Why 169.254.0.0 appears by default in the routing table?

Question

I defined a subnet on eth0 in /etc/network/interfaces with:

# interfaces(5) file used by ifup(8) and ifdown(8)  
auto lo  
iface lo inet loopback  

auto eth0  
iface eth0 inet static  
        address 192.168.1.1  
        netmask 255.255.255.0  
        network 192.168.1.0  

This subnet will be used to communicate to a Raspberry Pi connected on my ethernet port. Internet connection is not needed because already provided by wifi.

I was expecting my laptop to get the static ip address 192.168.1.1 when I run sudo ifup eth0 and it's the case. However, route -n shows:

Kernel IP routing table  
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface  
... (bunch of things) ...  
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0  

1. Why do I have a 169.254.0.0 line?
2. I believe it's unnecessary. How do I remove it?

Answer 1

From the quoted link:

If a DHCP client attempts to get an address, but fails to find a DHCP server after the timeout and retries period it will randomly assume an address from this network. This allows communication with hosts that have failed to obtain a DHCP address.

However, your interfaces file requests a static IP address that is likely impossible, so no valid address was given.

192.168.1.1 is likely the address of the router or other access point to which you are attempting to connect. Therefore, the address will not be granted. Moreover, if you expect to connect to the internet, you will need to specify DNS nameservers. May I suggest:

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 8.8.8.8 192.168.1.1

Of course verify the exact details before you proceed.

Restart the interface:

sudo ifdown eth0 && sudo ifup -v eth0

Check:

ping -c3 www.ubuntu.com

Answer 2

Per RFC 3927 (emphasis mine):

To participate in wide-area IP networking, a host needs to be
configured with IP addresses for its interfaces, either manually by
the user or automatically from a source on the network such as a
Dynamic Host Configuration Protocol (DHCP) server. Unfortunately,
such address configuration information may not always be available.
It is therefore beneficial for a host to be able to depend on a
useful subset of IP networking functions even when no address
configuration is available. This document describes how a host may
automatically configure an interface with an IPv4 address within the 169.254/16 prefix that is valid for communication with other devices connected to the same physical (or logical) link.

So, to summarize, the 169.254/16 subnet was reserved to allow for communications between two physically (or logically, e.g. through a switch) connected devices without having to rely on DHCP or manual assignment of static IP addresses.

To quote the Wikipedia article on private networks,

If a host on an IEEE 802 (Ethernet) network cannot obtain a network address via DHCP, an address from 169.254.1.0 to 169.254.254.255 may be assigned pseudorandomly.

As an example of its usage, say you have two embedded devices that need to communicate with one another via a direct Ethernet connection, but you want one or both devices to first look for DHCP so they also have the possibility of being connected into a local area network. When the devices are connected directly together and there is no DHCP available, the devices may use a protocol such as multicast DNS to discover each other automatically, but each device must first have some arbitrary IP address assigned so they can actually talk to each other over the Ethernet link. Thus, the operating system automatically assigns an IP address to the Ethernet device from the 169.254/16 network.

---

Now, as for why that route is there in the routing table by default...if you look at /etc/networks, you can see that the 'link-local' network is defined as being in this subnet:

$ cat /etc/networks
# symbolic names for networks, see networks(5) for more information
link-local 169.254.0.0

If you take this file's advice and check out man networks, you'll see:

This file is read by the route(8) and netstat(8) utilities.

So the route is created automatically on boot after route reads that file.

---

The route is only necessary if you plan to actually use link-local communications and don't want to assign a static IP address to the connected devices. But it doesn't do any harm to leave it in if you don't.

However, if you really want to remove it, you can do so with

sudo route del -net 169.254.0.0 gw 0.0.0.0

Or to remove it permanently, you should be able to just comment out its entry in /etc/networks and reboot.

Answer 3

As others have mentioned, 169.254.0.0 is present in your routing table because ifup is setting up the link-local network.

To avoid this, consider using ifconfig instead of ifup.

If necessary, run ifdown first.

sudo ifdown eth0
sudo ifconfig eth0 192.168.1.1

ifconfig does less work and does not read /etc/network/interfaces.

It should be possible to share internet access with the Pi. You need to turn on packet forwarding in the kernel on your laptop. I think this can be done without rebooting. I have done it in the past, but do not know how to do it off the top of my heard. This might help:

How do I bridge network interfaces?