我创建了两个站点：

1. foo.bar（用于asp.net用户界面），主机名为空白;
2. hostname：api.foo.bar for API.
   然后我把rewrite-rule放到api-site的web.config中：

<rewrite>
    <rules>
      <remove name="ReverseProxyInboundRule1" />
      <rule name="ReverseProxyInboundRule1" patternSyntax="Wildcard" stopProcessing="true">
          <match url="*" />
          <action type="Rewrite" url="http://localhost:8080/{R:0}" />
      </rule>
    </rules>
</rewrite>

一切都正常