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(): _BSD_SOURCE || _SVID_SOURCE
54 herror(), hstrerror() (since glibc 2.8): _BSD_SOURCE || _SVID_SOURCE ||
56 _GNU_SOURCE
57
59 The gethostbyname*() and gethostbyaddr*() functions are obsolete.
60 Applications should use getaddrinfo(3) and getnameinfo(3) instead.
61 The gethostbyname() function returns a structure of type hostent for
63 the given host name. Here name is either a hostname, or an IPv4
64 address in standard dot notation (as for inet_addr(3)), or an IPv6
65 address in colon (and possibly dot) notation. (See RFC 1884 for the
66 description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no
67 lookup is performed and gethostbyname() simply copies name into the
68 h_name field and its struct in_addr equivalent into the h_addr_list[0]
69 field of the returned hostent structure. If name doesn't end in a dot
70 and the environment variable HOSTALIASES is set, the alias file pointed
71 to by HOSTALIASES will first be searched for name (see hostname(7) for
72 the file format). The current domain and its parents are searched
73 unless name ends in a dot.
74 The gethostbyaddr() function returns a structure of type hostent for
76 the given host address addr of length len and address type type. Valid
77 address types are AF_INET and AF_INET6. The host address argument is a
78 pointer to a struct of a type depending on the address type, for exam‐
79 ple a struct in_addr * (probably obtained via a call to inet_addr(3))
80 for address type AF_INET.
81 The sethostent() function specifies, if stayopen is true (1), that a
83 connected TCP socket should be used for the name server queries and
84 that the connection should remain open during successive queries. Oth‐
85 erwise, name server queries will use UDP datagrams.
86 The endhostent() function ends the use of a TCP connection for name
88 server queries.
89 The (obsolete) herror() function prints the error message associated
91 with the current value of h_errno on stderr.
92 The (obsolete) hstrerror() function takes an error number (typically
94 h_errno) and returns the corresponding message string.
95 The domain name queries carried out by gethostbyname() and gethost‐
97 byaddr() use a combination of any or all of the name server named(8), a
98 broken out line from /etc/hosts, and the Network Information Service
99 (NIS or YP), depending upon the contents of the order line in
100 /etc/host.conf. The default action is to query named(8), followed by
101 /etc/hosts.
102 The hostent structure is defined in <netdb.h> as follows:
104 struct hostent {
106 char *h_name; /* official name of host */
107 char **h_aliases; /* alias list */
108 int h_addrtype; /* host address type */
109 int h_length; /* length of address */
110 char **h_addr_list; /* list of addresses */
111 }
112 #define h_addr h_addr_list[0] /* for backward compatibility */
113 The members of the hostent structure are:
115 h_name The official name of the host.
117 h_aliases
119 An array of alternative names for the host, terminated by a NULL
120 pointer.
121 h_addrtype
123 The type of address; always AF_INET or AF_INET6 at present.
124 h_length
126 The length of the address in bytes.
127 h_addr_list
129 An array of pointers to network addresses for the host (in net‐
130 work byte order), terminated by a NULL pointer.
131 h_addr The first address in h_addr_list for backward compatibility.
133
135 The gethostbyname() and gethostbyaddr() functions return the hostent
136 structure or a NULL pointer if an error occurs. On error, the h_errno
137 variable holds an error number. When non-NULL, the return value may
138 point at static data, see the notes below.
139
141 The variable h_errno can have the following values:
142 HOST_NOT_FOUND
144 The specified host is unknown.
145 NO_ADDRESS or NO_DATA
147 The requested name is valid but does not have an IP address.
148 NO_RECOVERY
150 A nonrecoverable name server error occurred.
151 TRY_AGAIN
153 A temporary error occurred on an authoritative name server. Try
154 again later.
155
157 /etc/host.conf
158 resolver configuration file
159 /etc/hosts
161 host database file
162 /etc/nsswitch.conf
164 name service switch configuration
165
167 POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(),
168 endhostent(), gethostent(), and h_errno; gethostbyname(), gethost‐
169 byaddr(), and h_errno are marked obsolescent in that standard.
170 POSIX.1-2008 removes the specifications of gethostbyname(), gethost‐
171 byaddr(), and h_errno, recommending the use of getaddrinfo(3) and get‐
172 nameinfo(3) instead.
173
175 The functions gethostbyname() and gethostbyaddr() may return pointers
176 to static data, which may be overwritten by later calls. Copying the
177 struct hostent does not suffice, since it contains pointers; a deep
178 copy is required.
179 In the original BSD implementation the len argument of gethostbyname()
181 was an int. The SUSv2 standard is buggy and declares the len argument
182 of gethostbyaddr() to be of type size_t. (That is wrong, because it
183 has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t,
184 which is OK.) See also accept(2).
185 The BSD prototype for gethostbyaddr() uses const char * for the first
187 argument.
188 System V/POSIX Extension
190 POSIX requires the gethostent() call, that should return the next entry
191 in the host data base. When using DNS/BIND this does not make much
192 sense, but it may be reasonable if the host data base is a file that
193 can be read line by line. On many systems a routine of this name reads
194 from the file /etc/hosts. It may be available only when the library
195 was built without DNS support. The glibc version will ignore ipv6
196 entries. This function is not reentrant, and glibc adds a reentrant
197 version gethostent_r().
198 GNU Extensions
200 Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
201 permits to specify the address family to which the address must belong.
202 Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(),
204 gethostbyname_r() and gethostbyname2_r(). The caller supplies a hos‐
205 tent structure ret which will be filled in on success, and a temporary
206 work buffer buf of size buflen. After the call, result will point to
207 the result on success. In case of an error or if no entry is found
208 result will be NULL. The functions return 0 on success and a nonzero
209 error number on failure. In addition to the errors returned by the
210 nonreentrant versions of these functions, if buf is too small, the
211 functions will return ERANGE, and the call should be retried with a
212 larger buffer. The global variable h_errno is not modified, but the
213 address of a variable in which to store error numbers is passed in
214 h_errnop.
215
217 gethostbyname() does not recognize components of a dotted IPv4 address
218 string that are expressed in hexadecimal.
219
221 getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3),
222 resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
223
225 This page is part of release 3.25 of the Linux man-pages project. A
226 description of the project, and information about reporting bugs, can
227 be found at http://www.kernel.org/doc/man-pages/.
228 2009-12-03 GETHOSTBYNAME(3)