DNS : Quick Definition

DNS is the tool which associate an IP Address with a name, an URL. So using name instead IP Address can also introduce failures because if your DNS down , you can’t resolve anymore IP addresses. But it can be very helpful in the case of a DR mode ( where you need to point the server to a different IP address)

Network Address Translation – NAT

To work correctly with your voice flows, a NAT device must inspect Layer3 and Layer4 protocol ,port information and sometimes the embedded payload.

Cisco IOS NAT ALG can support the following voip protocols :

  • H323v2
  • H225
  • H245
  • RAS
  • SIP
  • SCCP

So if you change the default port number of these protocols, then you have to change it also in the configuration with the following command ip nat service <<protocol>> tcp port <<port-number>>.

Pay attention also that NAT doesn’t support voice signaling encryption

Firewalls and Application Layer Gateway – ALG

Firewalls are commonly used to protect networks, including those that transport voice traffic. Firewalls inspect the header and sometimes can inspect also the payload of packets. Stateful firewalls also maintain state information so that they know which traffic forms part of permitted flows and should be allowed.

For voice traffic, mandatory traffic are required as MGCP,H323,SIP,SCCP,RTP and RTCP. In case of RTP and RTCP, the amount of ports must be larger as it represents thousands ports.

Be also cautious where you put your VPN gateway because if the VPN gateway is placed in front of a firewall, the VPN IPSec will be added or removed so then you can inspect your traffic. But in the case where your VPN gateway is behind the firewall, this one can’t inspect anymore the traffic so it could be a backdoor for intruder.Same thing for the placement of NAT in your firewall.

Note also that ALG doesn’t work with signaling encryption

So pay attention that by default, Cisco firewalls/ALG don’t support in native Unity, IPCC Express, IPCC Enterprise, Attendant Console and SCCP video. It is then require to configure manually your firewall manually to authorize the traffic for these ports.

TCP/UDP Port List

Here is as a reminder a collection of TCP/ UDP Ports used in an IP Telephony network:

PORT PURPOSE
TCP/1433 SQL
TCP/2552 Call Manager database change notifications
TCP/2555 RIS database server
TCP/2556 RIS database client
UDP/3000 Receive change notification from Call Manager database
UDP/3001 Database change notification from publisher to applications
UDP/3020 Dialed Number Analyzer plug-in database
TCP/3372 SQL Distributed Transaction Coordinator
TCP/7727 Application database change notification, CTI, voicemail, and so on
TCP/8001 Client database change notification
TCP/8002 Intracluster Communication Service
TCP/8003 Intracluster Communication Service
TCP/8009 Internal Tomcat requests
TCP/8111 IPMA Web requests
TCP/8222 Extension Mobility web requests
TCP/8333 Webdialer web requests
TCP/8444 Extension Mobility service requests
TCP/8555 Apache-SOAP web requests
TCP/8666 IPMA web requests for nondefault locales
TCP/8777 Tomcat manager web requests
TCP/9007 CDR Aalysis and Recording web requests
TCP/102 Directory Access Protocol (DAP) for DC Directory
TCP/8404 Local Directory Access Protocol (LDAP) for DC Directory
TCP/8405 LDAPS for DC Directory
TCP/389 LDAP query to external directory (AD, Netscape)
TCP/636 LDAPS query to external directory (AD, Netscape)
TCP/2000 SCCP
TCP/2443 Secure SCCP (SCCPS)
TCP/3804 CAPF
UDP/16384 – 32768 RTP,SRTP
TCP/4224 Cisco VT Advantage
UDP/1718 Gatekeeper H225 Discovery
UDP/1719 Gatekeeper H225 RAS
TCP/1720 H225 signaling services for H323 gateways and Inter-Cluster Trunk (ICT)
UDP/2427 MGCP gateway control
TCP/2428 MGCP gateway backhaul
TCP/UDP/5060 SIP Gateway and Inter-Cluster Trunk
TCP/2444 CTL Provider
TCP/2748 CTI Application Server
TCP/2789 JTAPI Application Server
TCP/2912 IPMA Server
TCP/1099-1129 Attendant Console
UDP/3223 Attendant Console
UDP/4321 Attendant Console

Protecting Voice Media and Signaling Traffic

There is two types of security that you can enable with Call Manager

  • Mixed mode : In this mode, depending the security configured on each phone, you can have secure calls when both devices are security-enabled and when one of the phones is missing security, your call will be nonsecure.
  • Nonsecure mode : As all phones are not set up with security (default configuration), all calls are nonsecure.

When you device to put security on phones , they can support the three following levels:

  • Nonsecure mode : secure calls are not supported
  • Authenticated mode : the phone will be able to authenticate calls
  • Encrypted mode : the phone will be able to support encrypted calls

If you enable the authentication and the encrytion on your network , you are then able to secure the media traffic as well the voice signaling.

If you want to have security on the media flow, it is then mandatory to secure also the signaling as the keys which are used to secure the media traffic are exchanged during the signaling phase.

SCCP messages sent by IP Phones and Call Manager can be secured using TLS, it is the signaling part. Then for the protection of the media traffic so the RTP packets , you will use the Secure RTP which is providing a framework for encryption and authentications of your stream.

SRTP will be also use between your MGCP gateway and your IP Phone but you need to know that your SRTP keys are exchanged in cleartext session between the MGCP gateway and the Call Manager.

Managing Certificates in an IPT network

Typically , in a PKI, there is a singme Certificate Authority (CA) or hierarchy of CA to issue certificates. However, several other elements can play this role too:

  • Self-signed certificates : These are certificates self-signed by the Call Manager,TFTP and the Certificate Authority Proxy Function (CAPF)
  • Certificates signed by the CAPF or external CA : These are issued as Locally Significant Certificates (LSC) to IP phones
  • Certificates signed by the Cisco CA : Some IP phones models are shipped with manufacturing installed certificates (MIC)

All these certificates types are necessary to carry out functions such as authentication and encryption of voice signaling and media traffic, authentication of images and authentication of configurations files.

To facilitate the distribution of the certificates to IP Phones, you can create a Cisco Trust List (CTL) which is created by a plugin that you install on a Windows server or a desktop. This CTL client will collapse all informations of trusted certificates entities to be issue in a file.This file will be signed by the Cisco Site Administrator Security Token (SAST). Then once the phone will initiate their boot-up sequence , they will be able to use this CTL list from the TFTP to validate server certificates and security tokens. This will also enabled secure communications and file authentications. Here are what you can find as elements in your CTL file:

  • Call Manager or TFTP
  • Co-resident Call Manager and TFTP
  • CAPF
  • Alternate TFTP server
  • SAST

So the informations that you will find for these entries will be

  • PKI
  • Server function
  • IP Address

As we have already mentioned it , don’t forget that new IP Phones models come with an existing certificates , it is installed at the factory , this is called MIC. So even if these certificates are good, it is recommended to replace them by LSC so that you ensure a global solution for all your phones and not a mixed solution. LSC can be issued by the CAPF or an external CA with a transiting CAPF as it will act as a proxy when phones enroll.

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