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| <h4 class="subsection">28.2.2 Actions in the NSS configuration</h4> |
| |
| <p>The second item in the specification gives the user much finer control |
| on the lookup process. Action items are placed between two service |
| names and are written within brackets. The general form is |
| |
| <pre class="display"> <code>[</code> ( <code>!</code>? <var>status</var> <code>=</code> <var>action</var> )+ <code>]</code> |
| </pre> |
| <p class="noindent">where |
| |
| <pre class="smallexample"> <var>status</var> ⇒ success | notfound | unavail | tryagain |
| <var>action</var> ⇒ return | continue |
| </pre> |
| <p>The case of the keywords is insignificant. The <var>status</var> |
| values are the results of a call to a lookup function of a specific |
| service. They mean |
| |
| <dl> |
| <dt>‘<samp><span class="samp">success</span></samp>’<a name="index-success-3275"></a><dd>No error occurred and the wanted entry is returned. The default action |
| for this is <code>return</code>. |
| |
| <br><dt>‘<samp><span class="samp">notfound</span></samp>’<a name="index-notfound-3276"></a><dd>The lookup process works ok but the needed value was not found. The |
| default action is <code>continue</code>. |
| |
| <br><dt>‘<samp><span class="samp">unavail</span></samp>’<a name="index-unavail-3277"></a><dd><a name="index-DNS-server-unavailable-3278"></a>The service is permanently unavailable. This can either mean the needed |
| file is not available, or, for DNS, the server is not available or does |
| not allow queries. The default action is <code>continue</code>. |
| |
| <br><dt>‘<samp><span class="samp">tryagain</span></samp>’<a name="index-tryagain-3279"></a><dd>The service is temporarily unavailable. This could mean a file is |
| locked or a server currently cannot accept more connections. The |
| default action is <code>continue</code>. |
| </dl> |
| |
| <p class="noindent">If we have a line like |
| |
| <pre class="smallexample"> ethers: nisplus [NOTFOUND=return] db files |
| </pre> |
| <p class="noindent">this is equivalent to |
| |
| <pre class="smallexample"> ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue |
| TRYAGAIN=continue] |
| db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue |
| TRYAGAIN=continue] |
| files |
| </pre> |
| <p class="noindent">(except that it would have to be written on one line). The default |
| value for the actions are normally what you want, and only need to be |
| changed in exceptional cases. |
| |
| <p>If the optional <code>!</code> is placed before the <var>status</var> this means |
| the following action is used for all statuses but <var>status</var> itself. |
| I.e., <code>!</code> is negation as in the C language (and others). |
| |
| <p>Before we explain the exception which makes this action item necessary |
| one more remark: obviously it makes no sense to add another action |
| item after the <code>files</code> service. Since there is no other service |
| following the action <em>always</em> is <code>return</code>. |
| |
| <p><a name="index-nisplus_002c-and-completeness-3280"></a>Now, why is this <code>[NOTFOUND=return]</code> action useful? To understand |
| this we should know that the <code>nisplus</code> service is often |
| complete; i.e., if an entry is not available in the NIS+ tables it is |
| not available anywhere else. This is what is expressed by this action |
| item: it is useless to examine further services since they will not give |
| us a result. |
| |
| <p><a name="index-nisplus_002c-and-booting-3281"></a><a name="index-bootstrapping_002c-and-services-3282"></a>The situation would be different if the NIS+ service is not available |
| because the machine is booting. In this case the return value of the |
| lookup function is not <code>notfound</code> but instead <code>unavail</code>. And |
| as you can see in the complete form above: in this situation the |
| <code>db</code> and <code>files</code> services are used. Neat, isn't it? The |
| system administrator need not pay special care for the time the system |
| is not completely ready to work (while booting or shutdown or |
| network problems). |
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