Hardware firewall question (syn atack)

I have a strange problem with my new modem-router.
(d link 2640 b e1)
I test it with grc shields up.
Its firewall has many settins such as protect from dos atacks and port scanning.
I have no problem with its all options but when i enable the protection from syn flood atack i have a strange problem.
With flood atack protection enabled in in grc shield up test i take as result many ports as not stealth but as closed, but with it disabled i take all of them as stealth.
Does anyone knows why when i enable protection from syn atack i take as result many ports as closed(red colour) instead of stealth?
It is better to disable this protection?

 

I beleive the sync flood attack and stealth settings on your router are mutually exclusive. If you have the ports stealted, a syn flood attack is impossible since no one can see your ports to perform a sync flood attack.

I believe the sync flood attack setting will set your port to closed versus stealth as evidenced by the GRC Shields Up test. You can get a send opinion port scan on the PCFlack web site: http://pcflank.com/.

I would just use the slealth setting on your router firewall. If it has a separate DOS(denial of service) setting, I would also set that on. Leave the sync flood setting off which I assume is its default setting?

 

Defaults are nat firewall + spi firewall.
And advanced options it has dos and port scanning protection which are:
syn atack,fin/urg/psh atack,ping atack,xmas tree atack,tcp reset atack,null scan atack,ping of death atack,syn/rst syn/fin atack.

i checked my firewall with pcflank with all of them and i have closed ports then i
enabled all of them except the syn atack and all my port are stealth.
-http://i47.tinypic.com/fa0rk9.jpg