gethostbyname(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

GETHOSTBYNAME(3)        Linux Programmer's Manual       GETHOSTBYNAME(3)

NAME         top

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

SYNOPSIS         top

       #include <netdb.h>

       extern int h_errno;

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

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

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

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

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

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

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

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       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 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 2.19:
               _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
           Before glibc 2.12:
               none

DESCRIPTION         top

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

       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 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() 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)).

   Historical
       The nsswitch.conf(5) file is the modern way of controlling the
       order of host lookups.

       In glibc 2.4 and earlier, the order keyword was used to control
       the order of host lookups as defined in /etc/host.conf
       (host.conf(5)).

       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
              An array of alternative names for the host, terminated by
              a null pointer.

       h_addrtype
              The type of address; always AF_INET or AF_INET6 at
              present.

       h_length
              The length of the address in bytes.

       h_addr_list
              An array of pointers to network addresses for the host (in
              network byte order), terminated by a null pointer.

       h_addr The first address in h_addr_list for backward
              compatibility.

RETURN VALUE         top

       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         top

       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         top

       /etc/host.conf
              resolver configuration file

       /etc/hosts
              host database file

       /etc/nsswitch.conf
              name service switch configuration

ATTRIBUTES         top

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

       ┌───────────────────┬───────────────┬────────────────────────────┐
       │Interface          Attribute     Value                      │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │gethostbyname()    │ Thread safety │ MT-Unsafe race:hostbyname  │
       │                   │               │ env locale                 │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │gethostbyaddr()    │ Thread safety │ MT-Unsafe race:hostbyaddr  │
       │                   │               │ env locale                 │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │sethostent(),      │ Thread safety │ MT-Unsafe race:hostent env │
       │endhostent(),      │               │ locale                     │
       │gethostent_r()     │               │                            │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │herror(),          │ Thread safety │ MT-Safe                    │
       │hstrerror()        │               │                            │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │gethostent()       │ Thread safety │ MT-Unsafe race:hostent     │
       │                   │               │ race:hostentbuf env locale │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │gethostbyname2()   │ Thread safety │ MT-Unsafe race:hostbyname2 │
       │                   │               │ env locale                 │
       ├───────────────────┼───────────────┼────────────────────────────┤
       │gethostbyaddr_r(), │ Thread safety │ MT-Safe env locale         │
       │gethostbyname_r(), │               │                            │
       │gethostbyname2_r() │               │                            │
       └───────────────────┴───────────────┴────────────────────────────┘
       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.

CONFORMING TO         top

       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         top

       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.

   System V/POSIX extension
       POSIX requires the gethostent() call, which should return the
       next entry in the host data base.  When using DNS/BIND this does
       not make much sense, but it may be reasonable if the host data
       base is a file that can be read line by line.  On many systems, a
       routine of this name reads from the file /etc/hosts.  It may be
       available only when the library was built without DNS support.
       The glibc version will ignore ipv6 entries.  This function is not
       reentrant, and glibc adds a reentrant version gethostent_r().

   GNU extensions
       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 gethostent_r(),
       gethostbyaddr_r(), gethostbyname_r(), and gethostbyname2_r().
       The caller supplies a hostent structure ret which will be filled
       in on success, and a temporary work buffer buf of size buflen.
       After the call, result will point to the result on success.  In
       case of an error or if no entry is found result will be NULL.
       The functions return 0 on success and a nonzero error number on
       failure.  In addition to the errors returned by the nonreentrant
       versions of these functions, if buf is too small, the functions
       will return ERANGE, and the call should be retried with a larger
       buffer.  The global variable h_errno is not modified, but the
       address of a variable in which to store error numbers is passed
       in h_errnop.

BUGS         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

                               2021-03-22               GETHOSTBYNAME(3)

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