Datatek IPv6 Transformer User Manual - Manual (Page 18)

S E C T I O N 2 O V E R V I E W
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advertised in the router advertisements to create a temporary address that is difficult to eavesdrop
due to its changing nature
DNS Server
The Transformer uses a DNS Application Layer Gateway (ALG) to act as a proxy DNS Server
for the IPv4 legacy device. As a proxy, the Transformer processes DNS lookups and reverse
lookups sent from the legacy device. A DNS lookup requests the IP address for a given domain
name. A DNS reverse lookup requests the domain name for a given IP address. Henceforth, an
A-record query will refer to a DNS lookup of an IPv4 address for a given domain name and an
AAAA-record query will refer to a lookup for an IPv6 address for a given domain name.
DNS Lookup or A/AAAA-record Query
The Transformer receives A-record queries from the IPv4 legacy device and translates them into
AAAA-record queries before forwarding them to the network’s DNS Server. The DNS Server
either responds with an IPv6 address or not. If an IPv6 address is received in response, the IPv6
address is mapped to an available IPv4 address from the IPv4
Address pool . The AAAA-
record response is then translated into an A-record response containing the IPv4 address and
forwarded to the legacy device. To the IPv4 legacy device, the IPv4 address looks as if it came
from the DNS Server directly.
If the network’s DNS Server does not respond to the AAAA-record query, the Transformer tries
to get an IPv4 address for the domain name by sending the DNS Server an A-record query. The
A-record response is then forwarded to the IPv4 legacy device without translation.
Reverse DNS Lookup or PTR-record Query
The DNS ALG also supports reverse lookups. Henceforth, Pointer (PTR) record will refer to
reverse lookups that ask for the host and domain name of a given IP address. If the Transformer
receives a PTR-record query for a given IPv4 address, it checks if it is mapped to an IPv6 address.
A mapping may exist if an IPv6 host on the network initiated a session with the IPv4 legacy host
and the packet received from the network contained the IPv6 address of the remote IPv6 host as
the source address. This IPv6 source address would have been bound to an available IPv4
address from the
Address pool before the packet was forwarded to the legacy device. The
legacy device could then send a PTR-record query for the IPv4 address from the address pool. In
this case the Transformer must translate the IPv4 PTR-record query into an IPv6 PTR-record
query for the domain name of the IPv6 address to which the IPv4 address is mapped. The IPv6
PTR-record query is sent to the DNS Server. The IPv6 PTR-record response from the DNS
Server contains a domain name that is then translated into an IPv4 response containing the same
domain name unchanged.
It is also possible for the Transformer to receive a PTR-record query for an IPv4 address that is
not bound to an IPv6 address. In the case of
Passthrough , which is used for communication
between two IPv4 hosts, there is no IPv4/IPv6 binding. The Transformer forwards a PTR-
record query from the legacy device for an unbound IPv4 address directly to the DNS Server.
The response is forwarded directly to the legacy device.