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VoIP network with ocserv

Author: Nikos Mavrogiannopoulos

In this scenario we describe a VPN server which is setup to provide access to an existing VoIP network, i.e., to a SIP server (e.g., asterisk or freeswitch). We will not get into details of setting up the SIP server; we assume it is there and it works. The VPN server will allow remote users to access the SIP server in a secure and efficient way for VoIP telephony. We assume the following setup.
       |SIP Server|
       |VPN Server|
       /            \
      /              \
 ----------------    ---------------
 |OpenWRT router|    |VPN/SIP Phone|
 ----------------    ---------------
 |SIP Phone|


The following instructions require at least ocserv 0.11.15. Two setups will be presented. The first will use OpenWRT as an intermediate to access the SIP server over VPN, while the second will use SIP phones which include support for OpenConnect.

For the first setup, the hardware that is needed is:

While the latter setup requires:

  • CISCO SPA525G or SPA525G2 (these models include an OpenConnect client)
  • Cisco 7800, 8800, 8900 and 9900 series of IP-Phones


VPN server

Nothing particularly interesting is needed on the server setup. The server only needs to provide access to the SIP server. For authentication, it is recommended to allow password authentication (either via radius, PAM or plain), since it will simplify the setup of the clients described in the next sections.

split-dns =
ipv4-network =

route =

The 'ipv4-network' entry contains the addresses that the VPN clients will be assigned to. The 'route' entry adds the routes that are directly routed by the VPN server; in our case it must route to the SIP server.

For certain newer CISCO Enterprise IP phones, you will need to enable a special compatibility mode in ocserv's configuration as follows.

cisco-svc-client-compat = true

Some hints for the rest of configuration options which relate to VoIP, follow. It is recommended to set compression = false since attempting to compress will increase latency which is undesirable for VoIP. If compression is required, utilize the no-compress-limit option and adjust it to the average size of RTP packets across the network. The default value of 256-bytes should be sufficient for common codecs.

The default value of output-buffer in ocserv is adjusted for an average performance in VoIP and throughput (downloading). If you experience latency issues, you may want to experiment with different values. The lower the value of output-buffer, the lower the latency, as well as throughput.

The hardware of the server is also important for latency. If the server provides hardware accelerated AES (e.g., x86-64 with AES-NI). It is best to ensure that the version of gnutls used in the server includes acceleration for the particular hardware. A way to check is the following.

$ GNUTLS_DEBUG_LEVEL=4 gnutls-cli -v

The output will be something like:

gnutls[2]: Enabled GnuTLS 3.5.5 logging...
gnutls[2]: getrandom random generator was detected
gnutls[2]: Intel SSSE3 was detected
gnutls[2]: Intel AES accelerator was detected
gnutls[2]: Intel GCM accelerator (AVX) was detected

VPN/SIP Phone setup

Setting up VPN on the phone

The CISCO SPA525G series of phones can only be configured with a fixed username-password pair, and as such the server must be configured to allow plain username and password combinations.

To set it up, on the phone menu (after clicking the button that looks like a 'file') select: Network configuration -> VPN

Then on the VPN Server field set the VPN server's external name (e.g.,, and then set the User name and Password fields. For servers that are available in alternative to 443 ports, it may be possible to set the server name as, though I have not test this setup.

Then select Enable Connection and if that works check Connect on bootup.

Setting up SIP on the phone

Once the VPN connection is established on the phone you should setup the SIP accounts. These are available through the web (administrator) interface. As this information depends on the SIP server we skip that step.

Setting up Cisco Enterprise IP-Phones

Some additional configuration may be necessary. See the README in ocserv source code.

OpenWRT Router setup

Setting up OpenWRT

There are two ways to setup OpenWRT as an openconnect VPN client. Via the luci-proto-openconnect package or manually via the the openconnect package. In this section we describe the former method via the web interface. For the manual configuration see the openconnect package documentation.

We assume basic knowledge of OpenWRT and accessing its web interface.

After installing the luci-proto-openconnect package, go to the web interface at the Network -> Firewall menu. Click on the 'Add new zone' option, name it something related to VPN and enable forwarding to and from the LAN zone.

Then save and switch to the Network -> Interfaces menu. At this page click the 'Add new interface' option and you will be redirected to a new page. There you must name the interface (use something related to VPN), and on the Protocol field select the 'OpenConnect (CISCO Anyconnect compatible)' option.

After submitting you will be asked to set the VPN server name (, port, which typically is 443, and username and password of the VPN user. You must also fill the 'VPN Server's certificate SHA1 hash' option. This can be done the following way. One your Linux PC type the following commands.

$ /usr/sbin/openconnect
Certificate from VPN server "" failed verification.
Reason: signer not found
Enter 'yes' to accept, 'no' to abort; anything else to view: 

Type enter and you will see more information about the server's certificate. What you need is the server's key hash which is displayed on the last lines.

Server key hash: sha1:6c4a751c55b59756fbcdb0de0a27598b0b34be48

Then copy paste the sha1:6c4a751c55b59756fbcdb0de0a27598b0b34be48 to the 'VPN Server's certificate SHA1 hash' field. If you are an administrator making instructions for users to setup their routers, a more secure alternative is to use the command certtool --key-id --infile server-cert.pem on the server certificate, and provide the output value (with 'sha1:' prepended to it), to the users setting OpenWRT up.

After entering all the information above on the 'General Setup' tab click on the firewall settings, select the zone previously created, and click 'Save and Apply'.

If everything was entered correctly, you should be able to ping the SIP server's internal IP, i.e., ping or ping should work.

Setting up the SIP phone

Once the VPN connection is established on the OpenWRT router any SIP phone connected behind the OpenWRT router should be able to connect to the SIP server on as if it was on the same LAN.