load-tester Plugin


The a load-tester plugin for libcharon does stability testing and performance optimizations of the charon daemon. This plugin allows to set up thousands of tunnels concurrently against the daemon itself or a remote host.

The plugin is disabled by default and can be enabled with the ./configure option

Never enable the load-tester plugin on productive systems. It provides preconfigured credentials and allows an attacker to authenticate as any user.

To make sure you are aware of this risk, an additional enable switch in strongswan.conf is required to load the plugin.


The load-tester plugin is configured using the following options in the charon.plugins.load-tester section of strongswan.conf:

Key Default Description


Subsection that contains key/value pairs with address pools (in CIDR notation) to use for a specific network interface e.g. eth0 =



Whether to keep dynamic addresses installed even after the associated SA got terminated



Network prefix length to use when installing dynamic addresses. If set to -1 the full address is used (i.e. 32 or 128)


Directory to load (intermediate) CA certificates from



Seconds to start CHILD_SA rekeying after setup


URI to a CRL to include as certificate distribution point in generated certificates



Delay between initiatons for each thread



Delete an IKE_SA as soon as it has been established



Digest algorithm used when issuing certificates



DPD delay to use in load test



Base port to be used for requests (each client uses a different port)



EAP secret to use in load test. [default-pwd]



Enable the load testing plugin



CHILD_SA proposal to use for load tests. [aes128-sha1]



Fake the kernel interface to allow load-testing against self



Seconds to start IKE_SA rekeying after setup



Global limit of concurrently established SAs during load test



Address to initiate from. []



Authentication method(s) the intiator uses. [pubkey]


Initiator ID used in load test


Initiator ID to match against as responder


Traffic selector on initiator side, as proposed by initiator


Traffic selector on responder side, as proposed by initiator



Number of concurrent initiator threads to use in load test


Path to the issuer certificate (if not configured a hard-coded default value is used)


Path to private key that is used to issue certificates (if not configured a hard-coded default value is used)



Number of IKE_SAs to initiate by each initiator in load test



IPsec mode to use, one of tunnel, transport, or beet. [tunnel]


Provide virtual IPs from a named pool



Preshared key to use in load test. [<default-psk>]



IKE proposal to use in load test. [aes128-sha1-modp768]



Request an INTERNAL_IPV4_ADDR and INTERNAL_IPV6_ADDR (since version 5.9.1) from the server



Address to initiation connections to. []



Authentication method(s) the responder uses


Responder ID used in load test



Traffic selector on initiator side, as narrowed by responder. [initiator_tsi]



Traffic selector on responder side, as narrowed by responder. [initiator_tsr]



Shutdown the daemon after all IKE_SAs have been established



Socket provided by the load-tester plugin. [unix://${piddir}/charon.ldt]



IKE version to use (0 means use IKEv2 as initiator and accept any version as responder)

Testing against Self

In the simplest case, the the daemon initiates IKE_SAs against self using the loopback interface. This will actually establish the doubled number of IKE_SAs, as the daemon is initiator and responder for each IKE_SA at the same time. Installation of IPsec SAs would fail, as each SA gets installed twice. To simulate the correct behavior, a faked kernel interface can be enabled which does not install the IPsec SAs at the kernel level.

A simple loop-back configuration in strongswan.conf might look like this:

charon {
  # create a new IKE_SA for each CHILD_SA to simulate different clients
  reuse_ikesa = no
  # turn off denial of service protection
  dos_protection = no

  plugins {
    load-tester {
      # enable the plugin
      enable = yes
      # use 4 threads to initiate connections simultaneously
      initiators = 4
      # each thread initiates 1000 connections
      iterations = 1000
      # delay each initiation in each thread by 20ms
      delay = 20
      # fake the kernel interface to avoid SA conflicts
      fake_kernel = yes

This will initiate 4000 IKE_SAs within 20 seconds. You may increase the delay value if your box can not handle that much load, or decrease it to put more load on it. If the daemon starts retransmitting messages, your box probably can not handle all connection attempts.

Testing against Remote Host

The plugin also allows to test against a remote host. This might help to test against a real world configuration. A connection setup to do stress testing of a gateway might look like this:

charon {
  reuse_ikesa = no
  threads = 32

  plugins {
    load-tester {
      # enable the plugin
      enable = yes
      # 10000 connections, ten in parallel
      initiators = 10
      iterations = 1000
      # use a delay of 100ms, overall time is: iterations * delay = 100s
      delay = 100
      # address of the gateway (releases before 5.0.2 used the "remote" keyword!)
      responder =
      # IKE-proposal to use
      proposal = aes128-sha1-modp1024
      # use faster PSK authentication instead of 1024bit RSA
      initiator_auth = psk
      responder_auth = psk
      # request a virtual IP using configuration payloads
      request_virtual_ip = yes
      # disable IKE_SA rekeying (default)
      ike_rekey = 0
      # enable CHILD_SA every 60s
      child_rekey = 60
      # do not delete the IKE_SA after it has been established (default)
      delete_after_established = no
      # do not shut down the daemon if all IKE_SAs established
      shutdown_when_complete = no

Configuration Details

For public key authentication, the responder uses the

"CN=srv, OU=load-test, O=strongSwan"

identity. The initiator, each connection attempt uses a different identity in the form

"CN=c1-r1, OU=load-test, O=strongSwan"

where the first number inidicates the client number, the second the authentication round (if multiple authentication is used).

For PSK authentication, FQDN identities are used. The server uses srv.strongswan.org, the client uses an identity in the form c1-r1.strongswan.org.

For EAP authentication, the client uses a NAI in the form 100000000010001@strongswan.org.

To configure multiple authentication, concatenate multiple methods using, e.g.

initiator_auth = pubkey|psk|eap-md5|eap-aka

The responder uses a hardcoded certificate based on a 1024-bit RSA key (see src/libcharon/plugins/load_tester/load_tester_creds.c). This certificate additionally serves as CA certificate. A peer uses the same private key, but generates client certificates on demand signed by the CA certificate. Install the Responder/CA certificate on the remote host to authenticate all clients.

To speed up testing, the load tester plugin implements a special Diffie-Hellman implementation called modpnull. By setting

proposal = aes128-sha1-modpnull

this wickedly fast DH implementation is used. It does not provide any security at all, but allows to run tests without DH calculation overhead.

Custom Credentials

load-tester can use a custom set of certificates for authentication. Certificates are still issued on demand and the issuer_key and isser_cert options define the path to load the CA certificate and private key to load for issuing certificates. To load additional certificates to verify the trust chain (above issuer_cert), the ca_dir option takes a directory to load trusted certificates from.

By default, the on-demand generated peer certificates are issued with RSA signatures using SHA1. To use a different hashing algorithm, the digest option can be used, which accepts hash algorithms, such as md5, sha1, sha256 or sha512.

To use other peer identities than those hard-coded, the initiator_id and responder_id options can be used. These options can contain up to two %d printf specifiers to replace. The first one is replaced by a unique number for each tunnel established, starting from 1. The second is replaced by the authentication round for this connection, starting at 1. Take care to use %d identifiers only and just two of them, as the format string is not validated. To include % characters in an identity, prefix it with an additional % as you would do it in a printf format string. As a responder, a specific initiator_id wouldn’t match to a custom initiator_id configured on the client. Hence the initiator_match option is introduced which can be defined to an identity with wildcards that should match to all identities the initiator generates from the initiator_id template.

The identities generated from the configured templates are included in the on-demand issued certificates. Distinguished Name identities are encoded as subject. FQDN, email, IPv4/v6 identities are encoded as subjectAltName. If a subjectAltName gets encoded, the subject Distinguished Name of the certificate is a single Common Name Relative Distinguished Name equal to the subjectAltName.

An example of a load-test to itself with custom credentials could look like this:

charon {
  # ...
  plugins {
    load-tester {
      enable = yes
      initiators = 1
      iterations = 100
      fake_kernel = yes

      # initiator authenticates twice with on-demand
      # generated certificates. Responder authenticates
      # once only.
      initiator_auth = pubkey|pubkey
      initiator_id = conn-%d-round-%d@strongswan.org
      initiator_match = *@strongswan.org
      responder_id = srv.strongswan.org
      digest = sha256
      issuer_cert = /path/to/ca.crt
      issuer_key = /path/to/ca.key
      ca_dir = /path/to/trustchain/certs

Traffic Selectors

Custom traffic selectors can be defined. The initiator_tsi,initiator_tsr, responder_tsi and responder_tsr define a traffic selector to propose as initiator or narrow down as responder. Only a single CIDR style subnet definition is allowed, protocol/port selectors are currently not supported.

On-Demand Installed External Addresses

To generate load against a responder that simulates more closely the real world, load-tester can use a unique external tunnel address for each established IKE SA. The IPs are installed on-demand just before tunnel establishment on one or more interfaces. The addrs sections takes key/value pairs of address pools to use, the addrs_prefix defines the network prefix how the address is installed on the interface. An example could look like this:

charon {
  # ...
  plugins {
    load-tester {
      # ...
      addrs {
        # install ~32K addresses on eth0
        eth0 =
        # and then ~32K addresses on eth1
        eth1 =
      # install all addresses with a /16 prefix
      addrs_prefix = 16
Make sure to shut down unused network services when installing hundreds of addresses. Some services don’t scale very well to this many addresses, the Avahi daemon for example can significantly slow down the system when running such tests.

Triggered Batch Initiations

Tunnels can not only be initiated on daemon startup with the initiators/ iterations keywords, but also while the daemon is running. The load-tester tool takes as first argument the number of tunnels, and as second argument the delay between each initiation. Multiple clients can trigger initiation at the same time.

The load-tester tool prints the status of the initiation with some simple characters to the console:

. : Initiation of a single tunnel started
! : Initiation of a tunnel failed
+ : A tunnel has been established successfully
- : A tunnel failed to establish during the exchange
* : A message has been retransmitted

A session might look like this:

> ipsec load-tester initiate 100 2

100 tunnels have been initiated and established successfully, one packet has been lost and was retransmitted. If many packets get retransmitted, this can be an indication that one of the systems is overloaded.