在nutch2.2.1中，有两份配置文件：nutch-default.xml与nutch-site.xml。
其中前者是nutch自带的默认属性，一般情况下不要修改。
如果需要修改默认属性，可以在nutch-site.xml中增加一个同名的属性，并修改其值。nutch-site.xml中的属性值会覆盖nutch-default.xml中的值。
1、db.ignore.external.links
若为true，则只抓取本域名内的网页，忽略外部链接。
可以在 regex-urlfilter.txt中增加过滤器达到同样效果，但如果过滤器过多，如几千个，则会大大影响nutch的性能。
<property> <name>db.ignore.external.links</name> <value>true</value> <description>If true, outlinks leading from a page to external hosts will be ignored. This is an effective way to limit the crawl to include only initially injected hosts, without creating complex URLFilters. </description> </property>
2、fetcher.parse
能否在抓取的同时进行解释：可以，但不 建议这样做。
<property> <name>fetcher.parse</name> <value>false</value> <description>If true, fetcher will parse content. NOTE: previous releases would default to true. Since 2.0 this is set to false as a safer default.</description> </property>
官方解释
N.B. In a parsing fetcher, outlinks are processed in the reduce phase (at least when outlinks are followed). If a fetcher‘s reducer stalls you may run out of memory or disk space,
usually after a very long reduce job. Behaviour typical to this is usually observed in
this situation.
In summary, if it is possible, users are advised not to use a parsing fetcher as it is heavy on IO and often leads to the above outcome.
Nutch2.2.1配置文件nutch-site.xml