I have a problem with sudo. Every time when I trying to use sudo I got this problem:

sudo: unable to resolve host ubuntu-server

My /etc/hosts file:

127.0.0.1       localhost
127.0.1.1       ubuntu-server

# The following lines are desirable for IPv6 capable hosts

::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
FF02::2 ip6-allrouters

/etc/hostname gives:

ubuntu-server

I think there is some other setting that causes the unable to resole host, but I cannot find these.