NG_ETF(4) FreeBSD Kernel Interfaces Manual NG_ETF(4)


ng_etfEthertype filtering netgraph node type


#include < netgraph.h>
#include < netgraph/ng_etf.h>


The etf node type multiplexes and filters data between hooks on the basis of the ethertype found in an Ethernet header, presumed to be in the first 14 bytes of the data. Incoming Ethernet frames are accepted on the downstream hook and if the ethertype matches a value which the node has been configured to filter, the packet is forwarded out the hook which was identified at the time that value was configured. If it does not match a configured value, it is passed to the nomatch hook. If the nomatch hook is not connected, the packet is dropped.

Packets travelling in the other direction (towards the downstream hook) are also examined and filtered. If a packet has an ethertype that matches one of the values configured into the node, it must have arrived in on the hook for which that value was configured, otherwise it will be discarded. Ethertypes of values other than those configured by the control messages must have arrived via the nomatch hook.


This node type supports the following hooks:
Typically this hook would be connected to a ng_ether(4) node, using the lower hook.
Typically this hook would also be connected to an ng_ether(4) type node using the upper hook.
< any legal name>
Any other hook name will be accepted and can be used as the match target of an ethertype. Typically this hook would be attached to a protocol handling node that requires and generates packets with a particular set of ethertypes.


This node type supports the generic control messages, plus the following:
NGM_ETF_GET_STATUS ( getstatus)
This command returns a struct ng_etfstat containing node statistics for packet counts.
NGM_ETF_SET_FILTER ( setfilter)
Sets the a new ethertype filter into the node and specifies the hook to and from which packets of that type should use. The hook and ethertype are specified in a structure of type struct ng_etffilter:

struct ng_etffilter { 
    char matchhook[NG_HOOKSIZ]; /* hook name */ 
    uint16_t ethertype;  /* this ethertype to this hook */ 


Using ngctl(8) it is possible to set a filter in place from the command line as follows:

cat <<DONE >/tmp/xwert 
# Make a new ethertype filter and attach to the Ethernet lower hook. 
# first remove left over bits from last time. 
shutdown ${ETHER_IF}:lower 
mkpeer ${ETHER_IF}: etf lower downstream 
# Give it a name to easily refer to it. 
name ${ETHER_IF}:lower etf 
# Connect the nomatch hook to the upper part of the same interface. 
# All unmatched packets will act as if the filter is not present. 
connect ${ETHER_IF}: etf: upper nomatch 
ngctl -f /tmp/xwert 
# something to set a hook to catch packets and show them. 
echo "Unrecognised packets:" 
nghook -a etf: newproto & 
# Filter two random ethertypes to that hook. 
ngctl 'msg etf: setfilter { matchhook="newproto" ethertype=${MATCH1} } 
ngctl 'msg etf: setfilter { matchhook="newproto" ethertype=${MATCH2} }


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


The ng_etf node type was implemented in FreeBSD 5.0.


Julian Elischer <julian@FreeBSD.org>
November 13, 2012 FreeBSD