Fallback Label Configuration Example

One of the new features added to the 2.6.25 release of the Linux Kernel was the ability to specify fallback peer labels using NetLabel. This made it possible for system administrators to specify a peer label to be used in the absence of a peer labeling protocol such as CIPSO or Labeled IPsec. In this post I'll try to provide a quick introduction on how to configure NetLabel to provide fallback peer labels.

The first step is to make sure you have all the right kernel and userspace bits in place. Any standard Linux distribution kernel 2.6.25 or greater that has NetLabel support should work. In addition to the kernel you will need to make sure the userspace utility, netlabelctl, supports the new configuration options; this requires netlabel_tools version 0.18 or greater. Once you have verified that you have the right versions installed and running, you can verify everything by running the following commands.

# netlabelctl -p mgmt version
NetLabel protocol version : 2
# netlabelctl -h
NetLabel Control Utility, version 0.18 (libnetlabel 0.18)
 Usage: netlabelctl [<flags>] <module> [<commands>]

   -h        : help/usage message
   -p        : make the output pretty
   -t <secs> : timeout
   -v        : verbose mode

 Modules and Commands:
  mgmt : NetLabel management
  map : Domain/Protocol mapping
    add default|domain:<domain> protocol:<protocol>[,<extra>]
    del default|domain:<domain>
  unlbl : Unlabeled packet handling
    accept on|off
    add default|interface:<DEV> address:<ADDR>[/<MASK>]
    del default|interface:<DEV> address:<ADDR>[/<MASK>]
  cipsov4 : CIPSO/IPv4 packet handling
    add trans doi:<DOI> tags:<T1>,<Tn>
    add pass doi:<DOI> tags:<T1>,<Tn>
    del doi:<DOI>
    list [doi:<DOI>]

The first command checks to see that the kernel speaks version 2 of the NetLabel control protocol which means that it understands the new fallback peer label configuration options. The second command verifies that you have netlabelctl version 0.18 installed and that it supports the fallback configuration commands that we will be using. If everything looks okay it is time to move on to the next step, building a test tool that we can use to verify the configuration. Obviously this isn't a strict requirement for configuring the fallback label mechanism but it is a nice way to verify that your configuration is correct. The test tool I will be using here is a simple little test program I wrote some time ago to test basic peer label functionality for IPv4 TCP sockets, I call it getpeercon_server. Once you have downloaded the C source file you will need to compile it with the following command, if you are on a Fedora system make sure you have the libselinux-devel package installed.

# gcc -o getpeercon_server getpeercon_server.c -lselinux
# ./getpeercon_server
usage: ./getpeercon_server <port>

As you can see the tool is very simple and takes a single argument, the TCP port to bind to a listen for connections. If you start the server on port 5000 and connect to it with telnet, netcat, or some other simple TCP application you should see something similar to what is shown below.

# ./getpeercon_server 5000
-> creating socket ... ok
-> listening on TCP port 5000 ... ok
-> waiting ... connect(,NO_CONTEXT)
Hello NetLabel Fans!
-> connection closed
-> waiting ...

In the example above we can see that a client connected from localhost,, and there was no peer label information provided with the connection, NO_CONTEXT. Now lets configure a fallback peer label for localhost and try it again. Adding a fallback label is relatively simple and can be done with a single command. However, the example below executes two commands. The first command adds a fallback label, "system_u:object_r:netlabel_peer_t:s0", to all traffic coming over the loopback interface, "lo", from localhost, The second command displays all of configured fallback labels; not a necessary step but helpful to ensure that you configured everything correctly.

# netlabelctl unlbl add interface:lo address: label:system_u:object_r:netlabel_peer_t:s0
# netlabelctl -p unlbl list
Accept unlabeled packets : on
Configured NetLabel address mappings (1)
 interface: lo
    label: "system_u:object_r:netlabel_peer_t:s0"

Now, lets try our simple connection test again with our newly established fallback label configuration.

# ./getpeercon_server 5000
-> creating socket ... ok
-> listening on TCP port 5000 ... ok
-> waiting ... connect(,system_u:object_r:netlabel_peer_t:s0)
Fallback Labels Are Really Cool!
-> connection closed
-> waiting ...

If you look at the output you will notice almost everything is the same except for one important thing, instead of NO_CONTEXT the test tool is displaying the fallback label we just configured. The kernel treats the NetLabel fallback label just the same as a normal peer label taken from a CIPSO tag or Labeled IPsec Security Association. This means that not only is the fallback label passed to applications that request it, it is also used when enforcing the LSM's network security policy; in this case SELinux. However, it is important to remember that the fallback labels are overridden by peer labeling protocols. As a result, if both fallback peer label information and Labeled IPsec peer label information is available then the kernel will use the Labeled IPsec peer label information and ignore the fallback peer labels. Fallback peer labels can be configured based on the network interface, network address, and netmask or just network address and netmask when the default network interface is chosen.

Before you start making use of the NetLabel fallback labels, there are a few things you should take into consideration. First, while the fallback functionality was included in Linux Kernel 2.6.25 there was a small bug which prevented some IPv6 fallback labels from being displayed correctly using the "netlabelctl -p unlbl list" command; this has been fixed in kernel 2.6.26. Second, Fedora has not yet adopted version 0.18 of netlabel_tools which means that you will need to download and build netlabelctl separately to take advantage of the new fallback functionality. A Red Hat bugzilla entry has been filed to get the latest netlabel_tools package included in Fedora.