Table Of Contents
Best Practices Guide
Cisco CallManager Express Security
What Is Cisco CallManager Express?
The Cisco® CallManager Express is an optional software feature in Cisco IOS® Software that enables Cisco routers to deliver Key System or Hybrid PBX functionality for Enterprise Branch Offices or Small Businesses. Cisco CallManager Express is ideal for customers for data connectivity requirements that also have a need for a telephony solution. Whether offered through a service provider's managed Services offering, or purchased directly by a corporation, Cisco CallManager Express offers many of the core telephony features required in the small office as well as many advanced features not available on traditional telephony solutions. Being able to deliver IP Telephony and data routing on a single converged solutions allows customers to optimize their operations and maintenance costs, resulting in a very cost-effective solution to meet their office needs.
Cisco CallManager Express delivers integrated IP Communications and call processing solutions in Cisco IOS Software on the Cisco Voice Gateway Routers. As a result, the same security best practices that are recommended for the Cisco IOS Software gateways are applicable to Cisco CallManager Express. In addition, the following Cisco CallManager Express specific security practices can be implemented to provide additional security protections.
Cisco CallManager Express on Cisco IOS Software Router or Voice Gateway
Cisco CallManager Express, a software feature added to Cisco IOS Software, delivers integrated IP Communications and call processing solutions in Cisco IOS Software on the Cisco Voice Gateway Routers.
Cisco CallManager Express is vulnerable to attacks to the Cisco IOS Software Router and Voice Gateways. As a result, the same security best practices that are recommended for the Cisco IOS Software routers and gateways are applicable to Cisco CallManager Express. In addition, the following Cisco CallManager Express-specific security practices can be implemented to provide additional levels of security control and security protection.
It's not the scope of this document to cover Cisco IOS Software gateway VPN/IP Sec/3DES/AES feature set because it's generic to the voice gateways and is not specific to Cisco CallManager Express.
System Access Locally and Remotely
When in EXEC mode, config t, the telephony-service command takes a user into the Cisco CallManager Express configuration mode. The show running-configure and show telephony-service commands show all registered phones/users, extension numbers, and username/passwords for Cisco CallManager Express GUI access. The first step of security control is at the system access level. Password encryption, user authentication, and command auditing are very critical to prevent security breaches and holes.
Enable Secret and Encrypt Passwords
Enable password is cleartexted to provide access control to EXEC mode of the router. Use Enable Secret to encrypt the enable password:
The enable secret command takes precedence over the enable password command if both are configured; they cannot be used simultaneously.
To increase security access, passwords can be encrypted to prevent any unauthorized users from viewing the passwords when packets are examined by protocol analyzers:
Create Multiple Privilege Levels
By default, Cisco IOS Software has two levels of access to commands: EXEC mode (level 1) and privileged EXEC mode (level 15). Configuring up to 16 privilege levels (from 0, the most restricted level, to 15, the least restricted level) to protect the system from unauthorized access.
Restrict Access to vty
Allow only certain users/locations to Telnet to the router via vty by defining and applying an access list for permitting or denying remote Telnet sessions.
Using AAA to Secure Access
An authentication server can be used to validate user access to the system. The following commands allow an AAA server, TACACS+ server, to be used for authentication services.
Command Accounting/Auditing on AAA
The following commands use a TACACS+ server for command accounting and auditing purposes.
The sample command log shows the information contained in a TACACS+ command accounting record for privilege level 1.
Configure Local User Authentication When AAA Is Not Available
Always require login—even though the external AAA server is unreachable.
Configure SSH Access
Use the following command to generate RSA key pairs for the router.
By default the vty's transport is Telnet. The following command disabled Telnet and supports only SSH to the vty lines.
ACLs for SNMP Access
The community access string can be set up to permit access to the Simple Network Management Protocol (SNMP). The following example assigns the "changeme-rw" string to SNMP, allowing read-write access and specifies that IP access list 10 can use the community string:
Because "read" and "write" are two common community strings for read and write access, respectively, change the community strings to different ones.
Disable CDP Unless Needed
Because CDP automatically discovers the neighboring network devices supporting CDP, disable CDP in an untrusted domain so that Cisco CallManager Express routers won't show in the CDP table of other devices.
If CDP is needed, then consider disabling CDP on a per-interface basis.
Cisco CallManager Express Security for IP Telephony
IP Phone Registration Control
Cisco CallManager Express should be configured to allow IP phones in the trusted domain for registration. Assuming that the local segment is a trusted domain, use the strict-match option in the ip source-address command, so that only locally attached IP phones will be able to register to the Cisco CallManager Express router and get telephony services.
You'd want to block port 2000 access from the WAN side to prevent external SCCP phones from registering with Cisco CallManager Express. Use the following access-list to block port 2000 access from WAN interfaces
Note: Unknown phones or phones that are not configured in Cisco CallManager Express are allowed to register with Cisco CallManager Express by default for ease of management, but they don't get dial tones until you configure them by associating the buttons with ephone-dns or configuring auto assign dns under telephony service.
Cisco IP Phone Registration Monitory
Cisco CallManager Express 3.0 has added the following syslog messages to generate and display all registration/deregistration events:
The following message indicates that a phone has registered and is not part of the explicit router configuration—ephone configuration has not been created:%IPPHONE-6-REGISTER_NEW: ephone-3:SEP003094C38724 IP:188.8.131.52 Socket:1 DeviceType:Phone has registered.
Cisco CallManager Express allows unconfigured phones to register in order to make provisioning of the Cisco CallManager Express system more convenient. By default, phones designated as "new" are not assigned phone lines and cannot make calls.
You can use the following configuration to enable syslogging to a router's buffer/console or a syslog server:
Call Activity Monitoring—Call History Logging
The Cisco CallManager Express GUI provides call history table information so that a network administrator can monitor the call history information for unknown callers and use this information to disallow calling activities based on select calling patterns. The call history log should be configured to perform forensics and accounting and allow the administrator to track down fraudulent calling patterns.
HTTPS for Cisco CallManager Express GUI Management
HTTP over SSL (HTTPS) provides Secure Socket Layer (SSL) version 3.0 support for the HTTP 1.1 server and HTTP 1.1 client within Cisco IOS Software. SSL provides server authentication, encryption, and message integrity to allow secure HTTP communications. SSL also provides HTTP client authentication. This feature is supported in only Cisco IOS software images that support SSL. Specifically, SSL is supported in IPSec 56 and IPSec 3DES images (contains "k8" or "k9" in the image name) in Cisco IOS Software Release 12.2(15)T.
Currently IP phones do not serve as HTTPS clients. If HTTPS is enabled on the Cisco CallManager Express router, IP phones will still attempt to connect to port 80. Because the SSL default port is 443, the phones will not be able to display local directory and system speed-dials.
Use the following command to generate an RSA usage key pair with a length of 1024 bits or greater:
If you do not generate an RSA usage key pair manually, an RSA usage key pair with a length of 768 bits will be generated automatically when you connect to the HTTPS server for the first time. These automatically generated RSA keys are not saved to the startup configuration; therefore they will be lost when the device is rebooted unless you save the configuration manually.
You should obtain an X.509 digital certificate with digital signature capabilities for the device from a certification authority (CA). If you do not obtain a digital certificate in advance, the device creates a self-signed digital certificate to authenticate itself.
If you change the device hostname after obtaining a device digital certificate, HTTPS connections to the device fail because the hostname does not match the hostname specified in the digital certificate. Obtain a new device digital certificate using the new hostname to fix this problem.
The ip http secure-server command will prevent cleartext passwords across the wires when a Cisco CallManager Express administrator/customer administrator logs into the Cisco CallManager Express GUI. However, communication between the phone and the router will still stay unsecured. A signed digital signature is required in the phoneload and Cisco IOS Software for secure connection.
The following are the suggested best practices for using HTTP's interactive access to the Cisco CallManager Express router:
1. Use ip http access-class command to restrict IP packets connecting to Cisco CallManager Express.
2. Use ip http authentication with a central TACACS+ or RADIUS server for authentication purposes. Configuring authentication for the HTTP and HTTPS servers adds additional security to communication between clients and the HTTP and HTTPS servers on the device.
3. Do not use the same enable password as an HTTP/Cisco CallManager Express login password. (To prevent a regular user from gaining administrator's right.)
COR for Incoming/Outgoing Calls—To Prevent Toll Fraud
The configuration example below illustrates Class of Restriction (COR). There are two classes of service in the configuration: user and super-user along with various permissions allowed such as local calling, long distance calling, 911 access, and 411 access. In this example, "superuser" has access to everything and "user" has access to all resources with the exception of toll "1900", directory assistance "411", and international calling.
After-hours Blocking to Restrict Outgoing Calling Pattern—Toll Fraud
After-hours blocking can be added to restrict incoming calls after certain hours. After-hours blocking can also be used to restrict calls to numbers/area codes known as fraudulent calling patterns. The configuration example below is used to restrict calls to certain area codes:
Cisco CallManager Express with Firewall
Cisco CallManager Express with NAT
The Cisco CallManager Express router's LAN interface (Ethernet interface) is used as the source IP address that IP phones and the Cisco CallManager Express router communicate with. The IP addresses of the IP phones are internal addresses to the Cisco CallManager Express router and are in a different segment that is not visible by the external devices or callers. Other devices including Cisco gateways or gatekeeper use the Cisco CallManager Express router's IP address to communicate instead of directly communicating with the IP phones. The Cisco CallManager Express router translates IP addresses back and forth for the traffic to route to the IP phones or outside of the network area. Therefore, no NAT configuration is needed when talking to the IP phones locally attached to the Cisco CallManager Express router. However, in the case when IP phones need to talk to other devices outside the firewall of the Cisco CallManager Express network, NAT needs to be configured on the Cisco CallManager Express router.
Cisco CallManager Express with Cisco IOS Firewall
Overview of Cisco IOS Firewall with Cisco CallManager Express
The Cisco IOS Firewall, running on Cisco IOS Software routers, provides a network-based firewall solution with the functionality of CBAC (Context-based Access Control) or Protocol Inspection, IDS (Intrusion Detection System), Authentication Proxy, and URL Filtering. A firewall provides access control between internal and external networks. It identifies networks as "inside" (private) or "outside" (public) in which packets can get from the inside to the outside, be blocked by default from outside to inside, and packets associated with an inside-originated connection are allowed to pass in. Many firewalls work only if all outside traffic originates from well-known sockets and don't handle asymmetric traffic (i.e., UDP media). Cisco IOS Firewalls allow the packets/traffic to pass through based on their source and destination IP addresses and the configured firewall policy.
The Cisco CallManager Express is a software feature added to the Cisco IOS Software routers that provides call processing for IP phones using Skinny Client Control Protocol (SCCP) for branch/SMB, and managed SP environments. There will be cases in SMB/branch offices, where only one router will be deployed and will be required to provide Internet access, IP Telephony service, and Cisco IOS Firewall functions. Cisco CallManager Express requires that all IP phones attach to the Cisco CallManager Express router locally. Thus SCCP support on the Cisco IOS Firewall is needed for locally generated Skinny traffic.
Problems on Cisco CallManager Express with Cisco IOS Firewall
SCCP is a Cisco proprietary small version of H.323. H.323 traffic can be classified into call signaling, call control, and media communication. H.323 uses Q.931, H.225, and H.245 to set up, manage/control, and tear down calls. When running Cisco CallManager Express with H.323/SCCP protocols, we need to consider how signaling and media streams are affected by the Cisco IOS firewall.
An H.323 call requires a TCP connection for H.245 signaling that does not have a well-known port associated with it. The H.245 port is dynamically assigned. Because this port is not known ahead of time and can't be configured when defining firewall policy, the Cisco IOS Firewall will block the H.245 message and the call signaling procedure will fail. When NAT is used in the H.323 signaling path, inside IP address (behind NAT), not known to the rest of the world, will be used as "calling party" information element in the H.225 signaling stream, thus an incoming call (attempts to make an H.225 connection back to that address) will fail.
Media Streams (RTP streams)
RTP streams run on top of UDP and do not have any fixed ports associated with them. Each type of media stream has one or more channels with dynamically assigned source/destination/port numbers, which are not known ahead of time and cannot be preconfigured in the firewall policy. For the media stream to traverse the firewall, the firewall needs to open many UDP ports with source and destination pairs for each call session, thus inducing vulnerabilities to the network behind the firewall.
In short, due to the fact that Cisco IOS Firewall doesn't allow outside traffic to transverse to the inside, VoIP calls (inbound calls) will fail. Furthermore, dynamic RTP/RTCP ports used by the endpoints are not automatically opened and allowed without modification of the security policy. The problems are summarized as follows:
•The firewall only looks at Layer 3 addresses
–VoIP signaling protocols embed IP addresses at Layer
–RTP/RTCP works at Layer 5
–By default, firewalls don't allow outside to inside traffic
–Cisco IOS firewall feature set and NAT and PIX have application functionality called Application Layer GW (ALG) or fixup Protocol which helps in resolving these issues
•The VoIP application is composed of a dynamic set of protocols
–SIP, MGCP, H.323, and SCCP for signaling
–SDP, H.225, and H.245 for capability exchange
–RTP/RTCP for control and audio media
–RTP/RTCP both use a dynamic port for the audio media ranging from 16384 to 32767 for all Cisco products
DDTS CSCdx39135 was opened by MCEBU to track and resolve the problem.
Currently, Cisco IOS Firewall does not support Skinny inspection, because outgoing packets will be converted to H323 or SIP, there is no need for Skinny inspection. However, for incoming Skinny packets inspection, ACLs can always be used to filter out unwanted packets/traffic. However, Cisco IOS Firewall will add H.323 inspection support for any locally generated traffic as a bug fix for DDTS CSCdx39135, which will be released in 12.3(4th)T/pi4 available in 2004.
The following are four alternative solutions to provide security to the Cisco CallManager Express users:
•Running Cisco IOS Firewall on a different router
•Set up maximum number of connections in the Cisco CallManager Express. This is available with the regular H.323 implementation in Cisco IOS Software and can help control the maximum number of H.323 (H225 setup Inbound + Outbound) calls that will be processed (e.g., dial-peer voice 10 voip; max-conn 5—limits to only 5 connections)
•Set up ACLs to accept H.225 connections only from the Gatekeeper (GK) if the GK in the network is using Routed Signaling
•Use H.235 security to authenticate the callers and provide additional call security
•Cisco IOS Software Security Documentation
•Cisco IOS Firewall Documentation
•Cisco IOS Software Configuration Guide on Cisco.com
•Cisco IOS Software Command Reference on Cisco.com