cURL cannot connect to localhost but browser can - ruby-on-rails

Question

I'm running a Rails app locally (Thin server), and I can connect locally from the browser (localhost:3000), but when I try to use curl, I get:
curl -H 'id:1' -i 'http://localhost:3000/api/data' -v
* Hostname was NOT found in DNS cache
* Trying ::1...
* Adding handle: conn: 0x7fd121808200
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x7fd121808200) send_pipe: 1, recv_pipe: 0
* Connection failed
* connect to ::1 port 3000 failed: Connection refused
* Trying fe80::1...
* Connection failed
* connect to fe80::1 port 3000 failed: Connection refused
* Failed to connect to localhost port 3000: Connection refused
* Closing connection 0
curl: (7) Failed to connect to localhost port 3000: Connection refused
This used to work just fine, but I recently updated to Mavericks, which I suspect may have broken something. I can also curl successfully from the web.

Answer 1

This is a curl bug (a strange one), where curl fails to fall back to IPv4 if there's an IPv6 entry in /etc/hosts that doesn't respond.
You can force it to use IPv4 via the -4 option.

Answer 2

Instead of
curl localhost:3000
try
curl 0.0.0.0:3000
It works.
If it doesn't, check the output when you start your rails server:
=> Booting WEBrick
=> Rails 3.2.13 application starting in development on http://0.0.0.0:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
[2014-10-28 15:30:08] INFO WEBrick 1.3.1
[2014-10-28 15:30:08] INFO ruby 2.0.0 (2014-02-24) [x86_64-darwin12.5.0]
[2014-10-28 15:30:08] INFO WEBrick::HTTPServer#start: pid=4004 port=3000
Use the url at the end of line 2 instead of ENV['HOST']

Answer 3

Are you behind a proxy? Then use the below to bypass the proxy altogether.
curl -x "" "http://127.0.0.1:3000"

Answer 4

Assuming that you have not touched /etc/hosts and scutil reports are positive:
$ scutil -r 127.0.0.1
Reachable,Local Address
$ scutil -r localhost
Reachable,Local Address
then my guess is that the Firewall is active and not accepting connections from curl.
Try adding curl to the list accepted applications under (I'm guessing at the language alternatives since my machine is set to use Swedish) Preference --> Security --> Firewall alternatives --> plus sign --> (serach for curl and add it)
Note: make sure that you add the curl that you are actually using in your shell.
$ type -a curl
curl is /opt/local/bin/curl
curl is /usr/bin/curl

Answer 5

Another cause of this issue can be a poorly configured system with a proxy address. Run
export http_proxy=
and rerun the curl command, to eliminate this as a contributing factor. It fixed this issue for me.

Answer 6

Restarting the computer can sometimes fix odd connection issues. I'm not sure why.