Yes, it's a bad idea, but an awful lot of bad ideas were the common practice a long time ago, when vendors had to deal with limitations of the HTTP/1.0 and 1.1

Yes, it's a bad idea, but an awful lot of bad ideas were the common practice a long time ago, when vendors had to deal with limitations of the HTTP/1.0 and 1.1 RFCs. In fact, we aren't talking about application headers. The HTTP RFC makes a distinction between application headers and hop-by-hop headers. There are mechanisms that proxies are required to implement, but they still don't. For instance, a proxy must declare itself by appending its signature to a "Via" header - however most of them don't. The RFC about forwarded HTTP extension is unclear about how a hop should deal with those headers, there is no one-size-fit-all situations: http://tools.ietf.org/html/rfc7239#section-8.1 http://tools.ietf.org/html/rfc7239#section-8.1