freebsd-src/lib/libc/net/gethostbyname.3

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.Dd June 27, 2022
.Dt GETHOSTBYNAME 3
.Os
.Sh NAME
.Nm gethostbyname ,
.Nm gethostbyname2 ,
.Nm gethostbyaddr ,
.Nm gethostent ,
.Nm sethostent ,
.Nm endhostent ,
.Nm herror ,
.Nm hstrerror ,
.Nm gethostbyname_r ,
.Nm gethostbyname2_r ,
.Nm gethostbyaddr_r
.Nd get network host entry
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In netdb.h
.Vt int h_errno ;
.Ft struct hostent *
.Fn gethostbyname "const char *name"
.Ft struct hostent *
.Fn gethostbyname2 "const char *name" "int af"
.Ft struct hostent *
.Fn gethostbyaddr "const void *addr" "socklen_t len" "int af"
.Ft struct hostent *
.Fn gethostent void
.Ft void
.Fn sethostent "int stayopen"
.Ft void
.Fn endhostent void
.Ft void
.Fn herror "const char *string"
.Ft const char *
.Fn hstrerror "int err"
.Ft int
.Fn gethostbyname_r "const char *name" "struct hostent *he" "char *buffer" "size_t buflen" "struct hostent **result" "int *h_errnop"
.Ft int
.Fn gethostbyname2_r "const char *name" "int af" "struct hostent *he" "char *buffer" "size_t buflen" "struct hostent **result" "int *h_errnop"
.Ft int
.Fn gethostbyaddr_r "const void *addr" "socklen_t len" "int af" "struct hostent *hp" "char *buf" "size_t buflen" "struct hostent **result" "int *h_errno"p
.Sh DESCRIPTION
.Bf -symbolic
The
.Xr getaddrinfo 3
and
.Xr getnameinfo 3
functions are preferred over the
.Fn gethostbyname ,
.Fn gethostbyname2 ,
and
.Fn gethostbyaddr
functions.
.Ef
.Pp
The
.Fn gethostbyname ,
.Fn gethostbyname2
and
.Fn gethostbyaddr
functions
each return a pointer to an object with the
following structure describing an internet host
referenced by name or by address, respectively.
.Pp
The
.Fa name
argument passed to
.Fn gethostbyname
or
.Fn gethostbyname2
should point to a
.Dv NUL Ns -terminated
hostname.
The
.Fa addr
argument passed to
.Fn gethostbyaddr
should point to an address which is
.Fa len
bytes long,
in binary form
(i.e., not an IP address in human readable
.Tn ASCII
form).
The
.Fa af
argument specifies the address family
(e.g.\&
.Dv AF_INET , AF_INET6 ,
etc.) of this address.
.Pp
The structure returned contains either the information obtained from the name
server, broken-out fields from a line in
.Pa /etc/hosts ,
or database entries supplied by the
.Xr yp 8
system.
The order of the lookups is controlled by the
.Sq hosts
entry in
.Xr nsswitch.conf 5 .
.Bd -literal
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 from name server */
};
#define	h_addr  h_addr_list[0]	/* address, for backward compatibility */
.Ed
.Pp
The members of this structure are:
.Bl -tag -width h_addr_list
.It Va h_name
Official name of the host.
.It Va h_aliases
A
.Dv NULL Ns -terminated
array of alternate names for the host.
.It Va h_addrtype
The type of address being returned; usually
.Dv AF_INET .
.It Va h_length
The length, in bytes, of the address.
.It Va h_addr_list
A
.Dv NULL Ns -terminated
array of network addresses for the host.
Host addresses are returned in network byte order.
.It Va h_addr
The first address in
.Va h_addr_list ;
this is for backward compatibility.
.El
.Pp
When using the nameserver,
.Fn gethostbyname
and
.Fn gethostbyname2
will search for the named host in the current domain and its parents
unless the name ends in a dot.
If the name contains no dot, and if the environment variable
.Dq Ev HOSTALIASES
contains the name of an alias file, the alias file will first be searched
for an alias matching the input name.
See
.Xr hostname 7
for the domain search procedure and the alias file format.
.Pp
The
.Fn gethostbyname2
function is an evolution of
.Fn gethostbyname
which is intended to allow lookups in address families other than
.Dv AF_INET ,
for example
.Dv AF_INET6 .
.Pp
The
.Fn sethostent
function
may be used to request the use of a connected
.Tn TCP
socket for queries.
Queries will by default use
.Tn UDP
datagrams.
If the
.Fa stayopen
flag is non-zero, a
.Tn TCP
connection to the name server will be used.
It will remain open after calls to
.Fn gethostbyname ,
.Fn gethostbyname2
or
.Fn gethostbyaddr
have completed.
.Pp
The
.Fn endhostent
function
closes the
.Tn TCP
connection.
.Pp
The
.Fn herror
function writes a message to the diagnostic output consisting of the
string argument
.Fa string ,
the constant string
.Qq Li ":\ " ,
and a message corresponding to the value of
.Va h_errno .
.Pp
The
.Fn hstrerror
function returns a string which is the message text corresponding to the
value of the
.Fa err
argument.
.Pp
Functions with the
.Em _r
suffix provide reentrant versions of their respective counterparts.
The caller must supply five additional parameters: a
.Vt struct hostent
variable to be filled on success, a
.Va buffer
of
.Va buflen
bytes in size, a
.Vt struct hostent
.Va result
variable that will point to the result on success or be set to
.Dv NULL
on failure or if the name is not found.
The
.Va h_errnop
variable will be filled with the error code if any.
All these functions return 0 on success.
.Sh FILES
.Bl -tag -width /etc/nsswitch.conf -compact
.It Pa /etc/hosts
.It Pa /etc/nsswitch.conf
.It Pa /etc/resolv.conf
.El
.Sh EXAMPLES
Print out the hostname associated with a specific IP address:
.Bd -literal -offset indent
const char *ipstr = "127.0.0.1";
struct in_addr ip;
struct hostent *hp;

if (!inet_aton(ipstr, &ip))
	errx(1, "can't parse IP address %s", ipstr);

if ((hp = gethostbyaddr((const void *)&ip,
    sizeof ip, AF_INET)) == NULL)
	errx(1, "no name associated with %s", ipstr);

printf("name associated with %s is %s\en", ipstr, hp->h_name);
.Ed
.Sh DIAGNOSTICS
Error return status from
.Fn gethostbyname ,
.Fn gethostbyname2
and
.Fn gethostbyaddr
is indicated by return of a
.Dv NULL
pointer.
The integer
.Va h_errno
may then be checked to see whether this is a temporary failure
or an invalid or unknown host.
The routine
.Fn herror
can be used to print an error message describing the failure.
If its argument
.Fa string
is
.Pf non- Dv NULL ,
it is printed, followed by a colon and a space.
The error message is printed with a trailing newline.
.Pp
The variable
.Va h_errno
can have the following values:
.Bl -tag -width HOST_NOT_FOUND
.It Dv HOST_NOT_FOUND
No such host is known.
.It Dv TRY_AGAIN
This is usually a temporary error
and means that the local server did not receive
a response from an authoritative server.
A retry at some later time may succeed.
.It Dv NO_RECOVERY
Some unexpected server failure was encountered.
This is a non-recoverable error.
.It Dv NO_DATA
The requested name is valid but does not have an IP address;
this is not a temporary error.
This means that the name is known to the name server but there is no address
associated with this name.
Another type of request to the name server using this domain name
will result in an answer;
for example, a mail-forwarder may be registered for this domain.
.El
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr getnameinfo 3 ,
.Xr inet_aton 3 ,
.Xr resolver 3 ,
.Xr hosts 5 ,
.Xr hostname 7
.Sh HISTORY
The
.Fn herror
function appeared in
.Bx 4.3 .
The
.Fn endhostent ,
.Fn gethostbyaddr ,
.Fn gethostbyname ,
.Fn gethostent ,
and
.Fn sethostent
functions appeared in
.Bx 4.2 .
The
.Fn gethostbyname2
function first appeared in
.Tn BIND
version 4.9.4.
The
.Fn gethostbyname_r
function first appeared in
.Fx 6.2 .
.Sh CAVEATS
The
.Fn gethostent
function
is defined, and
.Fn sethostent
and
.Fn endhostent
are redefined,
when
.Lb libc
is built to use only the routines to lookup in
.Pa /etc/hosts
and not the name server.
.Pp
The
.Fn gethostent
function
reads the next line of
.Pa /etc/hosts ,
opening the file if necessary.
.Pp
The
.Fn sethostent
function
opens and/or rewinds the file
.Pa /etc/hosts .
If the
.Fa stayopen
argument is non-zero,
the file will not be closed after each call to
.Fn gethostbyname ,
.Fn gethostbyname2
or
.Fn gethostbyaddr .
.Pp
The
.Fn endhostent
function
closes the file.
.Sh BUGS
These functions use a thread-specific data storage;
if the data is needed for future use, it should be
copied before any subsequent calls overwrite it.
.Pp
Though these functions are thread-safe,
still it is recommended to use the
.Xr getaddrinfo 3
family of functions, instead.
.Pp
Only the Internet
address format is currently understood.