iptables rule order

[SB CH]

Hello, netfilters!

I read that the iptables rule order is important.
In linux firewalls 2nd edition writeen by ziegler wrote like this.

1. blocking traffic rules must come before the rules allowing traffic to 
specific service.

2. obviously, the FTP data channel rules must come near the endof the rule 
list, even thought you'd want thr rules to be near the top of the list 
because FTP transfers tend to be large.

But I can't understand why those orders should.

Please let me know about this reason.

Thanks in advance.




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