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2.15 Bridge

A bridge is a layer 2 router that's sometimes acts as a firewall.

2.16 Proxies

A proxy receives requests from a client and sends them to the destination host presuming itself would be the real source of the request. It differs to a router in acting on the layers 4–6 (TCP/UDP) till up to layer 7 (application) instead of playing on layer 3 like a router.

Most proxies additionally have the possibility to deeply understand the protocol they are working on. This way they can suppress other protocols that a client may try to speak over its port and to filter dangerous/unwanted contents like spam and malware. Furthermore a proxy could force a user to authenticate by password or smart card before he or she is allowed to use its service.

Normally a proxy must explicitly be configured by the user. A web proxy, for example, gets inserted into a browser's configuration, but a special kind of proxy exists where a router or firewall (Sect. 2.18) automatically redirects a connection through a proxy without a user realizing it. Such a proxy is called transparent proxy. Most internet service providers nowadays use such a kind of proxy at least on HTTP ports for performance reasons. The proxy caches all static web contents like images and videos on its hard disk. In some countries transparent proxies are also used to censor and observe the internet access.

Some web proxies insert a PROXY-VIA entry into the HTTP header and such let a user know that his connection flows over this proxies and which IP address the proxy has. The existence of this header in transparent proxy is unlikely and may be a hint for misconfiguration or a slacky sysadmin.

Interested reader could, for example, use the following script to get an overview of all HTTP information sent by its browser to every web server they use

2.17 Virtual Private Networks

Virtual Private Networks (VPN) is a collection of security mechanisms, which only have in common the protection of a connection by using encryption and/or authentication. Nearly all VPNs support the possibility to secure the access to a whole network and thanks to powerful cryptology also protect against spionage and manipulation. Therefore it operates on the protocol stack either on layer 3, 4 or 7. It can be commonly said that the deeper the VPN intercepts the connection the more secure it can be, because it can prevent attacks on each layer.

Typical protocols or protocol stacks are IPsec, PPTP and OpenVPN. Mostly they are used to connect outside-agencies and to integrate roadrunner (Employees, which connect to the company network through a mobile internet connection).

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