I know that I can set the access level of a directory using chmod, but I need to specify a default level of access for every new file that is ever created in a directory, until the end of time.
Is there some way to accomplish this? chmod'ing every single file every time it gets generated in this directory isn't practical in a production environment, I need to make all files created in this directory default to 777.
Perhaps a little OT for StackOverflow.
Couple of options really, depending on what filesystem you've got.
Some filesystems support ACLs. http://linux.about.com/library/cmd/blcmdl1_setfacl.htm
Standard Unix won't allow you to force users to create mode 777, but you can set group setuid on a directory, such that all created files in that directory are owned by that group. If your default umask includes group write, that may do the trick.
On some filesystems, you can use inotify to detect changes and trigger a binary (like chmod).
Related
Say I have a folder tree like:
root/
ro/
symlink-to-ro/
my question is two-fold:
(a) is there a way to make all files in the ro directory read-only, but if the files are accessed by way of the symlink, make them writable?
(b) the reverse of (a): is there a way to make the files writable only if they are accessed directly?
This is just for *nix/MacOS
No. Permissions are assigned to inodes, not to directory entries; so the same set of permissions is checked regardless of the path you used to access the file.
EDIT: Scratch that. I just remembered there is a way: while files and folders don't carry permissions, mounts can be set to be readonly. If you were on Linux, a read-only bind mount would be exactly what you are looking for. AFAIK OSX can't do that, so you can fake it with an NFS mount (not as nice).
I have a REST server whose purpose is to organize files generated by various users. To keep things simple, both the server and the users have access to a shared network filesystem.
The workflow is as follows: the user generates the file in a temp folder. He then notifies the server who then puts the file in a place of its own and stores some metadata in a database. The server should then own the files and take care of their deletion as needed.
My problem is the following: since the files can be quite big, I'd like to avoid a costly copy and instead simply move the files from the temp folder to their final destination. However, moving the files prevents the server from changing their ownership (see here for example).
Is there a way around this, without 1) copying the file, and 2)running the server as root?
EDIT: a couple precisions:
The file to be moved can be a directory with a hierarchy of files
It would be nice to have the server own the files in the final location to restrict access to other users.
If you create a separate user just to handle the chown, you can give that user the CAP_CHOWN capability, and you can have a single executable owned by that user that has the setuid bit set on it (so it executes as that user).
For security, this executable should do as little as possible, with as many checks as possible.
It should do the chown for the server user after the server user does the move. It should exist in a directory that is not writable by other users; it can do checks to insure that it is happy with all the attributes of the files it is asked to chown (current owner, location, etc.), it can have the server user hard-coded (so nobody else can use it), etc.
This will probably have to be a small C program, since most systems don't let you use setuid with scripts. You can find several small example programs on the web that do chown -- one is here
You should use a user group for all users and the server. Make the temp directory owned by that group and set it group-writable and sgid.
chown :groupname /path/to/temp
chmod g+s /path/to/temp
chmod 770 /path/to/temp
Then the server can adopt ownership of the file easily. Of course this means users can write other users' files, but I guess this is not a concern because they stay there a very short time?
I'm a beginner in linux.I have a question on filesystem that is it possible to make all files under a directory read-only without changing permissions?
No it is not possible. The write permission grants the ability to write a file. So that you need to change it anyway to make the file read-only for specific user or group of users.
You'd probably like to read this at spare time.
If you have suitable privileges, and if the filesystem is not critical to the machine (do not try this with "/", your root filesystem), you can remount the filesystem read-only. The details differ slightly from one system to another, but a useful discussion is found here:
https://askubuntu.com/questions/296331/how-to-mount-a-hard-disk-as-read-only-from-the-terminal
that applies to Linux and BSDs.
Because this does not actually modify the files, you can undo this by remounting the filesystem again, with read/write permissions, e.g., "rw" where the "ro" option was used. For specific information you should read the manpage(s) for mount and fstab for the system you are using.
I have user:nobody and group:nogroup set for apache in httpd.conf.
Since I also use my own user to manage files on ssh through Samba, I would like to have access to the www folder for read/write, and also allow apache to read these files.
Some folders should have apache's write permissions.
Should I leave apache as nobody|nogroup?
I was thinking I should set my own user under a group called say "webadmins" and set apache a new user called say "apache" under the same group. Then allow the group to read from all files, but only my user will have write files. Whenever apache would need a write permission inside a folder, I would manually change that. Is this a fair enough approach or am I missing something?
Thanks!
usually any daemon will need to access a number of ressources.
it is therefore good practice to run each daemon under a special user:group, rather than nobody:nogroup.
traditionally (e.g. on Debian systems) apache runs as www-data:www-data.
finally, user permissions take precedence over group permissions (which in turn take precedence over other permissions).
this means that a directory where the user does not have write perms but the user's group can write is effectively r/o for that user (but not for other members of the group)
I have a folder on my server on which I have changed the permissions to 777 (read, write and execute all) to allow users to upload their pictures.
So I want to know, what are the security risks involved in this?
I have implemented code to restrict what file formats can be uploaded, but what would happen if someone was to find the location of the directory, can this pose any threat to my server?
Can they start uploading any files they desire?
Thanks.
When users are uploading files to your server through a web form and some PHP script, the disk access on the server happens with the user id the web server is running under (usually nobody, www-data, apache, _httpd or even root).
Note here, that this single user id is used, regardless of which user uploads the file.
As long as there are no local users accessing the system by other means (ssh, for example), setting the upload directories permissions to 0777 would make not much of a difference -- appart from somebody exploiting a security vulnerability somewhere else in your system there's no one those permissions apply to anyway, and such an attacker would probably just use /tmp.
It is always good practice to set only those permissions on a file or directory that are actually needed. In this case that means probably something like:
drwxrws--- 5 www-data www-data 4096 Nov 17 16:44 upload/
I'm assuming that other local users besides the web server will want to access those files, like the sysadmin or a web designer. Add those users to the group your web server runs under and they don't need sudo or root privileges to access that directory. Also, the +s means that new files and directories in upload/ will automatically be owned by the same group.
As to your last question: just because an attacker knows where the directory is, doesn't mean he can magically make files appear there. There still has to be some sort of service running that accepts files and stores them there... so no, setting the permissions to 0777 doesn't directly make it any less safe.
Still, there are several more dimensions to "safety" and "security" that you cannot address with file permissions in this whole setup:
uploaders can still overwrite each others files because they all work with the same user id
somebody can upload a malicious PHP script to the upload directory and run it from there, possibly exploit other vulnerabilities on your system and gain root access
somebody can use your server to distribute child porn
somebody could run a phishing site from your server after uploading a lookalike of paypal.com
...and there are probably more. Some of those problems you may have addressed in your upload script, but then again, understanding of unix file permissions and where they apply comes usually waaaay at the beginning when learning about security issues, which shows that you are probably not ready yet to tackle all of the possible problems.
Have your code looked at by somebody!
By what means are these users uploading their pictures? If it's over the web, then you only need to give the web server or the CGI script user access to the folder.
The biggest danger here is that users can overwrite other users files, or delete other users files. Nobody without access to this folder will be able to write to it (unless you have some kind of guest/anonymous user).
If you need a directory that everyone can create files in, what you want is to mimic the permissions of the /tmp directory.
$ chown root:root dir; chmod 777 dir; chmod +t dir;
This way any user can create a file, but they cannot delete files owned by other users.
Contrary to what others have said, the executable bit on a directory in unix systems means you can make that directory your current directory (cd to it). It has nothing to do with executing (execution of a directory is meaningless). If you remove the executable bit, nobody will be able to 'cd' to it.
If nothing else, I would remove the executable permissions for all users (if not owner and group as well). With this enabled, someone could upload a file that looks like a picture but is really an executable, which might cause no end of damage.
Possibly remove the read and write permissions for all users as well and restrict it to just owner and group, unless you need anonymous access.
You do not want the executable bit on. As far as *nix goes, the executable bit means you can actually run the file. So, for example, php scripts can be uploaded as type JPEG, and then someone can run that script if they know the location and it's within the web directory.