This may be a bit of a noobish question, but I was taking a look at /etc/hosts on my new Xubuntu install and saw this: localhost myhostname

On most 'nixes I've used, the second line is omitted, and if I want to add my hostname to the hosts file, I'd just do this: localhost myhostname

Is there a difference between these two files in any practical sense?

  • I wonder can i just delete the second line or will some process in debian/ubuntu regenerate the line and add it back?
    – simgineer
    Apr 4, 2017 at 20:45
  • I still don't understand do we put the same hostname to the right of and or should the hostnames be different?? Or maybe localhost and actual_hostname. Apr 27, 2023 at 12:05

5 Answers 5


There isn't a great deal of difference between the two; 127/8 (eg: => are all bound to the loopback interface.

The reason why is documented in the Debian manual in Ch. 5 Network Setup - 5.1.1. The hostname resolution.

Ultimately, it is a bug workaround; the original report is 316099.

  • 10
    The actual reason: "Associating the system hostname with the latter had the unwanted effect of making 'localhost.localdomain' the canonical hostname associated with the system hostname. That is, 'hostname --fqdn' returned 'localhost.localdomain'."
    – cmroanirgo
    May 29, 2015 at 20:58
  • 1
    I actually traced it back to this doc lists.debian.org/debian-boot/2005/06/msg00938.html and it appears that its because Thomas was feeling the need to use 1.1 instead of 0.1, which they are equivalent, correct me if im wrong, but that means this answer is really only a breadcrumb? Aug 2, 2016 at 4:56
  • 1
    @BrianThomas that message suggests why they're not equivalent. They wanted localhost and myhostname to be distinct - otherwise, one would be an alias of the other. Note this doesn't work in IPv6 where there is only one loopback address. It can be avoided by using an NSS module as mentioned in the thread, because it can be more flexible than /etc/hosts and return different canonical names despite them having the same IP address. I know this because my system is configured using such an NSS module.
    – sourcejedi
    Jul 21, 2017 at 17:09
  • what happen if I put my_host_name?
    – Davide
    Oct 30, 2019 at 11:49
  • 1
    It caused dnsmasq to return clients so I had to remove it.
    – Deepstop
    Jul 2, 2020 at 14:55

To sum up the linked-to information:

  • It is (debatably) useful to have an entry in your /etc/hosts translating the machine's fully-qualified domain name into its permanent IP address.
  • debian-installer, and more specifically, its netcfg component, currently (up to March 2013 at least) creates this entry.
  • If the machine is not known to have a permanent IP address, the debian-installer still wants it to have that kind of entry.
  • The address uses the loopback interface, answered by your own machine, just like but is a distinct entry in /etc/hosts which can be considered separately from if/when necessary.

Thomas Hood explains adding this entry as follows:

[This] will ensure that if the UNIX hostname is resolved then it will always be its own canonical hostname


In the long run the UNIX hostname should not be put in /etc/hosts at all.

  • 3
    .. and so... ? must I user for my fdqn? or or the static lan ip?
    – realtebo
    Aug 27, 2018 at 21:46
  • 1
    @realtebo: 1. It's useful, and thus it's added by the installer. 2. There is no "must" here; and it's only relevant when your FQDN doesn't have a permanent address.
    – einpoklum
    Aug 27, 2018 at 22:29
  • @einpoklum On ubuntu 20 it seems to automatically add this entry to /etc/hosts at reboot even if I already have my hostname there with it's actual ip, and this screws up our services (as they listen on the wrong interface). Is there any way of preventing ubuntu from messing about with /etc/hosts on reboot?
    – CpnCrunch
    Jun 18, 2021 at 16:34
  • 1
    @CpnCrunch: I'm 90% certain there's some configuration setting (in /etc/default maybe?) for disabling this behavior. That's what I'd try and do.
    – einpoklum
    Jun 18, 2021 at 16:37
  • @einpoklum possibly, but I don't see any mention of it anywhere there, or in the man page for networkd-dispatcher or its scripts in /usr/lib/networkd-dispatcher (which would seem to be the obvious culprit).
    – CpnCrunch
    Jun 18, 2021 at 16:51

I was curious myself, and i didnt like any of the other answer because they didnt seem to answer what i was looking for atleast.

The Answer: Looking back at this doc it almost appears as if Thomas was stating is "giving it another dedicated ip on the loopback allows it to be canonical".

Both point to your loopback. Using the following is an actual IP, on the loopback, whereas is either the device itself, or another ip on the loopback. Both end up on the same subnet, representing the loopback, but are separated by ip. They are equivalent dns wise, but separated because of having dedicated ip.

The point being, you can have all your entries on one line like this localhost localhost.domain www.myfakednsname.com myakednsname.com 

If your hostname is local, meaning doesn't have a global internet DNS entry mapped to an actual internet ip, then in this case Thomas was saying you NEED TO have the 2nd entry line, like this to dedicate it there (to canonical). localhost localhost.localdomain myfakednsname
  • in all fairness, after i finally composed this to a way that made sense to me, i see thats pretty much what einpoklum was trying to explain it looks like in hindsight. Aug 2, 2016 at 5:15
  • 2
    OK negative vote, not sure why. I still believe THIS is the BEST answer, or i wouldnt have put it, after landing here. the original answer helped, but didnt capture all the details. I would go with This one. Aug 10, 2016 at 20:05

I read several answers around, and I'm frankly a bit confused.

I first tried to understand what is canonical hostname here.

What I can say is that @cmroanirgo is right in saying that on my Ubuntu laptop, invoking

hostname --fqdn 

with /etc/hosts configured like this   localhost   laptop

returns laptop, whereas changing the file like follows   localhost laptop

returns localhost.

What impacts this can have, I have no idea, except the fact that if you have software that binds to hostname, and you need to talk to it internally to your machine, don't expect to be able to use the "localhost" hostname to do the job, in this setup, since they end up resolving two distinct separate addresses.

  • 2
    One example implication: unattended-upgrades uses socket.getfqdn() (in python) for the mail subject. I want the UNIX hostname there to distinguish different machines (not localhost), so the variant with seems correct or at least better to me. Mar 26, 2020 at 11:49
  • Using has downsides too! You cannot telnet laptop 25 when thesmtp service is configured to listen on localhost or even listen on lo (the interface). With /etc/hosts having two separate lines for localhost and laptop that both map to, this will work and hostname --fqdn will still return laptop just fine. So In that regard, there's no need to fiddle with at all! Indeed, the outlier is python's socket.getfqdn() (as used by unattended-upgrades ) that will insist on reporting localhost for such a setup.
    – cueedee
    Jun 9, 2023 at 12:50

Antique question, but adding a different answer. If you use a line: my_fqdn my_short_name

eg server.domain.com server

The (on Ubuntu at least) Apache will pick up your correct hostname without having to enter it explicitly in the apache config. This is probably similar to comment in the answer above.

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