11.17 getnameinfo
Function
This function is the complement of
getaddrinfo: It takes a socket address and returns a
character string describing the host and another character string
describing the service. This function provides this information in
a protocol-independent fashion; that is, the caller does not care
what type of protocol address is contained in the socket address
structure, as that detail is handled by the function.
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#include <netdb.h>
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int getnameinfo (const struct sockaddr
*sockaddr, socklen_t
addrlen, char
*host, socklen_t
hostlen, char
*serv, socklen_t
servlen, int flags) ;
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Returns: 0 if OK, nonzero on error (see
Figure 11.7)
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sockaddr points
to the socket address structure containing the protocol address to
be converted into a human-readable string, and addrlen is the length of this structure. This
structure and its length are normally returned by accept,
recvfrom, getsockname, or
getpeername.
The caller allocates space for the two
human-readable strings: host and
hostlen specify the host string,
and serv and servlen specify the service string. If the
caller does not want the host string returned, a hostlen of 0 is specified. Similarly, a
servlen of 0 specifies not to
return information on the service.
The difference between sock_ntop and
getnameinfo is that the former does not involve the DNS
and just returns a printable version of the IP address and port
number. The latter normally tries to obtain a name for both the
host and service.
Figure
11.20 shows the six flags that
can be specified to change the operation of
getnameinfo.
NI_DGRAM should be specified when the
caller knows it is dealing with a datagram socket. The reason is
that given only the IP address and port number in the socket
address structure, getnameinfo cannot determine the
protocol (TCP or UDP). There are a few port numbers that are used
for one service with TCP and a completely different service with
UDP. An example is port 514, which is the rsh service with
TCP, but the syslog service with UDP.
NI_NAMEREQD causes an error to be
returned if the hostname cannot be resolved using the DNS. This can
be used by servers that require the client's IP address to be
mapped into a hostname. These servers then take this returned
hostname and call getaddrinfo, and then verify that one of
the returned addresses is the address in the socket address
structure.
NI_NOFQDN causes the returned hostname
to be truncated at the first period. For example, if the IP address
in the socket address structure was 192.168.42.2,
gethostbyaddr would return a name of
aix.unpbook.com. But if this flag was specified to
getnameinfo, it would return the hostname as just
aix.
NI_NUMERICHOST tells
getnameinfo not to call the DNS (which can take time).
Instead, the numeric representation of the IP address is returned
as a string, probably by calling inet_ntop. Similarly,
NI_NUMERICSERV specifies that the decimal port number is
to be returned as a string instead of looking up the service name,
and NI_NUMERICSCOPE specifies that the numeric form of the
scope identifier is to be returned instead of its name. Servers
should normally specify NI_NUMERICSERV because the client
port numbers typically have no associated service name鈥攖hey are
ephemeral ports.
The logical OR of multiple flags can be
specified if they make sense together (e.g., NI_DGRAM and
NI_NUMERICHOST).
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