一共关系到cache_peer/always_direct/never_direct/hierarchy_stoplist/prefer_direct等配置项。
squid的使用指南上，关于always_direct和never_direct这么写到：
Squid checks all always_direct tags before it checks any never_direct tags. If a matching always_direct tag is found, Squid will not check the never_direct tags, but decides which cache to talk to immediately....If the line used the deny keyword instead of allow, Squid would have simply skipped on to checking the never_direct lines
squid的配置说明上，关于hierarchy_stoplist这么写到：
use this to not query neighbor caches for certain objects....never_direct overrides this option
squid的邮件列表上，Amos这么解释：
always_direct *prevents* peers being used. It does not force them. " hierarchy_stoplist ? " is the directive preventing the peer being used. 看起来挺让人晕头转向的。 其实就是说： always_direct allow的优先级高于never_direct，但deny（包括allow !）时则不。 hierarchy_stoplist强制请求通过域名解析回源，但never_direct又优先于它。 prefer_direct用于所有cache_peer都down了时，never_direct会报错，而prefer会转入dns解析。