1GETHOSTBYNAME(3)         BSD Library Functions Manual         GETHOSTBYNAME(3)
2

NAME

4     gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror
5     — get network host entry
6

SYNOPSIS

8     #include <netdb.h>
9
10     extern int h_errno;
11
12     struct hostent *
13     gethostbyname(char *name);
14
15     struct hostent *
16     gethostbyname2(char *name, int af);
17
18     struct hostent *
19     gethostbyaddr(char *addr, int len, type);
20
21     struct hostent *
22     gethostent();
23
24     sethostent(int stayopen);
25
26     endhostent();
27
28     herror(char *string);
29

DESCRIPTION

31     Gethostbyname(), gethostbyname2(), and gethostbyaddr() each return a
32     pointer to a hostent structure (see below) describing an internet host
33     referenced by name or by address, as the function names indicate.  This
34     structure contains either the information obtained from the name server,
35     named(8), or broken-out fields from a line in /etc/hosts.  If the local
36     name server is not running, these routines do a lookup in /etc/hosts.
37
38           struct  hostent {
39                   char    *h_name;        /* official name of host */
40                   char    **h_aliases;    /* alias list */
41                   int     h_addrtype;     /* host address type */
42                   int     h_length;       /* length of address */
43                   char    **h_addr_list;  /* list of addresses from name server */
44           };
45
46           #define h_addr  h_addr_list[0]  /* address, for backward compatibility */
47
48     The members of this structure are:
49
50     h_name       Official name of the host.
51
52     h_aliases    A zero-terminated array of alternate names for the host.
53
54     h_addrtype   The type of address being returned; usually AF_INET.
55
56     h_length     The length, in bytes, of the address.
57
58     h_addr_list  A zero-terminated array of network addresses for the host.
59                  Host addresses are returned in network byte order.
60
61     h_addr       The first address in h_addr_list; this is for backward com‐
62                  patibility.
63
64     When using the nameserver, gethostbyname() will search for the named host
65     in each parent domain given in the “search” directive of resolv.conf(5)
66     unless the name contains a dot (“.”).  If the name contains no dot, and
67     if the environment variable HOSTALIASES contains the name of an alias
68     file, the alias file will first be searched for an alias matching the
69     input name.  See hostname(7) for the domain search procedure and the
70     alias file format.
71
72     Gethostbyname2() is an evolution of gethostbyname() intended to allow
73     lookups in address families other than AF_INET, for example, AF_INET6.
74     Currently, the af argument must be specified as AF_INET else the function
75     will return NULL after having set h_errno to NETDB_INTERNAL.
76
77     Sethostent() may be used to request the use of a connected TCP socket for
78     queries.  If the stayopen flag is non-zero, this sets the option to send
79     all queries to the name server using TCP and to retain the connection
80     after each call to gethostbyname() or gethostbyaddr().  Otherwise,
81     queries are performed using UDP datagrams.
82
83     Endhostent() closes the TCP connection.
84

ENVIRONMENT

86     HOSTALIASES    Name of file containing (host alias, full hostname) pairs.
87

FILES

89     /etc/hosts     See hosts(5).
90

DIAGNOSTICS

92     Error return status from gethostbyname() and gethostbyaddr() is indicated
93     by return of a null pointer.  The external integer h_errno may then be
94     checked to see whether this is a temporary failure or an invalid or
95     unknown host.  The routine herror() can be used to print an error message
96     describing the failure.  If its argument string is non-NULL, it is
97     printed, followed by a colon and a space.  The error message is printed
98     with a trailing newline.
99
100     h_errno can have the following values:
101
102           NETDB_INTERNAL    This indicates an internal error in the library,
103                             unrelated to the network or name service.  errno
104                             will be valid in this case; see perror(3).
105
106           HOST_NOT_FOUND    No such host is known.
107
108           TRY_AGAIN         This is usually a temporary error and means that
109                             the local server did not receive a response from
110                             an authoritative server.  A retry at some later
111                             time may succeed.
112
113           NO_RECOVERY       Some unexpected server failure was encountered.
114                             This is a non-recoverable error, as one might
115                             expect.
116
117           NO_DATA           The requested name is valid but does not have an
118                             IP address; this is not a temporary error.  This
119                             means that the name is known to the name server
120                             but there is no address associated with this
121                             name.  Another type of request to the name server
122                             using this domain name will result in an answer;
123                             for example, a mail-forwarder may be registered
124                             for this domain.
125

SEE ALSO

127     hosts(5), hostname(7), named(8), resolver(3), resolver(5).
128

CAVEAT

130     Gethostent() is defined, and sethostent() and endhostent() are redefined,
131     when libc is built to use only the routines to lookup in /etc/hosts and
132     not the name server:
133
134
135           Gethostent() reads the next line of /etc/hosts, opening the file if
136           necessary.
137
138           Sethostent() is redefined to open and rewind the file.  If the
139           stayopen argument is non-zero, the hosts data base will not be
140           closed after each call to gethostbyname() or gethostbyaddr().
141
142           Endhostent() is redefined to close the file.
143

BUGS

145     All information is contained in a static area so it must be copied if it
146     is to be saved.  Only the Internet address format is currently under‐
147     stood.
148
1494th Berkeley Distribution        June 23, 1990       4th Berkeley Distribution
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