What is IP Address ::1 ?

Sam Varshavchik mrsam at courier-mta.com
Thu Mar 18 02:12:29 UTC 2010


Jonathan Ryshpan writes:

> Fetchmail produced a number of errors like:
>         connection to localhost:smtp [::1/25] failed: Connection
>         refused.
> These appeared to be caused by this /etc/hosts file:
>         127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
>         ::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
> which has two possible resolutions for localhost, one of which, namely "::1",
> isn't recognized.  I attempted to fix things by eliminating the "::1"
> line, to produce the hosts file:
>         127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
> But using this file causes mounts on /media to fail (in my case a
> detachable hard drive and a camera).
> Both problems are cured with this hosts file:
>         127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
>         ::1         localhost6 localhost6.localdomain6
> 
> All this seems related to IPV6 configuration, but how isn't clear to me.
> Can anyone elucidate?  Should I file a bug against the
> original /etc/hosts ?

No. The ::1 entry is correct. Put it back. This is the IPv6 loopback 
address.

It seems that the current convention is that localhost4 and 
localhost4.localdomain4 are the fake hostnames for the IPv4 loopback 
address, localhost6 and localhost6.localdomain6 for the IPv6 loopback 
address, and localhost and localhost.localdomain is both. That seems 
reasonable to me.

fetchmail is reporting an error connecting to the local smtp daemon. 
Whatever you're running, postfix or sendmail, or something else, is not 
binding to the IPv6 port 25. I don't know if fetchmail tries IPv6 or IPv4 
second, or the other way around. Either fetchmail is trying IPv6 first, 
fails to connect, then tries IPv4 and succeeds, or you don't have anything 
listening on port 25, either IPv4 and IPv6, and fetchmail is whining about 
both, and you're zeroing in on the IPv6 complaint.

In either case, the issue is not the hosts file, but either fetchmail or 
your smtp server configuration.

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