$ ffmpeg -f x11grab -r 25 -s 1024x768 -i :0 -preset medium http://localhost:8090/feed1.ffm
ffmpeg version 0.10.11-7:0.10.11-1~saucy1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Feb  6 2014 16:55:15 with gcc 4.8.1
  configuration: --arch=amd64 --disable-stripping --enable-pthreads --enable-runtime-cpudetect --extra-version='7:0.10.11-1~saucy1' --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --enable-bzlib --enable-libdc1394 --enable-libfreetype --enable-frei0r --enable-gnutls --enable-libgsm --enable-libmp3lame --enable-librtmp --enable-libopencv --enable-libopenjpeg --enable-libpulse --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-vaapi --enable-vdpau --enable-libvorbis --enable-libvpx --enable-zlib --enable-gpl --enable-postproc --enable-libcdio --enable-x11grab --enable-libx264 --shlibdir=/usr/lib/x86_64-linux-gnu --enable-shared --disable-static
  libavutil      51. 35.100 / 51. 35.100
  libavcodec     53. 61.100 / 53. 61.100
  libavformat    53. 32.100 / 53. 32.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100
[x11grab @ 0x19f1640] device: :0 -> display: :0 x: 0 y: 0 width: 1024 height: 768
[x11grab @ 0x19f1640] shared memory extension found
[x11grab @ 0x19f1640] Estimating duration from bitrate, this may be inaccurate
Input #0, x11grab, from ':0':
  Duration: N/A, start: 1392120578.914875, bitrate: 629145 kb/s
    Stream #0:0: Video: rawvideo (BGRA / 0x41524742), bgra, 1024x768, 629145 kb/s, 25 tbr, 1000k tbn, 25 tbc
Incompatible pixel format 'bgra' for codec 'libx264', auto-selecting format 'yuv420p'
[buffer @ 0x19dc9e0] w:1024 h:768 pixfmt:bgra tb:1/1000000 sar:0/1 sws_param:
[buffersink @ 0x1a092e0] auto-inserting filter 'auto-inserted scale 0' between the filter 'src' and the filter 'out'
[scale @ 0x1a09b80] w:1024 h:768 fmt:bgra -> w:1024 h:768 fmt:yuv420p flags:0x4
[libx264 @ 0x1a00a60] broken ffmpeg default settings detected
[libx264 @ 0x1a00a60] use an encoding preset (e.g. -vpre medium)
[libx264 @ 0x1a00a60] preset usage: -vpre <speed> -vpre <profile>
[libx264 @ 0x1a00a60] speed presets are listed in x264 --help
[libx264 @ 0x1a00a60] profile is optional; x264 defaults to high
Output #0, ffm, to 'http://localhost:8090/feed1.ffm':
    Stream #0:0: Video: h264, yuv420p, 1024x768, q=2-31, 200 kb/s, 1000k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo -> libx264)
Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height