GETHOSTBYNAME

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

#include <netdb.h>
extern int h_errno;

struct hostent *gethostbyname(const char *name);

#include <sys/socket.h>        /* for AF_INET */
struct hostent *gethostbyaddr(const char *addr,
  int len, int type);

void sethostent(int stayopen);

void endhostent(void);

void herror(const char *s);

const char *hstrerror(int err);

/* GNU extensions */


struct hostent *gethostbyname2(const char *name, int af);

int gethostbyname_r (const char *name,
  struct hostent *ret, char *buf, size_t buflen,
  struct hostent **result, int *h_errnop);

int gethostbyname2_r (const char *name, int af,
  struct hostent *ret, char *buf, size_t buflen,
  struct hostent **result, int *h_errnop);

The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a host name, or an IPv4 address in standard dot notation, or an IPv6 address in colon (and possibly dot) notation. (See RFC 1884 for the description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no lookup is performed and gethostbyname() simply copies name into the h_name field and its struct in_addr equivalent into the h_addr_list[0] field of the returned hostent structure. If name doesn't end in a dot and the environment variable HOSTALIASES is set, the alias file pointed to by HOSTALIASES will first be searched for name (see hostname(7) for the file format). The current domain and its parents are searched unless name ends in a dot.

The gethostbyaddr() function returns a structure of type hostent for the given host address addr of length len and address type type. The only valid address type is currently AF_INET.

The sethostent() function specifies, if stayopen is true (1), that a connected TCP socket should be used for the name server queries and that the connection should remain open during successive queries. Otherwise, name server queries will use UDP datagrams.

The endhostent() function ends the use of a TCP connection for name server queries.

The (obsolete) herror() function prints the error message associated with the current value of h_errno on stderr.

The (obsolete) hstrerror() function takes an error number (typically h_errno) and returns the corresponding message string.

The domain name queries carried out by gethostbyname() and gethostbyaddr() use a combination of any or all of the name server named(8), a broken out line from /etc/hosts, and the Network Information Service (NIS or YP), depending upon the contents of the order line in /etc/host.conf. (See resolv+(8)). The default action is to query named(8), followed by /etc/hosts.

The hostent structure is defined in <netdb.h> as follows:

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 */
}
#define h_addr  h_addr_list[0]  /* for backward compatibility */

The members of the hostent structure are:

h_name

The official name of the host.

h_aliases

A zero-terminated array of alternative names for the host.

h_addrtype

The type of address; always AF_INET at present.

h_length

The length of the address in bytes.

h_addr_list

A zero-terminated array of network addresses for the host in network byte order.

h_addr

The first address in h_addr_list for backward compatibility.

The gethostbyname() and gethostbyaddr() functions return the hostent structure or a NULL pointer if an error occurs. On error, the h_errno variable holds an error number.

The variable h_errno can have the following values:

HOST_NOT_FOUND

The specified host is unknown.

NO_ADDRESS or NO_DATA

The requested name is valid but does not have an IP address.

NO_RECOVERY

A non-recoverable name server error occurred.

TRY_AGAIN

A temporary error occurred on an authoritative name server. Try again later.

/etc/host.conf

resolver configuration file

/etc/hosts

host database file

BSD 4.3.

The SUS-v2 standard is buggy and declares the len parameter of gethostbyaddr() to be of type size_t. (That is wrong, because it has to be int, and size_t is not. POSIX 1003.1-2001 makes it socklen_t, which is OK.)

The functions gethostbyname() and gethostbyaddr() may return pointers to static data, which may be overwritten by later calls. Copying the struct hostent does not suffice, since it contains pointers - a deep copy is required.

Glibc2 also has a gethostbyname2() that works like gethostbyname(), but permits to specify the address family to which the address must belong.

Glibc2 also has reentrant versions gethostbyname_r() and gethostbyname2_r(). These return 0 on success and nonzero on error. The result of the call is now stored in the struct with address ret. After the call, *result will be NULL on error or point to the result on success. Auxiliary data is stored in the buffer buf of length buflen. (If the buffer is too small, these functions will return ERANGE.) No global variable h_errno is modified, but the address of a variable in which to store error numbers is passed in h_errnop.

POSIX 1003.1-2001 marks gethostbyaddr() and gethostbyname() legacy, and introduces

struct hostent *getipnodebyaddr (const void *restrict addr,
  socklen_t len, int type, int *restrict error_num);

struct hostent *getipnodebyname (const char *name,
  int type, int flags, int *error_num);

resolver(3), hosts(5), hostname(7), resolv+(8), named(8)