Cisco PIX 525 Spezifikationen Seite 200

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5-24
Cisco PIX Firewall and VPN Configuration Guide
78-15033-01
Chapter 5 Configuring Application Inspection (Fixup)
Voice Over IP
Overview
SIP works with Session Description Protocol (SDP) for call signalling. SDP specifies the ports for the
media stream. Using SIP, the PIX
Firewall can support any SIP Voice over IP (VoIP) gateways and VoIP
proxy servers. SIP and SDP are defined in the following RFCs:
SIP: Session Initiation Protocol, RFC 2543
SDP: Session Description Protocol, RFC 2327
You can use the fixup command to change the default TCP port assignment for the Session Initiation
Protocol (SIP). The command syntax is as follows.
[no] fixup protocol sip <udp> [port[-port]]
Note PAT support for SIP is provided by PIX Firewall Version 6.2 or higher. Only static NAT and dynamic
NAT are supported in earlier versions.
To change the default port assignments from 5060 use the port option. Use the -port option to apply SIP
application inspection to a range of port numbers.
To view the current timeout value for SIP connections, enter the following command:
show timeout sip
To view the state of SIP connections, enter the following command:
show conn state sip
To support SIP calls through the PIX Firewall, signaling messages for the media connection addresses,
media ports, and embryonic connections for the media must be inspected, because while the signaling is
sent over a well-known destination port (UDP/TCP 5060), the media streams are dynamically allocated.
Also, SIP embeds IP addresses in the user-data portion of the IP packet. SIP inspection applies NAT for
these embedded IP addresses.
With SIP application inspection enabled, the PIX Firewall does support connectivity between a SIP
phone and a Music on Hold (MOH) server. The specific scenario that has been tested is with a phone on
the more secure network connected to an MOH server with the SIP proxy on the less secure network.
Note If a remote endpoint tries to register with a SIP proxy on a network protected by PIX Firewall, the
registration will fail if the To field in the request does not specify the port number and if the SIP proxy
is configured with PAT.
Allowing Outside Phones to Place an Inside Phone on Hold
When an outbound call is made by an IP phone using SIP and the outside phone tries to put the inside
phone on hold, the operation fails. This is because a new connection is initiated to send the INVITE
packet from the outside phone and the PIX
Firewall drops the packet.
To solve this problem, do one of the following:
Configure an access list to allow the Re-INVITE packet to the inside gateway using port 5060
Use the established command, as in the following example:
established udp 5060 permitto udp 5060 permitfrom udp 0
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