Avaya Configuring IP Utilities Uživatelský manuál stáhnout pdf (Strana 63)

Overview of IP Utilities

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How the DNS Proxy Server Works

Clients on a LAN typically use DNS servers to resolve a host name to an IP

address. For example, a client might request the service,

“www.baynetworks.com”. Since the client cannot connect to a name service, it

must translate this name to an IP address so that it can communicate over the

network. DNS is the mechanism that resolves the host name to an IP address.

Clients are typically configured with a list of DNS name servers to contact to

resolve host names. Due to network infrastructure changes, Internet service

providers and network administrators often change the IP addresses of these

statically configured name servers. When the IP addresses of these name servers

change, every network client must change its local configuration for the IP address

of the new name servers. Using a DNS proxy server minimizes the work for the

system administrator. Each client uses the IP address of the DNS proxy server

instead of the true DNS servers. The DNS proxy server contains the list of real

name servers. If the DNS servers change, only the server list on the DNS proxy

server must change, not every client.

Typically, a network client has a default route specified to a local attached router.

Careful network planning can allow the DNS proxy server to be the same as this

default route. Setting up the DNS proxy server this way simplifies the task for a

network administrator, who does not need to know the list of DNS servers when

configuring new clients.

By default, the DNS proxy listens on UDP port 53 (standard DNS server port) for

the IP interface on which it is configured. (You can, however, configure a different

port number.) When the DNS proxy receives a valid request, it forwards the packet

to the DNS server on the proxy’s list. Once the DNS proxy server receives a

response from the DNS server, the DNS proxy forwards the packet to the

requesting client and puts the response into its local cache.

The DNS proxy lets you configure timeout intervals and the number of

retransmissions allowed. If the first DNS server contacted times out, the DNS

proxy tries the next server on the list, and so on, until it receives a response. If all

the servers time out, the DNS proxy returns a serv_fail error to the client. You can

configure up to three DNS servers per proxy interface.

DNS responses can contain several answers to the client question, although most

DNS clients use only the first answer in the list. Optionally, you can configure the

DNS proxy to truncate the number of answers returned to the client. Any answers

beyond the maximum are omitted from the DNS response message.

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Avaya Configuring IP Utilities Uživatelský manuál Strana 63