[2024-feb-29] Sad news: Eric Layton aka Nocturnal Slacker aka vtel57 passed away on Feb 26th, shortly after hospitalization. He was one of our Wiki's most prominent admins. He will be missed.
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slackbook:filesystem_permissions [2012/09/09 15:14 (UTC)] – [chmod, chown, and chgrp] update document with original text and formatting mfillpot | slackbook:filesystem_permissions [2012/10/15 22:28 (UTC)] (current) – [SUID, SGID, and the Sticky Bit] gerardo.zamudio | ||
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case, the permissions are rwxr-xr-x, the user is root and the group is | case, the permissions are rwxr-xr-x, the user is root and the group is | ||
also root. The permissions section, while grouped together, is really | also root. The permissions section, while grouped together, is really | ||
- | three seperate | + | three separate |
permissions granted to the user that owns the file. The second set of | permissions granted to the user that owns the file. The second set of | ||
three are those granted to the group owner, and the final three are | three are those granted to the group owner, and the final three are | ||
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|Others |r-x |Everyone else may read and execute| | |Others |r-x |Everyone else may read and execute| | ||
- | he permissions are pretty self explainatory | + | The permissions are pretty self explanatory |
files. Read, write, and execute allow you to read a file, write to it, | files. Read, write, and execute allow you to read a file, write to it, | ||
or execute it. But what do these permissions mean for directories? | or execute it. But what do these permissions mean for directories? | ||
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By adding these values together, we can reach any number between 0 and | By adding these values together, we can reach any number between 0 and | ||
7 and specify all possible permission combinations. For example, to | 7 and specify all possible permission combinations. For example, to | ||
- | grant both read and write privilages | + | grant both read and write privileges |
use the number 6. The number 3 would grant write and execute | use the number 6. The number 3 would grant write and execute | ||
permissions, | permissions, | ||
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**// | **// | ||
- | < | + | <key>'+'</ |
While this may be easier to | While this may be easier to | ||
remember, it's often easier to use the octal permissions. | remember, it's often easier to use the octal permissions. | ||
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group, and //" | group, and //" | ||
adding or removing permissions with the //" | adding or removing permissions with the //" | ||
- | sets can be changed at once by seperating | + | sets can be changed at once by separating |
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===== SUID, SGID, and the " | ===== SUID, SGID, and the " | ||
- | We're not quite done with permissions just yet. There are three other " | + | We're not quite done with permissions just yet. There are three other |
+ | //" | ||
+ | SUID, SGID, and the sticky bit. When a file has one or more of these | ||
+ | permissions set, it behaves in special ways. The SUID and SGID | ||
+ | permissions change the way an application is run, while the sticky bit | ||
+ | restricts deletion of files. These permissions are applied with | ||
+ | **//chmod//** | ||
+ | like read, write, and execute, but with a twist. | ||
+ | |||
+ | |||
+ | SUID and SGID stand for //"Set User ID"// and //"Set Group ID"// respectively. | ||
+ | When an application with one of these bits is set, the application runs | ||
+ | with the user or group ownership permissions of that application | ||
+ | regardless of what user actually | ||
+ | executed it. Let's take a look at a common SUID application, | ||
+ | **// | ||
- | SUID and SGID stand for "Set User ID" and "Set Group ID" respectively. When an application with one of these bits is set, the application runs with the user or group ownership permissions of that application regardless of what user actually executed it. Let's take a look at a common SUID application, | ||
< | < | ||
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</ | </ | ||
- | Notice the permissions on passwd. Instead of an x in the user's execute slot, we have an s. This tells us that passwd is a SUID program, and when we run it, the process will run as the user " | ||
- | So what about the sticky bit? The sticky bit restricts the ability to move or delete files and is only ever set on directories. Non-root users cannot move or delete any files under a directory with the sticky bit set unless they are the owner of that file. Normally anyone with write permission to the file can do this, but the sticky bit prevents it for anyone but the owner (and of course, root). Let's take a look at a common " | + | Notice the permissions on **// |
+ | an < | ||
+ | < | ||
+ | **// | ||
+ | it, the process will run as the user //" | ||
+ | that actually executed it. The reason for this is readily apparent as | ||
+ | soon as you look at the two files it modifies. Neither | ||
+ | ''/ | ||
+ | are writable by anyone other than root. Since users need to change | ||
+ | their personal information, | ||
+ | run as root in order to modify those files. | ||
+ | |||
+ | |||
+ | So what about the sticky bit? The sticky bit restricts the ability to | ||
+ | move or delete files and is only ever set on directories. Non-root | ||
+ | users cannot move or delete any files under a directory with the sticky | ||
+ | bit set unless they are the owner of that file. Normally anyone with | ||
+ | write permission to the file can do this, but the sticky bit prevents | ||
+ | it for anyone but the owner (and of course, root). Let's take a look at | ||
+ | a common | ||
< | < | ||
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</ | </ | ||
- | Naturally, being a directory for the storage of temporary files sytem wide, /tmp needs to be readable, | + | |
+ | Naturally, being a directory for the storage of temporary files system | ||
+ | wide, '' | ||
+ | executable by anyone and everyone. Since any user is likely to have a | ||
+ | file or two stored here at any time, it only makes good sense to | ||
+ | prevent other users from deleting those files, so the sticky bit has | ||
+ | been set. You can see it by the presence of the < | ||
+ | place of the < | ||
**Table 10.5. SUID, SGID, and " | **Table 10.5. SUID, SGID, and " | ||
- | ^Permission | + | ^Permission Type ^Octal Value ^Letter Value| |
|SUID |4 |s| | |SUID |4 |s| | ||
|SGID |2 |s| | |SGID |2 |s| | ||
|Sticky |1 |t| | |Sticky |1 |t| | ||
- | When using octal permissions, | + | When using octal permissions, |
+ | octal value. For example, to recreate the permission on | ||
+ | '' | ||
+ | permissions on '' | ||
+ | Essentially, | ||
+ | **//chmod//** assumes its value to be 0. | ||
< | < | ||
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</ | </ | ||
- | Using the alphabetic permission values is slightly different. Assuming the two files above have permissions of 0000 (no permissions at all), here is how we would set them. | + | |
+ | Using the alphabetic permission values is slightly different. Assuming | ||
+ | the two files above have permissions of 0000 (no permissions at all), | ||
+ | here is how we would set them. | ||
< | < | ||
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</ | </ | ||
+ | ====== Chapter Navigation ====== | ||
+ | |||
+ | **Previous Chapter: [[slackbook: | ||
+ | |||
+ | **Next Chapter: [[slackbook: | ||
====== Sources ====== | ====== Sources ====== | ||
<!-- If you copy information from another source, then specify that source --> | <!-- If you copy information from another source, then specify that source --> | ||
- | * Original source: [[http:// | + | |
<!-- Authors are allowed to give credit to themselves! --> | <!-- Authors are allowed to give credit to themselves! --> | ||
- | < | + | |
- | <!-- * Contrbutions | + | <!-- * Contributions |
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