Is there a way to designate a file append only?

abefroman · 11-23-2014, 07:54 AM

pan64 · 11-23-2014, 08:45 AM

can you explain it better? "file append" is not good enough?

abefroman · 11-23-2014, 08:58 AM

Quote:

Originally Posted by pan64

can you explain it better? "file append" is not good enough?

Forgot a comma, sorry.

Is there a way to designate a file, as append only? IE. it can only be appended to.

ntubski · 11-23-2014, 10:51 AM

As far as I know, the Linux/ Unix filesystem does not distinguish between appending and writing, so there is no way to do this directly. I stand corrected

You could allow write permission only for a dedicated user, who would run a program that appends the data it reads from a socket/pipe. Effectively, write permission on the pipe == append permission on the file.

rknichols · 11-23-2014, 01:52 PM

With SElinux you can grant append-only capability if you are willing to go through the work to create a new type and set up all the needed access permissions. Of course you would have to be running in enforcing mode for that to be effective.

Kenhelm · 11-24-2014, 08:40 AM

Setting the 'a' attribute with 'chattr' can make a file append-only.
chattr +a filename

http://en.wikipedia.org/wiki/Chattr

Quote:

chattr and lsattr utilities on Linux and the attributes they manipulate were originally specific to the Second Extended Filesystem family (ext2, ext3, ext4), and are available as part of the e2fsprogs package. The functionality has since been extended to many other Linux file systems, including XFS, ReiserFS, JFS, OCFS2 and others, although not all file systems support all flags.

http://www.linuxintheshell.org/2013/...tr-and-chattr/

Quote:

a - append only: Writing to a file will only allow the file to be opened in append mode for writing. That is you cannot redirect output to overwrite the file, only append to it. Most normal file edit operations, like opening the file with a text editor, will most likely fail as the program will attempt to overwrite the file with the changes and "permission denied" will be displayed. This attribute can only be set by an account with superuser privileges.

sundialsvcs · 11-25-2014, 06:59 AM

Also, generally speaking, this sort of file-access has pretty severe limitations. Consider, for example, the problem of what-to-do if more than one process wishes to append to the file at the same time. Given that the operating system will, of course, do it properly, the order of the records that would get written-to the file would of course be random. None of the programs could, so to speak, "see" what they are writing. Construction of any sort of data-structure would be impossible. Really, it would only be useful for some kind of log-file. And if you want to do that, a simple "logging daemon" that accepts network-connections or pipes is a much more satisfactory solution. As long as the daemon can be trusted to do its job correctly, it alone defines what sort of requests it will and won't respond-to.