Reverse proxy

In computer networks such as the internet, a reverse proxy is a common type of proxy server that is accessible from the public network. Large websites and content delivery networks use reverse proxies –together with other techniques– to balance the load between internal servers. Reverse proxies can keep a cache of static content, which further reduces the load on these internal servers and the internal network. It is also common for reverse proxies to add features such as compression or TLS encryption to the communication channel between the client and the reverse proxy.[1]

Example scenario: A client on the internet (cloud on the left) makes a request to a reverse proxy server (red oval in the middle). The proxy inspects the request, determines that it is valid and that it does not have the requested resource in its own cache. It then forwards the request to some internal web server (oval on the right). The internal server delivers the requested resource back to the proxy, which in turn delivers it to the client. The client on the internet is unaware of the internal network, and cannot tell whether it is communicating with a proxy or directly with a web server.

Reverse proxies are typically owned or managed by the web service, and they are accessed by clients from the public internet. In contrast, a forward proxy is typically managed by a client (or their company) who is normally restricted to a private, internal network. The client can, however, access the forward proxy, which then retrieves resources from the public internet on behalf of the client.

Reverse proxy servers are implemented in popular open-source web servers such as Apache, Nginx, and Caddy. This software can inspect HTTP headers, which, for example, allows it on a single IP address to relay requests to different internal servers based on the domain name of the HTTP request. Dedicated reverse proxy servers such as the open source software HAProxy and Squid are used by some of the biggest websites on the internet. Popular commercial providers of reverse proxy servers include Cloudflare and Imperva.

Uses of reverse proxies
  • Reverse proxies can hide the existence and characteristics of origin servers.
  • Application firewall features can protect against common web-based attacks, like a denial-of-service attack (DoS) or distributed denial-of-service attacks (DDoS). Without a reverse proxy, removing malware or initiating takedowns, for example, can be difficult.
  • In the case of secure websites, a web server may not perform TLS encryption itself, but instead offload the task to a reverse proxy that may be equipped with TLS acceleration hardware. (See TLS termination proxy.)
  • A reverse proxy can distribute the load from incoming requests to several servers, with each server supporting its own application area. In the case of reverse proxying web servers, the reverse proxy may have to rewrite the URL in each incoming request in order to match the relevant internal location of the requested resource.
  • A reverse proxy can reduce load on its origin servers by caching static content and dynamic content, known as web acceleration. Proxy caches of this sort can often satisfy a considerable number of website requests, greatly reducing the load on the origin server(s).
  • A reverse proxy can optimize content by compressing it in order to speed up loading times.
  • In a technique named "spoon-feeding",[2] a dynamically generated page can be produced all at once and served to the reverse proxy, which can then return it to the client a little bit at a time. The program that generates the page need not remain open, thus releasing server resources during the possibly extended time the client requires to complete the transfer.
  • Reverse proxies can operate wherever multiple web-servers must be accessible via a single public IP address. The web servers listen on different ports in the same machine, with the same local IP address or, possibly, on different machines with different local IP addresses. The reverse proxy analyzes each incoming request and delivers it to the right server within the local area network.
  • Reverse proxies can perform A/B testing and multivariate testing without placing JavaScript tags or code into pages.
  • A reverse proxy can add basic HTTP access authentication to a web server that does not have any authentication.[3]

Risks of reverse proxies

A HTTP/S reverse proxy can read and modify all traffic and IPs of web users going through it. In order to filter/cache/compress or otherwise modify the traffic, it must be able to decrypt and re-encrypt the HTTPS traffic and thus possess the TLS certificate's corresponding private key. Thus it can obviously log all passwords going through it or inject malware into the web sites, and might do so if compromised or run by a malicious party. Reverse proxies are also another single point of failure if there is no obvious way to access the server directly.

Using the reverse proxy of a third party (e.g. Cloudflare, Imperva) places the entire triad of Confidentiality, Integrity and Availability in the hands of said third party.

If a reverse proxy is fronting many different domains, its outage (e.g. by a misconfiguration or DDoS attack) could bring down all fronted domains.[4]

See also

References
  1. "Proxy servers and tunneling". MDN Web Docs. Retrieved 6 December 2020.
  2. "squid-cache wiki entry on "SpoonFeeding"". Francesco Chemolli. Retrieved 9 February 2011.
  3. "Possible to add basic HTTP access authentication via HAProxy?". serverfault.com.
  4. "Cloudflare outage knocks out major sites and services, including Discord". finance.yahoo.com. Retrieved 14 December 2020.

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