gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror,

       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(), endhostent(), │ Thread safety │ MT-Unsafe race:hostent env locale                       │
       │ gethostent_r()              │               │                                                         │
       ├─────────────────────────────┼───────────────┼─────────────────────────────────────────────────────────┤
       │ herror(), hstrerror()       │ Thread safety │ MT-Safe                                                 │
       ├─────────────────────────────┼───────────────┼─────────────────────────────────────────────────────────┤
       │ 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.

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

       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
       structin_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 structin_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)).

   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.

       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.

/etc/host.conf
              resolver configuration file

       /etc/hosts
              host database file

       /etc/nsswitch.conf
              name service switch configuration

sethostent()
       endhostent()
       gethostent()
              POSIX.1-2001.

       gethostbyname()
       gethostbyaddr()
       h_errno
              Marked  obsolescent  in  POSIX.1-2001.   Removed  in  POSIX.1-2008,  recommending   the   use   of
              getaddrinfo(3) and getnameinfo(3) instead.

       Standard C library (libc, -lc)

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

       The  functions  gethostbyname()  and  gethostbyaddr()  may  return  pointers to static data, which may be
       overwritten by later calls.  Copying the structhostent 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 constchar* for the first argument.

   SystemV/POSIXextension
       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().

   GNUextensions
       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.

       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.

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

Linux man-pages 6.9.1                              2024-05-02                                   gethostbyname(3)

sethostent()
       endhostent()
       gethostent()
              POSIX.1-2008.

       gethostent_r()
              GNU.

       Others:
              None.

#include<netdb.h>voidsethostent(intstayopen);voidendhostent(void);[[deprecated]]externinth_errno;[[deprecated]]structhostent*gethostbyname(constchar*name);[[deprecated]]structhostent*gethostbyaddr(constvoidaddr[.len],socklen_tlen,inttype);[[deprecated]]voidherror(constchar*s);[[deprecated]]constchar*hstrerror(interr);

       /* System V/POSIX extension */
       structhostent*gethostent(void);

       /* GNU extensions */
       [[deprecated]]structhostent*gethostbyname2(constchar*name,intaf);intgethostent_r(structhostent*restrictret,charbuf[restrict.buflen],size_tbuflen,structhostent**restrictresult,int*restricth_errnop);[[deprecated]]intgethostbyaddr_r(constvoidaddr[restrict.len],socklen_tlen,inttype,structhostent*restrictret,charbuf[restrict.buflen],size_tbuflen,structhostent**restrictresult,int*restricth_errnop);[[deprecated]]intgethostbyname_r(constchar*restrictname,structhostent*restrictret,charbuf[restrict.buflen],size_tbuflen,structhostent**restrictresult,int*restricth_errnop);[[deprecated]]intgethostbyname2_r(constchar*restrictname,intaf,structhostent*restrictret,charbuf[restrict.buflen],size_tbuflen,structhostent**restrictresult,int*restricth_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 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