text: MBUF_TAGS(9) BSD Kernel Manual MBUF_TAGS(9)
  mbuf_tags - a framework for generic packet attributes
  #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);
  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 headers. 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 respectively. 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 IP- sec 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 packet is already formatted for the appropriate security protocol. 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 interface 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. PACKET_TAG_PF_ROUTED Used to mark packets routed by the packet filter pf(4). Packets with this tag are not tested by pf more than once to prevent loops caused by subsequent matching routing rules. Contains no data. PACKET_TAG_PF_FRAGCACHE Used to mark fragmented packets cached by the packet filter pf(4). Packets with this tag have been cached by the frag- ment cache already and will short circuit it if processed again. If they were to re-enter the fragcache, they would be indistinguishable from a duplicate of a previous packet and would be dropped. Contains no data. PACKET_TAG_PF_QID Used by pf(4) for queueing. The tag contains the ID of the queue this packet should go to. PACKET_TAG_PF_TAG Used by pf(4) to tag packets and filtering on those later on. PACKET_TAG_PF_TRANSLATE_LOCALHOST Used by pf(4) to mark TCP and UDP packets redirected to loopback addresses. The functions tcp_input() and udp_input() reverse the order of lookups in in_pcblookup_listen(), when this tag is present, so un- specific listeners are matched before specific ones. This prevents external connections from appearing local to dae- mons such as portmap(8) listening on both unspecific and specific loopback sockets in order to grant higher privileges to local users. m_tag_free() de-allocates a tag. m_tag_find() finds 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.
  The tag-manipulating code is contained in the file sys/kern/uipc_mbuf2.c.
  bridge(4), gif(4), gre(4), ipsec(4), pf(4), malloc(9)
  The packet tags first appeared in OpenBSD 2.9 and were written by Angelos D. Keromytis <angelos@openbsd.org>. MirOS BSD #10-current June 25, 2001.