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| | #redirect [[Some_explanation_to_some_errors_and_warnings#Name_or_service_not_known]] |
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| ...an error, with an errno of -2.
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| A host name lookup problem, and one that can have ''many'' different causes.
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| Before digging in to diagnose the problem, check the same thing on a different computer. If any test fails on other computers on the same network, it's likely to be a problem external to you (e.g. DNS server or proxy failing, or such).
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| ==Possible causes / things to look at==
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| ===Nonsense lookup===
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| If a hostname lookup is being done that makes no sense, for example <tt>gethostbyname()</tt> on an URL instead of a hostname.
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| ===host can't look up itself===
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| If one of the following fails {{comment|(replacing hostx or your favorite resolving utility)}}:
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| hostx localhost
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| hostx `hostname`
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| ...then it's likely your <tt>/etc/hosts</tt> is broken, or possibly the hostname you have set (there are some extra details to this when you set an FQDN as your hostname).
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| ===nameserver / proxy trouble===
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| * If the above work but things like:
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| hostx google.com
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| hostx yourisp.com
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| ...all fail, then it might be DNS trouble - including not having a DNS server set (in *nices, look at <tt>/etc/resolv.conf</tt>) <!--(in rare cases a partial outage, e.g. your ISP being up but it being relatively severed from the rest of the internet somehow)-->
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| ===nsswitch===
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| * Another reason for things not to work -- or sometimes for them to have weird patterns of working and not working -- can be that your <tt>/etc/nsswitch.conf</tt> is malconfigured, or configured to include something that does not work consistently (see in particular its <tt>hosts:</tt> line).
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| <!--
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| ===Others===
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| * Perhaps IPv4/IPv6 confusion?
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| In my case, python seemed to be selectively failing. The following worked:
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| <code lang="bash">
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| python -c 'import socket; print socket.gethostbyname(socket.gethostname())'
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| </code>
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| Note: python code that somehow relies on mimetools (mail stuff) has a line like <tt>socket.gethostbyname(socket.gethostname())</tt>, so if that fials, so will it.
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| <code lang="bash">
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| python -c 'import socket; print socket.gethostbyname("google.com")'
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| </code>
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| ...failing points to localhost not being listed in /etc/hosts, and to general DNS trouble, respectively. Maybe <tt>/etc/resolv.conf</tt> isn't pointing to something useful?
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| In my case, the problem was revealed by python failing to do something specific (HTTP fetching), showing:
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| urllib2.URLError: <urlopen error [Errno -2] Name or service not known>
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| ...or...
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| socket.gaierror: (-2, 'Name or service not known')
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| ...depending on the specific code that brought this about.
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| None of the tests mentioned above failed or misbehaved, and even an access on a local URL:
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| <code lang="python">
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| import urllib2,socket
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| urllib2.urlopen( "http://%s/"%socket.gethostbyname(socket.gethostname()))
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| </code>
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| ...but failing on:
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| <code lang="python">
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| import urllib2
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| urllib2.urlopen( "http://google.com/" )
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| </code>
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| This in itself tells you little
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| -->
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| [[Category:Networking]] | |
| [[Category:Warnings and errors]]
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