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| <h4 class="subsection">28.2.3 Notes on the NSS Configuration File</h4> |
| |
| <p>Finally a few more hints. The NSS implementation is not completely |
| helpless if <samp><span class="file">/etc/nsswitch.conf</span></samp> does not exist. For |
| all supported databases there is a default value so it should normally |
| be possible to get the system running even if the file is corrupted or |
| missing. |
| |
| <p><a name="index-default-value_002c-and-NSS-3283"></a>For the <code>hosts</code> and <code>networks</code> databases the default value is |
| <code>dns [!UNAVAIL=return] files</code>. I.e., the system is prepared for |
| the DNS service not to be available but if it is available the answer it |
| returns is definitive. |
| |
| <p>The <code>passwd</code>, <code>group</code>, and <code>shadow</code> databases are |
| traditionally handled in a special way. The appropriate files in the |
| <samp><span class="file">/etc</span></samp> directory are read but if an entry with a name starting |
| with a <code>+</code> character is found NIS is used. This kind of lookup |
| remains possible by using the special lookup service <code>compat</code> |
| and the default value for the three databases above is |
| <code>compat [NOTFOUND=return] files</code>. |
| |
| <p>For all other databases the default value is |
| <code>nis [NOTFOUND=return] files</code>. This solution give the best |
| chance to be correct since NIS and file based lookup is used. |
| |
| <p><a name="index-optimizing-NSS-3284"></a>A second point is that the user should try to optimize the lookup |
| process. The different service have different response times. |
| A simple file look up on a local file could be fast, but if the file |
| is long and the needed entry is near the end of the file this may take |
| quite some time. In this case it might be better to use the <code>db</code> |
| service which allows fast local access to large data sets. |
| |
| <p>Often the situation is that some global information like NIS must be |
| used. So it is unavoidable to use service entries like <code>nis</code> etc. |
| But one should avoid slow services like this if possible. |
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