MBUF_TAGS(9) BSD Kernel Manual MBUF_TAGS(9)NAMEmbuf_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 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.
CODE REFERENCES
The tag-manipulating code is contained in the file sys/kern/uipc_mbuf2.c.
SEE ALSObridge(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>.
MirOS BSD #10-current June 25, 2001 3