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<HTML
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NAME="NET-COMMON-TCPIP-MANPAGES-GETNAMEINFO">getnameinfo</H1
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>GETNAMEINFO(3) System Library Functions Manual GETNAMEINFO(3)
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NAME
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getnameinfo - address-to-nodename translation in protocol-independent
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manner
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SYNOPSIS
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#include <sys/types.h>
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#include <sys/socket.h>
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#include <netdb.h>
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int
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getnameinfo(const struct sockaddr *sa, socklen_t salen, char *host,
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size_t hostlen, char *serv, size_t servlen, int flags);
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DESCRIPTION
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The getnameinfo() function is defined for protocol-independent address-
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to-nodename translation. Its functionality is a reverse conversion of
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getaddrinfo(3), and implements similar functionality with
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gethostbyaddr(3) and getservbyport(3) in more sophisticated manner.
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This function looks up an IP address and port number provided by the
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caller in the DNS and system-specific database, and returns text strings
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for both in buffers provided by the caller. The function indicates suc-
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cessful completion by a zero return value; a non-zero return value indi-
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cates failure.
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The first argument, sa, points to either a sockaddr_in structure (for
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IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address
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and port number. The salen argument gives the length of the sockaddr_in
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or sockaddr_in6 structure.
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The function returns the nodename associated with the IP address in the
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buffer pointed to by the host argument. The caller provides the size of
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this buffer via the hostlen argument. The service name associated with
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the port number is returned in the buffer pointed to by serv, and the
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servlen argument gives the length of this buffer. The caller specifies
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not to return either string by providing a zero value for the hostlen or
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servlen arguments. Otherwise, the caller must provide buffers large
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enough to hold the nodename and the service name, including the terminat-
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ing null characters.
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Unfortunately most systems do not provide constants that specify the max-
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imum size of either a fully-qualified domain name or a service name.
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Therefore to aid the application in allocating buffers for these two
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returned strings the following constants are defined in <netdb.h>:
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#define NI_MAXHOST MAXHOSTNAMELEN
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#define NI_MAXSERV 32
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The first value is actually defined as the constant MAXDNAME in recent
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versions of BIND's <arpa/nameser.h> header (older versions of BIND define
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this constant to be 256) and the second is a guess based on the services
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listed in the current Assigned Numbers RFC.
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The final argument is a flag that changes the default actions of this
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function. By default the fully-qualified domain name (FQDN) for the host
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is looked up in the DNS and returned. If the flag bit NI_NOFQDN is set,
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only the nodename portion of the FQDN is returned for local hosts.
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If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
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located in the DNS, the numeric form of the host's address is returned
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instead of its name (e.g., by calling inet_ntop() instead of
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gethostbyaddr()). If the flag bit NI_NAMEREQD is set, an error is
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returned if the host's name cannot be located in the DNS.
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If the flag bit NI_NUMERICSERV is set, the numeric form of the service
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address is returned (e.g., its port number) instead of its name. The two
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NI_NUMERICxxx flags are required to support the -n flag that many com-
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mands provide.
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A fifth flag bit, NI_DGRAM, specifies that the service is a datagram ser-
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vice, and causes getservbyport() to be called with a second argument of
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"udp" instead of its default of "tcp". This is required for the few
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ports (512-514) that have different services for UDP and TCP.
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These NI_xxx flags are defined in <netdb.h>.
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Extension for scoped IPv6 address
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The implementation allows experimental numeric IPv6 address notation with
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scope identifier. IPv6 link-local address will appear as string like
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``fe80::1%ne0'', if NI_WITHSCOPEID bit is enabled in flags argument.
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Refer to getaddrinfo(3) for the notation.
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EXAMPLES
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The following code tries to get numeric hostname, and service name, for
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given socket address. Observe that there is no hardcoded reference to
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particular address family.
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struct sockaddr *sa; /* input */
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char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
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if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
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sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
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errx(1, "could not get numeric hostname");
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/*NOTREACHED*/
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}
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printf("host=%s, serv=%s\n", hbuf, sbuf);
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The following version checks if the socket address has reverse address
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mapping.
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struct sockaddr *sa; /* input */
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char hbuf[NI_MAXHOST];
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if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
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NI_NAMEREQD)) {
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errx(1, "could not resolve hostname");
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/*NOTREACHED*/
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}
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printf("host=%s\n", hbuf);
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DIAGNOSTICS
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The function indicates successful completion by a zero return value; a
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non-zero return value indicates failure. Error codes are as below:
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EAI_AGAIN The name could not be resolved at this time. Future
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attempts may succeed.
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EAI_BADFLAGS The flags had an invalid value.
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EAI_FAIL A non-recoverable error occurred.
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EAI_FAMILY The address family was not recognized or the address
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length was invalid for the specified family.
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EAI_MEMORY There was a memory allocation failure.
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EAI_NONAME The name does not resolve for the supplied parameters.
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NI_NAMEREQD is set and the host's name cannot be
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located, or both nodename and servname were null.
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EAI_SYSTEM A system error occurred. The error code can be found
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in errno.
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SEE ALSO
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getaddrinfo(3), gethostbyaddr(3), getservbyport(3), hosts(5),
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resolv.conf(5), services(5), hostname(7), named(8)
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R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface
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Extensions for IPv6, RFC2553, March 1999.
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Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
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Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-02.txt,
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work in progress material.
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Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of
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the freenix track: 2000 USENIX annual technical conference, June 2000.
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HISTORY
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The implementation first appeared in WIDE Hydrangea IPv6 protocol stack
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kit.
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STANDARDS
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The getaddrinfo() function is defined IEEE POSIX 1003.1g draft specifica-
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tion, and documented in ``Basic Socket Interface Extensions for IPv6''
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(RFC2553).
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BUGS
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The current implementation is not thread-safe.
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The text was shamelessly copied from RFC2553.
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OpenBSD intentionally uses different NI_MAXHOST value from what RFC2553
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suggests, to avoid buffer length handling mistakes.
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BSD May 25, 1995 BSD
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</PRE
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