Squid proxy how does it work

Understanding the nature of workloads is important to the managing system capacity. The benefits of hierarchical caching namely, reduced network bandwidth consumption, reduced access latency, and improved resiliency come at a price. Caches higher in the hierarchy must field the misses of their descendents.

If this second level cache has most of the documents, it is usually still a win, but if higher level caches often don't have the document, or become overloaded, then they could actually increase access latency, rather than reduce it. Please see the Firewalls mailing list and FAQ information site. The LRU expiration age is a dynamically-calculated value. Any objects which have not been accessed for this amount of time will be removed from the cache to make room for new, incoming objects.

Another way of looking at this is that it would take your cache approximately this many days to go from empty to full at your current traffic levels. As your cache becomes more busy, the LRU age becomes lower so that more objects will be removed to make room for the new ones. Ideally, your cache ill have an LRU age value in the range of at least 3 days.

If the LRU age is lower than 3 days, then your cache is probably not big enough to handle the volume of requests it receives. By adding more disk space you could increase your cache hit ratio. Consider a pair of caches named A and B. It may be the case that A can reach B, and vice-versa, but B has poor reachability to the rest of the Internet. In this case, we would like B to recognize that it has poor reachability and somehow convey this fact to its neighbor caches.

Squid will track the ratio of failed-to-successful requests over short time periods. When the failed-to-successful ratio exceeds 1. No, you must send a HUP signal to have Squid re-read its configuration file, including access control lists. An easy way to do this is with the -k command line option: squid -k reconfigure. Performing the unlink operation in an external process opens up some race-condition problems for Squid. If we are not careful, the following sequence of events could occur: An object with swap file number S is removed from the cache.

We want to unlink file F which corresponds to swap file number S , so we write pathname F to the unlinkd socket. We also mark S as available in the filemap. We have a new object to swap out. It is allocated to the first available file number, which happens to be S. Squid opens file F for writing. The unlinkd process reads the request to unlink F and issues the actual unlink call. So, the problem is, how can we guarantee that unlinkd will not remove a cache file that Squid has recently allocated to a new object?

The approach we have taken is to have Squid keep a stack of unused but not deleted! The stack size is hard-coded at entries.

We only give unlink requests to unlinkd when the unused file number stack is full. Thus, if we ever have to start unlinking files, we have a pool of file numbers to choose from which we know will not be removed by unlinkd. In terms of implementation, the only way to send unlink requests to the unlinkd process is via the storePutUnusedFileno function. Unfortunately there are times when Squid can not use the unlinkd process but must call unlink 2 directly.

To be able to restart Squid, damaged caches must be deleted. Removing Squid from the system does not remove the cache hierarchy and log files. Setting up a local DNS server makes sense even if it does not manage its own domain. It then simply acts as a caching-only name server and is also able to resolve DNS requests via the root name servers without requiring any special configuration see Section How this can be done depends on whether you chose dynamic DNS during the configuration of the Internet connection.

This way, Squid can always find the local name server when it starts. With static DNS, no automatic DNS adjustments take place while establishing a connection, so there is no need to change any sysconfig variables. Defines settings in regard to cache memory, maximum and minimum object size, and more.

Define paths to access, cache, and cache store log files in addition with connection timeouts and client lifetime. To start Squid for the first time, no changes are necessary in this file, but external clients are initially denied access. The proxy is available for localhost. The default port is Many entries are commented and therefore begin with the comment character. The relevant specifications can be found at the end of the line.

The given values usually correlate with the default values, so removing the comment signs without changing any of the parameters usually has no effect. If possible, leave the commented lines as they are and insert the options along with the modified values in the line below. This way, the default values may easily be recovered and compared with the changes.

Sometimes, Squid options are added, removed, or modified. Therefore, if you try to use the old squid. The following is a list of a selection of configuration options for Squid.

It is not exhaustive. This is the port on which Squid listens for client requests. The default port is , but is also common. This option allows creating a network of caches that work together. The cache peer is a computer that also hosts a network cache and stands in a relationship to your own. The type of relationship is specified as the TYPE.

The type can either be parent or sibling. To make Squid behave like a Web browser instead of like a proxy server, prohibit the use of the ICP protocol. You can do so by appending the options default and no-query. This option defines the amount of memory Squid can use for very popular replies. The default is 8 MB. This does not specify the memory usage of Squid and may be exceeded.

Directory-based storage types: ufs , aufs the default , diskd. All three are variations of the storage format ufs. However, while ufs runs as part of the core Squid thread, aufs runs in a separate thread, and diskd uses a separate process.

Database-based storage systems: rock. In the following, only the parameters for storage types based on ufs will be discussed.

These values should only be increased with caution, because creating too many directories can lead to performance problems. These three options specify the paths where Squid logs all its actions. Normally, nothing needs to be changed here. If Squid is burdened by heavy usage, it might make sense to distribute the cache and the log files over several disks.

This option allows masking IP addresses of clients in the log files by applying a subnet mask. For example, to set the last digit of the IP address to 0 , specify This option allows setting the password that Squid should use for anonymous FTP login. Specify a valid e-mail address here, because some FTP servers check these for validity. If it unexpectedly crashes, Squid sends a message to this e-mail address. The default is webmaster. If you run squid -k rotate , squid can rotate log files.

The files are numbered in this process and, after reaching the specified value, the oldest file is overwritten. The default value is 10 which rotates log files with the numbers 0 to 9. Usually, your own domain is specified here, so specifying www in the browser accesses your own Web server.

If this option is set to on , it adds a line to the header similar to this:. If these options are set, Squid will cache some types of failures, such as responses.

The time taken is minimized significantly when server directly from the proxy server. However, if the requested object is not available, the proxy server requests the web server to deliver the content to the client. The efficiency of the proxy server is determined whether it is able to recognize the repeated requests and keep the corresponding objects in it or not.

Squid Proxy Server — Squid needs to be installed on the web server or on a separate server. However, in the process, it saves the copy of the requested object such that if the same client or other clients request for the same object in the near future, it can respond to the request directly. Therefore, the time duration from Squid proxy server passing on the request to the web server and the web server responding back with the internet object back to the proxy server becomes nil.

Therefore, the response time becomes fast. Similarly, due to the request not passing on to the web server, the bandwidth stays free both from sending and receiving.

Therefore, the bandwidth can be better utilized for requests that cannot be served by the proxy server. This is significant because there are thousands and thousands of requests to serve every second. As a matter of fact, ISPs are using Squid proxy servers for decades to improve the download speed and reduce latency. Server Overview Hyperscale Docs. Tip Prior to editing the configuration file, you should make a copy of the original file and protect it from writing so you will have the original settings as a reference, and to re-use as necessary.

This was formerly known in Squid as async-io. A single entry occupies one or more slots.