D-Link NetDefend DFL-210 User Manual page 231

6.2.8. The SIP ALG
To understand how to set up SIP scenarios with NetDefendOS, it is important to first understand the
SIP proxy Record-Route option. SIP proxies have the Record-Route option either enabled or
disabled. When it is switched on, a proxy is known as a Stateful proxy. When Record-Route is
enabled, a proxy is saying it will be the intermediary for all SIP signalling that takes place between
two clients.
When a SIP session is being set up, the calling client sends an INVITE message to its outbound SIP
proxy server. The SIP proxy relays this message to the remote proxy server responsible for the
called, remote client's contact information. The remote proxy then relays the INVITE message to the
called client. Once the two clients have learnt of each other's IP addresses, they can communicate
directly with each other and remaining SIP messages can bypass the proxies. This facilitates scaling
since proxies are used only for the initial SIP message exchange.
The disadvantage of removing proxies from the session is that NetDefendOS IP rules must be set up
to allow all SIP messages through the NetDefend Firewall, and if the source network of the
messages is not known then a large number of potentially dangerous connections must be allowed
by the IP rule set. This problem does not occur if the local proxy is set up with the Record-Route
option enabled. In this mode, all SIP messages will only come from the proxy.
The different rules required when the Record-Route option is enabled and disabled can be seen in
the two different sets of IP rules listed below in the detailed description of Scenario 1
Protecting local clients - Proxy located on the Internet.
IP Rules for Media Data
When discussing SIP data flows there are two distinct types of exchanges involved:
• The SIP session which sets up communication between two clients prior to the exchange of
media data.
• The exchange of the media data itself, for example the coded voice data which constitute a VoIP
phone call.
In the SIP setups described below, IP rules need only be explicitly defined to deal with the first of
the above, the SIP exchanges needed for establishing client-to-client communications. No IP rules
or other objects need to be defined to handle the second of the above, the exchange of media data.
The SIP ALG automatically and invisibly takes care of creating the connections required
(sometimes described as SIP pinholes) for allowing the media data traffic to flow through the
NetDefend Firewall.
SIP Usage Scenarios
NetDefendOS supports a variety of SIP usage scenarios. The following three scenarios cover nearly
all possible types of usage:
• Scenario 1
Protecting local clients - Proxy located on the Internet
The SIP session is between a client on the local, protected side of the NetDefend Firewall and a
client which is on the external, unprotected side. The SIP proxy is located on the external,
unprotected side of the NetDefend Firewall. Communication typically takes place across the
public Internet with clients on the internal, protected side registering with a proxy on the public,
unprotected side.
• Scenario 2
Tip
Make sure there are no preceding rules already in the IP rule set disallowing or
allowing the same kind of traffic.
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Chapter 6. Security Mechanisms