The SIP ALG – VOIP in FortiOS 6


In most cases you should use the SIP Application Layer Gateway (ALG) for processing SIP sessions. The SIP ALG provides the same basic SIP support as the SIP session helper. Additionally, the SIP ALG provides a wide range of features that protect your network from SIP attacks, apply rate limiting to SIP sessions, check the syntax of SIP and SDP content of SIP messages, and provide detailed logging and reporting of SIP activity.

By default all SIP traffic is processed by the SIP ALG. If the policy that accepts the SIP traffic includes a VoIP profile, the SIP traffic is processed by that profile. If the policy does not include a SIP profile the SIP traffic is processed by the SIP ALG using the default VoIP profile.

If a FortiGate or a VDOM has been configured to use the SIP session helper, you can change this behavior to the default configuration of using the SIP ALG with the following command:

config system settings set default-voip-alg-mode proxy-based set sip-helper disable


From the GUI you can only configure VoIP security profiles and add them to security policies if VoIP is turned on under System > Feature Visibility. However, you can always configure VoIP profiles and add them to security profiles from the CLI. And if the default-voip-alg mode is set to proxy-based the default SIP profile will still be used even if VoIP security profiles are not visible from the GUI.

As shown in the figure below, the FortiGate SIP ALG intercepts SIP packets after they have been routed by the routing module, accepted by a security policy and passed through DoS and IPS Sensors (if DoS and IPS are enabled). The ALG raises SIP packets to the application layer, analyzes the SIP and SDP addressing information in the SIP messages, makes adjustments (for example, NAT) to this addressing if required, and then sends the packets out the egress interface to their destination.

The SIP ALG provides:

  • All the same features as the SIP session helper including NAT and SIP and RTP Pinholes.
  • In addition for the ALG you can enable or disable RTP pinholing, SIP register pinholing and SIP contact pinholing. In a signaling only environment where the RTP stream bypasses the FortiGate, you can disable RTP pinholing to improve performance. l SIP TCP and UDP support l SIP Message order checking l Configurable Header line length maximums


SIP session helper diagnose commands

The SIP ALG works at the application level after ingress packets are accepted by a security policy


IP routing and forwarding
IPsec VPN encryption, decryption

Rate limiting and message blocking
Stateful SIP tracking
Message, header, and SDP syntax checking
Network surveillance
NAT and IP topology Hiding
Logging and debugging

Intrusion detection and prevention
Defined by Fortinet and enterprise signatures
SIP decoder identifies SIP sessions

Security policy
IPsec VPN encryption, decryption
Access control

Native (D)DoS prevention
Anomaly detection and prevention
  • Message fragment assembly (TCP) l If SIP messages are fragmented across multiple packets, the FortiGate assembles the fragments, does inspection and pass the message in its entirety to the SIP server as one packet. This offloads the server from doing all the TCP processing of fragments. l L4 Protocol Translation l Message Flood Protection l Protects a SIP server from intentional or unintentional DoS of flooding INVITE, REGISTER, and other SIP methods by allowing control of the rate that these massages pass through the FortiGate.
  • SIP message type filtering l The FortiGate can prevent specified SIP message types from passing through the FortiGate to a SIP server. For example In a voice only SIP implementation, there may be no need to permit a SUBSCRIBE message to ever make it’s way to the SIP call processor. Also, if a SIP server cannot process some SIP message types you can use SIP message type filtering to block them. For example, a SIP server could have a bug that prevents it from processing certain SIP messages. In this case you can temporarily block these message types until problem with the SIP server has been fixed. l SIP statistics and logging l SIP over IPv6 l SIP over SSL/TLS
  • Deep SIP message syntax checking (also called deep SIP header inspection or SIP fuzzing protection). Prevents attacks that use malformed SIP messages. Can check many SIP headers and SDP statements. Configurable bypass and modification options.
  • Hosted NAT traversal, Resolves IP address issue in SIP and SDP lines due to NAT-PT in far end firewall. Important feature for VoIP access networks.
  • SIP High Availability (HA), including active-passive clustering and session pickup (session failover) for SIP sessions.
  • Geographical Redundancy. In an HA configuration, if the active SIP server fails (missing SIP heartbeat messages or SIP traffic) SIP sessions can be redirected to a secondary SIP server in another location. l SIP per request method message rate limitation with configurable threshold for SIP message rates per request method. Protects SIP servers from SIP overload and DoS attacks.
  • RTP Bypass, Supports configurations with and without RTP pinholing. May inspect and protect SIP signaling only.
  • SIP NAT with IP address conservation. Performs SIP and RTP aware IP Network Address translation. Preserves the lost IP address information in the SDP profile i= line for later processing/debugging in the SIP server. See NAT with IP address conservation on page 80.
  • IP topology hiding l The IP topology of a network can be hidden through NAT and NAPT manipulation of IP and SIP level addressing. For example, see SIP NAT scenario: source address translation (source NAT) on page 65. l SIP inspection without address translation l The SIP ALG inspects SIP messages but addresses in the messages are not translated. This feature can be applied to a FortiGate operating in transparent mode or in NAT/Route mode. In transparent mode you add normal transparent mode security policies that enable the SIP ALG and include a VoIP profile that causes the SIP ALG to inspect SIP traffic as required. For an example configuration, see Configuration example: SIP in transparent mode on page 54.
  • For a FortiGate operating in NAT/Route mode, if SIP traffic can pass between different networks without requiring NAT because is supported by the routing configuration, you can add security policies that accept SIP traffic without enabling NAT. In the VoIP profile you can configure the SIP ALG to inspect SIP traffic as required.

Enabling VoIP support from the GUI

Before you begin to configure VoIP security profiles, including SIP, from the GUI you should go to System > Feature Visibility and turn on VoIP (under Additional Features). VoIP settings are visible in both Inspection modes (flow and proxy).

SIP ALG configuration overview

To apply the SIP ALG, you add a SIP VoIP profile to a security policy that accepts SIP sessions. All SIP sessions accepted by the security policy will be processed by the SIP ALG using the settings in the VoIP profile. The VoIP profile contains settings that are applied to SIP, Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions (SIMPLE) and Skinny Call Control Protocol (SCCP) sessions. All SCCP sessions accepted by the security policy are also processed by the ALG. You configure SIP and SCCP settings separately. SIP settings also apply to SIMPLE sessions.

VoIP profiles

You can customize the default VoIP profile or add new VoIP profiles.

To add a new VoIP profile from the GUI go to Security Profiles > VoIP and select Create New (the + button).

For SIP, from the GUI you can configure the VoIP profile to limit the number of SIP REGISTER and INVITE requests. Many additional options for configuring how the ALG processes SIP sessions are available from the CLI.

For SCCP you can limit the call setup time. Additional SCCP options are available from the CLI.

Use the following command to add a VoIP profile named VoIP_Pro_1 from the CLI:

config voip profile edit VoIP_Pro_1


FortiGates include two pre-defined VoIP profiles. On the GUI these profiles look identical. However, the CLI-only settings result in the following functionality.

default The most commonly used VoIP profile. This profile enables both SIP and SCCP and places the minimum restrictions on what calls will be allowed to negotiate. This profile allows normal SCCP, SIP and RTP sessions and enables the following security settings:

•               strict-register to open smaller more secure pinholes (see Enhancing SIP pinhole security on page 86).

•               block-long-lines to block SIP messages with lines that exceed maximum line lengths.

•               block-unknown to block unrecognized SIP request messages.

•               open-record-route-pinhole to open pinholes for Record-Route messages.

•               nat-trace (see NAT with IP address conservation on page 80).

•               contact-fixup perform NAT on the IP addresses and port numbers in SIP headers in SIP CONTACT messages even if they don’t match the session’s IP address and port numbers.

•               ips-rtp to enable IPS in security policies that also accept SIP sessions to protect the SIP traffic from SIP-based attacks.

strict This profile is available for users who want to validate SIP messages and to only allow SIP sessions that are compliant with RFC 3261. In addition to the settings in the default VoIP profile, the strict profile sets all SIP deep message inspection header checking options (for example, malformed-request-line and many others) to discard. So the strict profile blocks and drops SIP messages that contain malformed SIP or SDP lines that can be detected by the ALG. For more information about SIP deep header inspection, see Deep SIP message inspection on page 96.

Neither of the default profiles applies SIP rate limiting. To apply more ALG features to SIP sessions you can clone (copy) the pre-defined VoIP profiles and make your own modifications to them. You can clone VoIP profiles from the GUI or the CLI. For example, from the CLI, to clone the default profile and configure the limit for SIP NOTIFY request messages to 1000 messages per second per security policy and block SIP INFO request messages.

config voip profile clone default to my_voip_pro edit my_voip_pro config sip set notify-rate 1000 set block-info enable



Changing the port numbers that the SIP ALG listens on

Most SIP configurations use TCP or UDP port 5060 for SIP sessions and port 5061 for SIP SSL sessions. If your SIP network uses different ports for SIP sessions you can use the following command to configure the SIP ALG to listen on a different TCP, UDP, or SSL ports. For example, to change the TCP port to 5064, the UDP port to 5065, and the SSL port to 5066.

config system settings set sip-tcp-port 5064 set sip-udp-port 5065 set sip-ssl-port 5066


You also configure the SIP ALG to listen in two different TCP ports and two different UDP ports for SIP sessions. For example, if you receive SIP TCP traffic on port 5060 and 5064 and UDP traffic on ports 5061 and 5065 you can enter the following command to receive the SIP traffic on all of these ports:

config system settings set sip-tcp-port 5060 5064 set sip-udp-port 5061 5065


Disabling the SIP ALG in a VoIP profile

SIP is enabled by default in a VoIP profile. If you are just using the VoIP profile for SCCP you can use the following command to disable SIP in the VoIP profile.

config voip profile edit VoIP_Pro_2 config sip set status disable


SIP ALG diagnose commands

You can use the diagnose sys sip-proxy command to display diagnostic information for the SIP ALG. A number of options are available including:

Use the following command to list all active SIP calls being processed by the SIP ALG. You can also use the clear option to delete all active SIP calls being processed by the SIP ALG, the idle option to list idle SIP calls, and the invite option to list SIP invite transactions.

SIP ALG diagnose commands

diagnose sys sip-proxy calls {clear | list | idle | invite}

Use the following commands to employ filters to display specific information about the SIP ALG and the session that it is processing. You can build up a filter by including a number of options such as source address, VoIP profile, policy, and so on.

diagnose sys sip-proxy filter <filter_options> diagnose sys sip-proxy log-filter <filter_options>

Use the following command to display the active SIP rate limiting meters and their current settings.

diagnose sys sip-proxy meters list

Use the following command to display status information about the SIP sessions being processed by the SIP ALG. You can also clear all SIP ALG statistics.

diagnose sys sip-proxy stats {clear | list}


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