.\"	$KAME: getipnodebyname.3,v 1.6 2000/08/09 21:16:17 itojun Exp $
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.Dd June 27, 2022
.Dt GETIPNODEBYNAME 3
.Os
.\"
.Sh NAME
.Nm getipnodebyname ,
.Nm getipnodebyaddr ,
.Nm freehostent
.Nd nodename-to-address and address-to-nodename translation
.\"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netdb.h
.Ft "struct hostent *"
.Fn getipnodebyname "const char *name" "int af" "int flags" "int *error_num"
.Ft "struct hostent *"
.Fn getipnodebyaddr "const void *src" "size_t len" "int af" "int *error_num"
.Ft void
.Fn freehostent "struct hostent *ptr"
.\"
.Sh DESCRIPTION
The
.Fn getipnodebyname
and
.Fn getipnodebyaddr
functions are very similar to
.Xr gethostbyname 3 ,
.Xr gethostbyname2 3
and
.Xr gethostbyaddr 3 .
The functions cover all the functionalities provided by the older ones,
and provide better interface to programmers.
The functions require additional arguments,
.Fa af ,
and
.Fa flags ,
for specifying address family and operation mode.
The additional arguments allow programmer to get address for a nodename,
for specific address family
(such as
.Dv AF_INET
or
.Dv AF_INET6 ) .
The functions also require an additional pointer argument,
.Fa error_num
to return the appropriate error code,
to support thread safe error code returns.
.Pp
The type and usage of the return value,
.Li "struct hostent"
is described in
.Xr gethostbyname 3 .
.Pp
For
.Fn getipnodebyname ,
the
.Fa name
argument can be either a node name or a numeric address
string
(i.e., a dotted-decimal IPv4 address or an IPv6 hex address).
The
.Fa af
argument specifies the address family, either
.Dv AF_INET
or
.Dv AF_INET6 .
The
.Fa flags
argument specifies the types of addresses that are searched for,
and the types of addresses that are returned.
We note that a special flags value of
.Dv AI_DEFAULT
(defined below)
should handle most applications.
That is, porting simple applications to use IPv6 replaces the call
.Bd -literal -offset indent
hptr = gethostbyname(name);
.Ed
.Pp
with
.Bd -literal -offset indent
hptr = getipnodebyname(name, AF_INET6, AI_DEFAULT, &error_num);
.Ed
.Pp
Applications desiring finer control over the types of addresses
searched for and returned, can specify other combinations of the
.Fa flags
argument.
.Pp
A
.Fa flags
of
.Li 0
implies a strict interpretation of the
.Fa af
argument:
.Bl -bullet
.It
If
.Fa flags
is 0 and
.Fa af
is
.Dv AF_INET ,
then the caller wants only IPv4 addresses.
A query is made for
.Li A
records.
If successful, the IPv4 addresses are returned and the
.Li h_length
member of the
.Li hostent
structure will be 4, else the function returns a
.Dv NULL
pointer.
.It
If
.Fa flags
is 0 and if
.Fa af
is
.Li AF_INET6 ,
then the caller wants only IPv6 addresses.
A query is made for
.Li AAAA
records.
If successful, the IPv6 addresses are returned and the
.Li h_length
member of the
.Li hostent
structure will be 16, else the function returns a
.Dv NULL
pointer.
.El
.Pp
Other constants can be logically-ORed into the
.Fa flags
argument, to modify the behavior of the function.
.Bl -bullet
.It
If the
.Dv AI_V4MAPPED
flag is specified along with an
.Fa af
of
.Dv AF_INET6 ,
then the caller will accept IPv4-mapped IPv6 addresses.
That is, if no
.Li AAAA
records are found then a query is made for
.Li A
records and any found are returned as IPv4-mapped IPv6 addresses
.Li ( h_length
will be 16).
The
.Dv AI_V4MAPPED
flag is ignored unless
.Fa af
equals
.Dv AF_INET6 .
.It
The
.Dv AI_V4MAPPED_CFG
flag is exact same as the
.Dv AI_V4MAPPED
flag only if the kernel supports IPv4-mapped IPv6 address.
.It
If the
.Dv AI_ALL
flag is used in conjunction with the
.Dv AI_V4MAPPED
flag, and only used with the IPv6 address family.
When
.Dv AI_ALL
is logically or'd with
.Dv AI_V4MAPPED
flag then the caller wants all addresses: IPv6 and IPv4-mapped IPv6.
A query is first made for
.Li AAAA
records and if successful, the
IPv6 addresses are returned.
Another query is then made for
.Li A
records and any found are returned as IPv4-mapped IPv6 addresses.
.Li h_length
will be 16.
Only if both queries fail does the function
return a
.Dv NULL
pointer.
This flag is ignored unless af equals
AF_INET6.
If both
.Dv AI_ALL
and
.Dv AI_V4MAPPED
are specified,
.Dv AI_ALL
takes precedence.
.It
The
.Dv AI_ADDRCONFIG
flag specifies that a query for
.Li AAAA
records
should occur only if the node has at least one IPv6 source
address configured and a query for
.Li A
records should occur only if the node has at least one IPv4 source address
configured.
.Pp
For example, if the node has no IPv6 source addresses configured,
and
.Fa af
equals AF_INET6, and the node name being looked up has both
.Li AAAA
and
.Li A
records, then:
(a) if only
.Dv AI_ADDRCONFIG
is
specified, the function returns a
.Dv NULL
pointer;
(b) if
.Dv AI_ADDRCONFIG
|
.Dv AI_V4MAPPED
is specified, the
.Li A
records are returned as IPv4-mapped IPv6 addresses;
.El
.Pp
The special flags value of
.Dv AI_DEFAULT
is defined as
.Bd -literal -offset indent
#define  AI_DEFAULT  (AI_V4MAPPED_CFG | AI_ADDRCONFIG)
.Ed
.Pp
We noted that the
.Fn getipnodebyname
function must allow the
.Fa name
argument to be either a node name or a literal address string
(i.e., a dotted-decimal IPv4 address or an IPv6 hex address).
This saves applications from having to call
.Xr inet_pton 3
to handle literal address strings.
When the
.Fa name
argument is a literal address string,
the
.Fa flags
argument is always ignored.
.Pp
There are four scenarios based on the type of literal address string
and the value of the
.Fa af
argument.
The two simple cases are when
.Fa name
is a dotted-decimal IPv4 address and
.Fa af
equals
.Dv AF_INET ,
or when
.Fa name
is an IPv6 hex address and
.Fa af
equals
.Dv AF_INET6 .
The members of the
returned hostent structure are:
.Li h_name
points to a copy of the
.Fa name
argument,
.Li h_aliases
is a
.Dv NULL
pointer,
.Li h_addrtype
is a copy of the
.Fa af
argument,
.Li h_length
is either 4
(for
.Dv AF_INET )
or 16
(for
.Dv AF_INET6 ) ,
.Li h_addr_list[0]
is a pointer to the 4-byte or 16-byte binary address,
and
.Li h_addr_list[1]
is a
.Dv NULL
pointer.
.Pp
When
.Fa name
is a dotted-decimal IPv4 address and
.Fa af
equals
.Dv AF_INET6 ,
and
.Dv AI_V4MAPPED
is specified,
an IPv4-mapped IPv6 address is returned:
.Li h_name
points to an IPv6 hex address containing the IPv4-mapped IPv6 address,
.Li h_aliases
is a
.Dv NULL
pointer,
.Li h_addrtype
is
.Dv AF_INET6 ,
.Li h_length
is 16,
.Li h_addr_list[0]
is a pointer to the 16-byte binary address, and
.Li h_addr_list[1]
is a
.Dv NULL
pointer.
.Pp
It is an error when
.Fa name
is an IPv6 hex address and
.Fa af
equals
.Dv AF_INET .
The function's return value is a
.Dv NULL
pointer and the value pointed to by
.Fa error_num
equals
.Dv HOST_NOT_FOUND .
.Pp
The
.Fn getipnodebyaddr
function
takes almost the same argument as
.Xr gethostbyaddr 3 ,
but adds a pointer to return an error number.
Additionally it takes care of IPv4-mapped IPv6 addresses,
and IPv4-compatible IPv6 addresses.
.Pp
The
.Fn getipnodebyname
and
.Fn getipnodebyaddr
functions
dynamically allocate the structure to be returned to the caller.
The
.Fn freehostent
function
reclaims memory region allocated and returned by
.Fn getipnodebyname
or
.Fn getipnodebyaddr .
.\"
.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 DIAGNOSTICS
The
.Fn getipnodebyname
and
.Fn getipnodebyaddr
functions
returns
.Dv NULL
on errors.
The integer values pointed to by
.Fa error_num
may then be checked to see whether this is a temporary failure
or an invalid or unknown host.
The meanings of each error code are described in
.Xr gethostbyname 3 .
.\"
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr gethostbyaddr 3 ,
.Xr gethostbyname 3 ,
.Xr getnameinfo 3 ,
.Xr hosts 5 ,
.Xr nsswitch.conf 5 ,
.Xr services 5 ,
.Xr hostname 7
.Pp
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A W. Stevens
.%T Basic Socket Interface Extensions for IPv6
.%R RFC2553
.%D March 1999
.Re
.\"
.Sh STANDARDS
The
.Fn getipnodebyname
and
.Fn getipnodebyaddr
functions
are documented in
.Dq Basic Socket Interface Extensions for IPv6
(RFC2553).
.\"
.Sh HISTORY
The implementation first appeared in KAME advanced networking kit.
.\"
.Sh BUGS
The
.Fn getipnodebyname
and
.Fn getipnodebyaddr
functions
do not handle scoped IPv6 address properly.
If you use these functions,
your program will not be able to handle scoped IPv6 addresses.
For IPv6 address manipulation,
.Fn getaddrinfo 3
and
.Fn getnameinfo 3
are recommended.
.Pp
The text was shamelessly copied from RFC2553.