<script type="text/javascript">
<!--
        function AllMessages()
                {for(var x=0;x<document.form1.elements.length;x++)
                        {var y=document.form1.elements[x];
                         if(y.name!='ALLMSGS') y.checked=document.form1.checkall.checked;
                        }
                }
//-->
</script>

[...]
<td><input type="checkbox" name="host" value="HOSTNAME1" /> HOSTNAME1</td>
<td><input type="checkbox" name="host" value="HOSTNAME2" /> HOSTNAME2</td></tr>


<tr><td><input type="checkbox" name="checkall" onclick="AllMessages(this.form);" /> alle markieren</td></tr>