ERROR: Unable to download webpage: <urlopen error [Errno 64] Host is down> (caused by URLError(error(64, 'Host is down'),))

[Pysis868]

I am having a similar error to #5861, but the issue is still presenting itself to me after multiple attempts.
I am also able to view the webpage in my browser (Chrome 46).
I am not behind an HTTP or SOCKS proxy, nor using a VPN.
I am using Max OS X 10.11.1.
I have also tried "--no-check-certificate" from the old issue #5317 just in case, but that did not help either.
This is happening for the current video URL you may see in the command-line output. This is a sample video I have chosen to demonstrate the problem. I have tried this on other videos as well, and also encounter the same error.
Also, I don't know what happened with #384 to know if that is similar to my issue or not. :P

The Youtube-dl program has worked for me many times in the past, up to at least a week or 2 ago. It may have undergone an update since using it successfully the last time, but I think it still worked after that.
Trying the update now causes an error to be shown to the user.

Youtube-dl error command line input line and output

$ youtube-dl "https://www.youtube.com/watch?v=JMMlOyG-1ys" --verbose --no-check-certificate
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'https://www.youtube.com/watch?v=JMMlOyG-1ys', u'--verbose', u'--no-check-certificate']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.11.10
[debug] Python version 2.7.10 - Darwin-15.0.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 2.8, ffprobe 2.8
[debug] Proxy map: {}
[youtube] JMMlOyG-1ys: Downloading webpage
ERROR: Unable to download webpage: <urlopen error [Errno 64] Host is down> (caused by URLError(error(64, 'Host is down'),))
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/common.py", line 329, in _request_webpage
    return self._downloader.urlopen(url_or_request)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1873, in urlopen
    return self._opener.open(req, timeout=self._socket_timeout)
  File "/usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 431, in open
    response = self._open(req, data)
  File "/usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 449, in _open
    '_open', req)
  File "/usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/local/bin/youtube-dl/youtube_dl/utils.py", line 794, in https_open
    req, **kwargs)
  File "/usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 1197, in do_open
    raise URLError(err)

Brew Details

$ brew --version
0.9.5 (git revision 5ca6; last commit 2015-11-15)

Ping command attempt

ping -c 4 youtube.com
PING youtube.com (204.85.30.91): 56 data bytes
ping: sendto: Host is down
ping: sendto: Host is down
Request timeout for icmp_seq 0
ping: sendto: Host is down
Request timeout for icmp_seq 1
ping: sendto: Host is down
Request timeout for icmp_seq 2
^C
--- youtube.com ping statistics ---
4 packets transmitted, 0 packets received, 100.0% packet loss

For some reason I still had to use Ctrl+C, even with the -c option. O_o

Environment SSL Cert Details

echo $SSL_CERT_FILE

[Pysis868]

This did stop occurring after a system restart.
I believe my situation was that I was in the middle of a Little Snitch upgrade. I believe I had open rules for the Youtube-dl and other assistive command-line programs, so I'm chalking this one up to the pending restart it required, somehow strangely blocking some programs and not others.
I also issued the youtube-dl --update command, which did fail before the restart. Now that it works, I still think Little Snitch was the cause of the problem.

Youtube-dl update

$ youtube-dl --update
Updating to version 2015.11.27.1 ...
Updated youtube-dl. Restart youtube-dl to use the new version.

For fun, an excerpt of the program now working on my system:

$ youtube-dl "https://www.youtube.com/watch?v=JMMlOyG-1ys" --verbose
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'https://www.youtube.com/watch?v=JMMlOyG-1ys', u'--verbose']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.11.27.1
[debug] Python version 2.7.10 - Darwin-15.0.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 2.8, ffprobe 2.8
[debug] Proxy map: {}
[youtube] JMMlOyG-1ys: Downloading webpage
[youtube] JMMlOyG-1ys: Downloading video info webpage
[youtube] JMMlOyG-1ys: Extracting video information
[youtube] JMMlOyG-1ys: Downloading DASH manifest
[youtube] JMMlOyG-1ys: Downloading DASH manifest
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[debug] Invoking downloader on u'https://r1---sn-hjoj-gq0e.googlevideo.com/videoplayback?id=24c3253b21bed72b&itag=135&source=youtube&requiressl=yes&mm=31&mn=sn-hjoj-gq0e&mv=m&ms=au&pl=16&ratebypass=yes&mime=video/mp4&gir=yes&clen=53057012&lmt=1448522802188758&dur=479.800&upn=sRpQR1vyxcg&key=dg_yt0&mt=1448914609&signature=635E9E6B7145ABB3B97DDD89C343F16A6317C451.3E9C5E862FC90D630C20FDB1E04A461B76CC9350&sver=3&fexp=9407116,9408499,9408710,9409127,9416126,9417683,9420452,9421464,9422372,9422519,9422596,9422618,9422869,9423299,9423391,9423662,9423853,9423951,9423992,9424192,9424965,9425106,9425307&ip=152.7.224.7&ipbits=0&expire=1448936260&sparams=ip,ipbits,expire,id,itag,source,requiressl,mm,mn,mv,ms,pl,ratebypass,mime,gir,clen,lmt,dur'
[download] Destination: AMA 2015 Jennifer Lopez Full Performance-JMMlOyG-1ys.f135.mp4
[download]   7.9% of 50.60MiB at  2.39MiB/s ETA 00:19^C
ERROR: Interrupted by user

[Kobold]

As a note to others coming to this issue from google, [Errno 64] Host is down has been caused by Little Snitch for me — try turning it off and see if the problem goes away.

[geerlingguy]

I had the same issue as @Kobold, but instead of disabling Little Snitch (I don't like it gone for even a second :D), you can go into the network monitor and manually switch on 'Terminal via Python' then run the command again.