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Monthly Articles: Security
## Taking Advantage of TCP Wrappers
## Dan Langille <>

The tcp_wrapper port installs a great little daemon by the name of tcpd. This port can be used to enhance the security of your site. It is easy to use and provides a very useful log. It has become a basic part of what people use to secure incoming connections.

This article how I installed tcpd under FreeBSD. At it's lowest level, this port can be useful as a tool for monitoring incoming requests but it can also filter such requests.

What are TCP wrappers?

The port description contains the following excerpt:

	With this package you can monitor and filter incoming
	EXEC, TFTP, TALK, and other network services.

But I'm sure it can be used for more than just that.

tcpd is the daemon in question. It works on one-off programs like telnet which start running when inetd sees an incoming request on a port. It won't work for programs that run all the time. In short, tcpd is told by inetd what program to start. tcpd then checks starts other applications for you but first checks the security to see if the application should start.


For full port installation instructions, please see in the FreeBSD handbook.

Here's what I did:

	# cd /usr/ports/security/tcp_wrapper
	# make
	# make install


I recommend that you read the README file which should be located at /usr/ports/security/tcp_wrapper/work/tcp_wrappers_7.6. It will describe the port in detail and explain more fully the steps I will outline below.

I modified /etc/inetd.conf as follows:

	telnet  stream  tcp     nowait  root    /usr/libexec/telnetd telnetd


	telnet  stream  tcp     nowait  root    /usr/local/libexec/tcpd telnetd

Note that under recent installations (e.g. 3.1-STABLE), the file location was /usr/local/libexec/tcpd. Watch your make install for details on where the file was actually installed and amend /etc/inetd.conf as appropriate.

In order to test that the above change was working, I first did a test of the finger command. Try finger <user> where <user> is a user on your system. If you get "no such user", then perhaps the user has a .nofinger file in their home directory.

After testing that finger worked, I did a HUP to inetd. Then I did another finger. It still worked and thus proved the changes were successful. I did similar entries in /etc/inetd.conf for other daemons.


The main purpose of using tcp_wrapper is to provide log information. Where does that information go? Well, if you read the README file, you'll see that it goes to syslogd. I strongly urge you to read the manpage for syslog.conf before attempting to modify it.

Here's what you can expect to find if you have tcp wrappers running correctly for telnet:

  Jan 10 15:49:41 ngatoto telnetd[1758]: connect from wocker.dvl-software.com
  Jan 10 15:49:58 ngatoto login: login from wocker.dvl-software.com on ttyp2 as mike

The first line indicates that someone telnetted to the machine ngatoto. The second line indicates that they managed to login as mike. There are some issues associated with getting the above messages into your log file. I found it very difficult to get the above logging messages where I wanted them. The key is /etc/syslog.conf and the entries therein. The following file will put the messages into /var/log/auth.log.

	#       Spaces are NOT valid field separators in this file.
	#       Consult the syslog.conf(5) manpage.
	*.err;kern.debug;auth.notice;mail.crit          /dev/console
	*.notice;kern.debug;lpr.info;mail.crit;news.err /var/log/messages
	auth.*                                          /var/log/auth.log
	mail.info                                       /var/log/maillog
	lpr.info                                        /var/log/lpd-errs
	cron.*                                          /var/cron/log
	*.err                                           root
	*.notice;news.err                               root
	*.alert                                         root
	*.emerg                                         *
	# uncomment these if you're running inn
	# news.crit                                     /var/log/news/news.crit
	# news.err                                      /var/log/news/news.err
	# news.notice                                   /var/log/news/news.notice
	*.*                                             /var/log/slip.log
	*.*                                             /var/log/ppp.log

The only line I added is in bold and starts with "auth". This directs the log messages, as displayed in the example above, to the file /etc/auth.log. Remember to restart syslogd after making any change to /etc/syslog.conf. Also, you must manually create the log file because syslogd expects the file to exist. You can do this with this command:

	touch /var/log/auth.log

And because you have just created a new log file, you may wish to add a new entry to /etc/newsyslog.conf and have it rotated. See the man page for newsyslog for more details.

Restricting connections

There are two files should know about. Both are located in /usr/local/etc/hosts. These files are hosts.allow and hosts.deny. Here's what I have in my hosts.allow.

	ALL: theirdomain.com: ALLOW
	telnet: ALLOW

NOTE: Do not include the line numbers as indicated above. They are for reference only.

Line 1 allows all connections from from theirdomain.com.

Line 2 allows all machines in the 192.168.0.* network to telnet. Here's a extract from the man pages which explains the numbering system:

	An expression of the form `n.n.n.n/m.m.m.m' is
	interpreted as a `net/mask' pair.  A host address is
	matched if `net' is equal to the bitwise AND of the
	address and the `mask'.  For example, the net/mask
	pattern `' matches every
	address in the range `' through

Line 3 allows everyone to connect to the ftp server.

Line 4 denies access to everyone by default (if not explicity allowed above). Basic security practices dictate that you should deny everything and then explicity permit the things you want. The above example follows that advice. For more detail on this file, please see man 5 hosts_access.


tcpd comes with a couple of nice utilities, both of which I was recently informed off. They sound like very good companion tools. The following two sections contain description from the respective man pages for these tools.

These tools are found at /usr/local/sbin.


	tcpdchk examines your tcp wrapper configuration and reports
	all potential and real problems it can find.  The program
	examines the tcpd access control files (by default, these are
	/usr/local/etc/hosts.allow and /usr/local/etc/hosts.deny), and
	compares the entries in these files against entries in the
	inetd or  tlid network configuration files.

	tcpdchk reports problems such as non-existent pathnames; 
	services that appear in tcpd access control rules, but are not
	controlled by tcpd; services that should not be wrapped;
	non-existent host names or non-internet address forms;
	occurrences of host aliases instead of official host names;
	hosts with a name/address conflict; inappropriate use of
	wildcard patterns; inappropriate use of NIS netgroups or
	references to non-existent NIS netgroups; references to 
	non-existent options; invalid arguments to options; and so on.

	Where possible, tcpdchk provides a helpful suggestion to fix
	the problem.


	tcpdmatch predicts how the tcp wrapper would handle a specific
	request for service.  Examples are given below.

	The program examines the tcpd access control tables (default 
	/usr/local/etc/hosts.allow and /usr/local/etc/hosts.deny) and
	prints its conclusion.  For maximal accuracy, it extracts 
	additional information from your inetd or tlid network 
	configuration file.

	When tcpdmatch finds a match in the access control tables, it
	identifies the matched rule.  In addition, it displays the
	optional shell commands or options in a pretty-printed format;
	this makes it easier for you to spot any discrepancies between
	what you want and what the program understands.

Well, that was that.

This should get you started. Install the port. Monitor the logs. It should make your security issues much easier to deal with and allow you to concentrate on other issues.

- Dan

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Last modified: $Date: 1999/06/26 05:34:51 $
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