Topic

PXE Boot

Explanation

The Preboot eXecution Environment (PXE, also known as Pre-Execution
Environment;  sometimes pronounced  "pixie") is  an  environment to
boot  computers using  a  network interface  independently of  data
storage devices  (like hard disks) or  installed operating systems.
PXE was introduced as part of the Wired for Management framework by
Intel and is described in the specification (version 2.1) published
by Intel and  Systemsoft on September 20, 1999.[1]  It makes use of
several  network  protocols   like  Internet  Protocol  (IP),  User
Datagram Protocol (UDP), Dynamic Host Configuration Protocol (DHCP)
and  Trivial File  Transfer Protocol  (TFTP) and  of  concepts like
Globally  Unique Identifier  (GUID), Universally  Unique Identifier
(UUID)  and  Universal Network  Device  Interface  and extends  the
firmware of  the PXE  client (the computer  to be  bootstrapped via
PXE) with  a set  of predefined Application  Programming Interfaces
(APIs).


The PXE protocol  is approximately a combination of  DHCP and TFTP,
albeit with  subtle modifications to  both. DHCP is used  to locate
the appropriate boot server or  servers, with TFTP used to download
the initial bootstrap program  and additional files.  To initiate a
PXE bootstrap  session the  PXE firmware broadcasts  a DHCPDISCOVER
packet extended  with PXE-specific options  (extended DHCPDISCOVER)
to port  67/UDP (DHCP  server port). The  PXE options  identify the
firmware as  capable of PXE, but  they will be  ignored by standard
DHCP  servers.  If  the  firmware  receives  DHCPOFFERs  from  such
servers, it may  configure itself by requesting one  of the offered
configurations.

source : http://en.wikipedia.org/wiki/Preboot_Execution_Environment

Get The Hang

 // next, check what the packet type was
        opt = request(43,71);
        if(opt == NULL)
        {
                logger->Event(LEVEL_INFO, "MakeReply", 1,
                  "Received proxy DHCP packet");
                pkttype = 1;
        }
        else
        {
                delete[] opt->data;
                delete opt;
                logger->Event(LEVEL_INFO, "MakeReply", 1,
                  "Received PXE request packet");
                pkttype = 2;
        }

$time php bubble.php

real	0m7.404s
user	0m7.012s
sys	0m0.080s
$time php bubble.php

real	0m7.333s
user	0m7.044s
sys	0m0.068s
$time php bubble.php

real	0m7.393s
user	0m7.068s
sys	0m0.100s
$time php bubble.php

real	0m7.345s
user	0m7.024s
sys	0m0.104s
$time php bubble.php

real	0m7.545s
user	0m7.224s
sys	0m0.080s
$time php bubble.php

real	0m7.275s
user	0m6.992s
sys	0m0.072s
$time php bubble.php

real	0m7.340s
user	0m7.044s
sys	0m0.072s
$

noxious  means injurious  to physical  or mental  health; obnoxious
means very offensive, objectionable, or unpleasant

source : http://thesaurus.com/browse/obnoxious

$time php bubble.php

real	0m2.076s
user	0m1.736s
sys	0m0.052s
$time php bubble.php

real	0m1.8100s
user	0m1.680s
sys	0m0.044s
$time php bubble.php

real	0m1.935s
user	0m1.672s
sys	0m0.052s
$time php bubble.php

real	0m1.861s
user	0m1.676s
sys	0m0.040s
$time php bubble.php

real	0m2.076s
user	0m1.708s
sys	0m0.064s
$time php bubble.php

real	0m1.850s
user	0m1.640s
sys	0m0.060s
$time php bubble.php

real	0m1.872s
user	0m1.664s
sys	0m0.060s
$

The atime and noatime attribute


Linux records  information about when  files were created  and last
modified as  well as  when it  was last accessed.  There is  a cost
associated  with recording  the  last access  time.  The ext2  file
system of Linux has an attribute that allows the super-user to mark
individual  files   such  that  their  last  access   time  is  not
recorded. This may lead  to significant performance improvements on
often accessed  frequently changing files  such as the  contents of
the /var/spool/news directory.


Linux has a  special mount option for file  systems called noatime
that can be  added to each line that addresses  one file system in
the /etc/fstab file.  If a file system has  been mounted with this
option, reading accesses to the  file system will no longer result
in an  update to  the atime information  associated with  the file
like  we  have explained  above.  The  importance  of the  noatime
setting  is that  it eliminates  the need  by the  system  to make
writes  to  the file  system  for  files  which are  simply  being
read. Since writes  can be somewhat expensive, this  can result in
measurable performance gains. Note that the write time information
to a file will continue to  be updated anytime the file is written
to. In  our example below, we  will set the noatime  option to our
/chroot file system.

source : http://tldp.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/chap6sec73.html

$lsattr yt-chanrip
------------------- yt-chanrip
$ls -l yt-chanrip
-rw-r--r-- 1 jeffrin jeffrin 230 Sep  2 18:19 yt-chanrip
$chattr +A yt-chanrip
$lsattr yt-chanrip
-------A----------- yt-chanrip
$ls -l yt-chanrip
-rw-r--r-- 1 jeffrin jeffrin 230 Sep  2 18:19 yt-chanrip
$echo "   " >> yt-chanrip
$lsattr yt-chanrip
-------A----------- yt-chanrip
$ls -l yt-chanrip
-rw-r--r-- 1 jeffrin jeffrin 234 Nov 29 22:30 yt-chanrip
$time

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$chattr -A yt-chanrip
$echo "   " >> yt-chanrip
$ls -l yt-chanrip
-rw-r--r-- 1 jeffrin jeffrin 238 Nov 29 22:31 yt-chanrip
$

$time php bubble.php

real	0m0.846s
user	0m0.388s
sys	0m0.036s
$time php bubble.php

real	0m0.707s
user	0m0.368s
sys	0m0.032s
$time php bubble.php

real	0m0.718s
user	0m0.384s
sys	0m0.024s
$time php bubble.php

real	0m0.768s
user	0m0.372s
sys	0m0.036s
$time php bubble.php

real	0m0.556s
user	0m0.432s
sys	0m0.024s
$time php bubble.php

real	0m0.476s
user	0m0.372s
sys	0m0.040s
$time php bubble.php

real	0m0.438s
user	0m0.356s
sys	0m0.036s
$