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A transparent cache is so named because it works by intercepting the network traffic transparently to the browser.
In this mode, the cache short-circuits the retrieval process if the desired file is in the cache.
Transparent caches are especially useful because they require no browser setup modification.
Transparent caches are also the simplest way to use a cache internally on a network, because they do not require explicit coordination with other caches.
What is transparent caching?
The full explanation about the term "Transparent Caching and Transparent Proxying" depends on the context, but we can assume the context here is HTTP proxy/caches with transparent hijacking of port 80, which is the default HTTP traffic in the internet.
The difference is that the cache includes a cache, while the proxy only proxies without caching.
The term transparent is overloaded, having different meanings depending on the situation.
To some it means a setup that hijacks port 80 traffic where the client tried to go to other servers, to some it means a semantically transparent proxy that does not change the meaning or content of requests/replies.
There is no such thing as a truly transparent proxy, only semitransparent and certainly not such a thing as a truly transparent cache
Squid can be configured to act transparently.
In this mode, clients are not required to configure their browsers to access the cache, but Squid will transparently pick up the appropriate packets and cache requests.
This solves the biggest problem with caching: i.e. getting users to use the cache server.
Advantages of Transparent Caching
As might be expected, the advantages and disadvantages of transparent caching are largely the reverse of those cited for proxy caching. In the advantages category we have the following :
- Simplified administration - The browser does not need to be configured to talk to a cache.
- Central control - The user cannot change his/her browser to bypass the cache.
Disadvantages of Transparent Caching
- Not Robust
Because transparent caching relies on stable routed path between the client and the origin server which happens to pass through a "cached path," it is susceptible to routing changes in the Internet.
In other words, if a connection between a client and a cache is established and a routing change occurs which causes the client to take a path which no longer flows through the "diverting" network device, the session will break and the user will have to reload the page.
If routes in the Internet are flapping, then results will be even more unpredictable. - User control
Transparent caching takes control away from the user.
Many users have very strong biases about caching and will actually change ISPs to either avoid it or get it. - Browser dependency
For successful operation, many transparent caches rely on the browser supplying the host name of the origin server in the HTTP request header.
This is required because these caches cannot access the destination IP address of the origin server from the IP address of the packet.
Therefore, upon a cache miss, they cannot determine the origin server address to send the request to.
Some early browsers do not provide this information and therefore will not work properly with these transparent caches, but 90% of today's browsers satisfy the above.
In the real world, Many network providers have observed that a significant amount of HTTP requests are for non-cacheable content (as much as 35-45%).
The hit rate and performance of the cache is inversely proportional to the amount of non-cacheable content sent to the cache.
To enable proxy transparent feature, follow these steps
- On the left menu, choose web proxy
- select Network settings tab in the web proxy page
- Click on the Transparent proxy icon
- Switch to green the red circle and clik on edit button
