<appender name="SomeAppender" type="...RollingFileAppender">
    <HttpCodeError value="0" />
    <lockingmodel type="log4net.Appender.FileAppender+MinimalLock"/>
    <file value="some.log"/>
    <appendToFile value="true" />
    <encoding value="utf-8" />
    <rollingStyle value="Size" />
    <maxSizeRollBackups value="10" />
    <maximumFileSize value="1MB" />
    <staticLogFileName value="true" />
    <layout type="">
      <conversionPattern value="%date{d/M/yyyy H:mm:ss.fff};%-5level;%error%newline" />
    </layout>

     <filter type="log4net.Filter.StringMatchFilter">
          <stringToMatch value="notneed.some."/>
          <acceptOnMatch value="false" />
    </filter>    
    <filter type="log4net.Filter.StringMatchFilter">
          <stringToMatch value="need.some."/>
          <acceptOnMatch value="true" />
    </filter>    
    <filter type="log4net.Filter.LevelMatchFilter">
          <levelToMatch value="ERROR" />
    </filter>
    <filter type="log4net.Filter.DenyAllFilter" />
  </appender>