Cisco ACL's example to deny incoming SYN packets from outside network (Internet)

Below is an access list from a security book I am reading,

access-list 110 deny tcp any any established
access-list 110 permit tcp any any

interface s0
access-group 110 in  (this is the Internet interface)


If I want to deny incoming tcp traffic other than for established sessions should the access be as follows?

access-list 110 permit tcp any any established

interface s0
access-group 110 in

It seems to me the first access list will deny the returning packets and since a ACL will deny by default, why do I need the 'deny' at all in the first list?

Thanks,
Jerri
benje02Asked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

lrmooreCommented:
You are correct in that the first example is backward and is actually blocking what you want to permit (the return traffic to outbound requests). It appears to only permit unsolicited inbound tcp connection requests.

Your 2nd example will permit inbound tcp connections in response to outbound requests, but you will not be able to do things like dns name resolution and you will block icmp inreachables which can have unintended results.

This is they way I typically setup an access list

access-list 110 permit tcp any any established
access-list 110 permit udp any eq 53 any
access-list 110 permit icmp any any unreachables
access-list 110 deny ip any any log  <-- you have to add the line for the "log" keyword --helps in troubleshooting



0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
benje02Author Commented:
Thanks for the help.  You guys are great!  
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Routers

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.