ffmpeg - How to pass http headers?

Well, ffmpeg manual says to split multiple http-headers by CRLF. The problem is that you overwrite your first "-header" argument with the second "-header" as there can be only one "-header" argument.

For your example, you need to join User-Agent and X-Forwarded into one argument by valid CRLF like this:

-header "User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36"$'\r\n'"X-Forwarded-For: 13.14.15.66"$'\r\n'

For set x:1 and y:2 for header ffmpeg request, use this:

ffmpeg -headers $'x:1\r\ny:2\r\n' -i 'http://sample.com' -y 'sample.mp4' -v debug

Result:

[http @ 0x358be00] Setting default whitelist 'http,https,tls,rtp,tcp,udp,crypto,httpproxy'
[http @ 0x358be00] request: GET / HTTP/1.1
User-Agent: Lavf/57.76.100
Accept: */*
Range: bytes=0-
Connection: close
Host: example.com
Icy-MetaData: 1
x:1
y:2

Short Answer

Make sure you're using the latest ffmpeg, and use the -user-agent option.

Longer Answer

For debugging, I setup a BaseHTTPSever running at 127.0.0.1:8080 with do_GET() as:

def do_GET(self):
   try:
       f = open(curdir + sep + self.path, 'rb')
       self.send_response(200)
       self.end_headers()
       print("GET: "+ str(self.headers))
       self.wfile.write(f.read())
       f.close()
       return

   except IOError:
       self.send_error(404,'File Not Found: %s' % self.path)

With that running, this enabled me to run your command like:

ffmpeg  \
    -y \
    -timeout 5000000 \
    -map 0:0 \
    -an \
    -sn \
    -f md5 - \
    -headers "User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36" \
    -headers "X-Forwarded-For: 13.14.15.66" \
    -i "http://127.0.0.1:8080/some_video_file.mp4" \
    -v trace

When I do this, I see the following relevant output from ffmpeg:

Reading option '-headers' ... matched as AVOption 'headers' with argument 'User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36'.
Reading option '-headers' ... matched as AVOption 'headers' with argument 'X-Forwarded-For: 13.14.15.66'.

On the server, I saw:

User-Agent: Lavf/56.40.101
X-Forwarded-For: 13.14.15.66

So it looks like ffmpeg is setting it's own. But there is an option -user-agent to ffmpeg, and when I replaced -headers "User-Agent: <foo>" with -user-agent "<foo>", I then did see it too on the server, alongside the X-Forwarded-For header:

User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36

Last note. There are lots of discussions around headers bugs in trac for ffmpeg. What I have observed above (that essentially it is working, perhaps with a small command change) was with a fairly recent version:

ffmpeg version 2.8.1 Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
configuration: --enable-libx264 --enable-gpl --prefix=/usr/local --enable-shared --cc='gcc -fPIC'
libavutil      54. 31.100 / 54. 31.100
libavcodec     56. 60.100 / 56. 60.100
libavformat    56. 40.101 / 56. 40.101
libavdevice    56.  4.100 / 56.  4.100
libavfilter     5. 40.101 /  5. 40.101
libswscale      3.  1.101 /  3.  1.101
libswresample   1.  2.101 /  1.  2.101
libpostproc    53.  3.100 / 53.  3.100

So, your next move might be make sure you have the latest version of ffmpeg.