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Man Pages

Manual Reference Pages  -  GETHOSTBYADDR (3)


gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent, endhostent, herror, hstrerror - get network host entry


See Also


.Lb libc


.In netdb.h
.Vt int h_errno ; struct hostent * gethostbyname const char *name struct hostent * gethostbyname2 const char *name int af struct hostent * gethostbyaddr const void *addr socklen_t len int af struct hostent * gethostent void void sethostent int stayopen void endhostent void void herror const char *string const char * hstrerror int err


.Bf -symbolic The getaddrinfo(3) and getnameinfo(3) functions are preferred over the gethostbyname, gethostbyname2, and gethostbyaddr functions.

The gethostbyname, gethostbyname2 and gethostbyaddr functions each return a pointer to an object with the following structure describing an internet host referenced by name or by address, respectively.

The name argument passed to gethostbyname or gethostbyname2 should point to a NUL -terminated hostname. The addr argument passed to gethostbyaddr should point to an address which is len bytes long, in binary form (i.e., not an IP address in human readable ASCII form). The af argument specifies the address family (e.g. AF_INET, AF_INET6, etc.) of this address.

The structure returned contains either the information obtained from the name server, named(8), broken-out fields from a line in /etc/hosts, or database entries supplied by the yp(8) system. The order of the lookups is controlled by the 'hosts' entry in nsswitch.conf(5).

struct  hostent {
        char    *h_name;        /* official name of host */
        char    **h_aliases;    /* alias list */
        int     h_addrtype;     /* host address type */
        int     h_length;       /* length of address */
        char    **h_addr_list;  /* list of addresses from name server */
#define h_addr  h_addr_list[0]  /* address, for backward compatibility */

The members of this structure are:
h_name Official name of the host.
h_aliases A NULL -terminated array of alternate names for the host.
h_addrtype The type of address being returned; usually AF_INET.
h_length The length, in bytes, of the address.
  A NULL -terminated array of network addresses for the host. Host addresses are returned in network byte order.
h_addr The first address in h_addr_list; this is for backward compatibility.

When using the nameserver, gethostbyname and gethostbyname2 will search for the named host in the current domain and its parents unless the name ends in a dot. If the name contains no dot, and if the environment variable "HOSTALIASES" contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname(7) for the domain search procedure and the alias file format.

The gethostbyname2 function is an evolution of gethostbyname which is intended to allow lookups in address families other than AF_INET, for example AF_INET6.

The sethostent function may be used to request the use of a connected TCP socket for queries. If the stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to gethostbyname, gethostbyname2 or gethostbyaddr. Otherwise, queries are performed using UDP datagrams.

The endhostent function closes the TCP connection.

The herror function writes a message to the diagnostic output consisting of the string argument string, the constant string ": ", and a message corresponding to the value of h_errno.

The hstrerror function returns a string which is the message text corresponding to the value of the err argument.




Print out the hostname associated with a specific IP address:
const char *ipstr = "";
struct in_addr ip;
struct hostent *hp;

if (!inet_aton(ipstr, &ip))         errx(1, "can’t parse IP address %s", ipstr);

if ((hp = gethostbyaddr((const void *)&ip, sizeof ip, AF_INET)) == NULL)         errx(1, "no name associated with %s", ipstr);

printf("name associated with %s is %s\n", ipstr, hp->h_name);


Error return status from gethostbyname, gethostbyname2 and gethostbyaddr is indicated by return of a NULL pointer. The integer h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine herror can be used to print an error message describing the failure. If its argument string is non- NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.

The variable h_errno can have the following values:
  No such host is known.
TRY_AGAIN This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
NO_RECOVERY Some unexpected server failure was encountered. This is a non-recoverable error.
NO_DATA The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain.


getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3), hosts(5), hostname(7), named(8)


The gethostent function is defined, and sethostent and endhostent are redefined, when
.Lb libc is built to use only the routines to lookup in /etc/hosts and not the name server.

The gethostent function reads the next line of /etc/hosts, opening the file if necessary.

The sethostent function opens and/or rewinds the file /etc/hosts. If the stayopen argument is non-zero, the file will not be closed after each call to gethostbyname, gethostbyname2 or gethostbyaddr.

The endhostent function closes the file.


The herror function appeared in BSD 4.3 . The endhostent, gethostbyaddr, gethostbyname, gethostent, and sethostent functions appeared in BSD 4.2 . The gethostbyname2 function first appeared in BIND version 4.9.4.


These functions use a thread-specific data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it.

Though these functions are thread-safe, still it is recommended to use the getaddrinfo(3) family of functions, instead.

Only the Internet address format is currently understood.

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