Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

I disabled all my network connections and deleted 127.0.0.1 localhost in HOSTS file but cannot find what had I broken by it. My IIS and MS SQL Server 2008 R2 continues to resolve localhost just fine

Why does HOSTS file always contain 127.0.0.1 localhost ?
What had I broken by deleting this entry?

I am on Windows XP Pro SP3 writing here still without localhost in HOSTS file.
Should I put it back and how fast ?


The reasons of interest are many fold - for instance:


------ UPDATE05:

I am not changing the question! I add updates. Can I ask to stop deleting and editing it until I write that I fished with it? For ex., just now I wrote the same comment in all posts addressing the same point.

This is the essence of my question/doubt - that the DNS does not make any sense in relation to "localhost" or "127.0.0.222" or "(local)" names, synonyms, aliases, links, addresses, IDs, tokens, whatever.

They are hundreds synonyms to the same entity and they are internal and Windows-es know it without any resolutions since there is no sense to resolve between so many synonyms!

They are related to internal computer mechanisms while DNS is external (between various computers). How can internal IDs can depend on external ones?

All Windowses (including Home Editions) will have internal DNS server in order to function? and then replicate it when/if connected to network?

Well, the link from comments did not appear in Linked section, as I was told.

I forked a child subquestion:
http://stackoverflow.com/questions/3536351/is-localhost-host-resolved-to-127-0-0-1

share

locked by Gnoupi Aug 21 '10 at 10:55

1  
If you do something that results in something else breaking, it's generally a good idea to undo whatever you did :) Removing the host association to localhost will probably break things, even if nothing (yet) seems obviously broken. Note : Removing the entry in hosts doesn't take down the interface, it just breaks localhost resolving to that interface. –  Tim Post Aug 20 '10 at 13:12
2  
Related questions (according to the OP): superuser.com/questions/178187/… and superuser.com/questions/178279/… –  Gnoupi Aug 20 '10 at 14:54
2  
Stop changing the question. This is not a chatroom. –  random Aug 20 '10 at 15:20
2  
@vgv8 - people don't downvote without a reason. You have to balance the updates you make with the fact of keeping a clear question. A question with two pages of "updates" is impossible to read, and gets downvoted for not being clear. –  Gnoupi Aug 20 '10 at 15:49
2  
@vgv8 these sites are not about the people. Don't hold personal grudges against other users, care only about the individual content. –  Gnoupi Aug 21 '10 at 6:40
show 14 more comments

3 Answers

up vote 11 down vote accepted

The hosts file just associates canonical or fully qualified names to IP addresses.

For instance, I could have:

127.0.0.1  moes-bar-and-grill

Then anything connecting to moes-bar-and-grill would establish a connection to the loopback device, aka 127.0.0.1, commonly resolved as localhost.

I could also have (and this is quite common)

127.0.0.1  annoying-ad-server.com

Applications continue to work because they will connect to 127.0.0.1 (which is still a configured / up interface) if localhost does not resolve.

I'm not sure why you would want to disable the loopback address, but simply taking localhost out of your host file is not going to do that.

Edit

Well written software will make more than one attempt at resolving anything (resolving in a sense of working around problems, no pun intended) before it just dies and in some cases will continue to function even if things are not as expected. That does not mean that the software will work as advertised, it only means that it was written by a very defensive programmer.

Very defensive does not always mean helpful when it comes to telling the user that serious problems exist, for instance localhost not resolving. I can write stuff that passes tests no matter what a user does to their system, but that does nothing to promote the cause of "This won't work!". There is a stark difference between it runs and it works and you will only explore the difference between the two over time with every program that you run.

While everything seems to work, now, I think you may be headed for trouble later.

Disclaimer: I write software for a living.

share
    
@ Tim Post, Unfortunately, I do not have reputation to upvote your answer(it requires 15). Only marked it as answer though I have doubts/comments that, I believe, deserve NOT to be lost in comments –  Gennady Vanin Novosibirsk Aug 20 '10 at 16:40
    
Ok, i've got ability to upvote by accepting your answer. +1, it is pity that correct answers are frequently undervoted –  Gennady Vanin Novosibirsk Aug 20 '10 at 17:09
    
@Tim Post, I added my answer. Does it make sense to you? –  Gennady Vanin Novosibirsk Aug 20 '10 at 17:26
    
@vgv8 - I've updated my answer. –  Tim Post Aug 20 '10 at 20:04
    
@Tim, see my comments to other answers. I cannot get WTF internal computer mechanisms (and their labels, synonyms, IDs, addresses, links, whatever you may call them, for internal use) can depend on external ones (DNS)? –  Gennady Vanin Novosibirsk Aug 21 '10 at 3:20
show 3 more comments

I believe that Windows-es know by heart both 127.0.0.1 and localhost (and even more synonyms of themselves). And, the answer probably, that it does not even resolve anything internally.

I shall see after reboot if it had lost ability to resolve localhost

But why would it?
What is the sense in it?

MS SQL Server resolves (connects through) not only through localhost name, but also through(local) and . (single-symbol dot).

So, localhost is just convenience (to write letters instead of digits and dots).
Conversion from localhost to 127.0.0.1 is silly
because I have never heard that digital addresses (numbers) are resolved to names to be further used internally by computers.

Now, I can connect to my local MSSQLServer not only by 127.0.0.1 (or localhost) but also through any address from 127.0.0.* range (for ex., I checked now having connected to local MSSQLServer through 127.0.0.2)

Though, if it is so, WTF this HOSTS entry?

Thanks to Tim Post for putting me in the direction of right thought and getting me out of turmoil.

share
    
That is great for the services you listed, but what breaks? I know you can't possibly have installed everything that dies if 'localhost' can't be resolved. I still don't quite understand why you wanted to get rid of it anyway? Maybe I'm just dense, I just want to give you a good answer. –  Tim Post Aug 20 '10 at 19:41
    
The doubt is universal and conceptual WTF local names and their synonyms have to do with DNS? It is not comprehensible conceptually - if I unplug network the computer will stop working or shall all Windows head to having internal DNS server? –  Gennady Vanin Novosibirsk Aug 21 '10 at 3:15
    
Check my comment on ChrisF his answer. If you unplug the network the only thing that will stop functioning are the request to an external DNS server and external computers; thus the HOSTS file, DNS Client Cache and internal network services will remain functional. –  Tom Wijsman Aug 21 '10 at 10:27
add comment

The hosts file is a mapping between the IP address and the host name so that when you type the host name it resolves to the specified address.

By removing the line you are stopping Windows taking "localhost" and mapping it to "127.0.0.1".

I can think of two reasons why you are seeing it "work".

  1. You haven't rebooted the machine so the mapping is still in memory.
  2. The applications you've tried it in do the mapping for you.

As long as #2 applies you don't need it, but were you to use an application that didn't do the mapping for you it would fail.

So, on balance, you should put the line back. However, you don't have to do it as a matter of urgency.

share
    
That's why the colleagues are for! If I reboot and it stops, then, I shall need to reboot again. I'm rebooting once per day (when go to sleep and get up at morning), if not more rare, so it would take me 2+ days to see if I shall not forget about this issue on the way. I tried to enable/disable network connections, various caches but evreything seems to continue working for me. –  Gennady Vanin Novosibirsk Aug 20 '10 at 13:04
2  
@vgv8: Executing ipconfig /displaydns | findstr /C:localhost will show you that it is still in the cache. Executing ipconfig /flushdns will force a flush of the cache. If you won't, it will happen when the TTL expires. The default TTL is 86400 seconds, so after a day the localhost entry will expire and thus will not resolve anymore... –  Tom Wijsman Aug 20 '10 at 16:10
    
@TonyWij, I wrote that I made flushdns many times as well as much more already –  Gennady Vanin Novosibirsk Aug 20 '10 at 17:08
    
+1 to @TonyWij, for findstr. –  Gennady Vanin Novosibirsk Aug 20 '10 at 17:14
    
@TonyWij, I also wrote that localhost as well as 127.0.0.222 work with all network connections disabled as well as without network cards inserted in my computer at all. Which internal DNS on Windows XP? –  Gennady Vanin Novosibirsk Aug 20 '10 at 17:37
show 3 more comments

Not the answer you're looking for? Browse other questions tagged or ask your own question.