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.
大致意思就是：
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。