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server, and this operation is transparent. This is useful where a network administrator

wishes to have only one DHCP server across several physical and logical sub-

networks.

The relay works by forwarding all broadcast client requests to one or more known

DHCP servers. Server replies are then either broadcast or unicast back to the client

via the DHCP relay.

Note that DHCP implementation includes code from The Internet Software

Consortium.

DHCP requires certain properties of the network interfaces it uses. The interface

must be able to broadcast and receive DHCP packets before it has been allocated an

IP address.

The ethernet, Forum LAN Emulation (FLANE), and RFC1483 interfaces are all

suitable for use with DHCP.

The IP over ATM or Classical IP interfaces (PVC and SVC) are not suitable for use

with DHCP. They do not support IP broadcasts and this is an integral requirement of

DHCP. No standard exists for DHCP over IPoA. A proprietary solution may be

possible but would need to be operating at both ends of an IPoA connection. PPP

provides its own mechanism for IP address allocation, obviating the need for DHCP

support across this type of interface.

2.6 DNS Relay

DNS Relay is a software module that forwards DNS packets between a DNS resolver

and a DNS server. The DNS relay is capable of forwarding query packets from one

or more DNS resolvers to exactly one nominated DNS server. DNS responses

received from the server are then forwarded back to the DNS resolver that made the

original request. Both UDP and TCP traffic are supported.

From the point of view of a DNS resolver, the relay appears to behave exactly as a

DNS server. Indeed, the resolver will have its DNS server address configured to

match the IP address of the DNS relay. Conversely, from the point of view of the

DNS server, the DNS relay appears to be a normal DNS resolver.

The DNS relay does not bind itself to any one specific interface or interface type, but

rather will listen for traffic on all available IP interfaces. It relies on the well-known

UDP and TCP port number for a DNS server (port number 53) for receiving DNS

traffic. This value can be reconfigured, but this is not recommended.

2.7 NAT

The Network Address Translator (NAT) implements Port Address Translation (PAT)

and provides Network Address Port Translation (NAPT), also known as IP

Masquerading. NAT allows a single “real” IP address on the WAN side to be shared

among many devices on the LAN side, each of which have private addresses.

Incoming connections

Normally, NAT is used in a situation where clients on the private network make

outgoing connections to servers on the public network. Since the IP addresses on the

private network are not visible and cannot be routed to from the public network, it is

not possible for a client on the public network to originate a connection directly to a

server on the private network. NAT will normally reject any incoming packets that are

not in response to a previous outgoing packet.

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