HOWTO_static_ip_with_dhcp_router

Static IP

Sometimes you'll want to have a computer on the network that always has the same IP address. Usually a server of some sort or a machine that has a game on it that requires you to forward certain ports. Assigning a static IP address, an address that will never change, will save you from constantly updating details on your router.

Determining the Router's IP Address

The first thing that needs to be done is to determine your router's IP address. Route will help out:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
loopback        *               255.0.0.0       U     0      0        0 lo
default         192.168.1.1     0.0.0.0         UG    0      0        0 eth0

As can be seen, the default route is to 192.168.1.1, which is the router.

Assigning a Static IP Address

To assign a static IP address on a Linux machine, you'll need to edit /etc/conf.d/net.

You'll want to assign an address that is outside the DHCP IP Address Pool. (For example, on Titan's router, the DHCP pool has a range from 192.168.1.2 to 192.168.1.51.) Let's assume you want the address of a Linux machine on the network to be 192.168.1.101, edit /etc/conf.d/net like so:

config_eth0=( "192.168.1.101/24" )
routes_eth0=( "default gw 192.168.1.1" )

Or if you'd like to be more specific:

config_eth0=( "192.168.1.101 netmask 255.255.255.0 broadcast 192.168.1.255" )
routes_eth0=( "default gw 192.168.1.1" )

Defining the DNS Servers

Finally, you have to define your DNS server(s) in /etc/resolv.conf. The best thing to do is to define your ISP's DNS servers, but if you can't find that information, your router's IP address will most likely do the trick.

nameserver 192.168.1.1

Laptop: Static at Home and Dynamic Outside

This is a quick example of /etc/conf.d/net so that a Linux laptop can have a static IP address when you're at home and dynamically obtains an IP address when you're not at home. For more information, have a look at the article HOWTO Network profiles with arping. Here is the configuration example:

# interface eth2 and wpa_supplicant
modules=( "wpa_supplicant" )
wpa_supplicant_eth2="-Dwext -c /etc/wpa_supplicant/wpa_supplicant.conf"
wpa_timeout_eth2=60

# Enable arping mode
config_eth2=( "arping" )

# Define the gateways you want to configure
gateways_eth2="192.168.1.1"

# Define the IP and netmask,default route and DNS server when using gateway 192.168.1.1
# At home, you want IP 192.168.1.102
config_192168001001=( "192.168.1.102 netmask 255.255.255.0" )
routes_192168001001=( "default via 192.168.1.1" )
dns_servers_192168001001=( "192.168.1.1" )

# If any of the above profiles fail use DHCP
fallback_eth2=( "dhcp" )
iwconfig_eth2="mode managed"

DHCP

Now to modify your network configuration:

...
config_eth0=( "dhcp" )
dhcp_eth0="nodns nontp nonis"
...

This assumes DHCP and a few other options. For more information see section 8.b of the Gentoo handbook.

Check for a /etc/init.d/net.eth0 symbolic link:

ls -l /etc/init.d/net.eth0

If it doesn't exist, create it:

ln -s /etc/init.d/net.lo /etc/init.d/net.eth0

Now start it:

/etc/init.d/net.eth0 start

If you want it to start on boot, add it to the default runlevel:

rc-update add net.eth0 default

Fallback route (hotplugd)

This will setup you eth0 device to try dhcp and fall back to a static network. For me this works well because at work they use DHCP and at home I have a static setup.

# ETH0
hotplug_eth0="yes"
configure_eth0=( "dhcp ")
fallback_eth0=( "XXX.XXX.XXX.XXX netmask XXX.XXX.XXX.XXX broadcast XXX.XXX.XXX.XXX" )
fallback_route_eth0=( "default via XXX.XXX.XXX.XXX" )

Startup sequency for wired and wireless connections

If you want to use the same startup sequency and run-level for both wired and wireless connections, you will want to lower the DHCP timeout. That way it won't hang on your wired interface when you want to use wireless (default timeout is 60 seconds). To do this, open up /etc/conf.d/net and add the following lines. This assumes that eth0 is your wired and eth1 is your wireless interface. If the timeout of 5 seconds is too short for your particular network setup, simply increase the number.

config_eth0=( "dhcp" )
config_eth1=( "dhcp" )
dhcpcd_eth0="-t 5"
dhcpcd_eth1="-t 5"

See also

• Configure a wireless connection
• Gentoo networking guide
• Static IP Address Problems (SOLVED) on the Gentoo Forums