opkg update
opkg install fping
opkg install net-tools-arp
nano /root/getip.sh
#!/bin/ash
SERVERMACADDRESS="00:19:b9:22:ae:4a"
IP=`ifconfig eth1 | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}'`
NETMASK=`ifconfig eth1 | grep 'inet addr:'  | cut -d: -f4`
PREFIX=`/bin/ipcalc.sh $IP $NETMASK |grep 'PREFIX=' |sed  's/\(.\{7\}\)//'`
NETWORK=`/bin/ipcalc.sh $IP $NETMASK |grep 'NETWORK=' |sed  's/\(.\{8\}\)//'`
fping -c 1 -g $NETWORK/$PREFIX > /dev/null 2>&1
SERVERIP=`/usr/bin/arp -n | grep -i $SERVERMACADDRESS | awk '{print $1}'`
echo $SERVERIP | tee /root/serverip.txt
chmod 755  /root/getip.sh
/root/getip.sh

SERVERMACADDRESS="00:19:b9:22:ae:4a" is Yun's server

/root/serverip.txt hold server ip address at Yun's client

eth1: ethernet

wlan0: wifi