Files can be “hidden” on a filesystem in several ways; some are just attempts to “hide in plain sight.” This is not hiding data in files (such as steganography) but rather hiding the mere existence of files themselves.
There are several ways to do this. The basic ideas are:
- Make reporting tools lie about the existence of files. This falls into the realm of “rootkits.” Utilities such as ls (and even the kernel itself) may modified in order to not report the existance a particular file. A way to avoid this is to use programs (during examination) that are statically linked and precompiled and saved to read-only media: such programs are immune to future modifications, except for kernel resources (if the kernel is taken over, then all other resources are suspect).
- Make files “disappear” in file listings. Files of this sort include “.. ” (two dots followed by a space) and other non-standard characters. These files look normal but are not. One way to see files like these is to use the ls command and its -b and -q options. The -b option prints the octal equivalent of non-printing characters; the -q option uses the “?” character instead of non-printing characters. The -F option may also be of use: a file with a terminating space will show up when the -F places its “type” character at the end of the file name.
- Open the file, then delete it. As long as the file is in use, its disk blocks remain in use, and the file remains available. No other processes can access it – or even see it – but the program (or programs) that opened it still has access to it. Of course, this only works for running programs, but files disappear this way almost entirely. To see the files, use the utility lsof with the +L1 option. The +L option lists files with less than that many links; thus, +L1 lists files with less than one link (or zero links).
As mentioned previously, a static build of all relevant utilities to CDROM can be a very useful tool during investigation of a possible system break-in. All general reporting tools should be included: ls, ps, top, etc. Also included should be any desired programming interpreters: ksh, perl, ruby, tclsh, lua (whatever one finds useful). Also included should be any more intensive tools as well: tcpdump, nmap, etc.