MBUF_TAGS(9)                 OpenBSD Kernel Manual                MBUF_TAGS(9)

NAME
     packet tags - a framework for generic packet attributes

SYNOPSIS
     #include <sys/mbuf.h>

     struct m_tag *
     m_tag_get(int type, int len, int flags);

     void
     m_tag_free(struct m_tag *tag);

     struct m_tag *
     m_tag_find(struct mbuf *mbuf, int type, struct m_tag *tag);

     void
     m_tag_prepend(struct mbuf *mbuf, struct m_tag *tag);

     void
     m_tag_unlink(struct mbuf *mbuf, struct m_tag *tag);

     void
     m_tag_delete(struct mbuf *mbuf, struct m_tag *tag);

     struct m_tag *
     m_tag_copy(struct m_tag *tag);

     void
     m_tag_delete_chain(struct mbuf *mbuf, struct m_tag *tag);

     void
     m_tag_init(struct mbuf *mbuf);

     int
     m_tag_copy_chain(struct mbuf *mbuf, struct mbuf *mbuf2);

     struct m_tag *
     m_tag_first(struct mbuf *mbuf);

     struct m_tag *
     m_tag_next(struct mbuf *mbuf, struct m_tag *tag);

DESCRIPTION
     These functions allow the manipulation of generic packet attributes.
     They are used by the kernel to keep track of operations done or scheduled
     to happen to packets.  These attributes are attached to mbuf packet head-
     ers.

     m_tag_get() allocates a new tag of type type with len bytes of space fol-
     lowing the tag header itself.  The flag argument is passed directly to
     malloc(9). If successful, m_tag_get() returns a memory buffer of (len +
     sizeof (struct m_tag)) bytes.  The first sizeof(struct m_tag) bytes con-
     tain a struct m_tag:

     struct m_tag {
             SLIST_ENTRY(m_tag)      m_tag_link;     /* List of packet tags */
             u_int16_t               m_tag_id;       /* Tag ID */
             u_int16_t               m_tag_len;      /* Length of data */
     };

     The m_tag_link field is used to link tags together (see queue(3) for more
     details).  The m_tag_id and m_tag_len fields are set to type and len re-
     spectively.  Following this structure are len bytes of space that can be
     used to store tag-specific information.

     The currently defined tag types are:

     PACKET_TAG_NONE
                   This should never be used.

     PACKET_TAG_IPSEC_IN_DONE
                   Used by ipsec(4) to indicate successful processing per-
                   formed on an input packet.  The tag contains a struct
                   tdb_ident, as defined in sys/netinet/ip_ipsp.h, identifying
                   the security association under which the packet arrived.

     PACKET_TAG_IPSEC_OUT_DONE
                   Used by IPsec to indicate that an output packet has been
                   IPsec-processed.  The tag contains a struct tdb_ident iden-
                   tifying the security association applied to the packet.
                   This tag is primarily used to detect and avoid loops in
                   IPsec processing on output.

     PACKET_TAG_IPSEC_IN_CRYPTO_DONE
                   Used by network cards that implement on-board IPsec pro-
                   cessing to indicate that the crypto processing of an IPsec
                   packet has been done.  The tag contains a struct tdb_ident
                   identifying the security association under which the packet
                   arrived.

     PACKET_TAG_IPSEC_OUT_CRYPTO_NEEDED
                   Used by the IPsec stack to signal to network cards that im-
                   plement on-board IPsec processing that such processing is
                   needed.  The tag contains a struct tdb_ident identifying
                   the security association that should be applied.  The pack-
                   et is already formatted for the appropriate security proto-
                   col.

     PACKET_TAG_IPSEC_IN_COULD_DO_CRYPTO
                   Used by network cards that implement on-board IPsec pro-
                   cessing to indicate to the IPsec stack that cryptographic
                   processing could be deferred to hardware.  The tag contains
                   a struct tdb_ident identifying the security association
                   that could be offloaded to the network card.

     PACKET_TAG_IPSEC_PENDING_TDB
                   Used by the IPsec stack to keep track of IPsec processing
                   that should happen to the packet on output.  The tag con-
                   tains a struct tdb_ident identifying the security associa-
                   tion that should be applied at the next loop of IPsec pro-
                   cessing.

     PACKET_TAG_BRIDGE
                   Used by the bridge(4) code to detect loops in bridge pro-
                   cessing.  The tag contains a pointer to the bridge inter-
                   face that already forwarded the frame.

     PACKET_TAG_GIF
                   Used by the gif(4) interface to detect loops in processing.
                   The tag contains a pointer to the gif interface that al-
                   ready processed the packet.

     PACKET_TAG_GRE
                   Used by the gre(4) interface to detect loops in processing.
                   The tag contains a pointer to the gre interface that al-
                   ready processed the packet.

     PACKET_TAG_IN_PACKET_CHECKSUM
                   Used by network cards that can compute complete packet
                   checksums to pass that information to higher-level proto-
                   cols.  The tag contains the 2 byte checksum of the packet.

     PACKET_TAG_PF_GENERATED
                   Used to mark packets generated by the packet filter pf(4).
                   Packets with this tag are not tested by pf but passed un-
                   conditionally.  Contains no data.

     m_tag_free() de-allocates a tag.

     m_tag_find() finds the an instance of a tag of type type attached to
     packet mbuf. If tag is NULL, the first such tag is returned.  Otherwise,
     the first tag of type type after tag is returned.  If no such tag is
     found, NULL is returned.

     m_tag_prepend() adds the new tag tag at the head of the tag list for
     packet mbuf.

     m_tag_unlink() removes tag tag from the list of tags of packet mbuf.

     m_tag_delete() removes and then de-allocates tag tag from the list of
     tags of packet mbuf.

     m_tag_copy() creates an unlinked copy of tag tag.

     m_tag_delete_chain() deletes all tags attached to packet mbuf following
     tag tag. If tag is NULL, all tags are deleted.

     m_tag_init() initializes the tag storage for packet mbuf.

     m_tag_copy_chain() copies all tags from packet mbuf to packet mbuf2. On
     success, it returns 1.  Otherwise, it returns 0.

     m_tag_first() returns the first tag attached to packet mbuf.

     m_tag_next() returns the tag following tag in packet mbuf.

     The M_MOVE_PKTHDR() and M_MOVE_HDR() macros defined in sys/sys/mbuf.h
     move the tags from the old to the new mbuf.  The M_DUP_PKTHDR() and
     M_DUP_HDR() macros create copies of the tag chain for the new mbuf.

CODE REFERENCES
     The tag-manipulating code is contained in the file sys/kern/uipc_mbuf2.c.

SEE ALSO
     bridge(4), gif(4), gre(4), ipsec(4), pf(4), malloc(9)

HISTORY
     The packet tags first appeared in OpenBSD 2.9 and were written by Angelos
     D. Keromytis <angelos@openbsd.org>.

OpenBSD 3.0                      June 25, 2001                               3
