当我们配置Nutch抓取 http://yangshangchuan.iteye.com 的时候，抓取的所有页面内容均为：您的访问请求被拒绝 ...... 这是最简单的反爬虫策略（该策略简单地读取HTTP请求头User-Agent的值来判断是人（浏览器）还是机器爬虫），我们只需要简单地配置Nutch来模拟浏览器（simulate web browser）就可以绕过这种限制。
在nutch-default.xml中有5项配置是和User-Agent相关的：
http.agent.description Further description of our bot- this text is used in the User-Agent header. It appears in parenthesis after the agent name. http.agent.url A URL to advertise in the User-Agent header. This will appear in parenthesis after the agent name. Custom dictates that this should be a URL of a page explaining the purpose and behavior of this crawler. http.agent.email An email address to advertise in the HTTP 'From' request header and User-Agent header. A good practice is to mangle this address (e.g. 'info at example dot com') to avoid spamming. http.agent.name HTTP 'User-Agent' request header. MUST NOT be empty - please set this to a single word uniquely related to your organization. NOTE: You should also check other related properties: http.robots.agents http.agent.description http.agent.url http.agent.email http.agent.version and set their values appropriately. http.agent.version Nutch-1.7 A version string to advertise in the User-Agent header.
在类nutch1.7/src/plugin/lib-http/src/java/org/apache/nutch/protocol/http/api/HttpBase.java中可以看到这5项配置是如何构成User-Agent的：
this.userAgent = getAgentString( conf.get("http.agent.name"), conf.get("http.agent.version"), conf.get("http.agent.description"), conf.get("http.agent.url"), conf.get("http.agent.email") );
private static String getAgentString(String agentName, String agentVersion, String agentDesc, String agentURL, String agentEmail) { if ( (agentName == null) || (agentName.trim().length() == 0) ) { // TODO : NUTCH-258 if (LOGGER.isErrorEnabled()) { LOGGER.error("No User-Agent string set (http.agent.name)!"); } } StringBuffer buf= new StringBuffer(); buf.append(agentName); if (agentVersion != null) { buf.append("/"); buf.append(agentVersion); } if ( ((agentDesc != null) && (agentDesc.length() != 0)) || ((agentEmail != null) && (agentEmail.length() != 0)) || ((agentURL != null) && (agentURL.length() != 0)) ) { buf.append(" ("); if ((agentDesc != null) && (agentDesc.length() != 0)) { buf.append(agentDesc); if ( (agentURL != null) || (agentEmail != null) ) buf.append("; "); } if ((agentURL != null) && (agentURL.length() != 0)) { buf.append(agentURL); if (agentEmail != null) buf.append("; "); } if ((agentEmail != null) && (agentEmail.length() != 0)) buf.append(agentEmail); buf.append(")"); } return buf.toString(); }
在类nutch1.7/src/plugin/protocol-http/src/java/org/apache/nutch/protocol/http/HttpResponse.java中使用User-Agent请求头，这里的http.getUserAgent()返回的userAgent就是HttpBase.java中的userAgent：
String userAgent = http.getUserAgent();if ((userAgent == null) || (userAgent.length() == 0)) { if (Http.LOG.isErrorEnabled()) { Http.LOG.error("User-agent is not set!"); }} else { reqStr.append("User-Agent: "); reqStr.append(userAgent); reqStr.append("\r\n");}
通过上面的分析可知：在nutch-site.xml中只需要增加如下几种配置之一便可以模拟一个特定的浏览器（Imitating a specific browser）：
1、模拟Firefox浏览器：
http.agent.name Mozilla/5.0 (Windows NT 6.1; WOW64; rv:27.0) Gecko http.agent.version 20100101 Firefox/27.0
2、模拟IE浏览器：
http.agent.name Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident http.agent.version 6.0)
3、模拟Chrome浏览器：
http.agent.name Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.117 Safari http.agent.version 537.36
4、模拟Safari浏览器：
http.agent.name Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari http.agent.version 534.57.2
5、模拟Opera浏览器：
http.agent.name Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/32.0.1700.102 Safari/537.36 OPR http.agent.version 19.0.1326.59
后记：查看User-Agent的方法：
1、
2、
3、