NAME
inet6_option_space,
inet6_option_init,
inet6_option_append,
inet6_option_alloc,
inet6_option_next,
inet6_option_find
—
IPv6 Hop-by-Hop and Destination Options
manipulation
SYNOPSIS
#include <netinet/in.h>
int
inet6_option_space(
int
nbytes);
int
inet6_option_init(
void
*bp,
struct cmsghdr
**cmsgp,
int type);
int
inet6_option_append(
struct
cmsghdr *cmsg,
const
uint8_t *typep,
int
multx,
int plusy);
uint8_t *
inet6_option_alloc(
struct
cmsghdr *cmsg,
int
datalen,
int multx,
int plusy);
int
inet6_option_next(
const
struct cmsghdr *cmsg,
uint8_t **tptrp);
int
inet6_option_find(
const
struct cmsghdr *cmsg,
uint8_t **tptrp,
int type);
DESCRIPTION
Building and parsing the Hop-by-Hop and Destination options is complicated due
to alignment constraints, padding and ancillary data manipulation. RFC 2292
defines a set of functions to help the application. The function prototypes
for these functions are all in the
<netinet/in.h> header.
inet6_option_space
inet6_option_space() returns the number of bytes required to
hold an option when it is stored as ancillary data, including the
cmsghdr
structure at the beginning, and any padding at
the end (to make its size a multiple of 8 bytes). The argument is the size of
the structure defining the option, which must include any pad bytes at the
beginning (the value
y
in the alignment term
“
xn + y
”), the type byte, the length byte,
and the option data.
Note: If multiple options are stored in a single ancillary data object, which is
the recommended technique, this function overestimates the amount of space
required by the size of
N-1
cmsghdr
structures, where
N
is
the number of options to be stored in the object. This is of little
consequence, since it is assumed that most Hop-by-Hop option headers and
Destination option headers carry only one option (appendix B of [RFC 2460]).
inet6_option_init
inet6_option_init() is called once per ancillary data object
that will contain either Hop-by-Hop or Destination options. It returns
0
on success or
-1
on an
error.
bp is a pointer to previously allocated space that will
contain the ancillary data object. It must be large enough to contain all the
individual options to be added by later calls to
inet6_option_append() and
inet6_option_alloc().
cmsgp is a pointer to a pointer to a
cmsghdr
structure.
*cmsgp is
initialized by this function to point to the
cmsghdr
structure constructed by this function in the buffer pointed to by
bp.
type is either
IPV6_HOPOPTS
or
IPV6_DSTOPTS
. This
type is
stored in the
cmsg_type
member of the
cmsghdr
structure pointed to by
*cmsgp.
inet6_option_append
This function appends a Hop-by-Hop option or a Destination option into an
ancillary data object that has been initialized by
inet6_option_init(). This function returns
0
if it succeeds or
-1
on an
error.
cmsg is a pointer to the
cmsghdr
structure that must have been initialized by
inet6_option_init().
typep is a pointer to the 8-bit option type. It is assumed
that this field is immediately followed by the 8-bit option data length field,
which is then followed immediately by the option data. The caller initializes
these three fields (the type-length-value, or TLV) before calling this
function.
The option type must have a value from
2
to
255
, inclusive. (
0
and
1
are reserved for the
Pad1
and
PadN
options, respectively.)
The option data length must have a value between
0
and
255
, inclusive, and is the length of the option data
that follows.
multx is the value
x
in the
alignment term “
xn + y
”. It must have a
value of
1
,
2
,
4
, or
8
.
plusy is the value
y
in the
alignment term “
xn + y
”. It must have a
value between
0
and
7
,
inclusive.
inet6_option_alloc
This function appends a Hop-by-Hop option or a Destination option into an
ancillary data object that has been initialized by
inet6_option_init(). This function returns a pointer to the
8-bit option type field that starts the option on success, or
NULL
on an error.
The difference between this function and
inet6_option_append()
is that the latter copies the contents of a previously built option into the
ancillary data object while the current function returns a pointer to the
space in the data object where the option's TLV must then be built by the
caller.
cmsg is a pointer to the
cmsghdr
structure that must have been initialized by
inet6_option_init().
datalen is the value of the option data length byte for
this option. This value is required as an argument to allow the function to
determine if padding must be appended at the end of the option. (The
inet6_option_append() function does not need a data length
argument since the option data length must already be stored by the caller.)
multx is the value
x
in the
alignment term “
xn + y
”. It must have a
value of
1
,
2
,
4
, or
8
.
plusy is the value
y
in the
alignment term “
xn + y
”. It must have a
value between
0
and
7
,
inclusive.
inet6_option_next
This function processes the next Hop-by-Hop option or Destination option in an
ancillary data object. If another option remains to be processed, the return
value of the function is
0
and
*tptrp points to the 8-bit option type field (which is
followed by the 8-bit option data length, followed by the option data). If no
more options remain to be processed, the return value is
-1
and
*tptrp is
NULL
. If an error occurs, the return value is
-1
and
*tptrp is not
NULL
.
cmsg is a pointer to
cmsghdr
structure of which
cmsg_level
equals
IPPROTO_IPV6
and
cmsg_type
equals either
IPV6_HOPOPTS
or
IPV6_DSTOPTS
.
tptrp is a pointer to a pointer to an 8-bit byte and
*tptrp is used by the function to remember its place in
the ancillary data object each time the function is called. The first time
this function is called for a given ancillary data object,
*tptrp must be set to
NULL
.
Each time this function returns success,
*tptrp points to
the 8-bit option type field for the next option to be processed.
inet6_option_find
This function is similar to the previously described
inet6_option_next() function, except this function lets the
caller specify the option type to be searched for, instead of always returning
the next option in the ancillary data object.
cmsg is a
pointer to
cmsghdr
structure of which
cmsg_level
equals
IPPROTO_IPV6
and
cmsg_type
equals either
IPV6_HOPOPTS
or
IPV6_DSTOPTS
.
tptrp is a pointer to a pointer to an 8-bit byte and
*tptrp is used by the function to remember its place in
the ancillary data object each time the function is called. The first time
this function is called for a given ancillary data object,
*tptrp must be set to
NULL
.
~ This function starts searching for an option of the
specified type beginning after the value of
*tptrp. If
an option of the specified type is located, this function returns
0
and
*tptrp points to the 8-
bit option type field for the option of the specified type. If an option of
the specified type is not located, the return value is
-1
and
*tptrp is
NULL
. If an error occurs, the return value is
-1
and
*tptrp is not
NULL
.
EXAMPLES
RFC 2292 gives comprehensive examples in chapter 6.
DIAGNOSTICS
inet6_option_init() and
inet6_option_append() return
0
on
success or
-1
on an error.
inet6_option_alloc() returns
NULL
on
an error.
On errors,
inet6_option_next() and
inet6_option_find() return
-1
setting
*tptrp to non
NULL
value.
SEE ALSO
W. Stevens and M.
Thomas, Advanced Sockets API for IPv6,
RFC 2292, February
1998.
S. Deering and R.
Hinden, Internet Protocol, Version 6 (IPv6)
Specification, RFC 2460,
December 1998.
STANDARDS
The functions are documented in “Advanced Sockets API for IPv6” (RFC
2292).
HISTORY
The implementation first appeared in KAME advanced networking kit.
BUGS
The text was shamelessly copied from RFC 2292.