1GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
2
6 gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, her‐
7 ror, hstrerror - get network host entry
8
10 #include <netdb.h>
11 extern int h_errno;
12 struct hostent *gethostbyname(const char *name);
14 #include <sys/socket.h> /* for AF_INET */
16 struct hostent *
17 gethostbyaddr(const void *addr, int len, int type);
18 void sethostent(int stayopen);
20 void endhostent(void);
22 void herror(const char *s);
24 const char *hstrerror(int err);
26 /* System V/POSIX extension */
29 struct hostent *gethostent(void);
30 /* GNU extensions */
33 struct hostent *gethostbyname2(const char *name, int af);
34 int gethostent_r(
36 struct hostent *ret, char *buf, size_t buflen,
37 struct hostent **result, int *h_errnop);
38 int gethostbyname_r(const char *name,
40 struct hostent *ret, char *buf, size_t buflen,
41 struct hostent **result, int *h_errnop);
42 int gethostbyname2_r(const char *name, int af,
44 struct hostent *ret, char *buf, size_t buflen,
45 struct hostent **result, int *h_errnop);
46
48 The gethostbyname() function returns a structure of type hostent for
49 the given host name. Here name is either a host name, or an IPv4
50 address in standard dot notation, or an IPv6 address in colon (and pos‐
51 sibly dot) notation. (See RFC 1884 for the description of IPv6
52 addresses.) If name is an IPv4 or IPv6 address, no lookup is performed
53 and gethostbyname() simply copies name into the h_name field and its
54 struct in_addr equivalent into the h_addr_list[0] field of the returned
55 hostent structure. If name doesn't end in a dot and the environment
56 variable HOSTALIASES is set, the alias file pointed to by HOSTALIASES
57 will first be searched for name (see hostname(7) for the file format).
58 The current domain and its parents are searched unless name ends in a
59 dot.
60 The gethostbyaddr() function returns a structure of type hostent for
62 the given host address addr of length len and address type type. Valid
63 address types are AF_INET and AF_INET6. The host address argument is a
64 pointer to a struct of a type depending on the address type, for exam‐
65 ple a struct in_addr * (probably obtained via a call to inet_addr())
66 for address type AF_INET.
67 The sethostent() function specifies, if stayopen is true (1), that a
69 connected TCP socket should be used for the name server queries and
70 that the connection should remain open during successive queries. Oth‐
71 erwise, name server queries will use UDP datagrams.
72 The endhostent() function ends the use of a TCP connection for name
74 server queries.
75 The (obsolete) herror() function prints the error message associated
77 with the current value of h_errno on stderr.
78 The (obsolete) hstrerror() function takes an error number (typically
80 h_errno) and returns the corresponding message string.
81 The domain name queries carried out by gethostbyname() and gethost‐
83 byaddr() use a combination of any or all of the name server named(8), a
84 broken out line from /etc/hosts, and the Network Information Service
85 (NIS or YP), depending upon the contents of the order line in
86 /etc/host.conf. The default action is to query named(8), followed by
87 /etc/hosts.
88 The hostent structure is defined in <netdb.h> as follows:
90 struct hostent {
92 char *h_name; /* official name of host */
93 char **h_aliases; /* alias list */
94 int h_addrtype; /* host address type */
95 int h_length; /* length of address */
96 char **h_addr_list; /* list of addresses */
97 }
98 #define h_addr h_addr_list[0] /* for backward compatibility */
99 The members of the hostent structure are:
101 h_name The official name of the host.
103 h_aliases
105 An array of alternative names for the host, terminated by a NULL
106 pointer.
107 h_addrtype
109 The type of address; always AF_INET or AF_INET6 at present.
110 h_length
112 The length of the address in bytes.
113 h_addr_list
115 An array of pointers to network addresses for the host (in net‐
116 work byte order), terminated by a NULL pointer.
117 h_addr The first address in h_addr_list for backward compatibility.
119
121 The gethostbyname() and gethostbyaddr() functions return the hostent
122 structure or a NULL pointer if an error occurs. On error, the h_errno
123 variable holds an error number. When non-NULL, the return value may
124 point at static data, see the notes below.
125
127 The variable h_errno can have the following values:
128 HOST_NOT_FOUND
130 The specified host is unknown.
131 NO_ADDRESS or NO_DATA
133 The requested name is valid but does not have an IP address.
134 NO_RECOVERY
136 A non-recoverable name server error occurred.
137 TRY_AGAIN
139 A temporary error occurred on an authoritative name server. Try
140 again later.
141
143 /etc/host.conf
144 resolver configuration file
145 /etc/hosts
147 host database file
148 /etc/nsswitch.conf
150 name service switch configuration
151
153 4.3BSD, POSIX.1-2001.
154
156 POSIX requires the gethostent() call, that should return the next entry
157 in the host data base. When using DNS/BIND this does not make much
158 sense, but it may be reasonable if the host data base is a file that
159 can be read line by line. On many systems a routine of this name reads
160 from the file /etc/hosts. It may be available only when the library
161 was built without DNS support. The glibc version will ignore ipv6
162 entries. This function is not reentrant, and glibc adds a reentrant
163 version gethostent_r().
164
166 Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
167 permits to specify the address family to which the address must belong.
168 Glibc2 also has reentrant versions gethostbyname_r() and gethostby‐
170 name2_r(). These return 0 on success and non-zero on error. The result
171 of the call is now stored in the struct with address ret. After the
172 call, *result will be NULL on error or point to the result on success.
173 Auxiliary data is stored in the buffer buf of length buflen. (If the
174 buffer is too small, these functions will return ERANGE.) No global
175 variable h_errno is modified, but the address of a variable in which to
176 store error numbers is passed in h_errnop.
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 The SUS-v2 standard is buggy and declares the len parameter of gethost‐
185 byaddr() to be of type size_t. (That is wrong, because it has to be
186 int, and size_t is not. POSIX.1-2001 makes it socklen_t, which is OK.)
187 The BSD prototype for gethostbyaddr() uses const char * for the first
189 argument.
190 POSIX.1-2001 marks gethostbyaddr() and gethostbyname() obsolescent. See
192 getaddrinfo(3), getnameinfo(3), gai_strerror(3).
193
195 getaddrinfo(3), getipnodebyaddr(3), getipnodebyname(3), getnameinfo(3),
196 inet_ntop(3), inet_pton(3), resolver(3), hosts(5), nsswitch.conf(5),
197 hostname(7), named(8)
198 2004-10-31 GETHOSTBYNAME(3)