man3/gethostbyname.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-05-22, David Metcalfe
.\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
.\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
.\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2002-08-05, Michael Kerrisk
.\" Modified 2004-10-31, Andries Brouwer
.\"
.TH GETHOSTBYNAME 3 2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
h_errno,
herror, hstrerror,
gethostbyaddr_r,
gethostbyname2, gethostbyname2_r, gethostbyname_r,
gethostent_r \- get network host entry
.SH SYNOPSIS
.nf
.B #include <netdb.h>
.PP
.B extern int h_errno;
.PP
.BI "struct hostent *gethostbyname(const char *" name );
.BI "struct hostent *gethostbyaddr(const void *" addr ,
.BI "                              socklen_t " len ", int " type );
.PP
.BI "void sethostent(int " stayopen );
.B void endhostent(void);
.PP
.BI "void herror(const char *" s );
.BI "const char *hstrerror(int " err );
.PP
/* System V/POSIX extension */
.B struct hostent *gethostent(void);
.PP
/* GNU extensions */
.BI "struct hostent *gethostbyname2(const char *" name ", int " af );
.PP
.BI "int gethostent_r(struct hostent *restrict " ret ,
.BI "                 char *restrict " buf ", size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.PP
.BI "int gethostbyaddr_r(const void *restrict " addr ", socklen_t " len \
", int " type ,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char *restrict " buf ", size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.BI "int gethostbyname_r(const char *restrict " name ,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char *restrict " buf ", size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.BI "int gethostbyname2_r(const char *restrict " name ", int " af,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char *restrict " buf ", size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR gethostbyname2 (),
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
.BR gethostbyname2_r ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc up to and including 2.19:
        _BSD_SOURCE || _SVID_SOURCE
.fi
.PP
.BR herror (),
.BR hstrerror ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.8 to 2.19:
        _BSD_SOURCE || _SVID_SOURCE
    Before glibc 2.8:
        none
.fi
.PP
.BR h_errno :
.nf
    Since glibc 2.19
        _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
    Glibc 2.12 to 2.19:
        _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
    Before glibc 2.12:
        none
.fi
.SH DESCRIPTION
The
.BR gethostbyname* (),
.BR gethostbyaddr* (),
.BR herror (),
and
.BR hstrerror ()
functions are obsolete.
Applications should use
.BR getaddrinfo (3),
.BR getnameinfo (3),
and
.BR gai_strerror (3)
instead.
.PP
The
.BR gethostbyname ()
function returns a structure of type
.I hostent
for the given host
.IR name .
Here
.I name
is either a hostname or an IPv4 address in standard dot notation (as for
.BR inet_addr (3)).
If
.I name
is an IPv4 address, no lookup is performed and
.BR gethostbyname ()
simply copies
.I name
into the
.I h_name
field and its
.I struct in_addr
equivalent into the
.I h_addr_list[0]
field of the returned
.I hostent
structure.
If
.I name
doesn't end in a dot and the environment variable
.B HOSTALIASES
is set, the alias file pointed to by
.B HOSTALIASES
will first be searched for
.I name
(see
.BR hostname (7)
for the file format).
The current domain and its parents are searched unless \fIname\fP
ends in a dot.
.PP
The
.BR gethostbyaddr ()
function returns a structure of type \fIhostent\fP
for the given host address \fIaddr\fP of length \fIlen\fP and address type
\fItype\fP.
Valid address types are
.B AF_INET
and
.BR AF_INET6
(defined in
.IR <sys/socket.h> ).
The host address argument is a pointer to a struct of a type depending
on the address type, for example a \fIstruct in_addr *\fP (probably
obtained via a call to
.BR inet_addr (3))
for address type
.BR AF_INET .
.PP
The
.BR sethostent ()
function specifies, if \fIstayopen\fP is true (1),
that a connected TCP socket should be used for the name server queries and
that the connection should remain open during successive queries.
Otherwise, name server queries will use UDP datagrams.
.PP
The
.BR endhostent ()
function ends the use of a TCP connection for name
server queries.
.PP
The (obsolete)
.BR herror ()
function prints the error message associated
with the current value of \fIh_errno\fP on \fIstderr\fP.
.PP
The (obsolete)
.BR hstrerror ()
function takes an error number
(typically \fIh_errno\fP) and returns the corresponding message string.
.PP
The domain name queries carried out by
.BR gethostbyname ()
and
.BR gethostbyaddr ()
rely on the Name Service Switch
.RB ( nsswitch.conf (5))
configured sources or a local name server
.RB ( named (8)).
The default action is to query the Name Service Switch
.RB ( nsswitch.conf (5))
configured sources, failing that, a local name server
.RB ( named (8)).
.\"
.SS Historical
The
.BR nsswitch.conf (5)
file is the modern way of controlling the order of host lookups.
.PP
In glibc 2.4 and earlier, the
.I order
keyword was used to control the order of host lookups as defined in
.IR /etc/host.conf
.RB ( host.conf (5)).
.PP
The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
.PP
.in +4n
.EX
struct hostent {
    char  *h_name;            /* official name of host */
    char **h_aliases;         /* alias list */
    int    h_addrtype;        /* host address type */
    int    h_length;          /* length of address */
    char **h_addr_list;       /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
.EE
.in
.PP
The members of the \fIhostent\fP structure are:
.TP
.I h_name
The official name of the host.
.TP
.I h_aliases
An array of alternative names for the host, terminated by a null pointer.
.TP
.I h_addrtype
The type of address; always
.B AF_INET
or
.B AF_INET6
at present.
.TP
.I h_length
The length of the address in bytes.
.TP
.I h_addr_list
An array of pointers to network addresses for the host (in network byte
order), terminated by a null pointer.
.TP
.I h_addr
The first address in \fIh_addr_list\fP for backward compatibility.
.SH RETURN VALUE
The
.BR gethostbyname ()
and
.BR gethostbyaddr ()
functions return the
.I hostent
structure or a null pointer if an error occurs.
On error, the
.I h_errno
variable holds an error number.
When non-NULL, the return value may point at static data, see the notes below.
.SH ERRORS
The variable \fIh_errno\fP can have the following values:
.TP
.B HOST_NOT_FOUND
The specified host is unknown.
.TP
.BR NO_DATA
The requested name is valid but does not have an IP address.
Another type of request to the name server for this domain
may return an answer.
The constant
.BR NO_ADDRESS
is a synonym for
.BR NO_DATA .
.TP
.B NO_RECOVERY
A nonrecoverable name server error occurred.
.TP
.B TRY_AGAIN
A temporary error occurred on an authoritative name server.
Try again later.
.SH FILES
.TP
.I /etc/host.conf
resolver configuration file
.TP
.I /etc/hosts
host database file
.TP
.I /etc/nsswitch.conf
name service switch configuration
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.BR gethostbyname ()
T}	Thread safety	T{
MT-Unsafe race:hostbyname env
locale
T}
T{
.BR gethostbyaddr ()
T}	Thread safety	T{
MT-Unsafe race:hostbyaddr env
locale
T}
T{
.BR sethostent (),
.BR endhostent (),
.BR gethostent_r ()
T}	Thread safety	T{
MT-Unsafe race:hostent env
locale
T}
T{
.BR herror (),
.BR hstrerror ()
T}	Thread safety	MT-Safe
T{
.BR gethostent ()
T}	Thread safety	T{
MT-Unsafe race:hostent
race:hostentbuf env locale
T}
T{
.BR gethostbyname2 ()
T}	Thread safety	T{
MT-Unsafe race:hostbyname2
env locale
T}
T{
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
.BR gethostbyname2_r ()
T}	Thread safety	MT-Safe env locale
.TE
.hy
.ad
.sp 1
In the above table,
.I hostent
in
.I race:hostent
signifies that if any of the functions
.BR sethostent (),
.BR gethostent (),
.BR gethostent_r (),
or
.BR endhostent ()
are used in parallel in different threads of a program,
then data races could occur.
.SH CONFORMING TO
POSIX.1-2001 specifies
.BR gethostbyname (),
.BR gethostbyaddr (),
.BR sethostent (),
.BR endhostent (),
.BR gethostent (),
and
.IR h_errno ;
.BR gethostbyname (),
.BR gethostbyaddr (),
and
.IR h_errno
are marked obsolescent in that standard.
POSIX.1-2008 removes the specifications of
.BR gethostbyname (),
.BR gethostbyaddr (),
and
.IR h_errno ,
recommending the use of
.BR getaddrinfo (3)
and
.BR getnameinfo (3)
instead.
.SH NOTES
The functions
.BR gethostbyname ()
and
.BR gethostbyaddr ()
may return pointers to static data, which may be overwritten by
later calls.
Copying the
.I struct hostent
does not suffice, since it contains pointers; a deep copy is required.
.PP
In the original BSD implementation the
.I len
argument
of
.BR gethostbyname ()
was an
.IR int .
The SUSv2 standard is buggy and declares the
.I len
argument of
.BR gethostbyaddr ()
to be of type
.IR size_t .
(That is wrong, because it has to be
.IR int ,
and
.I size_t
is not.
POSIX.1-2001 makes it
.IR socklen_t ,
which is OK.)
See also
.BR accept (2).
.PP
The BSD prototype for
.BR gethostbyaddr ()
uses
.I "const char\ *"
for the first argument.
.SS System V/POSIX extension
POSIX requires the
.BR gethostent ()
call, which should return the next entry in the host data base.
When using DNS/BIND this does not make much sense, but it may
be reasonable if the host data base is a file that can be read
line by line.
On many systems, a routine of this name reads
from the file
.IR /etc/hosts .
.\" e.g., Linux, FreeBSD, UnixWare, HP-UX
It may be available only when the library was built without DNS support.
.\" e.g., FreeBSD, AIX
The glibc version will ignore ipv6 entries.
This function is not reentrant,
and glibc adds a reentrant version
.BR gethostent_r ().
.SS GNU extensions
Glibc2 also has a
.BR gethostbyname2 ()
that works like
.BR gethostbyname (),
but permits to specify the address family to which the address must belong.
.PP
Glibc2 also has reentrant versions
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
and
.BR gethostbyname2_r ().
The caller supplies a
.I hostent
structure
.I ret
which will be filled in on success, and a temporary work buffer
.I buf
of size
.IR buflen .
After the call,
.I result
will point to the result on success.
In case of an error
or if no entry is found
.I result
will be NULL.
The functions return 0 on success and a nonzero error number on failure.
In addition to the errors returned by the nonreentrant
versions of these functions, if
.I buf
is too small, the functions will return
.BR ERANGE ,
and the call should be retried with a larger buffer.
The global variable
.I h_errno
is not modified, but the address of a variable in which to store error numbers
is passed in
.IR h_errnop .
.SH BUGS
.BR gethostbyname ()
does not recognize components of a dotted IPv4 address string
that are expressed in hexadecimal.
.\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
.SH SEE ALSO
.BR getaddrinfo (3),
.\" .BR getipnodebyaddr (3),
.\" .BR getipnodebyname (3),
.BR getnameinfo (3),
.BR inet (3),
.BR inet_ntop (3),
.BR inet_pton (3),
.BR resolver (3),
.BR hosts (5),
.BR nsswitch.conf (5),
.BR hostname (7),
.BR named (8)
.\" .BR resolv+ (8)