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CLI-based Configuration Guide - IP Service

AR100, AR120, AR160, AR1200, AR2200, AR3200, and AR3600 V300R003

This document describes the concepts and configuration procedures of IP Service features on the device, and provides the configuration examples.
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NAT ALG

NAT ALG

NAT and NAPT can translate only IP addresses in IP datagram headers and port numbers in TCP/UDP headers. For some special protocols such as FTP, IP addresses or port numbers may be contained in the Data field of the protocol packets. Therefore, NAT cannot translate the IP addresses or port numbers. A good way to solve the NAT issue for these special protocols is to use the application level gateway (ALG) function. As a special translation agent for application protocols, the ALG interacts with the NAT device to establish states. It uses NAT state information to change the specific data in the Data field of IP datagrams and complete other necessary work, so that application protocols can run across private and public networks.

For example, when an FTP server with a private IP address sets up a session with a host on the public network, the server may need to send its IP address to the host. NAT cannot translate this IP address because the IP address is carried in the Data field. When the host on the public network attempts to use the received private IP address, it finds that the FTP server is unreachable.

DNS, FTP, PPTP and RTSP support the ALG function. Table 5-1 lists the NAT fields supported by different protocols.

Table 5-1  Fields supported by different protocols

Application Protocol

Field

DNS

IP and Port fields in a response packet

FTP

  • IP and Port fields in the payload of a Port request packet
  • IP and Port fields in the payload of a Passive response packet

SIP

  • Request line
  • From
  • To
  • Contact
  • Via
  • O
  • Connection information field (indicating an IP address) and media description field (indicating a port) in the Message body
  • record-router

PPTP

There are two scenarios:
  • PPTP client on the private network and PPTP server on the public network: Client-Call-ID field
  • PPTP server on the private network and PPTP client on the public network: Server-Call-ID field

RTSP

Port field in a setup/reply OK packet

Translation
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Updated: 2019-03-06

Document ID: EDOC1100069333

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