net/http: document that ProxyFromEnvironment special-cases localhost

[gopherbot]

by mislav.marohnic:

Go respects HTTP_PROXY environment variables, **except** when the original request is to
"localhost" or "127.0.0.1".

This special case is surprising (it's not documented) and undesired. I've come across
this when I tried to debug a go program that makes HTTP requests by passing all
communication through a HTTP debugging proxy:

  http_proxy=localhost:8888 myprog ...

However since this program makes HTTP requests to a *local test server*, my proxy
configuration was silently ignored.

The only way around this was to literally reimplement `http.ProxyFromEnvironment` in my
program without the needless special case for localhost.

go version 1.2

[bradfitz]

Comment 1:

I don't remember why we do that. (in func useProxy)
If we have to keep it, it at least needs to be documented.
Could you dig through the source history and figure out when it was added and why? Also,
do other tools do something like this?

Labels changed: added release-go1.3, repo-main.

Status changed to Accepted.

[adg]

Comment 2:

Maybe we have it because "localhost" will likely mean something different for a proxy?

[gopherbot]

Comment 3 by mislav.marohnic:

Maybe we have it because "localhost" will likely mean something different
That's a good point. However a HTTP proxy doesn't have to be on a remote
machine, like the use case here with a debugging proxy.
If a person wants to explicitly disable hitting the proxy for "localhost"
or 127.0.0.1, then can choose to do so by adding them to NO_PROXY.
https://code.google.com/p/go/source/browse/src/pkg/net/http/transport.go?name=go1.2#553

[bradfitz]

Comment 4:

Please see how far back our localhost exception goes. Perhaps it was a hack before we
had NO_PROXY support?

[gopherbot]

Comment 5 by mislav.marohnic:

@bradfitz: https://golang.org/issue/1589
There existed NO_PROXY support at the time of adding this.

[rsc]

Comment 7:

You can always use an alias for localhost, such as by putting a new entry in /etc/hosts
or often using localhost.localdomain.

[bradfitz]

Comment 8:

Labels changed: added documentation.

[gopherbot]

Comment 9:

CL https://golang.org/cl/97590043 mentions this issue.

[bradfitz]

Comment 10:

This issue was closed by revision f0bdee1.

Status changed to Fixed.