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NG_TAG(4)              FreeBSD Kernel Interfaces Manual              NG_TAG(4)

     ng_tag -- mbuf tags manipulating netgraph node type

     #include <netgraph/ng_tag.h>

     The tag node type allows mbuf packet tags (see mbuf_tags(9)) to be exam-
     ined, stripped or applied to data travelling through a Netgraph network.
     Mbuf tags are used in many parts of the FreeBSD kernel network subsystem,
     including the storage of VLAN tags as described in vlan(4), Mandatory
     Access Control (MAC) labels as described in mac(9), IPsec policy informa-
     tion as described in ipsec(4), and packet filter tags used by pf(4).  One
     should also consider useful setting or checking ipfw(8) tags, which are
     implemented as mbuf tags, too.

     Each node allows an arbitrary number of connections to arbitrarily named
     hooks.  With each hook is associated a tag which will be searched in the
     list of all tags attached to a packet incoming to this hook, a destina-
     tion hook for matching packets, a destination hook for non-matching pack-
     ets, a tag which will be appended to data leaving node through this hook,
     and various statistics counters.

     The list of incoming packet's tags is traversed to find a tag with speci-
     fied type and cookie values.  Upon match, if specified tag_len is non-
     zero, tag_data of tag is checked to be identical to that specified in the
     hook structure.  Packets with matched tags are forwarded to ``match''
     destination hook, or forwarded to ``non-match'' hook otherwise.  Either
     or both destination hooks can be an empty string, or may not exist, in
     which case the packet is dropped.

     Tag list of packets leaving the node is extended with a new tag specified
     in outgoing hook structure (it is possible to avoid appending a new tag
     to pass packet completely unchanged by specifying zero type and cookie
     values in the structure of the corresponding outgoing hook).  Addition-
     ally, a tag can be stripped from incoming packet after match if strip
     flag is set.  This can be used for simple tag removal or tag replacement,
     if combined with tag addition on outgoing matching hook.  Note that new
     tag is appended unconditionally, without checking if such a tag is
     already present in the list (it is up to user to check if this is a con-

     New hooks are initially configured to drop all incoming packets (as all
     hook names are empty strings; zero values can be specified to forward all
     packets to non-matching hook), and to forward all outgoing packets with-
     out any tag appending.

     Data payload of packets passing through the node is completely unchanged,
     all operations can affect tag list only.

     This node type supports any number of hooks having arbitrary names.  In
     order to allow internal optimizations, user should never try to configure
     a hook with a structure pointing to hooks which do not exist yet.  The
     safe way is to create all hooks first, then begin to configure them.

     This node type supports the generic control messages, plus the following:

             This command sets tag values which will be searched in the tag
             list of incoming packets on a hook.  The following structure must
             be supplied as an argument:

                 struct ng_tag_hookin {
                   char            thisHook[NG_HOOKSIZ];     /* name of hook */
                   char            ifMatch[NG_HOOKSIZ];      /* match dest hook */
                   char            ifNotMatch[NG_HOOKSIZ];   /* !match dest hook */
                   uint8_t         strip;                    /* strip tag if found */
                   uint32_t        tag_cookie;               /* ABI/Module ID */
                   uint16_t        tag_id;                   /* tag ID */
                   uint16_t        tag_len;                  /* length of data */
                   uint8_t         tag_data[0];              /* tag data */

             The hook to be updated is specified in thisHook.  Data bytes of
             tag corresponding to specified tag_id (type) and tag_cookie are
             placed in the tag_data array; there must be tag_len of them.
             Matching and non-matching incoming packets are delivered out the
             hooks named ifMatch and ifNotMatch, respectively.  If strip flag
             is non-zero, then found tag is deleted from list of packet tags.

             This command takes an ASCII string argument, the hook name, and
             returns the corresponding struct ng_tag_hookin as shown above.

             This command sets tags values which will be applied to outgoing
             packets.  The following structure must be supplied as an argu-

                 struct ng_tag_hookout {
                   char            thisHook[NG_HOOKSIZ];     /* name of hook */
                   uint32_t        tag_cookie;               /* ABI/Module ID */
                   uint16_t        tag_id;                   /* tag ID */
                   uint16_t        tag_len;                  /* length of data */
                   uint8_t         tag_data[0];              /* tag data */

             The hook to be updated is specified in thisHook.  Other variables
             mean basically the same as in struct ng_tag_hookin shown above,
             except used for setting values in a new tag.

             This command takes an ASCII string argument, the hook name, and
             returns the corresponding struct ng_tag_hookout as shown above.

             This command takes an ASCII string argument, the hook name, and
             returns the statistics associated with the hook as a struct

             This command takes an ASCII string argument, the hook name, and
             clears the statistics associated with the hook.

             This command is identical to NGM_TAG_GET_STATS, except that the
             statistics are also atomically cleared.

     Note: statistics counters as well as three statistics messages above work
     only if code was compiled with the NG_TAG_DEBUG option.  The reason for
     this is that statistics is rarely used in practice, but still consumes
     CPU cycles for every packet.  Moreover, it is even not accurate on SMP
     systems due to lack of syncronization between threads, as this is very

     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.

     It is possible to do a simple L7 filtering by using ipfw(8) tags in con-
     junction with ng_bpf(4) traffic analyzer.  Example below explains how to
     filter DirectConnect P2P network data traffic, which cannot be done by
     usual means as it uses random ports.  It is known that such data connec-
     tion always contains a TCP packet with 6-byte payload string "$Send|".
     So ipfw's netgraph action will be used to divert all TCP packets to an
     ng_bpf(4) node which will check for the specified string and return non-
     matching packets to ipfw(8).  Matching packets are passed to ng_tag(4)
     node, which will set a tag and pass them back to ng_bpf(4) node on a hook
     programmed to accept all packets and pass them back to ipfw(8).  A script
     provided in ng_bpf(4) manual page will be used for programming node.
     Note that packets diverted from ipfw(8) to Netgraph have no link-level
     header, so offsets in tcpdump(1) expressions must be altered accordingly.
     Thus, there will be expression ``ether[40:2]=0x244c &&
     ether[42:4]=0x6f636b20'' on incoming hook and empty expression to match
     all packets from ng_tag(4).

     So, this is ngctl(8) script for nodes creating and naming for easier

         /usr/sbin/ngctl -f- <<-SEQ
                 mkpeer ipfw: bpf 41 ipfw
                 name ipfw:41 dcbpf
                 mkpeer dcbpf: tag matched th1
                 name dcbpf:matched ngdc

     Now ``ngdc'' node (which is of type ng_tag) must be programmed to echo
     all packets received on the ``th1'' hook back, with the ipfw(8) tag 412
     attached.  MTAG_IPFW value for tag_cookie was taken from file
     <netinet/ip_fw.h> and value for tag_id is tag number (412), with zero tag

         ngctl msg ngdc: sethookin { thisHook=\"th1\" ifNotMatch=\"th1\" }
         ngctl msg ngdc: sethookout { thisHook=\"th1\" \
           tag_cookie=1148380143 \
           tag_id=412 }

     Don't forget to program ng_bpf(4) ``ipfw'' hook with the above expression
     (see ng_bpf(4) for script doing this) and ``matched'' hook with an empty

         ngctl msg dcbpf: setprogram { thisHook=\"matched\" ifMatch=\"ipfw\" \
           bpf_prog_len=1 bpf_prog=[ { code=6 k=8192 } ] }

     After finishing with netgraph(4) nodes, ipfw(8) rules must be added to
     enable packet flow:

         ipfw add 100 netgraph 41 tcp from any to any iplen 46
         ipfw add 110 reset tcp from any to any tagged 412

     Note: one should ensure that packets are returned to ipfw after process-
     ing inside netgraph(4), by setting appropriate sysctl(8) variable:

         sysctl net.inet.ip.fw.one_pass=0

     For manipulating any tags with data payload (that is, all tags with non-
     zero tag_len) one should care about non-portable machine-dependent repre-
     sentation of tags on the low level as byte stream.  Perhaps this should
     be done by another program rather than manually.

     The ng_tag node type was implemented in FreeBSD 6.2.

     netgraph(4), ng_bpf(4), ng_ipfw(4), ipfw(8), ngctl(8), mbuf_tags(9)

     Vadim Goncharov <>

FreeBSD 6.2                      June 10, 2006                     FreeBSD 6.2


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