16-ecp384,3des-sha1! dpdaction=clear dpddelay=300s authby=pubkey left=%any leftid=vpn.example.org leftsubnet=0.0.0.0/0 leftcert=vpnHostCert.der leftsendcert=always right=%any rightsourceip=10.42.42.0/24,2002:25f7:7489:3::/112 rightdns=220.127.116.11,2001:4860:4860::8888 conn IPSec-IKEv2 keyexchange=ikev2 auto=add
/etc/ipsec.conf file and create a symlink:
rm /etc/ipsec.conf ln -s /etc/strongswan.d/VPN.conf /etc/ipsec.conf
The configuration has settings for IKEv2 + RSA certificates. This is, as stated above, the most secure method. Older tutorials also set up IKEv1 (xauth) and username-password combo, but that is considered insecure.
Apple added support for IKEv2 in iOS 8, but it needs to be configured using a custom configuration profile. OS X 10.9 and lower do not support IKEv2.
Beginning with iOS 9, IKEv2 connections are natively supported. However, iOS9 only supports the use of certificates or username/password, but not both.
For iOS 9+ and OS X 10.10+ you need to make sure the
leftid= is the same as
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.
leftcert=vpnHostCert.der expands to the path
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
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
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:
Security Associations (0 up, 0 connecting): none
And a more elaborate status:
Status of IKE charon daemon (strongSwan 5.1.2, Linux 4.2.0-21-generic, x86_64): uptime: 4 minutes, since Dec 20 15:51:48 2015 malloc: sbrk 1814528, mmap 266240, used 758000, free 1056528 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 gcrypt af-alg fips-prf gmp agent 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-pam xauth-noauth tnc-imc tnc-tnccs tnccs-20 tnccs-11 tnccs-dynamic dhcp whitelist lookip error-notify certexpire led duplicheck radattr addrblock Virtual IP pools (size/online/offline): 10.42.42.0/24: 254/0/0 2002:25f7:7489:3::/112: 65534/0/0 Listening IP addresses: 18.104.22.168 Connections: IPSec-IKEv2: %any...%any IKEv2, dpddelay=300s IPSec-IKEv2: local: [vpn.raymii.nl] uses public key authentication IPSec-IKEv2: cert: "C=NL, O=Example Company, CN=vpn.raymii.nl" IPSec-IKEv2: remote: uses public key authentication IPSec-IKEv2: child: 0.0.0.0/0 === dynamic TUNNEL, dpdaction=clear Security Associations (0 up, 0 connecting): none
- StrongSwan Wiki and the
- StrongSwan ipsec.conf reference for most of the configuration.
- zeitgeist for the certificate setup.