Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary end This document is a tutorial and reference for turning a computer into an Internet gateway. This tutorial will explain how to set iptables to use NAT, allowing you to share a single broadband connection with an entire LAN.
This article is focused on security, since the gateway is connected directly to the Internet. It shouldn't run any services available to the outside world. Towards the LAN, it should only run gateway specific services. It should not run HTTPd, FTPd, Samba, NFSd, etc. those belong on a server on the LAN or DMZ (if you want to make these services available to the outside world) as they can introduce security flaws.
This document is it not suited to set up a shared connection between 2 PCs using cross-over cables. At the conclusion of this tutorial, the server will not be usable as a desktop computer for normal use. If you are looking for a solution to share internet using a desktop computer, see Internet share.
- 1 Hardware Requirements
- 2 Conventions
- 3 Installation
- 4 Network interface configuration
- 5 ADSL connection
- 6 DNS and DHCP
- 7 Connection sharing
- 8 Cleanup
- 9 Logrotate
- 10 Optional additions
- 10.1 Remote administration
- 10.2 Caching web proxy
- 10.3 Time server
- 10.4 Content filtering
- 10.5 Traffic shaping
- 10.6 Intrusion detection and prevention with snort
- 11 See also
- At least 1 GB of hard drive space. The base install will take up around 500MB of space and if you want to use a caching web proxy, you will need to reserve space for the cache as well.
- At least two physical network interfaces: a gateway connects two networks with each other. You will need to be able to connect those networks to the same physical computer. One interface must connect to the external network, while the other connects to the internal network.
- A hub, switch or UTP cable: You need a way to connect the other computers to the gateway
Conventions in this guide will be to use non-realistic interface names, to avoid confusion about which interface is which.
- intern0: the network card connected to the LAN. On an actual computer it will probably have the name eth0, eth1, etc.
- extern1: the network card connected to the external network (or WAN). It will probably have the name eth0, eth1, etc.
A fresh install of Arch Linux is the easiest to start from, as no configuration changes have been made and there is a minimal amount of packages installed. This is helpful when attempting to reduce security risk.
For security purposes, /var, /tmp and /home should be separate from the / partition. This prevents disk space from being completely used up by log files, daemons or the unprivileged user. It also allows different mount options for those partitions. If you have already partitioned your drive, the gparted livecd can be used to resize, move, or create new partitions.
Your home and root partitions can be much smaller than a regular install since this isn't a desktop machine. /var should be the largest partition - it's where databases, logs and long-term caches are stored. If you a lot of RAM, mounting /tmp as tmpfs is a good idea, so making a disk partition for it during the initial install is unnecessary.
After installation boot Arch and upgrade all the packages to their latest version:
# pacman -Syu
# pacman -S sudo
Now, add a normal user. Be sure to add the user to the wheel group. This will allow the user to use sudo. Logging in as root is unsafe, it is much better to use sudo.
After sudo is installed and configured, it's possible to lock to root account and prevent login directly to root.
# passwd -l root
Network interface configuration
When you let udev handle loading the modules, you'll notice your NIC's switch names: one boot your LAN NIC is eth0, the other boot it's eth1.
To fix this problem, see here.
Open /etc/rc.conf once more and scroll down to the network config section. Here's where you define how your network cards should obtain their IP. The LAN card will have a static IP, I'm going with 10.0.0.1 because it's easy to type. I'm building a gateway for a small student home with 4 rooms so I'm keeping the subnet fairly small: 4 bits allow 16 IP's.
16 - 3 IP's:
- one for the network address: 10.0.0.0
- the gateway: 10.0.0.1
- and the broadcast address: 10.0.0.15 leaves 13 IP's for computers on the LAN. This translates into:
lo="lo 127.0.0.1" intern1="eth0 10.0.0.1 netmask 255.255.255.240 broadcast 10.0.0.15" extern0="dhcp"
Using rp-pppoe, we can connect an ADSL modem to the eth1 of the firewall and have Arch manage the connection. Make sure you put the modem in bridged mode though, otherwise the modem will act as a router too.
# pacman -S rp-pppoe
The questions are all documented. You can select "no firewall" because we'll let Shorewall / iptables handle that part.
There's a bug in the package, so we need to manually create a symbolic link:
ln -s /usr/sbin/pppd /sbin/pppd
Everything should be in place.
DNS and DHCP
We'll use dnsmasq, a DNS and DHCP daemon for the LAN. It was specifically designed for small sites.
First, install dnsmasq:
# pacman -S dnsmasq
Now, dnsmasq needs to be configured. To do this:
Edit /etc/dnsmasq.conf and add the following lines
interface=intern1 # make dnsmasq listen for requests only on intern1 (our LAN) expand-hosts # add a domain to simple hostnames in /etc/hosts domain=foo.bar # allow fully qualified domain names for DHCP hosts (needed when # "expand-hosts" is used) dhcp-range=10.0.0.2,10.0.0.14,255.255.255.240,1h # defines a DHCP-range for the LAN: # from 10.0.0.2 to .14 with a subnet mask of 255.255.255.240 and a # DHCP lease of 1 hour (change to your own preferences)
Somewhere below, you'll notice you can also add "static" DHCP leases, i.e. assign an IP-address to the MAC-address of a computer on the LAN. This way, whenever the computer requests a new lease, it'll get the same IP. That's very useful for network servers with a DNS record. You can also deny certain MAC's from getting an IP. Evil!! ^_^ Now start dnsmasq
# /etc/rc.d/dnsmasq start
and add the daemon to the DAEMONS list in /etc/rc.conf.
Time to tie the two network interfaces to each other.
Shorewall, an iptables frontend, can be used as an easier alternative.
# pacman -S shorewall
Time to configure Shorewall! Open its config file in /etc/shorewall/shorewall.conf and start editing. The file is very well documented.
SUBSYSLOCK=/var/run IP_FORWARDING=On : it's a gateway, remember! ;) STARTUP_ENABLED=Yes # when you're done editing
After installing shorewall, run
$ pacman -Ql shorewall | grep Sample
to see where the sample files are. cd into the directory "two-interfaces" and copy the contents to the /etc/shorewall/ directory. Now use Shorewall's guide to set up the files correctly.
Read the document carefully. Take special care to change eth0 and eth1 (or ppp0 in if you're using PPPoE where appropriate in your config files as the Shorewall guide uses different names for the interfaces. When you've followed it thoroughly, make the following changes:
- /etc/shorewall/interfaces : add "dhcp" to the loc line to allow computers on the LAN to make use of our DHCP server
- /etc/shorewall/rules : add
ACCEPT loc $FW TCP 2367
but change 2367 into whatever port you have your SSH server listening on.
# /etc/rc.d/shorewall start
From here on, the Arch box is operational. Connect a hub or switch to eth0 and a computer to the LAN to test it.
Port forwarding (DNAT)
- /etc/shorewall/rules : here's an example for a webserver on our LAN with IP 10.0.0.85. You can reach it on port 5000 of our "external" IP.
DNAT net loc:10.0.0.85:80 tcp 5000
Now that the installation has been performed, it is necessary to remove as many packages as possible. Since we are making a gateway, keeping unneeded packages only "bloats" the system, and increases the number of security risks.
First, check for obsolete/deprecated packages (likely after a fresh install and massive series of updates):
$ pacman -Qm
Review the list of explicitly installed packages that are not dependencies and remove any that are unneeded. Having only needed packages installed is an important security consideration.
$ pacman -Qet
Completely remove the packages you don't need along with their configuration files and dependencies:
# pacman -Rsn package1 package2 package3
You should review the logrotate configuration to make sure the box isn't brought down by lack of diskspace due to logging.
Logrotate is installed by default, so you won't have to install it.
OpenSSH can be used to administer your router remotely. This is useful for running it "headless" (no monitor or input devices).
Caching web proxy
To use the router as a time server, see Network Time Protocol.
Then, configure shorewall or iptables to allow NTP traffic in and out.
# nano /etc/shorewall/rules
NTP/ACCEPT loc $FW NTP/ACCEPT $FW net
Install and configure DansGuardian if you need a content filtering solution.
Traffic shaping is very useful, especially when you're not the only one on the LAN. The idea is to assign a priority to different types of traffic. Interactive traffic (ssh, online gaming) probably needs the highest priority, while P2P traffic can do with the lowest. Then there's everything in between.
Traffic shaping with shorewall
Read Shorewall's Traffic Shaping/Control guide.
Here's my config as an example:
- /etc/shorewall/tcdevices : here's where you define the interface you want to have shaped and its rates. I've got a ADSL connection with a 4MBit down/256KBit up profile.
ppp0 4mbit 256kbit
- /etc/shorewall/tcclasses : here you define the minimum (rate) and maximum (ceil) throughput per class. You'll assign each one to a type of traffic to shape.
# interactive traffic (ssh) ppp0 1 full full 0 # online gaming ppp0 2 full/2 full 5 # http ppp0 3 full/4 full 10 # rest ppp0 4 full/6 full 15 default
- /etc/shorewall/tcrules : this file contains the types of traffic and the class it belongs to.
1 0.0.0.0/0 0.0.0.0/0 tcp ssh 2 0.0.0.0/0 0.0.0.0/0 udp 27000:28000 3 0.0.0.0/0 0.0.0.0/0 tcp http 3 0.0.0.0/0 0.0.0.0/0 tcp https 4 0.0.0.0/0 0.0.0.0/0 all
I've split it up my traffic in 4 groups:
- interactive traffic or ssh: although it takes up almost no bandwidth, it's very annoying if it lags due to leechers on the LAN. This get the highest priority.
- online gaming: needless to say you can't play when your ping sucks. ;)
- webtraffic: can be a bit slower
- everything else: every sort of download, they're the cause of the lag anyway.
Intrusion detection and prevention with snort
According to the site's homepage title, Snort is "the de facto standard for intrusion detection/prevention".
# pacman -S snort
The main configuration file is Template:Filename.
Read it carefully, as usual it's very well documented.
var HOME_NET 10.0.0.0/28 # Change to the subnet of your LAN. var EXTERNAL_NET !$HOME_NET var DNS_SERVERS $HOME_NET var SMTP_SERVERS $HOME_NET # Comment these if you're not running any servers on the LAN. var HTTP_SERVERS $HOME_NET var SQL_SERVERS $HOME_NET var TELNET_SERVERS $HOME_NET var HTTP_PORTS 80 var SHELLCODE_PORTS !80 var ORACLE_PORTS 1521 var AIM_SERVERS [22.214.171.124/24,126.96.36.199/24,188.8.131.52/24,184.108.40.206/24,220.127.116.11/24,18.104.22.168/ 24,22.214.171.124/24,126.96.36.199/24,188.8.131.52/24] var RULE_PATH /etc/snort/rules var HTTP_PORTS 80:5000 # For HTTPd's running on port 80 and 5000. Change appropriately # to the ports you are using on your LAN. config detection: search-method lowmem # If you're using a machine "with very limited resources".
At the bottom of the file, there's a list of includes. These define which rules you want to enforce. (Un)comment as you please. You should check that the corresponding file exists, as for me, none of the rules files were present.
groupadd snort mkdir -p /var/log/snort useradd -g snort -d /var/log/snort snort chown -R snort:snort /var/log/snort
SNORT_ARGS="-u snort -g snort -l /var/log/snort -K ascii -c /etc/snort/snort.conf -D -h 10.0.0.0/28 -A full
Replace 10.0.0.0/28 with the CIDR of your LAN.
Now Snort will run as user snort in group snort. Should improve security. The other options make it log to /var/log/snort in ASCII mode. Run snort -h to see other available options.
I've been running my router for 12 days now, and using the above snort options, I had around 120MB of logs! So I changed the -A switch to "-A none". This only logs alerts. I didn't know what to do with all the logs anyway.
Update the rules: Oinkmaster
If you want to be able to download Snort's latest rules, you'll need a subscription. This costs money. If you're happy enough with 5 days old rules, you just need to register for free. If you don't, the only updates you'll get are the new rules distributed with a new Snort release. Go ahead and register at Snort. If you really don't want to register, you can use the rules from BleedingSnort.com. They're bleeding edge, meaning they haven't been tested thoroughly.
A user has created a PKGBUILD for oinkmaster.
Edit Template:Filename and look for the URL section and uncomment the 2.4 line. Make sure to replace <oinkcode> by the Oink code you generated after logging into your Snort account. For Bleeding Snort rules, uncomment the appropriate line.
When you log into your new account, create an "Oink code". Another thing to change is
use_external_bins===1 # 1 uses wget, tar, gzip instead of Perl modules
The rest of the config file is fine.
oinkmaster.pl -o /etc/snort/rules
Create an executable script with the exact command and place it in /etc/cron.daily to update the rules daily automatically.