Re: Standard NBD URL syntax (again)

[Wouter Verhelst <w@uter.be>]

Hi,

On Mon, May 20, 2019 at 10:20:56AM +0100, Richard W.M. Jones wrote:
> 
> Previous discussion:
> https://lists.debian.org/nbd/2018/07/msg00002.html
> 
> Again I'm stuck because we need a URL syntax for NBD.
> 
> Various URLs have been proposed before:
> 
>   * Library      Hostname+port over TCP/IP
>                  Unix domain socket
> 		 Export name
>   - Link for further information
>   - Notes
> 
>   * libguestfs   nbd://example.com:port      (:port is optional)
>                  nbd://?socket=/socket
> 		 nbd://example.com:port/exportname
>                  nbd:///exportname?socket=/socket
>   - http://libguestfs.org/guestfish.1.html#a-nbd:-example.com-:port
>   - Cannot specify an export name without a leading '/' character
> 
>   * QEMU         nbd:example.com:port
>                  nbd:unix:/socket
> 		 nbd:example.com:port:exportname=/export
>   - https://qemu.weilnetz.de/doc/qemu-doc.html#Device-URL-Syntax
>   - Not a real URL!
> 
>   * Curl         nbd://example.com/
>                  nbd://example.com/exportname
>   - https://github.com/rwmjones/curl/commit/2aba3dd7d8f9b8e03a8c3435e7555ab9e195675a#diff-77ab3188c0ad08b7b9d8d9787c8e9d75
>   - Cannot specify an export name without a leading '/' character
>   - No Unix domain socket syntax was ever defined.
>   - Did not go upstream.

There's also dracut:

     kernel /vmlinuz-2.6.29.4-167.fc11.i686.PAE rw root=UUID=123456 netroot=nbd:192.168.0.4:2345:ext3 ip=eth0:dhcp

(from https://fedoraproject.org/wiki/QA:Testcase_Dracut_root%3Dnbd)

and the Debian initramfs-tools hooks:

nbdroot=<server>,<exportname>,<device>
nbdroot=<server>,<exportname> root=/dev/nbd0

(/usr/share/doc/nbd-client/README.Debian in the Debian nbd-client
package contains a few more examples)

these obviously aren't URLs currently, but the dracut syntax gets rather
close; and it might make sense to have a URL format that would allow
(eventually) to replace these.

> The leading problem with the "obvious" NBD URL format seems to be that
> it's impossible to specify empty export names or export names which
> don't start with a '/' character.

Not sure? We can just define the / as a separator character, rather than
as part of the name. That would then result in:

nbd://nbd-server.example.com:12345/foo

for "the export called 'foo' at nbd-server.example.com on port 12345"

nbd://nbd-server.example.com//foo

for "the export called '/foo' at nbd-server.example.com on the default
port"

Alternatively, we could URL-encode things, since the NBD spec allows way
more characters than do URLs:

nbds://nbd-server.example.com/%2Ffoo

for "the export called '/foo' at nbd-server.example.com, using STARTTLS"

That also allows to create a 

> This is why I'd be inclined to use an extra URL parameter instead,
> like:
> 
>   nbd://example.com[:port]/?export=exportname
> 
> and for Unix domain sockets:
> 
>   nbd:///?export=exportname&socket=/socket

I think I prefer the "nbd+unix" format here, that Eric suggested.

> The export would be optional, defaulting to empty string.  IPv6
> addresses would work in the obvious way for URLs, using "[...]"
> around the address part (ie as in RFC 2732).  All escaping issues
> would work as for standard URLs.

Right.

> I think we should standardize this once and for all in a document in
> the NBD repository.

+1. I have been planning to implement URL parsing in nbd-client, too;
having a standard URL format for that would definitely help.

-- 
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