Reverse proxy
A reverse proxy is a server that positions itself between client devices and backend web servers, intercepting incoming requests from clients and forwarding them to the appropriate backend server for processing, before returning the server's response to the client.[1][2] This setup allows the reverse proxy to act as an intermediary gateway, insulating the backend servers from direct exposure to the internet.[3] In operation, a reverse proxy receives a client's HTTP request, evaluates it based on configured rules, and routes it to one or more origin servers, often using protocols like HTTP or HTTPS.[4] It then collects the response from the backend—such as a web page or API data—and delivers it to the client, potentially modifying headers likeX-Forwarded-For to preserve original request information or Via to indicate proxy involvement.[4] This process enables the reverse proxy to handle tasks transparently without the client needing to know about the backend infrastructure.[2]
Key functions of reverse proxies include load balancing, which distributes traffic across multiple backend servers to prevent overload and ensure high availability; caching, where frequently requested static content like images is stored locally to reduce latency and backend load; and content compression to optimize data transfer sizes.[1][4] They also enhance security by concealing the IP addresses and details of origin servers from clients, thereby mitigating risks such as DDoS attacks, and can terminate SSL/TLS connections to offload encryption tasks from backends.[3][2]
Unlike a forward proxy, which operates on behalf of clients to hide their identities and facilitate access to external resources, a reverse proxy serves the interests of the backend servers by shielding them from direct client interactions and optimizing server-side performance.[1][4] This distinction makes reverse proxies essential for server-centric architectures, while forward proxies are more client-oriented for anonymity or content filtering.[2]
Common use cases for reverse proxies encompass web application acceleration in content delivery networks (CDNs), where they enable global load balancing across distributed servers; API gateway services that route and secure microservices traffic; and enterprise setups like those using software such as NGINX or Apache HTTP Server for scalable, protected deployments.[1][3] By centralizing these capabilities, reverse proxies support reliable and efficient web infrastructure for high-traffic environments.[2]
Definition and Purpose
Core Concept
A reverse proxy is a server that acts as an intermediary for requests from clients seeking resources from one or more backend servers, retrieving those resources on behalf of the client and returning them to the client while concealing the identity and details of the origin servers.[1][2] This setup positions the reverse proxy as the sole visible endpoint to clients, forwarding incoming requests to appropriate backend servers and relaying responses back without exposing the internal architecture.[5] According to IETF standards, a reverse proxy is an intermediary that acts as an origin server for the outbound connection but translates received requests and forwards them to backend servers.[6] The primary purpose of a reverse proxy is to create an abstraction layer between client-facing interfaces and backend services, facilitating centralized control over traffic management, security enforcement, and performance optimization.[7][8] By intercepting and processing requests at this intermediary point, it enables functionalities such as request routing, response modification, and resource protection, which collectively enhance system reliability and scalability.[1] At its core, a reverse proxy comprises three basic components: a front-end interface that receives and authenticates client requests, back-end connections that communicate with origin servers to fulfill those requests, and internal routing logic that determines how traffic is directed and processed.[5] These elements work in concert to ensure seamless operation, often incorporating features like load balancing to distribute requests across multiple backend servers for improved efficiency.[2]Distinction from Forward Proxy
A forward proxy, also known as a proxy server or web proxy, is a server that sits in front of a group of client machines within a network, intercepting outbound requests from those clients to external resources on the internet.[1] It acts on behalf of the clients by forwarding their requests to the destination servers and returning the responses, thereby concealing the clients' IP addresses and enabling functions such as anonymity, content filtering, or access control for outbound traffic.[9] For example, in corporate environments, forward proxies are commonly deployed to enforce web filtering policies, blocking access to certain sites and logging user activity to maintain compliance.[10] In contrast, the primary directional difference lies in traffic flow: reverse proxies handle inbound traffic directed toward internal backend servers, intercepting requests from external clients and distributing them appropriately to protect and optimize server resources, whereas forward proxies manage outbound traffic originating from internal clients seeking access to external services.[1] This inversion of roles means reverse proxies serve to shield servers from direct exposure to the internet, often aggregating multiple backend servers behind a single public-facing interface, while forward proxies focus on controlling and securing client-initiated connections to the wider network.[11] Use cases further highlight this divergence; reverse proxies are employed for server-side protection, such as concealing the existence and structure of multiple web servers from attackers, whereas forward proxies support client-side governance, like implementing firewalls in organizations to restrict access to prohibited external websites.[12] Architecturally, reverse proxies are positioned in front of web servers or application backends at the network edge, acting as an intermediary that receives all incoming traffic before it reaches the protected resources.[1] Forward proxies, however, are placed between clients and the broader internet, typically within the internal network perimeter, to mediate all egress communications.[9] This placement implies that reverse proxies enhance server scalability and security by centralizing request handling, while forward proxies promote network efficiency and policy enforcement from the client perspective, though both can incorporate security features like encryption termination, applied differently based on their directional focus.[10]Operational Mechanics
Architecture Overview
A reverse proxy operates within a topology where clients initiate connections to the proxy via a public IP address or domain name exposed to the internet, while the proxy forwards these requests to one or more backend servers, often using private IP addresses or integrating with internal load balancers to obscure the backend infrastructure.[13][14] This setup positions the reverse proxy as an intermediary gateway, shielding backend servers from direct client exposure and centralizing traffic management.[11] The core components of a reverse proxy include a listener module that accepts and terminates incoming client connections on designated ports, a routing engine that evaluates request attributes to select and direct traffic to suitable backend servers, and logging/monitoring interfaces that capture request metadata, errors, and performance metrics for operational oversight.[5][13] In implementations like NGINX, the routing is configured via directives such asproxy_pass to upstream server groups, while listeners are defined in server blocks to handle protocols like HTTP or HTTPS.[5]
Deployment models for reverse proxies vary to suit different environments: standalone hardware appliances provide dedicated performance for high-throughput scenarios, software solutions like NGINX or HAProxy run on general-purpose servers or virtual machines for flexible integration, and cloud-based services such as AWS Elastic Load Balancing offer managed, auto-scaling proxies without on-premises hardware.[13][15] These models allow adaptation to on-premises, hybrid, or fully cloud architectures, with cloud options emphasizing ease of provisioning and integration with other services.[14]
Scalability in reverse proxy architectures is achieved through horizontal scaling, where multiple proxy instances form clusters to distribute incoming load, often using DNS round-robin or dedicated load balancers at the proxy layer itself.[13][14] To handle stateful applications, session affinity—commonly known as sticky sessions—ensures subsequent requests from the same client are routed to the same backend server, preserving session data without compromising distribution.[11] This enables load balancing as a foundational architectural feature, supporting growth from single-server setups to handling thousands of concurrent connections.[5]
Request and Response Processing
When a client initiates an HTTP or HTTPS request to a reverse proxy, the proxy intercepts the incoming connection on its configured port and address.[5] The proxy then performs initial processing, such as authentication or validation if enabled through modules like access control lists, before determining the appropriate backend server.[3] Backend selection occurs based on predefined rules, including URL path matching—for instance, directing requests to/[api](/page/API) to a specific application server—or hashing algorithms like IP hash, which consistently routes requests from the same client IP to the same backend for session persistence. Once selected, the proxy forwards the request to the backend, often modifying it by adding headers such as [X-Forwarded-For](/page/X-Forwarded-For) to preserve original client information or [Host](/page/Host) to indicate the intended destination.[5]
Upon receiving a response from the backend server, the reverse proxy inspects the content for compliance or optimization, potentially modifying it—for example, by compressing the body using gzip if the original response is uncompressed and the client supports it.[5] If caching is configured as an optional response handling mechanism, the proxy may store suitable responses (like static assets) for future requests, though this is distinct from persistent storage management. The proxy then relays the processed response back to the client, ensuring headers like Content-Length are adjusted if modifications occurred, while maintaining the illusion of a direct connection.[5]
Reverse proxies support multiple protocols to handle diverse traffic, including HTTP/1.1 for basic compatibility, HTTP/2 for multiplexed streams and header compression on the client-facing side, and HTTP/3 (built on QUIC) in implementations like NGINX version 1.25 and later for improved performance over unreliable networks. A key feature is TLS termination, where the proxy decrypts incoming HTTPS traffic using its own certificates, offloading the computational burden of encryption from backend servers and allowing unencrypted HTTP connections to the upstream for efficiency.[16] Note that while client-side protocol upgrades like HTTP/2 and HTTP/3 are supported, backend communication typically remains at HTTP/1.1 unless explicitly configured otherwise in advanced setups.[17]
In cases of backend failures, such as timeouts or connection refusals, the reverse proxy generates protocol-specific error responses, notably the 502 Bad Gateway status code to indicate an invalid or unavailable upstream reply. To enhance reliability, many proxies support fallback mechanisms, such as designating backup servers in the upstream configuration that activate only when primary backends fail, ensuring continued service availability without client disruption.