Router: Basic

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Revision as of 23:15, 28 July 2010 by Ddffnn (Talk | contribs) (Firewall + IP Masquerading: Added shorewall settings)

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DRAFT

This article is a draft. It may be more helpful/secure to use Router until this article is more complete.

Description

If you'd like to build a router to forward connections to LAN client(s), you'll need the details of creating a basic router. A router is required in order to connect multiple machines to the Internet using the a single global IP address. This is almost always the case for residential Internet access. The Arch Linux machine will fulfill several roles required to connect machines in a local network to the Internet:

  • Firewall - Block unauthorized packets but allow authorized traffic
  • IP Masquerading - Manipulate IP addresses as packets cross between internal network and Internet
  • DHCP Server - Manage IP addresses of machines in the internal network
  • DNS Server - Accept DNS lookups from local machines and forward them to the Internet
  • Gateway - Translate between protocols of the internal network and your Internet Service Provide (optional)

Ethernet Devices

Installation

You'll need to have at least two Network Card Interfaces (NIC's) on the computer you plan to use as a router. Once installed see that they are recognized by the kernel:

ifconfig -a

If the NIC(s) don't show up, then either 1) the kernel module (driver) will need be loaded, 2) the kernel will need to be rebuilt with support for the hardware, or 3) the kernel may not have support for the driver yet.

If there is a kernel module for you NIC, the generic Arch Linux kernel will likely have support for it. You can add it by:

modprobe <device-module>

If there is support in the kernel, but not in the Arch kernel take a look at Kernel Compilation with ABS.

Names

Udev is the device manager for Arch Linux and can be used to manually choose names for each ethernet device. This should be done to make sure that each physical network connection always has the same name, and also for convenience during later configuration steps.

Create a Udev rule Template:Filename

SUBSYSTEM=="net", ATTR{address}=="aa:bb:cc:dd:ee:ff", NAME="wan"
SUBSYSTEM=="net", ATTR{address}=="ff:ee:dd:cc:bb:aa", NAME="lan"

You can easily find the address of an existing device:

udevadm info -a -p /sys/class/net/<device> | grep address

Just use the output as the second field in the rules file. Next time Udev assigns device names it will use these. This article assumes "wan" connects to the Internet and that "lan" connects to the local network.

IP Settings

Each ethernet device's IP configuration needs to be set in Template:Filename:

wan="dhcp"
lan="lan 192.168.0.0 netmask 255.255.255.0 broadcast 192.168.0.255"
INTERFACES=(wan lan)

The device wan will request a dynamic IP address from the ISP. The device lan will use a static IP address. Later on dnsmasq will be configured used to grant dhcp leases to other local machines in the same subnet, i.e. with address in the range 192.168.0.1-192.168.0.255 (but not 192.168.0.0 because the router has that local address).

LAN Setup

For connecting to/from your LAN client(s), you can have to either add to the router a DHCP server (which will build the LAN client's routes for you) or define a static-route(s) manually. There might be problems is both methods are used.

dnsmasq

Install dnsmasq

# pacman -S dnsmasq

Edit the dnsmasq configuration file Template:Filename:

# Only listen to routers' LAN NIC.  Doing so opens up tcp/udp port 53 to
# localhost and udp port 67 to world:
interface=lan

# dnsmasq will open tcp/udp port 53 and udp port 67 to world to help with
# dynamic interfaces (assigning dynamic ips). Dnsmasq will discard world
# requests to them, but the paranoid might like to close them and let the 
# kernel handle them:
bind-interfaces

# Dynamic range of IPs to make available to LAN pc
dhcp-range=192.168.0.1,192.168.0.255,12h

# If you’d like to have dnsmasq assign static IPs, bind the LAN computer's
# NIC MAC address:
dhcp-host=aa:bb:cc:dd:ee:ff,192.168.0.1

If you choose not to bind the interfaces, the domain port will need to be allowed in Template:Filename:

domain ALL : ALLOW

Static-Route

To assign a static-route (for example on a Arch Linux LAN client):

eth0="eth0 192.168.0.100 netmask 255.255.255.0 broadcast 192.168.0.255"
gateway="default gw 192.168.0.7"
ROUTES=(gateway)

IP Masquerading and Firewall

Kernel Settings

The kernel will need to be told it's allowed to forward packets to/from the LAN clients:

echo 1 > /proc/sys/net/ipv4/ip_forward

To permanently set this, enable ip forwarding in /etc/sysctl.conf:

net.ipv4.ip_forward=1

Shorewall

Shorewall is an iptables frontend. It is easier to setup than manually defining iptables rules. shorewall is available from the AUR. These settings are based on the two-interface documentation on the shorewall website.

Use the some example configuration files that come with the shorewall package

# cp /usr/share/shorewall/Samples/two-interfaces/* /etc/shorewall/

/etc/shorewall/interfaces

Change the interface settings to match the names used for our ethernet devices and to allow dhcp traffic on the local network. Edit Template:Filename

original

net     eth0            detect          dhcp,tcpflags,nosmurfs,routefilter,logmartians
loc     eth1            detect          tcpflags,nosmurfs,routefilter,logmartians

new

net     wan            detect          dhcp,tcpflags,nosmurfs,routefilter,logmartians
loc     lan            detect          dhcp,tcpflags,nosmurfs,routefilter,logmartians

/etc/shorewall/policy

Change the policy file to allow the router (this machine) to access the Internet. Edit Template:Filename

original

###############################################################################
#SOURCE         DEST            POLICY          LOG LEVEL       LIMIT:BURST

loc             net             ACCEPT
net             all             DROP            info
# THE FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info

new

###############################################################################
#SOURCE         DEST            POLICY          LOG LEVEL       LIMIT:BURST
$FW             net             ACCEPT
loc             net             ACCEPT
net             all             DROP            info
# THE FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info

/etc/shorewall/rules

DNS lookups are handled (actually forwarded) by dnsmasq, so shorewall needs to allow those connections. Add these lines to Template:Filename

#       Accept DNS connections from the local network to the firewall
#
DNS(ACCEPT)     loc              $FW

OPTIONAL: You can add these lines if you want to be able to SSH into the router from computers on the Internet

#       Accept SSH connections from the internet for administration
#
SSH(ACCEPT)     net             $FW

/etc/shorewall/shorewall.conf

When you are finished making above changes, enable shorewall by a change in it's config file Template:Filename:

original

STARTUP_ENABLED=No

new

STARTUP_ENABLED=Yes