linux box disallow incoming udp traffic ??

[M E Fieu]

Hi..I was playing with nmap function.  When I use nmap -sU (udp port scan) to another linux
machine, there is no result returned.  But when using nmap (tcp port scan) is okay.  If I nmap -sU
on the localhost itself nmap -sU 127.0.0.1, it has correct result returned.  And If I use nmap -sU
to another windows machine, it has correct result returned too.  There is no firewall in between
those host.  May I know why nmap -sU can't scan another linux PC ??  

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