HIR Issue 10: Setting up your home-net
by Axon
- I. Introduction
- II. Hardware Requirements
- III. Software Requirements
- IV. Intra-Net host setup (getting your server onto the 'net securely)
- V. Intra-Net workstations setup (getting your computers to talk)
- VI. Testing
- VII. Advanced Firewalling
- IIX. Closing
I. Introduction
This is going to be a general tutorial on how to be *REALLY* connected at home. This article is mostly geared toward the people with 2-10 (or more) computers at home, or for small businesses with a relatively tiny private network. This isn't guaranteed to work, and is based totally from my experience setting this system up in my own home. This includes cross-platform ability (We have Win95/98/Linux at my house).
II. Hardware Requirements:
You'll need:
- At least 2 computers
- A network card for each computer to be connected together (try to make them the same brand, and make sure they're all the same media. I prefer 10BaseT, but choose your own here)
- Cables to hook the computers together, and a hub (or a crossover cable if just using 2 computers)
- Maybe a crimping tool and hardware to crimp your own cables
- The Host Machine should be a 486/33 or faster with at least 32 megs of RAM. It should probably have at least 200 megs of hard drive space, but since it can also be used as the Intra-Net server and a workstation at the same time, it would be to your benefit to make this a more powerful machine. This will be running Linux. (This example will be done using Red Hat 5.2, but I've successfully done this with Other UNIX-like OS's such as FreeBSD, which is what I am currently using.)
- The workstations need to be able to run an OS capable of recognizing the network card and using a TCP/IP stack (Windows/Linux/MAC-OS all work)
III. Software Requirements
- The Workstations need an OS that can get a tcp/IP stack over a Network Card.
- The Host machine must be using some version of Linux (Redhat 5.2 works, and I'll be making reference to some tools that only RedHat has, but an experienced Linux user should be able to figure out how to get any other Linux Distro to get this to work. RedHat was chosen for this article because of it's ease of setup (with GUI tools) and the fact that the kernel was built with the proper options for masquerading. This would just be the "friendliest" way to set things up. I do not support any one distribution over the other as far as which one is "best", as it's all personal preference. I'm actually using FreeBSD right now (I know, it's not Linux), but it is a little more difficult to configure for this purpose, and requires a kernel recompile among starting and configuring natd (Network Address Translation) services. It's a different approach, but the results are even better. E-mail me if you want a copy of my FreeBSD kernel Configuration file.
IV. Intra-net host setup
The goals of our intra-net host will be the following:
- Act as a network management workstation
- Act as a network print server (if desired)
- Act as a network file server (if desired/needed)
- Access the Internet (through a modem/ppp in this example. Doing this through a cable-modem or DSL can be done, as well, but almost always requires a second Network Card)
- Act as a packet-filtering firewall to isolate intranet from the Internet.
- Act as a gateway, so that all intra-net computers have shared, secure Internet access when the Host system is online. Workstations should reside behind this machine as a firewall to prevent network attacks.
- Be as secure as possible, as the Host system will be your only line of defense. Once someone's compromised your Host, that's pretty much it, and they can do more damage once inside your intra-net.
In order to act as a network management workstation, the Host needs to have a few programs installed:
- ipfwadm (if you're using kernel 2.0.x. This comes with RedHat 5.x and most other kernel 2.0.x based linux distributions)
- ipchains (if you're using kernel 2.2.x. This comes with RedHat 6.0, and most other 2.2.x based linux distributions. As a side note, I would strongly recommend compiling a fairly new version of kernel 2.2.x, even on your RedHat 5.2 installation. If you don't know how to do this, just stick with RedHat's default kernel.)
- tcp-wrappers (also known as inetd. This allows us to restrict incoming access from remote networks). Comes with most distributions. You may take a serious look at a package called xinetd, as well. It has quite a few very tasty features (ident lookups on all connections, etc)
- Secure Shell 2.x, which will allow you to make secure connections to the host system from outside, if you must do so. If you're somewhere else on the Internet (I.E. at work) and you want access to to your home network, You will have to make a connection to the host system before you have access to any other system on your network.
- Some sniffers and analysis tools are nice to have, for troubleshooting the network. I recommend tcpdump (comes with many distributions) and iptraf (available all over the web, check around)
- Kernel must be compiled with IP Forwarding enabled, masquerade enabled, etc. I'm not going to quite get into setting up masquerading in the kernel. Read the IP-Masquerade mini-HOWTO, which is flawless at describing the basics of setting up masquerade stuff. We'll get around to some changes you'll make to the examples in a bit.
- "setip" is optional. It will upload a file called ".ip" to the ftp location of your choice. This is good if you want to be able to remotely access your network at home from a remote location, but have dynamic IP or DHCP, and therefore have no clue what IP you might have at any given moment.
- I also use the "Deception Finger Daemon" on my firewall/host at home. it can generate false user reports to people who attempt to use finger to determine what users are logged on or exist on your network. "dfingerd" also can make syslog entries, so that you know when you're someone's taking a peek at you. It's fairly easy to install, and the documentation that it comes with is very detailed.
The first thing we want to do is to get the host machine to be able to connect to the Internet. This can be done with RedHat's "netcfg" utility, used in the X-Window environment, or by manually setting up the chat script and ppp options for your ppp interface (these files are located differently between distributions) You will need to supply a phone number, user name, password, and you will probably want to make it so "Any user can control this interface", which is a checkbox in netcfg. This will be so that anyone on your network will be able to get the host system onto the Internet (and when we're done, this fact will mean that your entire network has 'Net access). Test out your Internet Connection. Telnet to something, or type "lynx hir.chewies.net" and surf the HiR Distro Site. =] Once that's working, you may want to add a user account for all people to use the home-server, or just a single account for the users, that is shared and can bring up and take down the I-Net connection. After that, you should be ready for the next step:
Setting up your ethernet card... Hopefully it's already installed, but if it isn't, halt your Host machine, install it, and power back up. We need to go back into X and use netcfg again (or whatever it is your distribution uses for setting up Network Cards). Add an "ethernet" interface (eth0), and set it up like this:
IP-Address: 192.168.1.1 (fake IP used for Intra-Nets)
Netmask: 255.255.255.0
gateway: (leave blank) In netcfg's "routing" tab, select "Network Packet Forwarding" and don't worry about a route, as the ppp interface will become the default route once it's connected.
You should now have created a class C intranet. Now to secure the server... The server is your golden goose. The rest of your machines are hidden from view from the outside world, and your server is the only machine that can be seen both by your side of the network and the rest of the world (Via the Internet connection). Once your server has been compromised, anyone who knows what they are doing, will see all your other machines, and potentially attack them, using your own server's recources! Potentially, they could figure out all the passwords that your users use, they could possibly erase all the files on open file shares, and maybe even abuse your paper and toner/ink supply on connected network printers. That's an insult.
The first thing we're going to do is lock down your open ports with tcp-wrappers. inetd is the tcp-wrapper program that checks for authorization to use a service. Not all of your services run through inetd (mostly sendmail, httpd, and secure shell). Inetd protects services such as telnet, finger, shell, login, auth, etc. When someone attempts to use any of these services, inetd checks /etc/hosts.allow and /etc/hosts.deny (respectively) for a line containing an inclusion for the service name (either the daemon executable name such as "in.telnetd", or "in.fingerd". "ALL" includes all service names).
My home server's hosts.allow file allows all the workstations behind the masquerade to access my hosts full potential, but locks the rest of the internet out, except for dfingerd, that fake finger daemon I was talking about earlier.
ALL : 192.168.1.0/255.255.255.0
dfingerd : ALL As soon as anyone connects to my machine, say with telnet, it checks hosts.allow to see if they have been granted access. If they're on the Internet (not on my intra-net), they aren't on this list, so it checks to see if they have been disallowed in hosts.deny.
My hosts.deny file is simple:
ALL : ALL This makes sure that only things granted in hosts.allow will make it to my system. Note that the deny ALL doesn't cover ssh, so I can still get back in over an encrypted connection if I know my IP address and have a user/password. Also notice if any remote machine tries to use dfingerd, they will be allowed, because inetd doesn't check hosts.deny if there is a rule in hosts.allow that applies to that connection.
If you're really in for security, I would advise trying to setup xinetd, which will attempt to find the username of the person who is connecting to the port my making use of their local machine's ident service (if available, which it is on most UNIXes and if they have mIRC running at the time, too). It's a little more difficult to set up, but it still uses the hosts.[allow|deny] convention.
Remember, anything that starts up outside of "inetd" is not protected by this modification, so don't rely specifically on it. Some daemons will try to read the hosts.allow/deny files as well, so there are a few exceptions. In general, you will want to keep things as closed up as possible, allowing telnet and ftp from machines on your private Intra-Net. Also, TCP-Wrappers are all considerably more open to attacks involving spoofed packets, and it may be possible for someone to make a packet with a source address of one of your trusted machines, but coming from the outside world. Since TCP Wrappers only has support for IP Address restrictions, they can't tell if the traffic is coming in from your network card on the trusted side of the firewall, or from a machine on the other end of the ppp connection through the modem link. This is where packet filters (such as ipfwadm and ipchains) come in.
Ipchains and ipfwadm are not the actual filters; but they are programs that control parts of the running kernel. They tell the kernel what kinds of packets you wish to let through, and what kinds of packets you wish to deny. Since filtering takes place at the kernel level, the filtering can potentially use any aspect of the networking structure to describe what packets to look for. This means that you can specify not only what addresses to allow or deny, but also what individual protocols, network interfaces, and many more things. This also means that you can manipulate the kernel into "repeating" or "routing" traffic between interfaces, allowing you to force the kernel into being a makeshift router for your home or small business' network. It looks for traffic that's not meant for any of the local machines, and tries to push it out over the modem link. If the connection is successful, one of your networked machines suddenly can access stuff THROUGH the server. Then, even more machines can do it at the same time, too. You could have as many connections as you wanted over one modem link (of course after 3 or 4 connections going at once, the link would become very slow). The Linux community calls this routing procedure "Masquerading". The *BSD community calls this "Network Address Translation", but like I said before, NAT is a little more than just kernel work.
Now, we need to use either ipfwadm or ipchains (Depending on your kernel) to add masqerade rules. Some people say to add a general policy to masquerade, and that's bad, because it would allow all sorts of evil stuff to happen, including maybe someone being able to hide themselves behind your masqerade and access all of your network resources without authorization. This is also known as "The Bad Thing". Also, I prefer to add masqerade rules on a per-machine basis (as in, one ipfwadm/ipchains line that tells the kernel to forward packets from workstation 1, workstation 2, etc... instead of telling it to forward from all hosts in 192.168.1.x. That's kind of my personal preference, but it's how I would suggest doing things. Ipfwadm or IPChains rules may be typed in at any time, but I would suggest putting the commands into your /etc/rc.d/rc.local file, which is the closest equivalent to an "autoexec.bat" for Linux.
If you're using IPChains on your Host Machine (with RedHat 6 or any other kernel 2.2.x based Linux system), this example should give your machines with IP Addresses 192.168.1.2 and 192.168.1.8 access to the internet when you're connected. Just add more "ipchains -A forward -s ..." lines to /etc/rc.d/rc.local and change the IP Addresses for each machine you need on the Internet. (These examples taken right from the IP-Masquerade mini HOWTO, by Ambrose Au and David Ranch)
ipchains -P forward DENY
ipchains -A forward -s 192.168.1.2/32 -j MASQ
ipchains -A forward -s 192.168.1.8/32 -j MASQ If using ipfwadm (as in with RedHat 5.x, or any kernel 2.0.x based linux), the equivalent to the above ipchains commands, using ipfwadm is:
ipfwadm -F -p deny
ipfwadm -F -a m -S 192.168.1.2/32 -D 0.0.0.0/0
ipfwadm -F -a m -S 192.168.1.8/32 -D 0.0.0.0/0 Again, these should just go at the end of /etc/rc.d/rc.local, and you can just add another "ipfwadm -F -a m -S ..." line for each ip you want to forward.
You'll also want to add modules for forwarding "interesting" traffic, where applicable. Strange protocols such as irc, cuseeme, and ftp do not like masquerading very much. A few kernel modules make the masquerade a little more friendly for these protocols. I just placed lines in rc.local again, using modprobe for the various modules. These are masq modules from the 2.2.5 kernel. I did not actually insert all of these modules. The title of them is fairly self-explanatory:
modprobe ip_masq_autofw.o
modprobe ip_masq_cuseeme.o
modprobe ip_masq_ftp.o
modprobe ip_masq_irc.o
modprobe ip_masq_mfw.o
modprobe ip_masq_portfw.o
modprobe ip_masq_quake.o
modprobe ip_masq_raudio.o
modprobe ip_masq_user.o
modprobe ip_masq_vdolive.o I'll go over advanced packet filtering toward the end of this article. There, I'll cover some real firewalling stuff.
Setting up a File Server using Session Message Block (SMB) Protocol.
SMB is the file sharing method used by Windows NT and the old LAN Manager. It is a fairly resilient protocol, and some advances have taken place in the recent past, that make it a little better (Specifically, the ability to go over TCP/IP instead of NetBEUI). If you use samba on your host, and configure it properly, you will have a nice framework for cross platform file sharing with Windows and many UNIXes. Also, using some third party software from Thursby software (www.thursby.com) called "DAVE", you can even use SMB shares from MacOS.
There's a lot of help docs out there that can help you set up Samba for print/file sharing. The default /etc/smb.conf file is so bloated and unnecessary, that I can't believe it got put in as an example of what it should look like. I'm actually running printer sharing off of the Windows 95/Linux dual boot workstation, for the rest of the family's convenience. This is what the smb.conf file looks like for my laptop (which shares all users' home dirs, a "public" world-writeable share, and the CD-ROM Drive):
[global]
workgroup = WORKGROUP
netbios name = ESCAPE_POD
server string = NEC Versa 4050C (Red Hat 6.0)
log file = /var/log/samba/log.%I
max log size = 50
security = share
socket options = TCP_NODELAY
dns proxy = no
local master = no
lm announce = yes
lm interval = 60
[homes]
comment = Home Directories
browseable = yes
public = no
writable = yes
create mask = 0755
follow symlinks = no
wide links = no
preserve case = yes
[public]
path = /usr/public
public = yes
only guest = yes
writable = yes
printable = no
available = yes
guest only = no
browseable = yes
only user = no
[cdrom]
path = /mnt/cdrom
public = yes
only guest = yes
writable = no
printable = no
available = yes
guest only = no
browseable = yes
only user = no From this, you should be able to see how shares are made up. Refer to the mainpage for "smb.conf", and it will help a lot from here, specifically with setting up a printer, if desired.
Now would be a great time to install the Ethernet Hub somewhere and plug the cable from your server into it.
IV. Intra-net workstation setup
The goals are simple: Use the resources of your home-server, and use it as a gateway to the 'net when it's connected. Also, you may want a something that makes it connect. If you used masqdial on the host machine, then you will need the masqdial client for the workstations. There's a Java one, one specifically for UNIXes, one for mac, Windows, and whatever else, pretty much. This is a good solution, in my opinion. It's difficult to set up though. Otherwise, you may want a script that telnets over, and launches the ppp connection.
Make sure that the Ethernet card is installed properly and that you have the drivers installed if necessary.
We need to set up IP connectivity first. This is platform dependant, and I don't have the time to tell you how to do it on every platform. Under RedHat, use netcfg, and Windows, use the "network" item in the control panel. I sequentially numbered my machines. The Windows/Linux dual boot machine is 192.168.1.2, my laptop is 192.168.1.3, and the list goes on and on. While you're setting up the IP over the ethernet card, make sure to include the IP of your host machine (in the example above, Setting up the host machine, we called it 192.168.1.1) as the "Gateway" or "default route", as our server will be functioning as the Internet connection point for all the workstations.
If you have any additional file sharing needs from the workstations, set them up, as well. Again, as the workstations can be almost any platform, I can't really help a lot here.
Network security on each of the workstations.
Now run Ethernet cable from the network cards of each workstation back to the hub, as well.
VI. Testing
Now it's time to test everything. First, go to your home-server and try to ping the IP addresses of the workstations. You should see a response listed in "milliseconds". IF you see no response, make sure your IP's are correct, and you have solid cable connections. Also, make sure the hub and the remote computer are turned on.
Next, go to each workstation and ping another workstation (if available) and the server's IP address. Then try to telnet to your home-server. IF you can do this, you're good so far. Start the Internet connection (usually "/sbin/ifup ppp0"). Wait for the home-server to connect. IF it does not, re-check your dialup settings. If it DOES connect, try pinging the outside world from the workstation. ping www.yahoo.com, or some other server out there. If you get a response, then your home-net works great for sharing an internet connection! If you can't ping the outside world from a workstation, try it from your home-server. If you can't, then re-check the internet dialup settings. If you can see the outside world from the home-server, but you can't see it from workstations, then it's a problem with your forwarding rules. Make sure you're using ipchains if you're running kernels 2.2.x, and make sure you're using ipfwadm for kernels 2.0.xx.
VII. Advanced firewalling
Sometimes hosts.allow and hosts.deny just don't cut it. For instance, you already know it usually only covers stuff that runs through tcp-wrappers. Also, if someone port-scans your home server, or any linux machine you're running, even though hosts.deny says not to allow them a connection, they still see an open port. ipchains and ipfwadm can eliminate the "look" of open ports on your machine, as well as locking it down even further, as it just blocks ports. This means that any other ports left un-protected by tcp-wrappers (such as SSH and Sendmail), can be protected as well. Here is a little quick-help guide to ipchains and ipfwadm I put together a while back. While yer messing with these rules, I suggest you port-scan the machine you're enforcing the rules on, to see how they would look from an outside attacker. The example that describes denying port to all hosts except the local network will probably be the most useful for you, just put multiple instances of ipchains or ipfwadm into rc.local, and change "23" to some other port that you want to lock down. My laptop is protected with ipchains. This is how I did it:
First I portscanned my box with nmap (from my other workstation) and I saw that ports 21, 22, 23, 79, 113, and 139 were open.
Then I went to my laptop, logged in, su'd to root, and began...
These are the ipchains rules I entered:
ipchains -A input -p tcp -d 0/0 21 -s ! 192.168.1.0/255.255.255.0 -j DENY
ipchains -A input -p tcp -d 0/0 23 -s ! 192.168.1.0/255.255.255.0 -j DENY
ipchains -A input -p tcp -d 0/0 79 -s ! 192.168.1.0/255.255.255.0 -j DENY
ipchains -A input -p tcp -d 0/0 113 -s ! 192.168.1.0/255.255.255.0 -j DENY
ipchains -A input -p tcp -d 0/0 139 -s ! 192.168.1.0/255.255.255.0 -j DENY Add firewall rules:
ipchains -A <input|output|all> -p <icmp|udp|tcp|all>
-i [!] <Interface-name>
-s [!] <src-ip/mask> [!] <(low)port:highport>
-d [!] <dest-ip/mask> [!] <(low)port:highport>
-j <DENY|ACCEPT|REJECT|MASQ|REDIRECT|RETURN> - Note: when using icmp protocol, ports are replaced with icmp types. If no port or icmp type is specified, it assumes that you meant "All ports or icmp types". If no source is specified, it assumes "all", and if no destination is specified, it assumes "all". Also, if ! is used before an ip or port rule, it means "anything except" the given port/ip.
ipfwadm -A <in|out|both> -P <udp|tcp|icmp|all>
-W [!] <Interface-name>
-S [!] <src-ip/mask> [!] <port [port] ...>
-D [!] <dest-ip/mask> [!] <port [port] ...>
-a <deny|reject|accept> Examples:
Deny port 23 (telnet) from all hosts not on the 192.168.1.xxx network:
ipchains -A input -p tcp -d 0/0 23 -s ! 192.168.1.0/255.255.255.0 -j DENY
ipfwadm -A in -P tcp -D 0/0 23 -S ! 192.168.1.0/255.255.255.0 -a deny Make machine stop responding to ping packets:
ipchains -A input -p icmp -s 0/0 echo-request -j DENY
ipchains -A input -P icmp -S 0/0 echo-request -a deny Deny ports 19-23 from the entire 198.248.53.xxx subnet:
ipchains -A input -p tcp -s 198.248.53.0/255.255.255.0 -d 0/0 19:23 -j DENY
ipfwadm -A in -P tcp -S 198.248.53.0/255.255.255.0 -D 0/0 19:23 -a deny Deny any outgoing icmp packets from your machine (I don't know why, it's just an example):
ipchains -A input -p icmp -d 0/0 -j DENY
ipfwadm -A in -P icmp -D 0/0 -a deny Deny all incoming tcp to port 23 if it's coming over interface "ppp0"
ipchains -A input -p tcp -D 0/0 23 -I ppp0 -j DENY
ipfwadm -A in -P tcp -D 0/0 23 -W ppp0 -a deny Delete firewall rules:
ipchains -D <input|output|all> [rulenum] (deletes one rule)
ipchains -F (flushes all rules)
ipfwadm -A -d [policy] (deletes the matching policy)
ipfwadm -A -f (flushes all rules) IIX. Closing
Well, I guess that about covers it, as far as I can tell. If nothing else, a quickie lesson about firewall rules for ya all. Having a home network for myself has been a great experience, and has allowed me to play with hacking in a new way: in my own home, as much as I want. I control the environment, and I don't have to worry about messing other people's connections up when doing it. I've done a lot of experimentation with firewalling rules since I've gotten my home-net up. Once mastered, these skills will aid you in being a very valuable asset to major corporations, and will help you understand network security a little more. This was by no means a "full attempt" to make people understand networking, but is only a primer, to get you guys thinking. I've found that very few things can be as rewarding to a hacker as having their own network to play around on as much as they like. I'd hope that many of you will take this information and use it to help you learn a little more. Then you might be able to help people with their own security problems.
These are just some basic networking implementations, and they're more than enough for the average person, but then again, most hackers aren't average... There's plenty more info on this topic, all out there on the 'net. If you're interested in this sort of thing, then go out and do it!
--Axon
