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