Using nc (netcat) to make an HTTP request

I must have had some reason for wanting to do this, although I can’t think of why right now. curl is an excellent tool for ad hoc HTTP requests.

On a server running Apache 2.4.6, first I tried:

# nc 80
GET / HTTP/1.1

Which returned a HTTP/1.1 400 Bad Request error.

Next I tried:

# printf "GET /index.html HTTP/1.1\r\n\r\n" | nc 80

Which also returned a HTTP/1.1 400 Bad Request error.

I decided to take a look at what curl was sending, since that was working:

# curl -v
* About to connect() to port 80 (#0)
* Trying
* Connected to ( port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.29.0
> Host:
> Accept: */*

I put the same headers (with a modified User-Agent) into my printf statement:

# printf "GET /index.html HTTP/1.1\r\nUser-Agent: nc/0.0.1\r\nHost:\r\nAccept: */*\r\n\r\n" | nc 80
HTTP/1.1 200 OK
Date: Sun, 28 Jan 2018 23:11:04 GMT
Server: Apache/2.4.6 (CentOS) PHP/5.4.16
Last-Modified: Sun, 28 Jan 2018 20:10:37 GMT
ETag: "78-563dbb912bfe0"
Accept-Ranges: bytes
Content-Length: 120
Content-Type: text/html; charset=UTF-8

<!DOCTYPE html>
<title>well that worked</title>
<h1>apache is running</h1>

That worked!

I eliminated the User-Agent the Accept headers and it still worked, so the missing Host header was the cause of my problems. I swear I’ve done this before without a Host header though.

I looked up the HTTP specification, and as described in section 5.2 of the RFC:

1. If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request MUST be ignored.

2. If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field value.

3. If the host as determined by rule 1 or 2 is not a valid host on the server, the response MUST be a 400 (Bad Request) error message.

Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested.

I could not get it to work with an absoluteURI, even using the example in the RFC. However I did find that I could ignore the Host header if I specified HTTP/1.0:

# printf "GET / HTTP/1.0\r\n\r\n" | nc 80

I also found that Apache didn’t care what the Host header was when using HTTP/1.1, just so long as something was there:

# printf "GET / HTTP/1.1\r\nHost: z\r\n\r\n" | nc 80

That’s a little odd. I did not specify a ServerName in my Apache config, but even after I specified ServerName in /etc/httpd/conf/httpd.conf and restarted Apache, it still required the Host header and it still didn’t care what the content of the Host header was (so long as it was not empty).