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.TH "Firewall mark classifier in tc" 8 "21 Oct 2015" "iproute2" "Linux"
.SH NAME
fw \- fwmark traffic control filter
.SH SYNOPSIS
.in +8
.ti -8
.BR tc " " filter " ... " fw " [ " classid
.IR CLASSID " ] [ "
.B action
.IR ACTION_SPEC " ]"
.SH DESCRIPTION
the
.B fw
filter allows one to classify packets based on a previously set
.BR fwmark " by " iptables .
If the masked value of the
.B fwmark
matches the filter's masked
.BR handle ,
the filter matches. By default, all 32 bits of the
.B handle
and the
.B fwmark
are masked.
.B iptables
allows one to mark single packets with the
.B MARK
target, or whole connections using
.BR CONNMARK .
The benefit of using this filter instead of doing the
heavy-lifting with
.B tc
itself is that on one hand it might be convenient to keep packet filtering and
classification in one place, possibly having to match a packet just once, and on
the other users familiar with
.BR iptables " but not " tc
will have a less hard time adding QoS to their setups.
.SH OPTIONS
.TP
.BI classid " CLASSID"
Push matching packets to the class identified by
.IR CLASSID .
.TP
.BI action " ACTION_SPEC"
Apply an action from the generic actions framework on matching packets.
.SH EXAMPLES
Take e.g. the following tc filter statement:
.RS
.EX
tc filter add ... handle 6 fw classid 1:1
.EE
.RE
will match if the packet's
.B fwmark
value is
.BR 6 .
This is a sample
.B iptables
statement marking packets coming in on eth0:
.RS
.EX
iptables -t mangle -A PREROUTING -i eth0 -j MARK --set-mark 6
.EE
.RE
Specific bits of the packet's
.B fwmark
can be set using the
.B skbedit
action. For example, to only set one bit of the
.B fwmark
without changing any other bit:
.RS
.EX
tc filter add ... action skbedit mark 0x8/0x8
.EE
.RE
The
.B fw
filter can then be used to match on this bit by masking the
.B handle:
.RS
.EX
tc filter add ... handle 0x8/0x8 fw action drop
.EE
.RE
This is useful when different bits of the
.B fwmark
are assigned different meanings.
.EE
.RE
.SH SEE ALSO
.BR tc (8),
.BR iptables (8),
.BR iptables-extensions (8),
.BR tc-skbedit (8)
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