18.7 Summary
The last socket address structure that we
encounter in this text is the sockaddr_dl structure, the
variable-length datalink socket address structure. Berkeley-derived
kernels associate these with interfaces, returning the interface
index, name, and hardware address in one of these structures.
Five types of messages can be written to a
routing socket by a process and 15 different messages can be
returned by the kernel asynchronously on a routing socket. We
showed an example where the process asks the kernel for information
on a routing table entry and the kernel responds with all the
details. These kernel responses contain up to eight socket address
structures and we have to parse this message to obtain each piece
of information.
The sysctl function is a general way to
fetch and store OS parameters. The information we are interested in
with sysctl is
The changes required by IPv6 to the sockets API
include four functions to map between interface names and their
indexes. Each interface is assigned a unique positive index.
Berkeley-derived implementations already associate an index with
each interface, so we are easily able to implement these functions
using sysctl.
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