send udp packet that cksum 0xffff.

udpffff

#!/bin/sh
 
DST=192.168.2.123
SRC=192.168.0.2
DSTPORT=11111
SRCPORT=12345

perl -e '
        sub xip{ pack("C4",map(~(0+$_),split("\\.",shift))) }
        sub xn{ pack("n",map(~(0+$_),shift)) }
        print xip(shift),xip(shift),xn(shift),xn(shift),xn(0x3175)
' $DST $SRC $DSTPORT $SRCPORT | socat - UDP:$DST:$DSTPORT,sourceport=$SRCPORT