Date: Tue, 15 Jul 2003 17:48:21 +0200 From: Tobias Roth <roth@iam.unibe.ch> To: freebsd-questions@freebsd.org Subject: getting arp cache entry reliably Message-ID: <20030715154821.GB10201@speedy.unibe.ch>
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Hi
i have the following task to do:
i need to get the mac adress of my first upstream router (i.e my default
router). there are a number of constraints, however:
1) detection has to be as fast as possible, but 100% reliable
2) i can only use tools from /bin and /sbin, nothing from under /usr.
3) it has to be done from a shell script without any external c hacks.
4) all information i have at this point is the fixed IP adress of the
router.
right know i am doing this:
i configure my interface with an arbitrary ip (any IP from within the
RFC 1918 adress space, notably NOT within the same subnet as the router).
i cannot just use an ip within the same subnet as the router is because i
do not want to disturb the network.
next i set my default route to my interface
then i ping the router
after that, i have an entry in my arp cache which i read with the route
command and parse with the sh builtin expr.
this looks at follows (my router is 192.168.1.1):
ifconfig fxp0 10.11.12.13
ifconfig fxp0 up
route add default -interface fxp0
ping -c 5 -t 5 -m 1 -n -q 192.168.1.1
route=` route -n -v get 192.168.1.1 `
macaddr=`expr "//${route}" : ".* 192.168.1.1 \(.*\) fxp0.*10.11.12.13$" `
my problem with the above sequence is: sometimes i get an arp cache entry,
sometimes i don't. i've tried varying the -c and -t flags of ping with
little success.
as there a better way of doing this, or are some other flags to ping more
promising?
thanks in advance, t.
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