IP Phone SSL VPN to ASA using AnyConnect

[Jason Burns]

• Purpose
• Functional Overview
• CUCM Places ASA Certificate Hash and VPN URL in Phone Config
• Phone Connects to ASA
• Software Versions
• Phone Models
• CUCM Configuration
• Phone Configuration
• ASA Configuration
• Additional Requirements:
• Troubleshooting Steps
• Common Issues

Purpose

Starting in CUCM 8.0.1 and IP Phone Firmware 9.X, IP Phones are now able to directly connect to an ASA using the AnyConnect VPN. This  document will help address some common issues encountered during intial configuration. This guide will act as a supplement to the Official IP Phone VPN Documentation.

Functional Overview

Before we get into versions and model numbers let's look at how the feature works.

CUCM Places ASA Certificate Hash and VPN URL in Phone Config

Before the phone is ready for VPN, it must first be provisioned using the internal network. This requires direct access to the CUCM TFTP server.

The first step after the ASA is fully configured is to take the ASA HTTPS Certificate and upload it to the CUCM server. This allows the CUCM server to build an IP phone config file that tells the phone how to get to the ASA. The CUCM requires some additional configuration to associate the uploaded certificate with a VPN Profile that can be assigned to the phone.

Here is an example of the IP Phone VPN section of a phone's config file after performing the required configuration:

jasburns@jasburns-gentoo /home/jasburns $ tftp 14.48.44.80
tftp> get SEP0011215A1AE3.cnf.xml.sgn
Received XXXX bytes in 0.0 seconds
jasburns@jasburns-gentoo /home/jasburns $ cat SEP0011215A1AE3.cnf.xml.sgn
..........
<vpnGroup>
[Some Lines Omitted]
<addresses>
<url1>https://X.X.X.X/PhoneVPN</url1>

</addresses>

<credentials>

<hashAlg>0</hashAlg>

<certHash1>1eD9l3VEI9DGWQGKlNBGE1bRhUg=</certHash1>

</credentials>

</vpnGroup>

Note that the URL is printed exactly as entered on the VPN Gateway Configuration page in CUCM. Make sure the IP Phone can resolve this address.

Even more interesting is the Cert Hash. The IP phone configuration does not contain the entire certificate, merely a SHA1 Base64 encoded hash of the certificate.

You can compare the certificate hash in the IP phone configuration file to the cert hash of the actual file on the ASA or CUCM if you copy it to a computer running OpenSSL (either Windows, Linux, or Mac)

$ cat r2800.cisco.com.pem

-----BEGIN CERTIFICATE-----

<Base64 value of the cert omitted>

-----END CERTIFICATE-----

openssl x509 -in r2800.cisco.com.pem -noout -fingerprint

SHA1 Fingerprint=D5:E0:FD:97:75:44:23:D0:C6:59:01:8A:94:D0:46:13:56:D1:85:48

This is the SHA1 Fingerprint in Hexadecimal form. In the configuration file this value is instead printed as the Base64 value. I used the following website to convert from Hex to Base64:

http://tomeko.net/online_tools/hex_to_base64.php

This method can be used to verify the certificate loaded onto and presented by the ASA matches the certificate hash loaded into the phone.

Phone Downloads Configuration

This part is extremely important. The phone must download the configuration (including the certificate hash in Base64) while it is inside the network and has direct access to the CUCM TFTP server.

The phone has to be provisioned inside the network before it can be moved outside the network and use the VPN feature.

Phone Connects to ASA

After internal provisioning has been completed, the phone can be moved to the external network for VPN access. Here the Corporate Phone has been moved to a Home location.

Depending on the phone's configuration it will either automatically attempt to connect to the VPN gateway, or will connect once manually initiated. If auto network detect is enabled, the phone will try to ping the TFTP server. If there is no response to this ping request the phone will automatically bring up the VPN process on the phone.

The phone connects on TCP port 443 over HTTPS to the ASA. The ASA responds back with the configured certificate, hopefully the same certificate uploaded to CUCM. In additional TCP 443 (Transport Layer Security, or TLS), the phone will also connect on UDP 443 for DTLS (Datagram Transport Layer Security).

Phone Verifies Presented Certificate

The phone console logs show us the hash of the certificate that the ASA presents in Hex form:

3943: INF 18:10:22.354209 VPNC: vpnc_save_to_file: wrote: </tmp/leaf.crt>, 479 bytes

3944: NOT 18:10:22.355351 VPNC: cert_vfy_cb: peer cert saved: /tmp/leaf.crt

3945: NOT 18:10:22.361892 SECD: Leaf cert hash = D5E0FD97754423D0C659018A94D0461356D18548

3946: NOT 18:10:22.362574 SECD: Hash was found in the trust list

3947: NOT 18:10:22.400294 VPNC: VPN cert chain trusted

These messages show us that the phone was able to validate the certificate that the ASA presented. The cert presented matched the hash in the configuration file.

At this point the phone will establish an SSL session with the ASA and continue setting up the VPN tunnel.

All communication will now flow between the phone and the ASA in an encrypted tunnel. Once the traffic reaches the ASA it will be decrypted and forwarded along to any location in the network that the phone would like to connect to.

The beauty of this solution is that the phone obtains an address on the Internal network that is typically not filtered. The phone can connect using SCCP, SIP, HTTP, HTTPS to any server inside the Corporate Network. This allows advanced phone services and features to function that might not work through ASA Phone Proxy.

Software Versions

CUCM >= 8.0.1.100000-4

IP Phone >= 9.0(2)SR1S - SCCP

ASA >= 8.0.4

Anyconnect VPN Pkg >= 2.4.1012

Note: A "Premium" license and an "AnyConnect for Cisco VPN Phone" license is required.  The part number for the "AnyConnect for Cisco VPN Phone" is L-ASA-AC-PH-55XX= where XX = 05,10,20,40,50,80.

Phone Models

7942 / 7962 / 7945 / 7965 / 7975 / 8961 / 9951 / 9971. For a complete list of supported phones in your CUCM version go to:

https://<CUCM Server IP Address>:8443/cucreports/systemReports.do

Unified CM Phone Feature List

Generate a new report

Feature: Virtual Private Network

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/security/8_5_1/secugd/secuvpn.html

CUCM Configuration

The following document provides a complete set of configuration  tasks required to configure CUCM for this feature:

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/security/8_5_1/secugd/secuvpn.html

Note: Please make sure the URL for the VPN Gateway contains the full and correct address to reach the IP Phone VPN tunnel-group on the ASA.

Phone Configuration

1. Use a supported phone model per the CUCM Supported Models / Features report.
2. Register the phone to the CUCM server on the Internal network
3. Configure the IP phone with a TFTP server manually.
4. Move the phone to the external network.

ASA Configuration

Configure Anyconnect VPN access on ASA to provide network access.

See  http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a00808efbd2.shtml  for example configuration.

The lateset CUCM Security Guide also provides sample ASA configuration.

Additional Requirements:

1. The ASA must have the AnyConnect for Cisco VPN Phone Licensed feature enabled.  Licensing info can be found using show version command.
2. Group-policy must not be configured with split tunnel or split exclude.  Only tunnel all is the supported tunneling policy
3. The tunnel-group used can not be the DefaultWEBVPNGroup.   Create another tunnel-group and use "group-url https://x.x.x.x/phonevpn enable to map to the correct tunnel-group.
4. DTLS must be enabled and negotiated for operation.  This  requires both tcp/443 and udp/443 to be open and allowed on all devices between the ASA and the phone.

 

Troubleshooting Steps

1. Plug the phone into the internal network.  This will test whether the phone's configuration works prior to adding VPN.
2. Connect with AnyConnect on a PC from the outside to the ASA.  This will confirm that the ASA is configured correctly for Anyconnect
3. From the connected PC try to ping the TFTP server and CUCM server.  This will test basic ip connectivity to the two servers.
4. From the PC try to download the TFTP config file for the phone in question "tftp -i <TFTP Server> GET  SEP<Mac Address>.cnf.xml"  This will test that the tftp service is reachable and serving files.
5. From the PC try to telnet to TCP Port 2000 on the CUCM server "telnet <CUCM IP> 2000". This should immediately come back with a new line and a blank cursor.  This will test connectivity to the CUCM SCCP port, for SIP registrations use port 5060 instead.

 

Common Issues

1. One-way or no voice.  The phone registers and makes calls but no audio is heard.  Confirm routing between the two phone/rtp stream endpoints.
2. Auto Network Detect does not reliably work in IP Phone Firmware 9.0(2), but does work as expected in 9.2(1).
   1. Auto Network Detect allows the phone to detect whether it is inside or outside the network. If outside it will bring up the VPN, if inside, it will connect directly.
   2. The phone uses a series of pings to the TFTP server to determine whether it is outside the network. If pings to the TFTP server fail, the VPN GUI will be brought up on the phone and the phone will attempt to access the VPN URL.
3. Username and Password authentication from the phone does NOT support the SPACE character in either the username or the password.

[Hythim Ali El Hadad]

Hi,

Thanks alot for this detailed mail

Only I need to confirm something,,

With CUCM 8, Do I need only premium license to have ip phone vpn feature on ASA, or also I need this license L-ASA-AC-PH-55XX= beside the premium

That I hear that with CUCM 8 we need only the premium license [Any connect include it] , and this license L-ASA-AC-PH-55XX= may be needed only with CUCME8 ?

If, I'll need this license L-ASA-AC-PH-55XX=

Will I order it per users or per session or per device ??

Thanks a lot and waiting for your reply

Best Regards

[Jay Young]

You need both the Premium license and the Phone license applied to the ASA.  The Premium license is a number of concurrent sessions license whereas the Phone license is a enable/disable feature license.

You can order the licenses separately but whether or not they are included with the purchase of CUCM 8 I do not know.  Perhaps you can contact your local sales rep to see if they can perform a bundle.

[Hythim Ali El Hadad]

Dear Jay,

Thank you very much for your concern and fast reply

Only one other question:

Is this required license L-ASA-AC-PH-55XX= is not included in this license: ASA-ANYCONN-CSD-K9

ASA-ANYCONN-CSD-K9 : is AnyConnect  client license and I read that there is a new license for ASA called AnyConnect and it include most of the needed features ?

Kindly correct my sentense above that I'm a voice engineer and have low level expertise in Security and tell me what is the AnyConnect license exactly

Thanks for your help and patience

Best Regards

[Jay Young]

The ASA-ANYCONN-CSD-K9 only allows anyconnect and csd.  You will want to order the both the premium license and the L-ASA-AC-PH-55XX=

[Hythim Ali El Hadad]

Hi Jay

Thanks a lot for your follow up

[calmichael]

Since the application is for a phone versus a workstation, is there any settings that should be tweaked?  e.g. Keepalive timers, dead peer detection timers, avoiding AES-256 encryption, etc

Sample:

ssl encryption aes128-sha1

group-policy <phone/UC> attributes

banner none

dns-server value <DNS1> <DNS2>

vpn-idle-timeout none

vpn-session-timeout none

vpn-tunnel-protocol svc

default-domain value <company>.com

address-pools value <DHCP/RAS POOL>

webvpn

  svc dtls enable

  svc keep-installer installed

  svc keepalive 120

  svc rekey time 4

  svc rekey method new-tunnel

  svc dpd-interval client none

  svc dpd-interval gateway 300

  svc compression deflate

Thanks

[Stoyan Stoitsev]

Hi,

I have a couple of questions:

If I'm using user and password authentication do I need to get the ASA cert to the phones?

After installing the ASA certificate in CUCM and applying it to the phone will I see it in the phone CTL?

If I am using certificates for authentication which one gets picked - MIC or LSC?

BR,

Stoyan

[Joseph Martini]

Even with username nad password authentication you do need the ASA certificate uploaded to call manager as a phone-vpn-trust, so that the phone can "get" the certificate from call manager and has it to verify the SSL handshake when connecting to the ASA.  There is no way currently to see the certificate on the phone (http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCtk00661) like you can with the CTL.  Either the MIC or LSC when using certificates for authentication too.

[calmichael]

Just a follow-up.  ASA code 8.2(4) was providing the unexpected result of random midcall failures using the configuration above.  ASA code 8.3(2)4 does seem to have corrected the issue.

[calmichael]

What happens when we need to replace the ASA's certificate?  e..g. The certificate on the ASA is revoked or ages out and simply needs to be replaced.  If I follow the procedure and "replace" the old certificate, won't my remote phones who were brought up using the legacy certifictate no longer be able to VPN in?

[Joseph Martini]

If you have to replace your ASA certificate if possible put the ASA certificate on call manager before applying it to the ASA, so that CUCM has a new phone-vpn-trust cert, apply that new certificate to the VPN gateway profile and then reset the phone, that way the phone gets the new certificate while still being connected to the VPN.  Alternatively, if the ASA cert expires the phone will not be able to connect to the VPN, what must be done in that case is an external TFTP has to be setup that has the phone's new config file with the new certificate that can be downloaded from call manager and then the phone has to be pointed to the external TFTP.  Otherwise you're correct the phone would have to be brought back inside the internal network again.

[fgasimzade]

Thank you for this usefull topic!

Got one question

What if I dont have this feature mentioned in my sh run

AnyConnect for Cisco VPN Phone : Disabled 

[Jay Young]

That feature won't be mentioned if you are running an older version of the ASA code.  You will need to upgrade to the minimum version as referenced about.

[madan.kumar]

Thank you for the Wonderfull Post.

I have congigured the CUCM/ASA as per the guidence given here and provioned the 7945 phone in the Inside network.

Now am trying to connect the phone from Public  network its prompts me for VPN username/password as soon as i give the creadentials it gives me "Authentication failed" and there is no logs in the orresponding time in ASA(Its not hitting the ASA?).I have tested by connecting PC to SSL VPN and accessing CUCM it works fine.

Any suggestiones here is very much appriciated

Thanks in Advance

[Joseph Martini]

If it's not hitting the ASA I would look at the phone console logs which can be accessed from putting in the phone's IP address into a browser.  Just to check something basic, is the URL configured as a hostname or IP address?  If it's a hostname does the phone have a DNS server that can resolve it?