Kerio Tech KERIO WINROUTE FIREWALL 6 User Manual download pdf (Page 103)

7.9 Media hairpinning

103

Example: Two SIP telephones in the LAN

Let us suppose two SIP telephones are located in the LAN. These telephones authenticate at

a SIP server in the Internet. The parameters may be as follows:

• IP addresses of the phones: 192.168.1.100 and 192.168.1.101

• Public IP address of the ﬁrewall: 195.192.33.1

• SIP server: sip.server.com

For the telephones, deﬁne corresponding traﬃc rules — see chapter

7.8 (as apparent from

ﬁgure7.39, simply specify Source of the Full cone NAT traﬃc rule by IP address of the other

telephone).

Both telephones will be registered on SIP server under the ﬁrewall’s public IP address

(195.192.33.1). If these telephones establish mutual connection, data packets (for voice

transmission) from both telephones will be sent to the ﬁrewall’s public IP address (and to the

port of the other telephone). Under normal conditions, such packets would be dropped. How-

ever, WinRoute is capable of using a corresponding record in the NAT table to recognize that

a packet is addressed to a client in the local network. Then it translates the destination IP

address and sends the packet back to the local network (as well as in case of port mapping).

This ensures that traﬃc between the two phones will work correctly.

Note:

1. Hairpinning requires traﬃc between the local network and the Internet being allowed (be-

fore processed by the ﬁrewall, packets use a local source address and an Internet destina-

tion address — i.e. this is an outgoing traﬃc from the local network to the Internet). In

default traﬃc rules created by the wizard (see chapter 7.1), this condition is met by the

NAT rule.

2. In principle, hairpinning does not require that Full cone NAT is allowed (see chapter 7.8).

However, in our example, Full cone NAT is required for correct functioning of the SIP

protocol.

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Kerio Tech KERIO WINROUTE FIREWALL 6 User Manual Page 103