Last updated: 2026-05-15

strongSwan TLS/IPsec Configuration Guide

This guide provides recommended IKEv2/IPsec settings for strongSwan to establish encrypted VPN tunnels using certificate-based authentication. strongSwan implements the IKEv2 protocol with modern cryptographic algorithms for secure site-to-site and remote access VPN connections.

Prerequisites#

• strongSwan 5.9 or later
• OpenSSL 1.1.1 or later (or a compatible crypto backend)
• X.509 certificates (server certificate, private key, and CA certificate)

RHEL/CentOS: Install with dnf install strongswan.

Debian/Ubuntu: Install with apt install strongswan strongswan-pki.

Certificate Setup#

Generate Certificates with strongSwan PKI#

# Generate a CA private key and certificate
pki --gen --type rsa --size 4096 --outform pem > /etc/strongswan/ipsec.d/private/ca-key.pem
pki --self --ca --lifetime 3650 --in /etc/strongswan/ipsec.d/private/ca-key.pem \
    --dn "CN=VPN CA" --outform pem > /etc/strongswan/ipsec.d/cacerts/ca-cert.pem

# Generate a server private key and certificate
pki --gen --type rsa --size 2048 --outform pem > /etc/strongswan/ipsec.d/private/server-key.pem
pki --req --type priv --in /etc/strongswan/ipsec.d/private/server-key.pem \
    --dn "CN=vpn.example.com" --san vpn.example.com --outform pem > server.csr
pki --issue --cacert /etc/strongswan/ipsec.d/cacerts/ca-cert.pem \
    --cakey /etc/strongswan/ipsec.d/private/ca-key.pem \
    --type pkcs10 --in server.csr --serial 01 --lifetime 365 \
    --flag serverAuth --flag ikeIntermediate \
    --san vpn.example.com --outform pem > /etc/strongswan/ipsec.d/certs/server-cert.pem

Using Existing Certificates#

If you have certificates from an external CA:

cp ca.crt /etc/strongswan/ipsec.d/cacerts/ca-cert.pem
cp server.crt /etc/strongswan/ipsec.d/certs/server-cert.pem
cp server.key /etc/strongswan/ipsec.d/private/server-key.pem

chmod 600 /etc/strongswan/ipsec.d/private/server-key.pem

Set Permissions#

chmod 600 /etc/strongswan/ipsec.d/private/*.pem
chown root:root /etc/strongswan/ipsec.d/private/*.pem

swanctl Configuration#

strongSwan 5.7+ uses swanctl.conf as the primary configuration format. The file is located at /etc/strongswan/swanctl/swanctl.conf.

IKEv2 Proposals#

Configure strong cryptographic proposals for the IKE (control) channel:

connections {
    vpn {
        version = 2

        proposals = aes256gcm16-sha384-x25519,aes256gcm16-sha384-ecp384,aes128gcm16-sha256-x25519,aes128gcm16-sha256-ecp256
    }
}

ESP (Data Channel) Proposals#

Configure the ESP proposals for encrypting tunnel traffic:

connections {
    vpn {
        children {
            net {
                esp_proposals = aes256gcm16-sha384,aes128gcm16-sha256,chacha20poly1305-sha256
            }
        }
    }
}

Configuration Explained#

• aes256gcm16 -- AES-256-GCM with 128-bit ICV (authenticated encryption)
• sha384 -- SHA-384 for PRF and integrity (IKE only; not used with GCM in ESP)
• x25519 -- Curve25519 Diffie-Hellman key exchange
• ecp384 -- NIST P-384 elliptic curve Diffie-Hellman
• chacha20poly1305 -- ChaCha20-Poly1305 authenticated encryption (good for devices without AES-NI)

Certificate Authentication#

Configure certificate-based authentication in the connection:

connections {
    vpn {
        local {
            auth = pubkey
            certs = server-cert.pem
            id = vpn.example.com
        }
        remote {
            auth = pubkey
            cacerts = ca-cert.pem
        }
    }
}

See RFC 8446 §4.3.2 for the TLS Certificate Request specification, and Wikipedia: Mutual authentication for a general overview.

Complete Configuration#

swanctl.conf#

connections {
    vpn {
        version = 2
        rekey_time = 86400
        dpd_delay = 30

        # IKE proposals (control channel)
        proposals = aes256gcm16-sha384-x25519,aes256gcm16-sha384-ecp384,aes128gcm16-sha256-x25519,aes128gcm16-sha256-ecp256

        # Local (server) authentication
        local {
            auth = pubkey
            certs = server-cert.pem
            id = vpn.example.com
        }

        # Remote (client) authentication
        remote {
            auth = pubkey
            cacerts = ca-cert.pem
        }

        # Child SA (tunnel)
        children {
            net {
                local_ts = 10.0.0.0/24
                esp_proposals = aes256gcm16-sha384,aes128gcm16-sha256,chacha20poly1305-sha256
                rekey_time = 3600
                dpd_action = restart
            }
        }
    }
}

secrets {
    private {
        file = server-key.pem
    }
}

Remote Access (Road Warrior)#

For remote access VPN with virtual IP assignment:

connections {
    roadwarrior {
        version = 2
        proposals = aes256gcm16-sha384-x25519,aes128gcm16-sha256-x25519

        pools = client_pool

        local {
            auth = pubkey
            certs = server-cert.pem
            id = vpn.example.com
        }
        remote {
            auth = pubkey
            cacerts = ca-cert.pem
        }

        children {
            roadwarrior {
                local_ts = 0.0.0.0/0
                esp_proposals = aes256gcm16-sha384,aes128gcm16-sha256
            }
        }
    }
}

pools {
    client_pool {
        addrs = 10.10.10.0/24
        dns = 10.0.0.1
    }
}

Client Configuration#

Generate Client Certificate#

pki --gen --type rsa --size 2048 --outform pem > client-key.pem
pki --req --type priv --in client-key.pem --dn "CN=client@example.com" --outform pem > client.csr
pki --issue --cacert /etc/strongswan/ipsec.d/cacerts/ca-cert.pem \
    --cakey /etc/strongswan/ipsec.d/private/ca-key.pem \
    --type pkcs10 --in client.csr --serial 02 --lifetime 365 \
    --flag clientAuth --outform pem > client-cert.pem

Export as PKCS#12 for import into clients:

openssl pkcs12 -export -in client-cert.pem -inkey client-key.pem \
    -certfile /etc/strongswan/ipsec.d/cacerts/ca-cert.pem \
    -out client.p12 -name "VPN Client"

Native Client Support#

• Windows 10/11 -- Import the .p12 file and CA certificate, then create an IKEv2 VPN connection
• macOS/iOS -- Import certificates via a .mobileconfig profile or manually in Keychain Access
• Android -- Import the .p12 file in Settings > Security, configure IKEv2/IPsec VPN
• Linux -- Use strongSwan or NetworkManager with the strongswan plugin

Security Notes#

strongSwan implements IKEv2/IPsec, not TLS. The cryptographic considerations are similar but the attack surface differs:

• POODLE / BEAST / FREAK / LOGJAM: Not applicable. These attacks exploit flaws in SSL/TLS protocol versions. IKEv2 uses its own proposal negotiation and algorithm selection, independent of SSL/TLS.
• Forward secrecy: Guaranteed by using ECDHE (x25519, ecp384) in the IKE proposals. Each session negotiates a unique Diffie-Hellman exchange.
• Weak algorithms excluded: The recommended proposals exclude DES, 3DES, MD5, and short-key RSA. Only AES-GCM (authenticated encryption) and SHA-384/SHA-256 are recommended.
• Sweet32 (CVE-2016-2183, 2016): 3DES/64-bit block ciphers are excluded from all IKE and ESP proposals.
• Downgrade attacks: IKEv2 proposal negotiation includes a cookie mechanism and strong proposal selection; the recommended configuration avoids legacy algorithms that could be used in a downgrade scenario.
• Replay attacks: IKEv2 includes anti-replay protection in the ESP data channel via sequence numbers.

The following are not applicable to IKEv2/IPsec:

• Heartbleed (CVE-2014-0160, 2014): Not applicable to IKEv2/IPsec. strongSwan does not use OpenSSL for the IKEv2 key exchange itself. If you expose an HTTPS management interface on the same host, ensure the web server's OpenSSL is patched.
• BREACH / CRIME (2012-2013): Not applicable. These target HTTP-level and TLS-level compression, respectively; IPsec tunnels do not use either.
• DROWN (CVE-2016-0800, 2016): Not applicable. strongSwan does not use SSLv2.

Verification#

Load the configuration and start the connection:

swanctl --load-all
swanctl --list-conns

Check active security associations:

swanctl --list-sas

View the negotiated algorithms:

swanctl --list-sas --raw

Check strongSwan logs for IKE negotiation details:

grep -i ike /var/log/syslog

Verify the connection from a client:

swanctl --initiate --child net
swanctl --list-sas