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Source Tracking in PF

I just committed code which adds support to track stateful connections
by source IP address. This allows a user to:
- Ensure that clients get a consistent IP mapping with load-balanced
  translation/routing rules
- Limit the number of simultaneous connections a client can make
- Limit the number of clients which can connect through a rule
As always, the more people who test this and provide feedback, the
happier I am. Read below for details.
The 'sticky-address' keyword applies to translation rules; the
translation address will be selected from the list, but once a client
makes a connection, all subsequent connections are translated the same
nat on $ext_if from $int_net to any -> \
        {, } round-robin sticky-address
Similarly in the following case, connections to a server are mapped
randomly, but all connections from a client are mapped the same way.
rdr on $ext_if from any to $ext_if -> \
        { } random sticky-address
In the case of tracking states per rule, the syntax is slightly
different. The 'source-track' state option is used to enable tracking,
limits can also be set there: 'max-src-states' limits how many states
from a single source can exist simultaneously and 'max-src-nodes'
limits how many source nodes can exist (effectively this is a limit
on how many clients can connect at once).
The following rule allows a maximum of 1000 source ip's to connect to a
webserver, each with a maximum of 3 simultaneous states:
pass in on $ext_if proto tcp to $webserver port www flags S/SA \
        keep state (source-track, max-src-states 3, max-src-nodes 10)
By default, the source node is removed when all the states which
reference it are removed. However the timeout 'src.track' regulates
how long the source node is retained after all it's states have died.
set timeout src.track 900
A list of src nodes can be retrived with pfctl -sS, pfctl -vsS;
pfctl -vsi provides statistics on src-node useage (similar to the
statistics provided for states)