INET6_OPT_INIT(3) FreeBSD Library Functions Manual INET6_OPT_INIT(3)


inet6_opt_init, inet6_opt_append, inet6_opt_finish, inet6_opt_set_val, inet6_opt_next, inet6_opt_find, inet6_opt_get_valIPv6 Hop-by-Hop and Destination Options manipulation


#include < netinet/in.h>

inet6_opt_init( void *extbuf, socklen_t extlen);

inet6_opt_append( void *extbuf, socklen_t extlen, int offset, u_int8_t type, socklen_t len, u_int8_t align, void **databufp);

inet6_opt_finish( void *extbuf, socklen_t extlen, int offset);

inet6_opt_set_val( void *databuf, int offset, void *val, socklen_t vallen);

inet6_opt_next( void *extbuf, socklen_t extlen, int offset, u_int8_t *typep, socklen_t *lenp, void **databufp);

inet6_opt_find( void *extbuf, socklen_t extlen, int offset, u_int8_t type, socklen_t *lenp, void **databufp);

inet6_opt_get_val( void *databuf, int offset, void *val, socklen_t vallen);


Building and parsing the Hop-by-Hop and Destination options is complicated. The advanced sockets API defines a set of functions to help applications create and manipulate Hop-by-Hop and Destination options. This man page describes the functions specified in IETF Draft RFC3542. These functions use the formatting rules specified in Appendix B in RFC2460, i.e., that the largest field is placed last in the option. The function prototypes for these functions are all contained in the < netinet/in.h> header file.


The inet6_opt_init() function returns the number of bytes needed for an empty extension header, one without any options. If the extbuf argument points to a valid section of memory then the inet6_opt_init() function also initializes the extension header's length field. When attempting to initialize an extension buffer passed in the extbuf argument, extlen must be a positive multiple of 8 or else the function fails and returns -1 to the caller.


The inet6_opt_append() function can perform two different jobs. When a valid extbuf argument is supplied it appends an option to the extension buffer and returns the updated total length as well as a pointer to the newly created option in databufp. If the value of extbuf is NULL then the inet6_opt_append() function only reports what the total length would be if the option were actually appended. The len and align arguments specify the length of the option and the required data alignment which must be used when appending the option. The offset argument should be the length returned by the inet6_opt_init() function or a previous call to inet6_opt_append().

The type argument is the 8-bit option type.

After inet6_opt_append() has been called, the application can use the buffer pointed to by databufp directly, or use inet6_opt_set_val() to specify the data to be contained in the option.

Option types of 0 and 1 are reserved for the Pad1 and PadN options. All other values from 2 through 255 may be used by applications.

The length of the option data is contained in an 8-bit value and so may contain any value from 0 through 255.

The align parameter must have a value of 1, 2, 4, or 8 and cannot exceed the value of len. The alignment values represent no alignment, 16 bit, 32 bit and 64 bit alignments, respectively.


The inet6_opt_finish() function calculates the final padding necessary to make the extension header a multiple of 8 bytes, as required by the IPv6 extension header specification, and returns the extension header's updated total length. The offset argument should be the length returned by inet6_opt_init() or inet6_opt_append(). When extbuf is not NULL the function also sets up the appropriate padding bytes by inserting a Pad1 or PadN option of the proper length.

If the extension header is too small to contain the proper padding then an error of -1 is returned to the caller.


The inet6_opt_set_val() function inserts data items of various sizes into the data portion of the option. The databuf argument is a pointer to memory that was returned by the inet6_opt_append() call and the offset argument specifies where the option should be placed in the data buffer. The val argument points to an area of memory containing the data to be inserted into the extension header, and the vallen argument indicates how much data to copy.

The caller should ensure that each field is aligned on its natural boundaries as described in Appendix B of RFC2460.

The function returns the offset for the next field which is calculated as offset + vallen and is used when composing options with multiple fields.


The inet6_opt_next() function parses received extension headers. The extbuf and extlen arguments specify the location and length of the extension header being parsed. The offset argument should either be zero, for the first option, or the length value returned by a previous call to inet6_opt_next() or inet6_opt_find(). The return value specifies the position where to continue scanning the extension buffer. The option is returned in the arguments typep, lenp, and databufp, which point to the 8-bit option type, the 8-bit option length and the option data, respectively. This function does not return any PAD1 or PADN options. When an error occurs or there are no more options, the return value is -1.


The inet6_opt_find() function searches the extension buffer for a particular option type, passed in through the type argument. If the option is found then the lenp and databufp arguments are updated to point to the option's length and data, respectively. The extbuf and extlen arguments must point to a valid extension buffer and give its length. The offset argument can be used to search from a location anywhere in the extension header.


The inet6_opt_get_val() function extracts data items of various sizes in the data portion of the option. The databuf is a pointer returned by the inet6_opt_next() or inet6_opt_find() functions. The val argument points where the data will be extracted. The offset argument specifies from where in the data portion of the option the value should be extracted; the first byte of option data is specified by an offset of zero.

It is expected that each field is aligned on its natural boundaries as described in Appendix B of RFC2460.

The function returns the offset for the next field by calculating offset + vallen which can be used when extracting option content with multiple fields. Robust receivers must verify alignment before calling this function.


All the functions return -1 on an error.


RFC3542 gives comprehensive examples in Section 23.

KAME also provides examples in the advapitest directory of its kit.


W. Stevens, M. Thomas, E. Nordmark, and T. Jinmei, Advanced Sockets API for IPv6, RFC3542, October 2002.

S. Deering and R. Hinden, Internet Protocol, Version 6 (IPv6) Specification, RFC2460, December 1998.


The functions are documented in “Advanced Sockets API for IPv6” (RFC3542).


The implementation first appeared in KAME advanced networking kit.
December 23, 2004 FreeBSD