Failed to send 300 byte long packet over fallback interface

1.0 Error

While configuring a server, the following errors were noted in the logfiles.

send_packet: Operation not permitted
dhclient.c:2386: Failed to send 300 byte long packet over fallback interface.
DHCPREQUEST on eth0  port 67 (xid=0x1580bfd0)
...

2.0 Solution

There are two possibilities depending upon whether DHCP is being used for getting the network IP address or not.

2.1 DHCP is not being used

There may be an error in the network configuration. For example, the file /etc/network/interfaces may be having entries like the following:

...
# The primary network interface
auto eth0
iface eth0 inet dhcp
...

This needs to be changed to something similar to what is given below:

...
auto eth0
iface eth0 inet static
# The primary network interface
    address 192.0.2.100/24
    gateway 192.0.2.1
    dns-nameservers 192.0.2.5 192.0.2.6
    dns-search example.com
    dns-options rotate
...

2.2 DHCP is being used

The problem could be because of firewall not permitting UDP packets on port 67 for DHCP. The problem can be resolved by allowing outgoing UDP packets on port 67. The following iptables rule may be added,

iptables -A OUTPUT -p udp  -o eth0 --dport 67 --sport 1024:65535 -j ACCEPT