NAME¶

gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostbyname2_r, gethostbyname_r, gethostent_r - get network host entry

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <netdb.h>

void sethostent(int stayopen);
void endhostent(void);

[[deprecated]] extern int h_errno;

[[deprecated]] struct hostent *gethostbyname(const char *name);
[[deprecated]] struct hostent *gethostbyaddr(const void addr[.len],
                                             socklen_t len, int type);

[[deprecated]] void herror(const char *s);
[[deprecated]] const char *hstrerror(int err);

/* System V/POSIX extension */
struct hostent *gethostent(void);

/* GNU extensions */
[[deprecated]]
struct hostent *gethostbyname2(const char *name, int af);

int gethostent_r(struct hostent *restrict ret,
                 char buf[restrict .buflen], size_t buflen,
                 struct hostent **restrict result,
                 int *restrict h_errnop);

[[deprecated]]
int gethostbyaddr_r(const void addr[restrict .len], socklen_t len,
                 int type,
                 struct hostent *restrict ret,
                 char buf[restrict .buflen], size_t buflen,
                 struct hostent **restrict result,
                 int *restrict h_errnop);
[[deprecated]]
int gethostbyname_r(const char *restrict name,
                 struct hostent *restrict ret,
                 char buf[restrict .buflen], size_t buflen,
                 struct hostent **restrict result,
                 int *restrict h_errnop);
[[deprecated]]
int gethostbyname2_r(const char *restrict name, int af,
                 struct hostent *restrict ret,
                 char buf[restrict .buflen], size_t buflen,
                 struct hostent **restrict result,
                 int *restrict h_errnop);

gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(), gethostbyname2_r():

    Since glibc 2.19:


        _DEFAULT_SOURCE


    glibc up to and including 2.19:


        _BSD_SOURCE || _SVID_SOURCE

herror(), hstrerror():

    Since glibc 2.19:


        _DEFAULT_SOURCE


    glibc 2.8 to glibc 2.19:


        _BSD_SOURCE || _SVID_SOURCE


    Before glibc 2.8:


        none

h_errno:

    Since glibc 2.19


        _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L


    glibc 2.12 to glibc 2.19:


        _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L


    Before glibc 2.12:


        none

DESCRIPTION¶

The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() functions are obsolete. Applications should use getaddrinfo(3), getnameinfo(3), and gai_strerror(3) instead.

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 gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a hostname or an IPv4 address in standard dot notation (as for inet_addr(3)). If name is an IPv4 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. Valid address types are AF_INET and AF_INET6 (defined in <sys/socket.h>). The host address argument is a pointer to a struct of a type depending on the address type, for example a struct in_addr * (probably obtained via a call to inet_addr(3)) for address type AF_INET.

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() rely on the Name Service Switch (nsswitch.conf(5)) configured sources or a local name server (named(8)). The default action is to query the Name Service Switch (nsswitch.conf(5)) configured sources, failing that, a local name server (named(8)).

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 */

RETURN VALUE¶

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. When non-NULL, the return value may point at static data, see the notes below.

ERRORS¶

The variable h_errno can have the following values:

HOST_NOT_FOUND

The specified host is unknown.

NO_DATA

The requested name is valid but does not have an IP address. Another type of request to the name server for this domain may return an answer. The constant NO_ADDRESS is a synonym for NO_DATA.

NO_RECOVERY

A nonrecoverable name server error occurred.

TRY_AGAIN

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

FILES¶

/etc/host.conf

resolver configuration file

/etc/hosts

host database file

/etc/nsswitch.conf

name service switch configuration

ATTRIBUTES¶

For an explanation of the terms used in this section, see attributes(7).

In the above table, hostent in race:hostent signifies that if any of the functions sethostent(), gethostent(), gethostent_r(), or endhostent() are used in parallel in different threads of a program, then data races could occur.

STANDARDS¶

POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(), endhostent(), gethostent(), and h_errno; gethostbyname(), gethostbyaddr(), and h_errno are marked obsolescent in that standard. POSIX.1-2008 removes the specifications of gethostbyname(), gethostbyaddr(), and h_errno, recommending the use of getaddrinfo(3) and getnameinfo(3) instead.

NOTES¶

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.

In the original BSD implementation the len argument of gethostbyname() was an int. The SUSv2 standard is buggy and declares the len argument of gethostbyaddr() to be of type size_t. (That is wrong, because it has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t, which is OK.) See also accept(2).

The BSD prototype for gethostbyaddr() uses const char * for the first argument.

BUGS¶

gethostbyname() does not recognize components of a dotted IPv4 address string that are expressed in hexadecimal.

SEE ALSO¶

getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3), resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)