1GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
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6 gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
7 h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostby‐
8 name2_r, gethostbyname_r, gethostent_r - get network host entry
9
11 #include <netdb.h>
12 extern int h_errno;
13 struct hostent *gethostbyname(const char *name);
15 #include <sys/socket.h> /* for AF_INET */
17 struct hostent *gethostbyaddr(const void *addr,
18 socklen_t len, int type);
19 void sethostent(int stayopen);
21 void endhostent(void);
23 void herror(const char *s);
25 const char *hstrerror(int err);
27 /* System V/POSIX extension */
29 struct hostent *gethostent(void);
30 /* GNU extensions */
32 struct hostent *gethostbyname2(const char *name, int af);
33 int gethostent_r(
35 struct hostent *ret, char *buf, size_t buflen,
36 struct hostent **result, int *h_errnop);
37 int gethostbyaddr_r(const void *addr, socklen_t len, int type,
39 struct hostent *ret, char *buf, size_t buflen,
40 struct hostent **result, int *h_errnop);
41 int gethostbyname_r(const char *name,
43 struct hostent *ret, char *buf, size_t buflen,
44 struct hostent **result, int *h_errnop);
45 int gethostbyname2_r(const char *name, int af,
47 struct hostent *ret, char *buf, size_t buflen,
48 struct hostent **result, int *h_errnop);
49 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
51 gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(),
53 gethostbyname2_r():
54 _BSD_SOURCE || _SVID_SOURCE
55 herror(), hstrerror():
57 Since glibc 2.8:
58 _BSD_SOURCE || _SVID_SOURCE || _GNU_SOURCE
59 Before glibc 2.8:
60 none
61
63 The gethostbyname*() and gethostbyaddr*() functions are obsolete.
64 Applications should use getaddrinfo(3) and getnameinfo(3) instead.
65 The gethostbyname() function returns a structure of type hostent for
67 the given host name. Here name is either a hostname, or an IPv4
68 address in standard dot notation (as for inet_addr(3)), or an IPv6
69 address in colon (and possibly dot) notation. (See RFC 1884 for the
70 description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no
71 lookup is performed and gethostbyname() simply copies name into the
72 h_name field and its struct in_addr equivalent into the h_addr_list[0]
73 field of the returned hostent structure. If name doesn't end in a dot
74 and the environment variable HOSTALIASES is set, the alias file pointed
75 to by HOSTALIASES will first be searched for name (see hostname(7) for
76 the file format). The current domain and its parents are searched
77 unless name ends in a dot.
78 The gethostbyaddr() function returns a structure of type hostent for
80 the given host address addr of length len and address type type. Valid
81 address types are AF_INET and AF_INET6. The host address argument is a
82 pointer to a struct of a type depending on the address type, for exam‐
83 ple a struct in_addr * (probably obtained via a call to inet_addr(3))
84 for address type AF_INET.
85 The sethostent() function specifies, if stayopen is true (1), that a
87 connected TCP socket should be used for the name server queries and
88 that the connection should remain open during successive queries. Oth‐
89 erwise, name server queries will use UDP datagrams.
90 The endhostent() function ends the use of a TCP connection for name
92 server queries.
93 The (obsolete) herror() function prints the error message associated
95 with the current value of h_errno on stderr.
96 The (obsolete) hstrerror() function takes an error number (typically
98 h_errno) and returns the corresponding message string.
99 The domain name queries carried out by gethostbyname() and gethost‐
101 byaddr() use a combination of any or all of the name server named(8), a
102 broken out line from /etc/hosts, and the Network Information Service
103 (NIS or YP), depending upon the contents of the order line in
104 /etc/host.conf. The default action is to query named(8), followed by
105 /etc/hosts.
106 The hostent structure is defined in <netdb.h> as follows:
108 struct hostent {
110 char *h_name; /* official name of host */
111 char **h_aliases; /* alias list */
112 int h_addrtype; /* host address type */
113 int h_length; /* length of address */
114 char **h_addr_list; /* list of addresses */
115 }
116 #define h_addr h_addr_list[0] /* for backward compatibility */
117 The members of the hostent structure are:
119 h_name The official name of the host.
121 h_aliases
123 An array of alternative names for the host, terminated by a NULL
124 pointer.
125 h_addrtype
127 The type of address; always AF_INET or AF_INET6 at present.
128 h_length
130 The length of the address in bytes.
131 h_addr_list
133 An array of pointers to network addresses for the host (in net‐
134 work byte order), terminated by a NULL pointer.
135 h_addr The first address in h_addr_list for backward compatibility.
137
139 The gethostbyname() and gethostbyaddr() functions return the hostent
140 structure or a NULL pointer if an error occurs. On error, the h_errno
141 variable holds an error number. When non-NULL, the return value may
142 point at static data, see the notes below.
143
145 The variable h_errno can have the following values:
146 HOST_NOT_FOUND
148 The specified host is unknown.
149 NO_ADDRESS or NO_DATA
151 The requested name is valid but does not have an IP address.
152 NO_RECOVERY
154 A nonrecoverable name server error occurred.
155 TRY_AGAIN
157 A temporary error occurred on an authoritative name server. Try
158 again later.
159
161 /etc/host.conf
162 resolver configuration file
163 /etc/hosts
165 host database file
166 /etc/nsswitch.conf
168 name service switch configuration
169
171 POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(),
172 endhostent(), gethostent(), and h_errno; gethostbyname(), gethost‐
173 byaddr(), and h_errno are marked obsolescent in that standard.
174 POSIX.1-2008 removes the specifications of gethostbyname(), gethost‐
175 byaddr(), and h_errno, recommending the use of getaddrinfo(3) and get‐
176 nameinfo(3) instead.
177
179 The functions gethostbyname() and gethostbyaddr() may return pointers
180 to static data, which may be overwritten by later calls. Copying the
181 struct hostent does not suffice, since it contains pointers; a deep
182 copy is required.
183 In the original BSD implementation the len argument of gethostbyname()
185 was an int. The SUSv2 standard is buggy and declares the len argument
186 of gethostbyaddr() to be of type size_t. (That is wrong, because it
187 has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t,
188 which is OK.) See also accept(2).
189 The BSD prototype for gethostbyaddr() uses const char * for the first
191 argument.
192 System V/POSIX extension
194 POSIX requires the gethostent() call, that should return the next entry
195 in the host data base. When using DNS/BIND this does not make much
196 sense, but it may be reasonable if the host data base is a file that
197 can be read line by line. On many systems a routine of this name reads
198 from the file /etc/hosts. It may be available only when the library
199 was built without DNS support. The glibc version will ignore ipv6
200 entries. This function is not reentrant, and glibc adds a reentrant
201 version gethostent_r().
202 GNU extensions
204 Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
205 permits to specify the address family to which the address must belong.
206 Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(),
208 gethostbyname_r() and gethostbyname2_r(). The caller supplies a hos‐
209 tent structure ret which will be filled in on success, and a temporary
210 work buffer buf of size buflen. After the call, result will point to
211 the result on success. In case of an error or if no entry is found
212 result will be NULL. The functions return 0 on success and a nonzero
213 error number on failure. In addition to the errors returned by the
214 nonreentrant versions of these functions, if buf is too small, the
215 functions will return ERANGE, and the call should be retried with a
216 larger buffer. The global variable h_errno is not modified, but the
217 address of a variable in which to store error numbers is passed in
218 h_errnop.
219
221 gethostbyname() does not recognize components of a dotted IPv4 address
222 string that are expressed in hexadecimal.
223
225 getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3),
226 resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
227
229 This page is part of release 3.53 of the Linux man-pages project. A
230 description of the project, and information about reporting bugs, can
231 be found at http://www.kernel.org/doc/man-pages/.
232 2010-10-04 GETHOSTBYNAME(3)