recvmsg() and sendmsg(). Setting the socket option specifies any of
the optional output fields:
setsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, len);
The fourth argument points to a buffer containing one or more
ancillary data objects, and the fifth argument is the total length of
all these objects. The application fills in this buffer exactly as
if the buffer were being passed to sendmsg() as control information.
The options set by calling setsockopt() for IPV6_PKTOPTIONS are
called "sticky" options because once set they apply to all packets
sent on that socket. The application can call setsockopt() again to
change all the sticky options, or it can call setsockopt() with a
length of 0 to remove all the sticky options for the socket.
The corresponding receive option
getsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, &len);
returns a buffer with one or more ancillary data objects for all the
optional receive information that the application has previously
specified that it wants to receive. The fourth argument points to
the buffer that is filled in by the call. The fifth argument is a
pointer to a value-result integer: when the function is called the
integer specifies the size of the buffer pointed to by the fourth
argument, and on return this integer contains the actual number of
bytes that were returned. The application processes this buffer
exactly as if the buffer were returned by recvmsg() as control
information.
To simplify this document, in the remaining sections when we say "can
be specified as ancillary data to sendmsg()" we mean "can be
specified as ancillary data to sendmsg() or specified as a sticky
option using setsockopt() and the IPV6_PKTOPTIONS socket option".
Similarly when we say "can be returned as ancillary data by
recvmsg()" we mean "can be returned as ancillary data by recvmsg() or
returned by getsockopt() with the IPV6_PKTOPTIONS socket option".
4.5.1. TCP Sticky Options
When using getsockopt() with the IPV6_PKTOPTIONS option and a TCP
socket, only the options from the most recently received segment are
retained and returned to the caller, and only after the socket option
has been set. That is, TCP need not start saving a copy of the
options until the application says to do so.
The application is not allowed to specify ancillary data in a call to
sendmsg() on a TCP socket, and none of the ancillary data that we
describe in this document is ever returned as control information by
recvmsg() on a TCP socket.
4.5.2. UDP and Raw Socket Sticky Options
The IPV6_PKTOPTIONS socket option can also be used with a UDP socket
or with a raw IPv6 socket, normally to set some of the options once,
instead of with each call to sendmsg().
Unlike the TCP case, the sticky options can be overridden on a per-
packet basis with ancillary data specified in a call to sendmsg() on
a UDP or raw IPv6 socket. If any ancillary data is specified in a
call to sendmsg(), none of the sticky options are sent with that
datagram.
5. Packet Information
There are four pieces of information that an application can specify
for an outgoing packet using ancillary data:
1. the source IPv6 address,
2. the outgoing interface index,
3. the outgoing hop limit, and
4. the next hop address.
Three similar pieces of information can be returned for a received
packet as ancillary data:
1. the destination IPv6 address,
2. the arriving interface index, and
3. the arriving hop limit.
The first two pieces of information are contained in an in6_pktinfo
structure that is sent as ancillary data with sendmsg() and received
as ancillary data with recvmsg(). This structure is defined as a
result of including the <netinet/in.h> header.
struct in6_pktinfo {
struct in6_addr ipi6_addr; /* src/dst IPv6 address */
unsigned int ipi6_ifindex; /* send/recv interface index */
};
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_PKTINFO, and the first byte of cmsg_data[] will be the first
byte of the in6_pktinfo structure.
This information is returned as ancillary data by recvmsg() only if
the application has enabled the IPV6_PKTINFO socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_PKTINFO, &on, sizeof(on));
Nothing special need be done to send this information: just specify
the control information as ancillary data for sendmsg().
(Note: The hop limit is not contained in the in6_pktinfo structure
for the following reason. Some UDP servers want to respond to client
requests by sending their reply out the same interface on which the
request was received and with the source IPv6 address of the reply
equal to the destination IPv6 address of the request. To do this the
application can enable just the IPV6_PKTINFO socket option and then
use the received control information from recvmsg() as the outgoing
control information for sendmsg(). The application need not examine
or modify the in6_pktinfo structure at all. But if the hop limit
were contained in this structure, the application would have to parse
the received control information and change the hop limit member,
since the received hop limit is not the desired value for an outgoing
packet.)
5.1. Specifying/Receiving the Interface
Interfaces on an IPv6 node are identified by a small positive
integer, as described in Section 4 of [RFC-2133]. That document also
describes a function to map an interface name to its interface index,
a function to map an interface index to its interface name, and a
function to return all the interface names and indexes. Notice from
this document that no interface is ever assigned an index of 0.
When specifying the outgoing interface, if the ipi6_ifindex value is
0, the kernel will choose the outgoing interface. If the application
specifies an outgoing interface for a multicast packet, the interface
specified by the ancillary data overrides any interface specified by
the IPV6_MULTICAST_IF socket option (described in [RFC-2133]), for
that call to sendmsg() only.
When the IPV6_PKTINFO socket option is enabled, the received
interface index is always returned as the ipi6_ifindex member of the
in6_pktinfo structure.
5.2. Specifying/Receiving Source/Destination Address
The source IPv6 address can be specified by calling bind() before
each output operation, but supplying the source address together with
the data requires less overhead (i.e., fewer system calls) and
requires less state to be stored and protected in a multithreaded
application.
When specifying the source IPv6 address as ancillary data, if the
ipi6_addr member of the in6_pktinfo structure is the unspecified
address (IN6ADDR_ANY_INIT), then (a) if an address is currently bound
to the socket, it is used as the source address, or (b) if no address
is currently bound to the socket, the kernel will choose the source
address. If the ipi6_addr member is not the unspecified address, but
the socket has already bound a source address, then the ipi6_addr
value overrides the already-bound source address for this output
operation only.
The kernel must verify that the requested source address is indeed a
unicast address assigned to the node.
When the in6_pktinfo structure is returned as ancillary data by
recvmsg(), the ipi6_addr member contains the destination IPv6 address
from the received packet.
5.3. Specifying/Receiving the Hop Limit
The outgoing hop limit is normally specified with either the
IPV6_UNICAST_HOPS socket option or the IPV6_MULTICAST_HOPS socket
option, both of which are described in [RFC-2133]. Specifying the
hop limit as ancillary data lets the application override either the
kernel's default or a previously specified value, for either a
unicast destination or a multicast destination, for a single output
operation. Returning the received hop limit is useful for programs
such as Traceroute and for IPv6 applications that need to verify that
the received hop limit is 255 (e.g., that the packet has not been
forwarded).
The received hop limit is returned as ancillary data by recvmsg()
only if the application has enabled the IPV6_HOPLIMIT socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_HOPLIMIT, &on, sizeof(on));
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_HOPLIMIT, and the first byte of cmsg_data[] will be the first
byte of the integer hop limit.
Nothing special need be done to specify the outgoing hop limit: just
specify the control information as ancillary data for sendmsg(). As
specified in [RFC-2133], the interpretation of the integer hop limit
value is
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
5.4. Specifying the Next Hop Address
The IPV6_NEXTHOP ancillary data object specifies the next hop for the
datagram as a socket address structure. In the cmsghdr structure
containing this ancillary data, the cmsg_level member will be
IPPROTO_IPV6, the cmsg_type member will be IPV6_NEXTHOP, and the
first byte of cmsg_data[] will be the first byte of the socket
address structure.
This is a privileged option. (Note: It is implementation defined and
beyond the scope of this document to define what "privileged" means.
Unix systems use this term to mean the process must have an effective
user ID of 0.)
If the socket address structure contains an IPv6 address (e.g., the
sin6_family member is AF_INET6), then the node identified by that
address must be a neighbor of the sending host. If that address
equals the destination IPv6 address of the datagram, then this is
equivalent to the existing SO_DONTROUTE socket option.
5.5. Additional Errors with sendmsg()
With the IPV6_PKTINFO socket option there are no additional errors
possible with the call to recvmsg(). But when specifying the
outgoing interface or the source address, additional errors are
possible from sendmsg(). The following are examples, but some of
these may not be provided by some implementations, and some
implementations may define additional errors:
ENXIO The interface specified by ipi6_ifindex does not exist.
ENETDOWN The interface specified by ipi6_ifindex is not enabled
for IPv6 use.
EADDRNOTAVAIL ipi6_ifindex specifies an interface but the address
ipi6_addr is not available for use on that interface.
EHOSTUNREACH No route to the destination exists over the interface
specified by ifi6_ifindex.
6. Hop-By-Hop Options
A variable number of Hop-by-Hop options can appear in a single Hop-
by-Hop options header. Each option in the header is TLV-encoded with
a type, length, and value.
Today only three Hop-by-Hop options are defined for IPv6 [RFC-1883]:
Jumbo Payload, Pad1, and PadN, although a proposal exists for a
router-alert Hop-by-Hop option. The Jumbo Payload option should not
be passed back to an application and an application should receive an
error if it attempts to set it. This option is processed entirely by
the kernel. It is indirectly specified by datagram-based
applications as the size of the datagram to send and indirectly
passed back to these applications as the length of the received
datagram. The two pad options are for alignment purposes and are
automatically inserted by a sending kernel when needed and ignored by
the receiving kernel. This section of the API is therefore defined
for future Hop-by-Hop options that an application may need to specify
and receive.
Individual Hop-by-Hop options (and Destination options, which are
described shortly, and which are similar to the Hop-by-Hop options)
may have specific alignment requirements. For example, the 4-byte
Jumbo Payload length should appear on a 4-byte boundary, and IPv6
addresses are normally aligned on an 8-byte boundary. These
requirements and the terminology used with these options are
discussed in Section 4.2 and Appendix A of [RFC-1883]. The alignment
of each option is specified by two values, called x and y, written as
"xn + y". This states that the option must appear at an integer
multiple of x bytes from the beginning of the options header (x can
have the values 1, 2, 4, or 8), plus y bytes (y can have a value
between 0 and 7, inclusive). The Pad1 and PadN options are inserted
as needed to maintain the required alignment. Whatever code builds
either a Hop-by-Hop options header or a Destination options header
must know the values of x and y for each option.
Multiple Hop-by-Hop options can be specified by the application.
Normally one ancillary data object describes all the Hop-by-Hop
options (since each option is itself TLV-encoded) but the application
can specify multiple ancillary data objects for the Hop-by-Hop
options, each object specifying one or more options. Care must be
taken designing the API for these options since
1. it may be possible for some future Hop-by-Hop options to be
generated by the application and processed entirely by the
application (e.g., the kernel may not know the alignment
restrictions for the option),
2. it must be possible for the kernel to insert its own Hop-by-Hop
options in an outgoing packet (e.g., the Jumbo Payload option),
3. the application can place one or more Hop-by-Hop options into a
single ancillary data object,
4. if the application specifies multiple ancillary data objects,
each containing one or more Hop-by-Hop options, the kernel must
combine these a single Hop-by-Hop options header, and
5. it must be possible for the kernel to remove some Hop-by-Hop
options from a received packet before returning the remaining
Hop-by-Hop options to the application. (This removal might
consist of the kernel converting the option into a pad option of
the same length.)
Finally, we note that access to some Hop-by-Hop options or to some
Destination options, might require special privilege. That is,
normal applications (without special privilege) might be forbidden
from setting certain options in outgoing packets, and might never see
certain options in received packets.
6.1. Receiving Hop-by-Hop Options
To receive Hop-by-Hop options the application must enable the
IPV6_HOPOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_HOPOPTS, &on, sizeof(on));
All the Hop-by-Hop options are returned as one ancillary data object
described by a cmsghdr structure. The cmsg_level member will be
IPPROTO_IPV6 and the cmsg_type member will be IPV6_HOPOPTS. These
options are then processed by calling the inet6_option_next() and
inet6_option_find() functions, described shortly.
6.2. Sending Hop-by-Hop Options
To send one or more Hop-by-Hop options, the application just
specifies them as ancillary data in a call to sendmsg(). No socket
option need be set.
Normally all the Hop-by-Hop options are specified by a single
ancillary data object. Multiple ancillary data objects, each
containing one or more Hop-by-Hop options, can also be specified, in
which case the kernel will combine all the Hop-by-Hop options into a
single Hop-by-Hop extension header. But it should be more efficient
to use a single ancillary data object to describe all the Hop-by-Hop
options. The cmsg_level member is set to IPPROTO_IPV6 and the
cmsg_type member is set to IPV6_HOPOPTS. The option is normally
constructed using the inet6_option_init(), inet6_option_append(), and
inet6_option_alloc() functions, described shortly.
Additional errors may be possible from sendmsg() if the specified
option is in error.
6.3. Hop-by-Hop and Destination Options Processing
Building and parsing the Hop-by-Hop and Destination options is
complicated for the reasons given earlier. We therefore define a set
of functions to help the application. The function prototypes for
these functions are all in the <netinet/in.h> header.
6.3.1. inet6_option_space
int inet6_option_space(int nbytes);
This function 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 (p. 33 of [RFC-1883]).)
6.3.2. inet6_option_init
int inet6_option_init(void *bp, struct cmsghdr **cmsgp, int
type);
This function 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.
6.3.3. inet6_option_append
int inet6_option_append(struct cmsghdr *cmsg, const uint8_t *typep,
int multx, int plusy);
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" described
earlier. It must have a value of 1, 2, 4, or 8.
plusy is the value y in the alignment term "xn + y" described
earlier. It must have a value between 0 and 7, inclusive.
6.3.4. inet6_option_alloc
uint8_t *inet6_option_alloc(struct cmsghdr *cmsg, int datalen,
int multx, int plusy);
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" described
earlier. It must have a value of 1, 2, 4, or 8.
plusy is the value y in the alignment term "xn + y" described
earlier. It must have a value between 0 and 7, inclusive.
6.3.5. inet6_option_next
int inet6_option_next(const struct cmsghdr *cmsg, uint8_t
**tptrp);
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.
6.3.6. inet6_option_find
int inet6_option_find(const struct cmsghdr *cmsg, uint8_t *tptrp,
int type);
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.
6.3.7. Options Examples
We now provide an example that builds two Hop-by-Hop options. First
we define two options, called X and Y, taken from the example in
Appendix A of [RFC-1883]. We assume that all options will have
structure definitions similar to what is shown below.
/* option X and option Y are defined in [RFC-1883], pp. 33-34 */
#define IP6_X_OPT_TYPE X /* replace X with assigned value */
#define IP6_X_OPT_LEN 12
#define IP6_X_OPT_MULTX 8 /* 8n + 2 alignment */
#define IP6_X_OPT_OFFSETY 2
struct ip6_X_opt {
uint8_t ip6_X_opt_pad[IP6_X_OPT_OFFSETY];
uint8_t ip6_X_opt_type;
uint8_t ip6_X_opt_len;
uint32_t ip6_X_opt_val1;
uint64_t ip6_X_opt_val2;
};
#define IP6_Y_OPT_TYPE Y /* replace Y with assigned value */
#define IP6_Y_OPT_LEN 7
#define IP6_Y_OPT_MULTX 4 /* 4n + 3 alignment */
#define IP6_Y_OPT_OFFSETY 3
struct ip6_Y_opt {
uint8_t ip6_Y_opt_pad[IP6_Y_OPT_OFFSETY];
uint8_t ip6_Y_opt_type;
uint8_t ip6_Y_opt_len;
uint8_t ip6_Y_opt_val1;
uint16_t ip6_Y_opt_val2;
uint32_t ip6_Y_opt_val3;
};
We now show the code fragment to build one ancillary data object
containing both options.
struct msghdr msg;
struct cmsghdr *cmsgptr;
struct ip6_X_opt optX;
struct ip6_Y_opt optY;
msg.msg_control = malloc(inet6_option_space(sizeof(optX) +
sizeof(optY)));
inet6_option_init(msg.msg_control, &cmsgptr, IPV6_HOPOPTS);
optX.ip6_X_opt_type = IP6_X_OPT_TYPE;
optX.ip6_X_opt_len = IP6_X_OPT_LEN;
optX.ip6_X_opt_val1 = <32-bit value>;
optX.ip6_X_opt_val2 = <64-bit value>;
inet6_option_append(cmsgptr, &optX.ip6_X_opt_type,
IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);
optY.ip6_Y_opt_type = IP6_Y_OPT_TYPE;
optY.ip6_Y_opt_len = IP6_Y_OPT_LEN;
optY.ip6_Y_opt_val1 = <8-bit value>;
optY.ip6_Y_opt_val2 = <16-bit value>;
optY.ip6_Y_opt_val3 = <32-bit value>;
inet6_option_append(cmsgptr, &optY.ip6_Y_opt_type,
IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);
msg.msg_controllen = cmsgptr->cmsg_len;
The call to inet6_option_init() builds the cmsghdr structure in the
control buffer.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = CMSG_LEN(0) = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_HOPOPTS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Here we assume a 32-bit architecture where sizeof(struct cmsghdr)
equals 12, with a desired alignment of 4-byte boundaries (that is,
the ALIGN() macro shown in the sample implementations of the
CMSG_xxx() macros rounds up to a multiple of 4).
The first call to inet6_option_append() appends the X option. Since
this is the first option in the ancillary data object, 2 bytes are
allocated for the Next Header byte and for the Hdr Ext Len byte. The
former will be set by the kernel, depending on the type of header
that follows this header, and the latter byte is set to 1. These 2
bytes form the 2 bytes of padding (IP6_X_OPT_OFFSETY) required at the
beginning of this option.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 28 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_HOPOPTS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ 8-octet field +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The cmsg_len member of the cmsghdr structure is incremented by 16,
the size of the option.
The next call to inet6_option_append() appends the Y option to the
ancillary data object.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 44 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_HOPOPTS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=3 | Option Type=X |Opt Data Len=12|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ 8-octet field +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PadN Option=1 |Opt Data Len=1 | 0 | Option Type=Y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opt Data Len=7 | 1-octet field | 2-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PadN Option=1 |Opt Data Len=2 | 0 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 bytes are appended by this function, so cmsg_len becomes 44. The
inet6_option_append() function notices that the appended data
requires 4 bytes of padding at the end, to make the size of the
ancillary data object a multiple of 8, and appends the PadN option
before returning. The Hdr Ext Len byte is incremented by 2 to become
3.
Alternately, the application could build two ancillary data objects,
one per option, although this will probably be less efficient than
combining the two options into a single ancillary data object (as
just shown). The kernel must combine these into a single Hop-by-Hop
extension header in the final IPv6 packet.
struct msghdr msg;
struct cmsghdr *cmsgptr;
struct ip6_X_opt optX;
struct ip6_Y_opt optY;
msg.msg_control = malloc(inet6_option_space(sizeof(optX)) +
inet6_option_space(sizeof(optY)));
inet6_option_init(msg.msg_control, &cmsgptr, IPPROTO_HOPOPTS);
optX.ip6_X_opt_type = IP6_X_OPT_TYPE;
optX.ip6_X_opt_len = IP6_X_OPT_LEN;
optX.ip6_X_opt_val1 = <32-bit value>;
optX.ip6_X_opt_val2 = <64-bit value>;
inet6_option_append(cmsgptr, &optX.ip6_X_opt_type,
IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);
msg.msg_controllen = CMSG_SPACE(sizeof(optX));
inet6_option_init((u_char *)msg.msg_control + msg.msg_controllen,
&cmsgptr, IPPROTO_HOPOPTS);
optY.ip6_Y_opt_type = IP6_Y_OPT_TYPE;
optY.ip6_Y_opt_len = IP6_Y_OPT_LEN;
optY.ip6_Y_opt_val1 = <8-bit value>;
optY.ip6_Y_opt_val2 = <16-bit value>;
optY.ip6_Y_opt_val3 = <32-bit value>;
inet6_option_append(cmsgptr, &optY.ip6_Y_opt_type,
IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);
msg.msg_controllen += cmsgptr->cmsg_len;
Each call to inet6_option_init() builds a new cmsghdr structure, and
the final result looks like the following:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 28 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_HOPOPTS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ 8-octet field +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 28 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_HOPOPTS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=1 | Pad1 Option=0 | Option Type=Y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opt Data Len=7 | 1-octet field | 2-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4-octet field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PadN Option=1 |Opt Data Len=2 | 0 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When the kernel combines these two options into a single Hop-by-Hop
extension header, the first 3 bytes of the second ancillary data
object (the Next Header byte, the Hdr Ext Len byte, and the Pad1
option) will be combined into a PadN option occupying 3 bytes.
The following code fragment is a redo of the first example shown
(building two options in a single ancillary data object) but this
time we use inet6_option_alloc().
uint8_t *typep;
struct msghdr msg;
struct cmsghdr *cmsgptr;
struct ip6_X_opt *optXp; /* now a pointer, not a struct */
struct ip6_Y_opt *optYp; /* now a pointer, not a struct */
msg.msg_control = malloc(inet6_option_space(sizeof(*optXp) +
sizeof(*optYp)));
inet6_option_init(msg.msg_control, &cmsgptr, IPV6_HOPOPTS);
typep = inet6_option_alloc(cmsgptr, IP6_X_OPT_LEN,
IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);
optXp = (struct ip6_X_opt *) (typep - IP6_X_OPT_OFFSETY);
optXp->ip6_X_opt_type = IP6_X_OPT_TYPE;
optXp->ip6_X_opt_len = IP6_X_OPT_LEN;
optXp->ip6_X_opt_val1 = <32-bit value>;
optXp->ip6_X_opt_val2 = <64-bit value>;
typep = inet6_option_alloc(cmsgptr, IP6_Y_OPT_LEN,
IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);
optYp = (struct ip6_Y_opt *) (typep - IP6_Y_OPT_OFFSETY);
optYp->ip6_Y_opt_type = IP6_Y_OPT_TYPE;
optYp->ip6_Y_opt_len = IP6_Y_OPT_LEN;
optYp->ip6_Y_opt_val1 = <8-bit value>;
optYp->ip6_Y_opt_val2 = <16-bit value>;
optYp->ip6_Y_opt_val3 = <32-bit value>;
msg.msg_controllen = cmsgptr->cmsg_len;
Notice that inet6_option_alloc() returns a pointer to the 8-bit
option type field. If the program wants a pointer to an option
structure that includes the padding at the front (as shown in our
definitions of the ip6_X_opt and ip6_Y_opt structures), the y-offset
at the beginning of the structure must be subtracted from the
returned pointer.
The following code fragment shows the processing of Hop-by-Hop
options using the inet6_option_next() function.
struct msghdr msg;
struct cmsghdr *cmsgptr;
/* fill in msg */
/* call recvmsg() */
for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
if (cmsgptr->cmsg_level == IPPROTO_IPV6 &&
cmsgptr->cmsg_type == IPV6_HOPOPTS) {
uint8_t *tptr = NULL;
while (inet6_option_next(cmsgptr, &tptr) == 0) {
if (*tptr == IP6_X_OPT_TYPE) {
struct ip6_X_opt *optXp;
optXp = (struct ip6_X_opt *) (tptr - IP6_X_OPT_OFFSETY);
<do whatever with> optXp->ip6_X_opt_val1;
<do whatever with> optXp->ip6_X_opt_val2;
} else if (*tptr == IP6_Y_OPT_TYPE) {
struct ip6_Y_opt *optYp;
optYp = (struct ip6_Y_opt *) (tptr - IP6_Y_OPT_OFFSETY);
<do whatever with> optYp->ip6_Y_opt_val1;
<do whatever with> optYp->ip6_Y_opt_val2;
<do whatever with> optYp->ip6_Y_opt_val3;
}
}
if (tptr != NULL)
<error encountered by inet6_option_next()>;
}
}
7. Destination Options
A variable number of Destination options can appear in one or more
Destination option headers. As defined in [RFC-1883], a Destination
options header appearing before a Routing header is processed by the
first destination plus any subsequent destinations specified in the
Routing header, while a Destination options header appearing after a
Routing header is processed only by the final destination. As with
the Hop-by-Hop options, each option in a Destination options header
is TLV-encoded with a type, length, and value.
Today no Destination options are defined for IPv6 [RFC-1883],
although proposals exist to use Destination options with mobility and
anycasting.
7.1. Receiving Destination Options
To receive Destination options the application must enable the
IPV6_DSTOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_DSTOPTS, &on, sizeof(on));
All the Destination options appearing before a Routing header are
returned as one ancillary data object described by a cmsghdr
structure and all the Destination options appearing after a Routing
header are returned as another ancillary data object described by a
cmsghdr structure. For these ancillary data objects, the cmsg_level
member will be IPPROTO_IPV6 and the cmsg_type member will be
IPV6_HOPOPTS. These options are then processed by calling the
inet6_option_next() and inet6_option_find() functions.
7.2. Sending Destination Options
To send one or more Destination options, the application just
specifies them as ancillary data in a call to sendmsg(). No socket
option need be set.
As described earlier, one set of Destination options can appear
before a Routing header, and one set can appear after a Routing
header. Each set can consist of one or more options.
Normally all the Destination options in a set are specified by a
single ancillary data object, since each option is itself TLV-
encoded. Multiple ancillary data objects, each containing one or
more Destination options, can also be specified, in which case the
kernel will combine all the Destination options in the set into a
single Destination extension header. But it should be more efficient
to use a single ancillary data object to describe all the Destination
options in a set. The cmsg_level member is set to IPPROTO_IPV6 and
the cmsg_type member is set to IPV6_DSTOPTS. The option is normally
constructed using the inet6_option_init(), inet6_option_append(), and
inet6_option_alloc() functions.
Additional errors may be possible from sendmsg() if the specified
option is in error.
8. Routing Header Option
Source routing in IPv6 is accomplished by specifying a Routing header
as an extension header. There can be different types of Routing
headers, but IPv6 currently defines only the Type 0 Routing header
[RFC-1883]. This type supports up to 23 intermediate nodes. With
this maximum number of intermediate nodes, a source, and a
destination, there are 24 hops, each of which is defined as a strict
or loose hop.
Source routing with IPv4 sockets API (the IP_OPTIONS socket option)
requires the application to build the source route in the format that
appears as the IPv4 header option, requiring intimate knowledge of
the IPv4 options format. This IPv6 API, however, defines eight
functions that the application calls to build and examine a Routing
header. Four functions build a Routing header:
inet6_rthdr_space() - return #bytes required for ancillary data
inet6_rthdr_init() - initialize ancillary data for Routing header
inet6_rthdr_add() - add IPv6 address & flags to Routing header
inet6_rthdr_lasthop() - specify the flags for the final hop
Four functions deal with a returned Routing header:
inet6_rthdr_reverse() - reverse a Routing header
inet6_rthdr_segments() - return #segments in a Routing header
inet6_rthdr_getaddr() - fetch one address from a Routing header
inet6_rthdr_getflags() - fetch one flag from a Routing header
The function prototypes for these functions are all in the
<netinet/in.h> header.
To receive a Routing header the application must enable the
IPV6_RTHDR socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RTHDR, &on, sizeof(on));
To send a Routing header the application just specifies it as
ancillary data in a call to sendmsg().
A Routing header is passed between the application and the kernel as
an ancillary data object. The cmsg_level member has a value of
IPPROTO_IPV6 and the cmsg_type member has a value of IPV6_RTHDR. The
contents of the cmsg_data[] member is implementation dependent and
should not be accessed directly by the application, but should be
accessed using the eight functions that we are about to describe.
The following constants are defined in the <netinet/in.h> header:
#define IPV6_RTHDR_LOOSE 0 /* this hop need not be a neighbor */
#define IPV6_RTHDR_STRICT 1 /* this hop must be a neighbor */
#define IPV6_RTHDR_TYPE_0 0 /* IPv6 Routing header type 0 */
When a Routing header is specified, the destination address specified
for connect(), sendto(), or sendmsg() is the final destination
address of the datagram. The Routing header then contains the
addresses of all the intermediate nodes.
8.1. inet6_rthdr_space
size_t inet6_rthdr_space(int type, int segments);
This function returns the number of bytes required to hold a Routing
header of the specified type containing the specified number of
segments (addresses). For an IPv6 Type 0 Routing header, the number
of segments must be between 1 and 23, inclusive. The return value
includes the size of the cmsghdr structure that precedes the Routing
header, and any required padding.
If the return value is 0, then either the type of the Routing header
is not supported by this implementation or the number of segments is
invalid for this type of Routing header.
(Note: This function returns the size but does not allocate the space
required for the ancillary data. This allows an application to
allocate a larger buffer, if other ancillary data objects are
desired, since all the ancillary data objects must be specified to
sendmsg() as a single msg_control buffer.)
8.2. inet6_rthdr_init
struct cmsghdr *inet6_rthdr_init(void *bp, int type);
This function initializes the buffer pointed to by bp to contain a
cmsghdr structure followed by a Routing header of the specified type.
The cmsg_len member of the cmsghdr structure is initialized to the
size of the structure plus the amount of space required by the
Routing header. The cmsg_level and cmsg_type members are also
initialized as required.
The caller must allocate the buffer and its size can be determined by
calling inet6_rthdr_space().
Upon success the return value is the pointer to the cmsghdr
structure, and this is then used as the first argument to the next
two functions. Upon an error the return value is NULL.
8.3. inet6_rthdr_add
int inet6_rthdr_add(struct cmsghdr *cmsg,
const struct in6_addr *addr, unsigned int flags);
This function adds the address pointed to by addr to the end of the
Routing header being constructed and sets the type of this hop to the
value of flags. For an IPv6 Type 0 Routing header, flags must be
either IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.
If successful, the cmsg_len member of the cmsghdr structure is
updated to account for the new address in the Routing header and the
return value of the function is 0. Upon an error the return value of
the function is -1.
8.4. inet6_rthdr_lasthop
int inet6_rthdr_lasthop(struct cmsghdr *cmsg,
unsigned int flags);
This function specifies the Strict/Loose flag for the final hop of a
Routing header. For an IPv6 Type 0 Routing header, flags must be
either IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.
The return value of the function is 0 upon success, or -1 upon an
error.
Notice that a Routing header specifying N intermediate nodes requires
N+1 Strict/Loose flags. This requires N calls to inet6_rthdr_add()
followed by one call to inet6_rthdr_lasthop().
8.5. inet6_rthdr_reverse
int inet6_rthdr_reverse(const struct cmsghdr *in, struct cmsghdr *out);
This function takes a Routing header that was received as ancillary
data (pointed to by the first argument) and writes a new Routing
header that sends datagrams along the reverse of that route. Both
arguments are allowed to point to the same buffer (that is, the
reversal can occur in place).
The return value of the function is 0 on success, or -1 upon an
error.
8.6. inet6_rthdr_segments
int inet6_rthdr_segments(const struct cmsghdr *cmsg);
This function returns the number of segments (addresses) contained in
the Routing header described by cmsg. On success the return value is
between 1 and 23, inclusive. The return value of the function is -1
upon an error.
8.7. inet6_rthdr_getaddr
struct in6_addr *inet6_rthdr_getaddr(struct cmsghdr *cmsg, int
index);
This function returns a pointer to the IPv6 address specified by
index (which must have a value between 1 and the value returned by
inet6_rthdr_segments()) in the Routing header described by cmsg. An
application should first call inet6_rthdr_segments() to obtain the
number of segments in the Routing header.
Upon an error the return value of the function is NULL.
8.8. inet6_rthdr_getflags
int inet6_rthdr_getflags(const struct cmsghdr *cmsg, int index);
This function returns the flags value specified by index (which must
have a value between 0 and the value returned by
inet6_rthdr_segments()) in the Routing header described by cmsg. For
an IPv6 Type 0 Routing header the return value will be either
IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.
Upon an error the return value of the function is -1.
(Note: Addresses are indexed starting at 1, and flags starting at 0,
to maintain consistency with the terminology and figures in [RFC-
1883].)
8.9. Routing Header Example
As an example of these Routing header functions, we go through the
function calls for the example on p. 18 of [RFC-1883]. The source is
S, the destination is D, and the three intermediate nodes are I1, I2,
and I3. f0, f1, f2, and f3 are the Strict/Loose flags for each hop.
f0 f1 f2 f3
S -----> I1 -----> I2 -----> I3 -----> D
src: * S S S S S
dst: D I1 I2 I3 D D
A[1]: I1 I2 I1 I1 I1 I1
A[2]: I2 I3 I3 I2 I2 I2
A[3]: I3 D D D I3 I3
#seg: 3 3 2 1 0 3
check: f0 f1 f2 f3
src and dst are the source and destination IPv6 addresses in the IPv6
header. A[1], A[2], and A[3] are the three addresses in the Routing
header. #seg is the Segments Left field in the Routing header.
check indicates which bit of the Strict/Loose Bit Map (0 through 3,
specified as f0 through f3) that node checks.
The six values in the column beneath node S are the values in the
Routing header specified by the application using sendmsg(). The
function calls by the sender would look like:
void *ptr;
struct msghdr msg;
struct cmsghdr *cmsgptr;
struct sockaddr_in6 I1, I2, I3, D;
unsigned int f0, f1, f2, f3;
ptr = malloc(inet6_rthdr_space(IPV6_RTHDR_TYPE_0, 3));
cmsgptr = inet6_rthdr_init(ptr, IPV6_RTHDR_TYPE_0);
inet6_rthdr_add(cmsgptr, &I1.sin6_addr, f0);
inet6_rthdr_add(cmsgptr, &I2.sin6_addr, f1);
inet6_rthdr_add(cmsgptr, &I3.sin6_addr, f2);
inet6_rthdr_lasthop(cmsgptr, f3);
msg.msg_control = ptr;
msg.msg_controllen = cmsgptr->cmsg_len;
/* finish filling in msg{}, msg_name = D */
/* call sendmsg() */
We also assume that the source address for the socket is not
specified (i.e., the asterisk in the figure).
The four columns of six values that are then shown between the five
nodes are the values of the fields in the packet while the packet is
in transit between the two nodes. Notice that before the packet is
sent by the source node S, the source address is chosen (replacing
the asterisk), I1 becomes the destination address of the datagram,
the two addresses A[2] and A[3] are "shifted up", and D is moved to
A[3]. If f0 is IPV6_RTHDR_STRICT, then I1 must be a neighbor of S.
The columns of values that are shown beneath the destination node are
the values returned by recvmsg(), assuming the application has
enabled both the IPV6_PKTINFO and IPV6_RTHDR socket options. The
source address is S (contained in the sockaddr_in6 structure pointed
to by the msg_name member), the destination address is D (returned as
an ancillary data object in an in6_pktinfo structure), and the
ancillary data object specifying the Routing header will contain
three addresses (I1, I2, and I3) and four flags (f0, f1, f2, and f3).
The number of segments in the Routing header is known from the Hdr
Ext Len field in the Routing header (a value of 6, indicating 3
addresses).
The return value from inet6_rthdr_segments() will be 3 and
inet6_rthdr_getaddr(1) will return I1, inet6_rthdr_getaddr(2) will