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Comment by lotyrin | Hacker News Reader

Ha, I remember back when I first had Jekyll tell me to browse to http://0.0.0.0:NNNN/ and thinking "This can't possibly be correct, certainly that's invalid. Oh, it worked."

Glad I wasn't losing my mind, but this certainly is something people use, I guess it should probably be allowed.

That said, I think a local server that's listening on all interfaces/addresses should probably feel confident to say "please open http://localhost:NNNN" I've definitely seen neophytes to various tech get anxious about numbers appearing where they expect a name, and it's less clear what 0.0.0.0 is, if maybe it's something special and different from the "localhost" they're accustomed to (because it is... at least in theory).

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Imagine you are developing inside a VM but use the browser on the host OS to check the results. If the bind address was localhost then you would need to tunnel your connection from host to guest to be able to reach your server.

By binding to 0.0.0.0:PORT the guest OS will make your service available on all enabled interfaces:PORT combinations which, hopefully will "just work" in most use cases.

I'm not recommending anyone change the address they listen on by default. Certainly the wildcard is a sensible default for the reasons you mention.

binding to localhost also prevents others on the lan to access your service, like you phone over wifi

The right way for a server to do this is to listen on 0.0.0.0 and then examine the socket to see what address(es) it is listening on, and report a real address to the user. It's just laziness (or ignorance) to print 0.0.0.0.

Yep, exactly this. I do that in a Go app that I posted in my top level comment.

The difference is that 0.0.0.0 represents all addresses, while localhost is /one of/ those addresses.

Right, so it might make sense to go in the listen parameter of an app, but probably not for me to enter into my browser for how I get to that app.

Yeah 0.0.0.0 is a listening address, not a target.

not all addresses but all interfaces, which can have one or more addresses plumbed in.

i was always under the impression that 127.0.0.1 is what you should use

That would account for the case where my local machine doesn't have a host entry for "localhost" but would break if I don't have an IPV4 loopback (if, say, I only have a IPV6 one).

Both of those are pretty odd cases, so I'm not sure how much they're worth worrying about.

I could be wrong, because networking is Not My Thing At All, Oh God, Make The Pain Stop, Why Does The Routing Table Have Bees In It, but shouldn't 127.0.0.0/8 be able to find the IPv6 loopback if the IPv4 one does not exist? Or am I misunderstanding yet another part of networking?

Nope, perfectly possible to not have an IPV4 address.

  $ sudo ifconfig lo 0
  $ ifconfig lo
  lo        Link encap:Local Loopback
            inet6 addr: ::1/128 Scope:Host
            UP LOOPBACK RUNNING [... the rest omitted for brevity]
  $ ping 127.0.0.1
  PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
  ^C
  --- 127.0.0.1 ping statistics ---
  32 packets transmitted, 0 received, 100% packet loss

Interesting. I guess I assumed it'd tack a bunch of zeroes on the front and try it as an IPv6 address. Thanks!

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