Authentication Gateway HOWTO
zornnh@musc.edu
Revision HistoryRevision 0.052002-05-10Revised by: nhzRevision 0.042002-02-28Revised by: nhzRevision 0.032001-09-28Revised by: nhzRevision 0.022001-09-28Revised by: KETRevision 0.012001-09-06Revised by: nhz
There are many concerns with the security of wireless networks and public access areas such as libraries or dormitories. These concerns are not met with current security implementations. A work around has been proposed by using an authentication gateway. This gateway addresses the security concerns by forcing the user to authenticate in order to use the network.
----; Table of Contents; 1. Introduction: ; 1.1. Copyright Information; 1.2. Disclaimer; 1.3. New Versions; 1.4. Credits; 1.5. Feedback; 2. What is needed: ; 2.1. Netfilter; 2.2. Software for dynamic Netfilter rules.; 2.3. DHCP Server; 2.4. Authentication mechanism; 2.5. DNS Server; 3. Setting up the Gateway Services: ; 3.1. Netfilter Setup; 3.2. Dynamic Netfilter rules.; 3.3. DHCP Server Setup; 3.4. Authentication Method Setup; 3.5. DNS Setup; 4. Using the authentication gateway; 5. Concluding Remarks; 6. Additional Resources; 7. Questions and Answers
With wireless networks and public acces areas it is very easy for an unauthorized user to gain access. Unauthorized users can look for a signal and grab connection information from the signal. Unauthorized users can plug their machine into a public terminal and gain access to the network. Security has been put in place such as WEP, but this security can be subverted with tools like !AirSnort?. One approach to solving these problems is to not rely on the wireless security features , and instead to place an authentication gateway in front of the wireless network or public access area and force users to authenticate against it before using the network. This HOWTO describes how to set up this gateway with Linux.
This document is copyrighted (c) 2001 Nathan Zorn. Permission is
granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.1 or any
later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts. A copy of the license is available at
http://www.gnu.org/copyleft/fdl.html
If you have any questions, please contact
`zornnh@musc.edub
No liability for the contents of this documents can be accepted. Use the concepts, examples and other content at your own risk. As this is a new edition of this document, there may be errors and inaccuracies, that may of course be damaging to your system. Proceed with caution, and although this is highly unlikely, the author(s) do not take any responsibility for that.
All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.
Naming of particular products or brands should not be seen as endorsements.
You are strongly recommended to take a backup of your system before major installation and backups at regular intervals.
The newest release of this document can be found at
. Related HOWTOs can be found at the
Linux Documentation Project homepage.
Jamin W. Collins
Kristin E Thomas
Logu
Feedback is most certainly welcome for this document. Without your submissions and input, this document wouldn't exist. Please send your additions, comments and criticisms to the following email address : `zornnh@musc.edub.
This section describes what is needed for the authentication gateway.
The authentication gateway uses Netfilter and iptables to manage the
firewall. Please see the
Netfilter HOWTO
.
One means to insert and remove Netfilter rules is to use pam_iptables. This is a pluggable authentication module (PAM) written by Nathan Zorn that can be found at
. This PAM module allows users to use ssh and telnet to authenticate to the gateway.
Another means to dynamically remove and create Netfilter rules is to use !NocatAuth?. !NocatAuth? can be found at
.
The authentication gateway will act as the dynamic host configuration protocol (DHCP) server for the public network. It only serves those requesting DHCP services on the public network. I used the
ISC DHCP Server
.
The gateway can use any means of PAM authentication. The authentication mechanism the Medical University of South Carolina uses is LDAP. Since LDAP was used for authentication, the pam modules on the gateway box were set up to use LDAP. More information can be found at
. PAM allows you to use many means of authentication. Please see the documentation for the PAM module you would like to use. For more information on other methods, see
pam modules
.
If !NocatAuth? is used, an authentication service needs to be setup. The !NocatAuth? authentication service supports authentication with LDAP,RADIUS,MySQL,and a password file. More information can be found at
.
The gateway box also serves as a DNS server for the public network. I installed Bind, and set it up as a caching nameserver. The rpm package caching-namserver was also used. This package came with Red Hat.
This section describes how to setup each piece of the authentication gateway. The examples used are for a public network in the 10.0.1.0 subnet. eth0 is the interface on the box that is connected to the internal network. eth1 is the interface connected to the public network. The IP address used for this interface is 10.0.1.1. These settings can be changed to fit the network you are using. Red Hat 7.1 was used for the gateway box, so a lot of the examples are specific to Red Hat.
To setup netfilter the kernel must be recompiled to include netfilter support. Please see the Kernel-HOWTO for more information on configuring and compiling your kernel.
This is what my kernel configuration looked like.
#
# CONFIG_PACKET=y
CONFIG_NETFILTER=y CONFIG_NETFILTER_DEBUG=y CONFIG_FILTER=y CONFIG_UNIX=y CONFIG_INET=y CONFIG_IP_MULTICAST=y
# CONFIG_IP_NF_CONNTRACK=y CONFIG_IP_NF_FTP=y CONFIG_IP_NF_IPTABLES=y CONFIG_IP_NF_MATCH_LIMIT=y CONFIG_IP_NF_MATCH_MAC=y CONFIG_IP_NF_MATCH_MARK=y CONFIG_IP_NF_MATCH_MULTIPORT=y CONFIG_IP_NF_MATCH_TOS=y CONFIG_IP_NF_MATCH_TCPMSS=y CONFIG_IP_NF_MATCH_STATE=y CONFIG_IP_NF_MATCH_UNCLEAN=y CONFIG_IP_NF_MATCH_OWNER=y CONFIG_IP_NF_FILTER=y CONFIG_IP_NF_TARGET_REJECT=y CONFIG_IP_NF_TARGET_MIRROR=y CONFIG_IP_NF_NAT=y CONFIG_IP_NF_NAT_NEEDED=y CONFIG_IP_NF_TARGET_MASQUERADE=y CONFIG_IP_NF_TARGET_REDIRECT=y CONFIG_IP_NF_NAT_FTP=y CONFIG_IP_NF_MANGLE=y CONFIG_IP_NF_TARGET_TOS=y CONFIG_IP_NF_TARGET_MARK=y CONFIG_IP_NF_TARGET_LOG=y CONFIG_IP_NF_TARGET_TCPMSS=y
Once netfilter has been configured, turn on IP forwarding by
executing this command.
echo 1 b /proc/sys/net/ipv4/ip_forward
To make sure ip forwarding is enabled when the machine restarts
add the following line to /etc/sysctl.conf.
net.ipv4.ip_forward = 1
If !NocatAuth? is being used, you can skip to the !NoCatAuth? gateway setup section.
iptables needs to be installed. To install iptables either use a package from your distribution or install from source. Once the above options were compiled in the new kernel and iptables was installed, I set the following default firewall rules.
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A INPUT -i eth0 -m state --state NEW, INVALID -j DROP iptables -A FORWARD -i eth0 -m state --state NEW, INVALID -j DROP iptables -I FORWARD -o eth0 -j DROP iptables -I FORWARD -s 10.0.1.0/24 -d 10.0.1.1 -j ACCEPT
The above commands can also be put in an initscript to start up when the server restarts. To make sure the rules have been added
iptables -v -t filter -L
To save these rules I used Red Hat's init scripts.
/etc/init.d/iptables save
/etc/init.d/iptables restart
Now the gateway box will be able to do network address translation (NAT), but it will drop all forwarding packets except those coming from within the public network and bound for the gateway.
This section describes how to setup the software needed to dynamically insert and remove Netfilter rules on the gateway.
The PAM session module that inserts the firewall rules is
needed to allow forwarding for the authenticated client. To set it up simply get the source and compile it by running the following commands.
gcc -fPIC -c pam_iptables.c
ld -x --shared -o pam_iptables.so pam_iptables.o
You should now have two binaries called
pam_iptables.so and pam_iptables.o. Copy pam_iptables.so to /lib/security/pam_iptables.so.
cp pam_iptables.so /lib/security/pam_iptables.so
Now install the firewall script to /usr/local/auth-gw.
mkdir /usr/local/auth-gw
cp insFwall /usr/local/auth-gw
The chosen authentication client for the gateway was ssh so we added the
following line to /etc/pam.d/sshd.
session required /lib/security/pam_iptables.so
Now, when a user logs in with ssh, the firewall rule will be added.
To test if the pam_iptables module is working perform the following
steps:
#
Log into the box with ssh.
# #
Check to see if the rule was added with the command iptables -L -v.
# #
Log out of the box to make sure the rule is removed.
This section describes the process of setting up the !!NocatAuth? gateway. To setup !!NocatAuth? get the source and install with the following steps.
Make sure gpgv is installed. gpgv is a PGP signature verifier.
It is part of gnupg and can be found at http://www.gnupg.org/download.html.
Unpack the !!NocatAuth? tar file.
tar xvzf !!NocatAuth?-x.xx.tar.gz
If you do not want !!NoCatAuth? to be in the directory /usr/local/nocat, edit the Makefile and change INST_PATH to the directory you would like !!NoCatAuth? to reside.
Next build the gateway.
cd !NoCatAuth?-x.xx
make gateway
Edit the /usr/local/nocat.conf file. Please see the INSTALL documentation for details on what is required in the conf
#
Verbosity 10
http://www.itlab.musc.edu/AuthServiceURL 
https://$!AuthServiceAddr/cgi-bin/login
https://$!AuthServiceAddr/forms/logout.htmlNow you should be able to start the gateway. If any problems occur, please see the INSTALL documentation in the unpacked
I installed DHCP using the following dhcpd.conf file.
subnet 10.0.1.0 netmask 255.255.255.0 {
option routers 10.0.1.1; option subnet-mask 255.255.255.0; option broadcast-address 10.0.1.255; option domain-name-servers 10.0.1.1; range 10.0.1.3 10.0.1.254; option time-offset -5; # Eastern Standard Time default-lease-time 21600; max-lease-time 43200; }
The server was then run using eth1 , the interface to the public net.
/usr/sbin/dhcpd eth1
Authentication with PAM and a !NoCatAuth? authentication service is described. Both examples are done with LDAP. Other means of authentication besides LDAP can be used. Please read the documentation for PAM and !NoCatAuth? to find the steps to use another authentication source.
As indicated in previous sections, I've set this gateway up to use LDAP for authenticating. However, you can use any means that PAM allows for authentication. See Section 2.4 for more information.
In order to get PAM LDAP to authenticate, I installed OpenLDAP and configured it with the following in /etc/ldap.conf.
- Your LDAP server. Must be resolvable without using LDAP.
host itc.musc.edu
base dc=musc,dc=edu ssl no
The following files were used to configure PAM to do the LDAP authentication. These files were generated by Red Hat's configuration utility.
; /etc/pam.d/system-auth was created and looked
auth required /lib/security/pam_env.so auth sufficient /lib/security/pam_unix.so likeauth nullok auth sufficient /lib/security/pam_ldap.so use_first_pass auth required /lib/security/pam_deny.so account required /lib/security/pam_unix.so account [default=ok user_unknown=ignore service_err=ignore system_err=ignore? /lib/security/pam_ldap.so password required /lib/security/pam_cracklib.so retry=3 password sufficient /lib/security/pam_unix.so nullok use_authtok password sufficient /lib/security/pam_ldap.so use_authtok password required /lib/security/pam_deny.so session required /lib/security/pam_limits.so session required /lib/security/pam_unix.so session optional /lib/security/pam_ldap.so
; Then the following
auth required /lib/security/pam_stack.so service=system-auth auth required /lib/security/pam_nologin.so account required /lib/security/pam_stack.so service=system-auth password required /lib/security/pam_stack.so service=system-auth session required /lib/security/pam_stack.so service=system-auth
session required /lib/security/pam_iptables.so debug session optional /lib/security/pam_console.so
It is recommended to install the !NoCatAuth? Service on another server besides the gateway. A seperate server was used in my examples. In order to setup a !NoCatAuth? Service, you will need the following software:
#
An SSL enabled webserver, preferably with a registered SSL cert. I used Apache + mod_ssl.
# #
Perl 5 (5.6 or better recommended)
# #
Net::LDAP, Digest::MD5, DBI, and DBD::MySQL perl modules (get them from CPAN) The module you need depends on what authentication source you are going to use. In my example Net::LDAP is used as the authentication means.
# #
Gnu Privacy Guard (gnupg 1.0.6 or better), available at
http://www.gnupg.org/download.html
#
To install unpack the tar file.
$ tar zvxf !NoCatAuth?-x.xx.tar.gz
If you would like to change the path that !NoCatAuth? resides , edit the
Makefile and change INST_PATH to the desired directory.
Next run the command:
make authserv This installs everything in /usr/local/nocat or what you changed INST_PATH to.
Then run make pgpkey
The defaults should be fine for most purposes. IMPORTANT: do NOT enter a passphrase! Otherwise, you will get strange messages when the auth service attempts to encrypt messages, and tries to read your passphrase from a non-existent tty
Edit /usr/local/nocat/nocat.conf to fit your situation.
#
http://www.itlab.musc.edu/#
#
Make sure /usr/local/nocat/pgp is owned by the web server user. (ie..nobody or www-data)
Add etc/authserv.conf to your apache httpd.conf file.
Include /usr/local/nocat/etc/authserv.conf
Copy your /usr/local/nocat/trustedkeys.pgp to the gateway. Restart apache and try it out. Please see the !NoCatAuth? documentation for more information. It can be found in docs/ in the unpacked !NoCatAuth? directory.
I installed the default version of Bind that comes with Red Hat 7.1, and the caching-nameserver RPM. The DHCP server tells the machines on the public net to use the gateway box as their nameserver.
To use the authentication gateway, configure your client machine to use DHCP. Install a ssh client on the box and ssh into the gateway. Once you are logged in, you will have access to the internal network.
As long as you stayed logged in, you will have access. Once you log out, access will be taken away.
To use the authentication gateway with !!NoCatAuth? installed, configure your client machine to use DHCP. Install a web browser such as Mozilla. Start up the web browser. The browser should be redirected to the authentication screen. Submit your username and password and a screen will pop up explaining that you are authenticated to the network and to keep the window open to remain authenticated. Click logout or close the window to end the session.
has a web based client.
This is just a collection of what I believe are the most common
questions people might have. Give me more feedback and I will turn this section into a proper FAQ.
I have not come up with a good answer or solution to this problem. Logu has contributed some modifications to pam_iptables and a set of other tools to solve this problem. These tools can be found in the contrib directory with pam_iptables.
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