Getting Started with the Linux Intrusion Detection System
With increasing usage of Linux in various computing environments, a lot of security vulnerabilities are being discovered in GNU/Linux-based systems. Due to the open nature of application development in the Open Source world, a lot of vulnerabilities are being addressed very quickly. But, it may happen that a patch is not addressed in a timely manner, and in the meantime, all the systems running the application are exposed. Malicious users can possibly gain root privileges and wreak havoc with these systems. This is where the Linux Intrusion Detection System (LIDS) comes to the rescue.
LIDS is a patch to the Linux kernel; it implements access control and a reference monitor. LIDS is configured with its two admin tools, lidsconf and lidsadm.
lidsadm is the utility that allows you to disable LIDS in a terminal, so that you can set various settings, which LIDS, when enabled, won't allow you to do, and you can view the current status of your LIDS installation with this tool.
lidsconf is the tool that allows you to add and remove access control to certain files, which can be binaries or any other files. LIDS refers to these files as objects, and the capabilities we allow or disallow are referred to as subjects. LIDS overrides things like filesystem permissions. You can literally use LIDS to make it impossible to access virtually any object, whether it's a file, raw device, memory or I/O, even if you're trying to access the object as the root user.
In short, LIDS is a complete security model implementation for the Linux kernel.
The developers of LIDS have included installation instructions in the INSTALL file. However, I describe the main tasks in this article.
The stable releases of LIDS are created against a vanilla source of the Linux kernel. It is recommended that the LIDS patch be applied only to the original kernel source, not to the distribution-specific source, as it may lead to various compilation errors, as most distributions customize the kernel for their own use. LIDS is known to have problems when used on non-i386 architectures.
For example, lids-2.2.1-2.6.13.tar.gz should be applied to the 2.6.13 kernel.
After patching the kernel with:
patch -p1 /dir_to_the_patch_file/patch-lids-2.2.1-2.6.13
You can run make [x/menu]config and select the LIDS options from the security section and compile the kernel with:
make make modules_install
(if you configured any parts of the kernel as modules).
Copy the bzImage from /kernelpath/arch/i386/boot to your /boot directory, and re-initialize your bootloader. Restart into your LIDS-enhanced kernel.
You can see the status of your LIDS installation by typing:
If you get an error, LIDS was not installed into the kernel; check your kernel configurations and recompile.
Before we set access controls for various server applications, here is the general syntax of lidsconf:
lidsconf -A [-s subject] -o object [-d] [-t from-to] [-i level] -j ACTION
The subject is a program upon which a capability is added. The object can be a binary, directory, socket name or a capability.
The -d switch tells LIDS that the domain is an exec domain. The -t lets you set a specific time dependency for the capability and -i defines the inheritance level.
The -j switch is an action that can be one of the following:
DENY: denies access to the object.
READONLY: sets the object to read-only.
APPEND: mostly used for logs, this allows a certain program to append only that file, not remove it.
WRITE: allows other binaries to write on the file.
GRANT: used in conjunction with a capability, used to grant the subject a capability.
IGNORE and DISABLE: two options that allow you to disable the setting of any permission on a certain object and disable some extension features, respectively.
The capabilities LIDS supports are the following, as can be seen by typing:
lidsadm -h|grep CAP
CAP_DAC_OVERRIDE: DAC access.
CAP_DAC_READ_SEARCH: DAC read.
CAP_FOWNER: owner ID, not equal user.
ID CAP_FSETID: effective user ID, not equal owner.
ID CAP_KILL: real/effective ID, not equal process.
ID CAP_SETGID: set*gid(2).
CAP_SETPCAP: transfer capability.
CAP_LINUX_IMMUTABLE: immutable and append file attributes.
CAP_NET_BIND_SERVICE: binding to ports below 1024.
CAP_NET_BROADCAST: broadcasting/listening to multicast.
CAP_NET_ADMIN: interface/firewall/routing changes.
CAP_NET_RAW: raw sockets.
CAP_IPC_LOCK: locking of shared memory segments.
CAP_IPC_OWNER: IPC-ownership checks.
CAP_SYS_MODULE: insertion and removal of kernel modules.
CAP_SYS_RAWIO: ioperm(2)/iopl(2) access.
CAP_SYS_PACCT: configuration of process accounting.
CAP_SYS_ADMIN: tons of admin stuff.
CAP_SYS_RESOURCE: sets resource limits.
CAP_SYS_TIME: sets system time.
CAP_SYS_TTY_CONFIG: tty configuration.
CAP_MKNOD: mknod operation.
CAP_LEASE: taking leases on files.
CAP_HIDDEN: hidden process.
CAP_KILL_PROTECTED: kill protected programs.
CAP_PROTECTED: protect the process from signals.
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