Are firewalls the same as CBAC on a router, and do they work by checking out going connections then checking the return traffic, , also say i had a web server, how does the pix make sure no other traffic or protocols get through ?
CBAC or Firewall Feature Set IOS does most of what PIX firewall does. However, PIX firewall is designed for complex and advanced configurations.
Moreover, PIX firewall are more robust because the IOS is design mainly for security and you have a hardware based firewall. Therefore, it really depends on the requiremetns to usually spec a PIX firewall or router with IOS firewall.
As for your question, PIX inspect all traffic comming from inside to outside and outside to inside. You can restrict traffic by using Access-list on the outside and inside interface. Moreover, there is security level on each interface 100 being for inside and 1 being from outside. This means everything from outside to inside is denied unless you implement an access list permitting specific type of traffic.
By default, everything from outside to inside will be denied. To allow Http access to the web server you have to do Static NAT (one to one NAT) for the webserver and create an access-list allowing http to the webserver public IP and apply to the outside interface.
IN this way, only http access will be allowed to your webserver.
one thing the other answers are missing is an explanation of the fixup command in PIX (6.3), in 7.x the fixup command has been replaced with policy maps. The pix will do deep packet inspection on (for example) http packets to make sure they are actually http packets and not something piggy backing over tcp/80 (or 443). CBAC to my knowledge doesn't do this.
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