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<H1>ARPD Daemon</H1>
<H2>Alexey Kuznetsov, <CODE>kuznet@ms2.inr.ac.ru</CODE></H2>some_negative_number, 20 Sep 2001
<HR>
<EM><CODE>arpd</CODE> is daemon collecting gratuitous ARP information, saving
it on local disk and feeding it to kernel on demand to avoid
redundant broadcasting due to limited size of kernel ARP cache. </EM>
<HR>
<P><B>Description</B></P>
<P>The format of the command is:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
arpd OPTIONS [ INTERFACE [ INTERFACE ... ] ]
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P> <CODE>OPTIONS</CODE> are:</P>
<P>
<UL>
<LI><CODE>-l</CODE> - dump <CODE>arpd</CODE> database to stdout and exit. Output consists
of three columns: interface index, IP address and MAC address.
Negative entries for dead hosts are also shown, in this case MAC address
is replaced by word <CODE>FAILED</CODE> followed by colon and time when the fact
that host is dead was proven the last time.
</LI>
<LI><CODE>-f FILE</CODE> - read and load <CODE>arpd</CODE> database from <CODE>FILE</CODE>
in text format similar dumped by option <CODE>-l</CODE>. Exit after load,
probably listing resulting database, if option <CODE>-l</CODE> is also given.
If <CODE>FILE</CODE> is <CODE>-</CODE>, <CODE>stdin</CODE> is read to get ARP table.
</LI>
<LI><CODE>-b DATABASE</CODE> - location of database file. Default location is
<CODE>/var/lib/arpd/arpd.db</CODE>.
</LI>
<LI><CODE>-a NUMBER</CODE> - <CODE>arpd</CODE> not only passively listens ARP on wire, but
also send brodcast queries itself. <CODE>NUMBER</CODE> is number of such queries
to make before destination is considered as dead. When <CODE>arpd</CODE> is started
as kernel helper (i.e. with <CODE>app_solicit</CODE> enabled in <CODE>sysctl</CODE>
or even with option <CODE>-k</CODE>) without this option and still did not learn enough
information, you can observe 1 second gaps in service. Not fatal, but
not good.
</LI>
<LI><CODE>-k</CODE> - suppress sending broadcast queries by kernel. It takes
sense together with option <CODE>-a</CODE>.
</LI>
<LI><CODE>-n TIME</CODE> - timeout of negative cache. When resolution fails <CODE>arpd</CODE>
suppresses further attempts to resolve for this period. It makes sense
only together with option <CODE>-k</CODE>. This timeout should not be too much
longer than boot time of a typical host not supporting gratuitous ARP.
Default value is 60 seconds.
</LI>
<LI><CODE>-R RATE</CODE> - maximal steady rate of broadcasts sent by <CODE>arpd</CODE>
in packets per second. Default value is 1.
</LI>
<LI><CODE>-B NUMBER</CODE> - number of broadcasts sent by <CODE>arpd</CODE> back to back.
Default value is 3. Together with option <CODE>-R</CODE> this option allows
to police broadcasting not to exceed <CODE>B+R*T</CODE> over any interval
of time <CODE>T</CODE>.
</LI>
</UL>
</P>
<P><CODE>INTERFACE</CODE> is name of networking inteface to watch.
If no interfaces given, <CODE>arpd</CODE> monitors all the interfaces.
In this case <CODE>arpd</CODE> does not adjust <CODE>sysctl</CODE> parameters,
it is supposed user does this himself after <CODE>arpd</CODE> is started.</P>
<P> Signals</P>
<P> <CODE>arpd</CODE> exits gracefully syncing database and restoring adjusted
<CODE>sysctl</CODE> parameters, when receives <CODE>SIGINT</CODE> or <CODE>SIGTERM</CODE>.
<CODE>SIGHUP</CODE> syncs database to disk. <CODE>SIGUSR1</CODE> sends some statistics
to <CODE>syslog</CODE>. Effect of another signals is undefined, they may corrupt
database and leave <CODE>sysctl</CODE> parameters in an unpredictable state.</P>
<P> Note</P>
<P> In order to <CODE>arpd</CODE> be able to serve as ARP resolver, kernel must be
compiled with the option <CODE>CONFIG_ARPD</CODE> and, in the case when interface list
is not given on command line, variable <CODE>app_solicit</CODE>
on interfaces of interest should be set in <CODE>/proc/sys/net/ipv4/neigh/*</CODE>.
If this is not made <CODE>arpd</CODE> still collects gratuitous ARP information
in its database.</P>
<P> Examples</P>
<P>
<OL>
<LI> Start <CODE>arpd</CODE> to collect gratuitous ARP, but not messing
with kernel functionality:
<BLOCKQUOTE><CODE>
<PRE>
arpd -b /var/tmp/arpd.db
</PRE>
</CODE></BLOCKQUOTE>
</LI>
<LI> Look at result after some time:
<BLOCKQUOTE><CODE>
<PRE>
killall arpd
arpd -l -b /var/tmp/arpd.db
</PRE>
</CODE></BLOCKQUOTE>
</LI>
<LI> To enable kernel helper, leaving leading role to kernel:
<BLOCKQUOTE><CODE>
<PRE>
arpd -b /var/tmp/arpd.db -a 1 eth0 eth1
</PRE>
</CODE></BLOCKQUOTE>
</LI>
<LI> Completely replace kernel resolution on interfaces <CODE>eth0</CODE>
and <CODE>eth1</CODE>. In this case kernel still does unicast probing to
validate entries, but all the broadcast activity is suppressed
and made under authority of <CODE>arpd</CODE>:
<BLOCKQUOTE><CODE>
<PRE>
arpd -b /var/tmp/arpd.db -a 3 -k eth0 eth1
</PRE>
</CODE></BLOCKQUOTE>
This is mode which <CODE>arpd</CODE> is supposed to work normally.
It is not default just to prevent occasional enabling of too aggressive
mode occasionally.
</LI>
</OL>
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