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Assuming the following is defined in .../hosts:

127.0.0.1 localhost

What, if any, are the actual differences between using 127.0.0.1 and localhost as the server name, especially when hitting processes running locally that are listening for connections?

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  • 26
    You could say localhost is protocol independent, both IPv4 and IPv6 will work. Commented Sep 12, 2011 at 2:05
  • 1
    Another different is that most browsers treat localhost as a secure origin, whereas 127.0.0.1 is not treated as secure. Commented Oct 22, 2020 at 22:46
  • @user10833119 chrome and edge do not seem to treat localhost as secure and ff says page is stored on your computer. Commented Mar 28, 2024 at 16:55
  • related (windows): serverfault.com/q/1019805 Commented Oct 31, 2024 at 10:31

5 Answers 5

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184

Well, the most likely difference is that you still have to do an actual lookup of localhost somewhere.

If you use 127.0.0.1, then (intelligent) software will just turn that directly into an IP address and use it. Some implementations of gethostbyname will detect the dotted format (and presumably the equivalent IPv6 format) and not do a lookup at all.

Otherwise, the name has to be resolved. And there's no guarantee that your hosts file will actually be used for that resolution (first, or at all) so localhost may become a totally different IP address.

By that I mean that, on some systems, a local hosts file can be bypassed. The host.conf file controls this on Linux (and many other Unices).

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5 Comments

The main difference is that the connection can be made via Unix Domain Socket, as stated here: stackoverflow.com/questions/3715925/localhost-vs-127-0-0-1
/etc/nsswitch.conf selects if hosts or DNS is used first for a host lookup, if nss is running.
@DonViegues That's MySQL specific and IMO bad design. It will see localhost and tries to use unix-socket instead of connecting using IP but for 127.0.0.1 it just uses IP.
@ArmanOrdookhani AFAIK if you are connecting to a remote host via SSH and then from there to a MySQL (like a SSH tunnel to reach the MySQL server) you have to use 127.0.0.1, as the whole thing goes over TC/IP. But if 2 processes running on the same machine want to connect, localhost (unix socket) is faster/has lees overhead. Thanks!
@DonViegues Yeah you are right unix socket has less overhead than TCP/IP. I just wanted to mention this behaviour is specific to MySQL and not something in OS or networking level.
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Wikipedia sums this up well:

On modern computer systems, localhost as a hostname translates to an IPv4 address in the 127.0.0.0/8 (loopback) net block, usually 127.0.0.1, or ::1 in IPv6.

The only difference is that it would be looking up in the DNS for the system what localhost resolves to. This lookup is really, really quick. For instance, to get to stackoverflow.com you typed in that to the address bar (or used a bookmarklet that pointed here). Either way, you got here through a hostname. localhost provides a similar functionality.

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This is actually important. localhost can resolve to an IPv6 address, whereas 127.0.0.1 cannot. Using localhost on an IPv6 only system can work, whereas 127.0.0.1 will not since IPv4 is not available.
On my Windows10 system the DNS lookup is really, really slow, about 1100 milli secs. So I switched to 127.0.0.1
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Some applications will treat localhost specially.

For example, the MySQL client will treat localhost as a request to connect to the local Unix domain socket instead of using TCP to connect to the server on 127.0.0.1. This may be faster, and may be in a different authentication zone.

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Well, by IP is faster.

Basically, when you call by server name, it is converted to original IP.

But it would be difficult to memorize an IP, for this reason the domain name was created.

Personally I use http://localhost instead of http://127.0.0.1 or http://username.

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localhost is faster, is does not use TCP/IP
@Don Viegues - what you say is contradicted to the other answers in this thread. Can you elaborate?
Yes, I will copy and paste other comment of mine: The main difference is that the connection can be made via Unix Domain Socket, as stated here: stackoverflow.com/questions/3715925/localhost-vs-127-0-0-1
@DonViegues that is specific to MySQL, which is handling localhost in a special way. Other applications will likely still lookup localhost.
If Mysql hosted on Windows computer, it could be a big difference. Connection by IP address could be much faster in that case.
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There is nothing different. One is easier to remember than the other. Generally, you define a name to associate with an IP address. You don't have to specify localhost for 127.0.0.1, you could specify any name you want.

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