HTTP: The Definitive Guide

Name

Via

Synopsis

The Via header is used to trace messages as they pass through proxies and gateways. It is an informational header that can be used to see what applications are handling requests and responses.

When a message passes through an HTTP application on its way to a client or a server, that application can use the Via header to tag the message as having gone via it. This is an HTTP/1.1 header; many older applications insert a Via-like string in the User-Agent or Server headers of requests and responses.

If the message passes through multiple in-between applications, each one should tack on its Via string. The Via header must be inserted by HTTP/1.1 proxies and gateways.

Type

General header

Basic Syntax

Via: 1# (received-protocol received-by [comment])^[11]

Example

Via: 1.1 joes-hardware.com ( Joes-Server/1.0)

The above says that the message passed through the Joes Server Version 1.0 software running on the machine joes-hardware.com. Joe’s Server was speaking HTTP 1.1. The Via header should be formatted like this:

HTTP-Version machine-hostname (Application-Name-Version)

^[11] See the HTTP/1.1 specification for the complete Via header syntax.