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A proxy usually controls HTTP(S) traffic going outbound. IE it can be blocked, and so forth. How is this different to a firewall working on port 80/443?

Also, if a firewall is to control outbound http(s) traffic, how can it block ads? Would that not be a reverse proxy?

  • To help you differentiate, you have to change your terms: a proxy controls the content outbound, not general traffic. Firewalls allow traffic, proxies allow content (which can sometime look like blocking traffic). – schroeder Oct 18 '14 at 22:05
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I would recommend doing a little reading on the OSI model. It might help shed a little light on this, but I will give you a brief overview so this makes more sense. Remember that these concepts can blend together and a firewall could have all of these features.

A traditional network firewall operates at layer 3 and layer 4 of the OSI model, which is IP addressing/ICMP and TCP/UDP. In its simplest sense, it's a hierarchical rule chain that blocks or allows specific packets which match a specific criteria. This type of firewall implementation has little to no awareness of "higher" layers of the OSI model.

An application proxy is typically said to exist at layer 7 of the OSI model, the application layer. These proxies must understand the protocols that they proxy, such as HTTP. A firewall can often have these features as well. A web proxy, with its awareness of web protocols, can have content filtering features which could filter ads. A device or software package described as a firewall could have these features. These are typically called application firewalls or layer 7 firewalls.

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A normal firewall typically works on Layer 3 and 4 of OSI model, a proxy can work on Layer 7. Also another thing that a proxy does is: anonymise the requests.

E.g.: A normal firewall can block based on destination / origin IP or TCP/UDP ports. A proxy can block based on protocol (https) and / or url (http://stackexchange.com)...

Now most of the modern firewalls can have the same functions of the proxies making hard to distinguish from them but all is related with power and logic...

To have a fast firewall in is work we should not have it doing more than what is necessary. this makes harder to cause a denial of service. Also having powerful firewalls can make us expend lots of more money.

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The word "firewall" just describes some device or software to separate security zones. It is not a description of a specific technical implementation. Implementation range from simple packet filters like iptables at OSI layer 3..4 up to application level gateways at OSI layer 7. (Application) proxies instead refer to forwarding a protocol at the application level (layer 7). Firewalls can also include proxies for deeper protocol inspection, like Sophos NGFW (formerly Astaro) does.

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