What are the SUID, SGID and the Sticky Bits

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                What are the SUID, SGID and the Sticky Bits?
            
         
        
      
   
  
  

  
  
   
      
        
         
            
              
               
Linux
                  has some special attributes associated with all files. Often
                  in X Windows when you check the properties of any file (by right
                  clicking on it and viewing its properties) you would get to
                  see 3 special attributes besides the common read/write/execute
                  rights for the owner/group/others . The 3 extra attributes are
                  known as SUID, SGID and Sticky Bits
                  
               
                  Sticky
                  Bit
                  
                  Lets start with Sticky bit first. Since this is the most simplest
                  to explain. Setting the sticky bit tells Unix that once the
                  concerned application is executed, it should remain in memory.
                  Remember that Unix is a multi-user OS and was mainly designed
                  so that multiple users can work simultaneously. Thus the logic
                  used is that a program that exists in memory requires lesser
                  time to start when a new user requests for the same program.
                  Thus when one user has just used a program and then a new user
                  wants to use the same program, the second user doesn't have
                  to face a time delay for the program to initialize itself. It
                  would be readily available to him. The concept of the sticky
                  bit was a very useful one, long back when fast disk access and
                  other memory access technologies weren't around. But in today's
                  age the concept of sticky bit is obsolete, since modern day
                  technology is advanced enough to reduce the time delay while
                  loading applications into the memory. Thus currently the sticky
                  bit is of very little significance. Sticky bit is only associated
                  with executables.
               
                  SUID (Set User ID) Bit
                  
                  Sometime you may faced an error while trying to run any application
                  stating that the application must be 'SUID root' . You might
                  have been confused that time, but now once you read this article
                  you would no longer find it confusing.
                  
                  SUID stands for Set User ID. This means that if the SUID
                  bit is set for any application then your user ID would be set
                  as that of the owner of application/file rather than the current
                  user, while running that application. That means in case
                  I have an application whose owner is ' root ' and it has its
                  SUID bit set, then when I run this application as a normal user,
                  that application would still run as root. Since the SUID
                  bit tells Linux that the the User ID root is set for this application
                  and whenever this application executes it must execute as if
                  root was executing it (since root owns this file).
                  
                  In case you have really understood the above you may be wondering
                  - isnt this a major security risk? If users are able to run
                  applications as root, then it must be definitely posing as a
                  threat to the security of the system. Actually the SUID is used
                  to increase the security in a way. Let me explain this with
                  my own example I use on my machine.
                  
                  
                  One way I use SUID on my machine
                  
                  I have a few files that I modify through Linux and then before
                  I shutdown Linux I have to transfer them to my Windows partition
                  for further use there. As a normal user I do not have write
                  access to the Windows partitions that I have mounted. So I have
                  to be the superuser to be able to write to that partition. I
                  have created a simple shell script that copies my files to the
                  Windows partitions. This script was created by root user and
                  the SUID bit was set. Access rights to this script have been
                  given to all users. Now whenever I want to copy my files I simply
                  run this script. Even though I have logged in as a normal user,
                  the SUID bit which is set causes this script to execute as if
                  the root was executing it and it allows me to write to the Windows
                  partitions.
                  
                  Had the SUID bit not been set, I would have to type ' su ' at
                  the prompt and get temporary superuser access to get write access
                  to the Windows partitions. Hope you got the point..
                  
                  Note : In case
                  you do not know how to access your Windows partitions through
                  Linux, refer to
Article No. 3
                  
                  You may be thinking that since these applications would run
                  as root they can do harmful things and destroy the system. The
                  concept behind SUID bit is that you as the superuser would be
                  able to allow certain applications / scripts to be run by the
                  users as if they were the superuser for the time being. What
                  these application / scripts do when they execute should be completely
                  known to you. Even though the users would be allowed to execute
                  these programs as root they would be able to do ONLY THOSE
                  things that these programs were designed to do. So in case
                  a script was designed to copy 5 files from one place to another.
                  Then the user who would run that script would be able to ONLY
                  copy those 5 files from one place to another. He would not be
                  able to modify that script in any way since he would not
                  have write access to the script. He would only be having
                  execute rights for that script. Hence its an excellent idea
                  to allow users to do some important backup using a script that
                  does only that and by setting the SUID bit for that script.
                  This way the users don't have to know the superuser password
                  but can still use some facilities that are only available to
                  the superuser
                  
                  Important
                  : Think twice before setting the SUID bit for scripts (owned
                  by root) that take arguments at the command line. Since you
                  never know what parameters a malicious user may pass to your
                  script. Since the script would run as root it could do great
                  damage if misused.
                  
                  
                  SGID (Set Group ID) bit
                  
                  Just like SUID, setting the SGID bit for a file sets your group
                  ID to the file's group while the file is executing. IT is really
                  useful in case you have a real multi-user setup where users
                  access each others files. As a single homeuser I haven't really
                  found a lot of use for SGID. But the basic concept is the same
                  as the SUID, the files whose SGID bit are set would be used
                  as if they belong to that group rather than to that user alone.
               
Note
                  : Making SUID and SGID programs completely safe is very
                  difficult (or maybe impossible) thus in case you are a system
                  administrator it is best to consult some professionals before
                  giving access rights to root owned applications by setting
                  the SUID bit. As a home user (where you are both the normal
                  user and the superuser) the SUID bit helps you do a lot of things
                  easily without having to log in as the superuser every now and
                  then.
               
  
              
         
        
      
   
  
  
               
               
               
               
               

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