Resolve hostname to IPv6 address in listen directive

Maxim Dounin mdounin at
Wed Aug 25 22:45:55 MSD 2010


On Wed, Aug 25, 2010 at 07:39:00PM +0200, Matthias-Christian Ott wrote:

> On Wed, Aug 25, 2010 at 03:04:44PM +0400, Maxim Dounin wrote:
> > On Wed, Aug 25, 2010 at 09:27:43AM +0200, Matthias-Christian Ott wrote:
> > 
> > > At the moment nignx does not allow IPv6 addresses to specified by
> > > hostname in a listen directive, that is the following will not work:
> > > 
> > >   listen;
> > >   listen [];
> > >   listen ipv6only=on;
> > >   listen [] ipv6only=on;
> > > 
> > > Though I see a potential security problem with hostnames here (this
> > > also applies to IPv4), because DNS replies can be manipulated if
> > > DNSSEC is not used, I think that this feature would be helpful and
> > > simplifies administration.
> > 
> > Note well: listen with hostname always uses *one* address returned 
> > by hostname lookup, the first one returned by gethostbyname().  It 
> > doesn't make sense to attempt to use it with hostname which 
> > resolves to multiple addresses.
> Of course you would have to use getaddrinfo() (gethostbyname() is
> removed in POSIX.1-2008 and is deprecated by RFC 3493). Multiple
> addresses for one hostname are necessary for IPv4/IPv6 dual stack.
> So I see no reason why nginx should only associate one IP address
> with a hostname.

Directive "listen" specifies exactly one socket (address:port pair 
for ipv4/ipv6, path for unix sockets) nginx will listen for 
connections (and sets options for this socket).  To listen on 
multiple sockets you have to write multiple listen directives.  
This is just how it works.

> > > Given that resolves to an IPv4 and IPv6 address, simply
> > > binding to both addresses with the following directive would break
> > > backwards compatibility: listen;
> > > 
> > > For backwards compatibility I propose the following to resolve the
> > > IPv6 addresses of a hostname and listen on them:
> > > 
> > > a) listen ipv6only=on;
> > > 
> > > b) listen [];
> > > 
> > > Solution b) has the disadvantage that it doesn't conform to RFC 3986.
> > 
> > Both are bad.  Attribute ipv6only serves completely different 
> > purpose: it disables implicit mapping of ipv6 listen sockets to 
> > ipv4 (for OSes where such mapping is on by default), i.e. 
> > instructs nginx to do setsockopt(IPV6_V6ONLY) on listen socket.  
> > See for details.
> I know what the option means and I'm aware of the fact that my
> proposed solution introduces a semantic corner case for [::] since we
> “overload” the semantics of ipv6only. But I though for backwards
> compatibility this would be a good idea. If you prefer a new syntax
> simply suggest “ipv4” and “ipv6” as in:
>   listen ipv4;
>   listen ipv6;
> This would be the cleanest solution I can come up with.

I don't see any reason why a hostname with ipv4 and ipv6 addresses 
should be handled differently from a hostname with two ipv4 

> Moreover, IPv4-compatible IPv6 address are deprecated by RFC 4291 and
> I think IPV6_V6ONLY should be the default option, so ipv6only could
> at least in 0.8 if not in 0.7 be removed or reused.

As I already said, this is for OSes which do implicitly open ipv4 
sockets for ipv6 binds.  It doesn't make sense to do extra work 
for normal ones, and it doesn't make sense to remove it while such 
OSes are still here.

> > I believe correct solution would be to make
> > 
> >     listen;
> > 
> > to use ipv6 address if no ipv4 addresses were found.
> No definitely not. This would make IPv4/IPv6 dual stack impossible. It
> should rather bind to all addresses returned by getaddrinfo() as any
> other server does. ipv4only/ipv6only or ipv4/ipv6 (depending on the
> final choice) could be used to listen only to IPv4 or IPv6 addresses
> of the hostname. This is pretty much the standard behaviour of every
> server that support IPv6 correctly.

This a) maintains backward compatibility, b) preserves "how it 
works" and c) allows to specify ipv6 address by name.  Again, 
nobody stops you from using multiple "listen" directives.

But actually:

1. I see no reason to use hostnames in "listen" directive.  From 
my experience this causes more harm than good.

2. This discussion is mostly pointless without patches.

3. It's up to Igor, so feel free to ignore me.

Maxim Dounin

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