1GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
2
6 gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_er‐
7 rno, 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 Since glibc 2.19:
55 _DEFAULT_SOURCE
56 Glibc versions up to and including 2.19:
57 _BSD_SOURCE || _SVID_SOURCE
58 herror(), hstrerror():
60 Since glibc 2.19:
61 _DEFAULT_SOURCE
62 Glibc 2.8 to 2.19:
63 _BSD_SOURCE || _SVID_SOURCE
64 Before glibc 2.8:
65 none
66 h_errno:
68 Since glibc 2.19
69 _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
70 Glibc 2.12 to 2.19:
71 _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
72 Before glibc 2.12:
73 none
74
76 The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() func‐
77 tions are obsolete. Applications should use getaddrinfo(3), getname‐
78 info(3), and gai_strerror(3) instead.
79 The gethostbyname() function returns a structure of type hostent for
81 the given host name. Here name is either a hostname or an IPv4 address
82 in standard dot notation (as for inet_addr(3)). If name is an IPv4 ad‐
83 dress, no lookup is performed and gethostbyname() simply copies name
84 into the h_name field and its struct in_addr equivalent into the
85 h_addr_list[0] field of the returned hostent structure. If name
86 doesn't end in a dot and the environment variable HOSTALIASES is set,
87 the alias file pointed to by HOSTALIASES will first be searched for
88 name (see hostname(7) for the file format). The current domain and its
89 parents are searched unless name ends in a dot.
90 The gethostbyaddr() function returns a structure of type hostent for
92 the given host address addr of length len and address type type. Valid
93 address types are AF_INET and AF_INET6. The host address argument is a
94 pointer to a struct of a type depending on the address type, for exam‐
95 ple a struct in_addr * (probably obtained via a call to inet_addr(3))
96 for address type AF_INET.
97 The sethostent() function specifies, if stayopen is true (1), that a
99 connected TCP socket should be used for the name server queries and
100 that the connection should remain open during successive queries. Oth‐
101 erwise, name server queries will use UDP datagrams.
102 The endhostent() function ends the use of a TCP connection for name
104 server queries.
105 The (obsolete) herror() function prints the error message associated
107 with the current value of h_errno on stderr.
108 The (obsolete) hstrerror() function takes an error number (typically
110 h_errno) and returns the corresponding message string.
111 The domain name queries carried out by gethostbyname() and gethost‐
113 byaddr() rely on the Name Service Switch (nsswitch.conf(5)) configured
114 sources or a local name server (named(8)). The default action is to
115 query the Name Service Switch (nsswitch.conf(5)) configured sources,
116 failing that, a local name server (named(8)).
117 Historical
119 The nsswitch.conf(5) file is the modern way of controlling the order of
120 host lookups.
121 In glibc 2.4 and earlier, the order keyword was used to control the or‐
123 der of host lookups as defined in /etc/host.conf (host.conf(5)).
124 The hostent structure is defined in <netdb.h> as follows:
126 struct hostent {
128 char *h_name; /* official name of host */
129 char **h_aliases; /* alias list */
130 int h_addrtype; /* host address type */
131 int h_length; /* length of address */
132 char **h_addr_list; /* list of addresses */
133 }
134 #define h_addr h_addr_list[0] /* for backward compatibility */
135 The members of the hostent structure are:
137 h_name The official name of the host.
139 h_aliases
141 An array of alternative names for the host, terminated by a null
142 pointer.
143 h_addrtype
145 The type of address; always AF_INET or AF_INET6 at present.
146 h_length
148 The length of the address in bytes.
149 h_addr_list
151 An array of pointers to network addresses for the host (in net‐
152 work byte order), terminated by a null pointer.
153 h_addr The first address in h_addr_list for backward compatibility.
155
157 The gethostbyname() and gethostbyaddr() functions return the hostent
158 structure or a null pointer if an error occurs. On error, the h_errno
159 variable holds an error number. When non-NULL, the return value may
160 point at static data, see the notes below.
161
163 The variable h_errno can have the following values:
164 HOST_NOT_FOUND
166 The specified host is unknown.
167 NO_DATA
169 The requested name is valid but does not have an IP address.
170 Another type of request to the name server for this domain may
171 return an answer. The constant NO_ADDRESS is a synonym for
172 NO_DATA.
173 NO_RECOVERY
175 A nonrecoverable name server error occurred.
176 TRY_AGAIN
178 A temporary error occurred on an authoritative name server. Try
179 again later.
180
182 /etc/host.conf
183 resolver configuration file
184 /etc/hosts
186 host database file
187 /etc/nsswitch.conf
189 name service switch configuration
190
192 For an explanation of the terms used in this section, see at‐
193 tributes(7).
194 ┌───────────────────┬───────────────┬───────────────────────────────┐
196 │Interface │ Attribute │ Value │
197 ├───────────────────┼───────────────┼───────────────────────────────┤
198 │gethostbyname() │ Thread safety │ MT-Unsafe race:hostbyname env │
199 │ │ │ locale │
200 ├───────────────────┼───────────────┼───────────────────────────────┤
201 │gethostbyaddr() │ Thread safety │ MT-Unsafe race:hostbyaddr env │
202 │ │ │ locale │
203 ├───────────────────┼───────────────┼───────────────────────────────┤
204 │sethostent(), │ Thread safety │ MT-Unsafe race:hostent env │
205 │endhostent(), │ │ locale │
206 │gethostent_r() │ │ │
207 ├───────────────────┼───────────────┼───────────────────────────────┤
208 │herror(), │ Thread safety │ MT-Safe │
209 │hstrerror() │ │ │
210 ├───────────────────┼───────────────┼───────────────────────────────┤
211 │gethostent() │ Thread safety │ MT-Unsafe race:hostent │
212 │ │ │ race:hostentbuf env locale │
213 ├───────────────────┼───────────────┼───────────────────────────────┤
214 │gethostbyname2() │ Thread safety │ MT-Unsafe race:hostbyname2 │
215 │ │ │ env locale │
216 ├───────────────────┼───────────────┼───────────────────────────────┤
217 │gethostbyaddr_r(), │ Thread safety │ MT-Safe env locale │
218 │gethostbyname_r(), │ │ │
219 │gethostbyname2_r() │ │ │
220 └───────────────────┴───────────────┴───────────────────────────────┘
221 In the above table, hostent in race:hostent signifies that if any of
222 the functions sethostent(), gethostent(), gethostent_r(), or endhos‐
223 tent() are used in parallel in different threads of a program, then
224 data races could occur.
225
227 POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(),
228 endhostent(), gethostent(), and h_errno; gethostbyname(), gethost‐
229 byaddr(), and h_errno are marked obsolescent in that standard.
230 POSIX.1-2008 removes the specifications of gethostbyname(), gethost‐
231 byaddr(), and h_errno, recommending the use of getaddrinfo(3) and get‐
232 nameinfo(3) instead.
233
235 The functions gethostbyname() and gethostbyaddr() may return pointers
236 to static data, which may be overwritten by later calls. Copying the
237 struct hostent does not suffice, since it contains pointers; a deep
238 copy is required.
239 In the original BSD implementation the len argument of gethostbyname()
241 was an int. The SUSv2 standard is buggy and declares the len argument
242 of gethostbyaddr() to be of type size_t. (That is wrong, because it
243 has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t,
244 which is OK.) See also accept(2).
245 The BSD prototype for gethostbyaddr() uses const char * for the first
247 argument.
248 System V/POSIX extension
250 POSIX requires the gethostent() call, which should return the next en‐
251 try in the host data base. When using DNS/BIND this does not make much
252 sense, but it may be reasonable if the host data base is a file that
253 can be read line by line. On many systems, a routine of this name
254 reads from the file /etc/hosts. It may be available only when the li‐
255 brary was built without DNS support. The glibc version will ignore
256 ipv6 entries. This function is not reentrant, and glibc adds a reen‐
257 trant version gethostent_r().
258 GNU extensions
260 Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
261 permits to specify the address family to which the address must belong.
262 Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(),
264 gethostbyname_r(), and gethostbyname2_r(). The caller supplies a hos‐
265 tent structure ret which will be filled in on success, and a temporary
266 work buffer buf of size buflen. After the call, result will point to
267 the result on success. In case of an error or if no entry is found re‐
268 sult will be NULL. The functions return 0 on success and a nonzero er‐
269 ror number on failure. In addition to the errors returned by the non‐
270 reentrant versions of these functions, if buf is too small, the func‐
271 tions will return ERANGE, and the call should be retried with a larger
272 buffer. The global variable h_errno is not modified, but the address
273 of a variable in which to store error numbers is passed in h_errnop.
274
276 gethostbyname() does not recognize components of a dotted IPv4 address
277 string that are expressed in hexadecimal.
278
280 getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3),
281 resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
282
284 This page is part of release 5.10 of the Linux man-pages project. A
285 description of the project, information about reporting bugs, and the
286 latest version of this page, can be found at
287 https://www.kernel.org/doc/man-pages/.
288 2020-12-21 GETHOSTBYNAME(3)