Here’s an interesting bit I stumbled upon while playing with Amazon Web Services (AWS) Elastic Load Balancing (ELB): HTTP requests must have their lines terminated with CRLF and not just a line feed. When using netcat to test a web server by speaking HTTP, it only sends LFs by default (\n). While RFC 2616 specifies:

… a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures …

Using netcat to connect to a web server typically works just fine. I’m inputting the HTTP requests by hand and [ENTER] is where I hit the enter key.

$ nc www.google.com 80
GET / HTTP/1.0[ENTER]
[ENTER]
HTTP/1.0 200 OK
Date: Fri, 09 Apr 2010 20:07:25 GMT
Expires: -1
[snip]

This works against Apache. However when connecting to an Apache server through ELB, one must run netcat with the -C option to send a CRLF instead of a lone LF upon return.

$ nc -C elb.example.org 80
GET / HTTP/1.0[ENTER]
[ENTER]
HTTP/1.1 302 Found
Content-Type: text/html; charset=iso-8859-1
Date: Fri, 09 Apr 2010 20:09:39 GMT
Location: http://elb.example.org/404/
Server: Apache
Vary: Accept-Encoding
Content-Length: 290
Connection: Close

Sans the -C option, the connection simply hangs. Which asks the question, what is Amazon doing with your HTTP traffic in between?