Re: sunrpc port allocation and IANA reserved list

[Chuck Lever]

On Nov 10, 2009, at 4:32 PM, Ben Hutchings wrote:

> On Tue, 2009-11-10 at 15:06 -0600, Chris Friesen wrote:
> > On 11/10/2009 02:26 PM, Trond Myklebust wrote:
> > > On Tue, 2009-11-10 at 12:37 -0600, Chris Friesen wrote:
> > > > On 11/10/2009 11:53 AM, Ben Hutchings wrote:
> > > > > On Tue, 2009-11-10 at 11:43 -0600, Chris Friesen wrote:
> > > >
> > > > > > Given that a userspace application can be stopped and restarted  
> > > > > > at any
> > > > > > time, and a sunrpc registration can happen at any time, what is  
> > > > > > the
> > > > > > expected mechanism to prevent the kernel from allocating a port  
> > > > > > for use
> > > > > > by sunrpc that reserved or well-known?
> > > > > >
> > > > > > Apparently Redhat and Debian have distro-specific ways of  
> > > > > > dealing with
> > > > > > this, but is there a standard solution?  Should there be?
> > > > > >
> > > > > > The current setup seems suboptimal.
> > > > >
> > > > > I believe both RH and Debian are using the same implementation:
> > > > > <http://cyberelk.net/tim/software/portreserve/>.
> > > >
> > > > That helps with the startup case, but still leaves a possible  
> > > > hole if an
> > > > app using a fixed port number is restarted at runtime.  During the
> > > > window where nobody is bound to the port, the kernel could randomly
> > > > assign it to someone else.
> > >
> > > Just use /proc/sys/sunrpc/{max,min}_resvport interface to restrict  
> > > the
> > > range used to a safer one. That's what it is for...
>
> Unless I'm much mistaken, that only affects in-kernel SunRPC users.
>
> > What constitutes a "safer range"?  IANA has ports assigned
> > intermittently all the way through the default RPC range.  The  
> > largest
> > unassigned range is 922-988 (since 921 is used by lwresd).  If  
> > someone
> > needs more than 66 ports, how are they supposed to handle it?
>
> I'm sure we could afford 128 bytes for a blacklist of privileged  
> ports.
> However, the problem is that there is no API for userland to request
> 'any free privileged port' - it has to just try binding to different
> ports until it finds one available.

bindresvport(3) and bindresvport_sa(3t) ?

> This means that the kernel can't
> tell whether a process is trying to allocate a specifically assigned
> port or whether the blacklist should be applied.

Such a blacklist would have to be managed by glibc or libtirpc.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com



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