Shrew VPN Client + Juniper SRX : "session terminated by gateway" (Autodisconnect)

If like me, you're trying to connect to a Juniper dynamic VPN with Shrew VPN Client, be aware that this not yet possible.

The connection works but the tunnel is constantly disconnected after 60 seconds.

I asked the core developer "Matthiew Grooms" about this and after few debug, it seems like a fix is needed in Shrew's code:

"It's pretty clear whats going on but it won't be possible to fix without 
a rewrite of the modecfg code on the Shrew Soft VPN client, which is probably 
needed anyway."

Full technical details are available at :
https://lists.shrew.net/pipermail/vpn-help/2012-December/014091.html

If anybody found out an alternative solution please share !

Finding less used wireless channel on Linux

Want to find the less used wireless channel around you ? This little one liner will give a summary of all wireless channels with the number of SSID associated.

 echo "Nb SSID - Channel" ; iwlist scan 2>/dev/null | grep "Channel:" | cut -d':' -f2 | sort -n | uniq -c   
 Nb SSID - Channel  
       4 1  
       3 2  
       3 3  
       1 5  
       5 6  
       4 7  
       3 9  
       4 11  
       3 12  

Right column is the Channel number and left column is the number of SSID found on this channel.

 Of course, this is just a basic overview, for real deployments you should use a more sophisticated tool like inSSIDer.

Also an interesting post on how to choose the right channel :
http://www.dslreports.com/faq/14250

Set scale in (non interactive) scripts with the bc command

If you need to do some kind of math operation in shell scripts you might want to write :

 $ echo "9/2" | bc  
 4  

By default bc truncate the result which is bit annonying...

Using bc -l option gives you a far too precise result :

 $ echo "9/2" | bc -l  
 4.50000000000000000000  

Fortunately, bc comes with the "scale" option to set the scale to whatever presision you wish.

To do it non interactively, you need to specify the scale in the echo before the operation.
For example, to set a precision of 2 digits :

 $ echo -e "scale=2 \n 9/2" | bc  
 4.50  

So the script code would look like :

 val=$(echo -e "scale=2 \n 9/2" | bc)  

Replace end of lines (\n) by any character

Just found out a classic UNIX command I never used before.

The command "paste" allows you to replace end of lines (i.e \n) by any character of your choice.

I normally used sed for this purpose but the syntax is quite...dirty. For example to replace all end of lines by a space with sed, the command is :

 $ sed ':a;N;$!ba;s/\n/ /g' /path/to/file  

With paste the syntax is much more human readable :

 $ paste -s /path/to/file  

By default "paste" replaces end of lines with tabs, to specify a delimiter use the -d option.

Replace end of lines with spaces :

 $ paste -s -d' ' /path/to/file  

Replace end of lines with commas :

 $ paste -s -d',' /path/to/file  

 Use '-' for reading from stdin :

 echo -e "a\nb\nc\nd" | paste -s -d',' -  
 a,b,c,d  

More info in the man as always.

This tips will not change your life but i found it quite useful !

Hope that helps !

Choosing RAID Level / Stripe Size

Below interesting articles on how to choose your RAID level / stripe size.

Good litterature on RAID :
http://www.fccps.cz/download/adv/frr/hdd/hdd.html

RAID Level Explained :
http://www.techrepublic.com/blog/datacenter/choose-a-raid-level-that-works-for-you/3237

RAID Stripe Explained :
http://www.anandtech.com/show/788/5

RAID Benchmarks :
https://raid.wiki.kernel.org/index.php/Performance

RAID Calculator :
http://www.z-a-recovery.com/art-raid-estimator.htm

In any case, always plan your workload type (Read/Writes, Sequential/Random, Large/Small File, Number of concurrent access).

Create large partitions on Linux / Bypass the 2TB partition Limit

The default partition schema (MBR based) limits partition to 2.2TB. With new hardrives this limit is easily reached.

In order to create partition bigger than 2.2TB you need to switch from MBR to GUID (GPT) partition table.
This can be done with the "parted" utility on Linux.

For exemple if you want to create a single big partition on /dev/sdb :

 # parted /dev/sdb  
 (parted) mklabel GPT  
 (parted) mkpart partition_name fstype 1 -1  
 (parted) print  
 Model: DELL PERC H700 (scsi)  
 Disk /dev/sdb: 4000GB  
 Sector size (logical/physical): 512B/512B  
 Partition Table: gpt  
 Number Start  End   Size  File system Name Flags  
  1   1049kB 4000GB 4000GB        data  

Note : I found out that partition name and fstype are quite useless.

You can then format the partition with the filesystem of your choice or create a LVM PV.

More info on GUID / MBR Limits :
http://en.wikipedia.org/wiki/GUID_Partition_Table

Parted official website :
http://www.gnu.org/software/parted/

More parted exemples :
http://www.thegeekstuff.com/2011/09/parted-command-examples/

Hope that helps !