[Beowulf] automount on high ports

Robert G. Brown rgb at phy.duke.edu
Wed Jul 2 11:12:05 EDT 2008


On Wed, 2 Jul 2008, Carsten Aulbert wrote:

> Which corresponds exactly to the maximum achievable mounts of 358 right
> now. Besides, I'm far from being an expert on TCP/IP, but is it possible
> for a local process to bind to a port which is already in use but to
> another host? I don't think so, but may be wrong.

AFAIK, no they don't.  The way TCP daemons that listen on a
well-known/privileged port work is that they accept a connection on that
port, then fork a connection on a higher unprivileged (>1023) port on
both ends so that the daemon can listen once again.  You can see this by
running e.g. netstat -a.  Many daemons have a limit that can be set on
the number of simultaneous connections they can manage.

However, this is for TCP ports that maintain a persistent connection.
UDP ports are "connectionless" and hence somewhat different.  They tend
to make a connection, receive a command/request for some service,
immediately deliver the result, and end the connection.  NFS used to be
built on top of UDP, and honestly I don't know what it does and how it
(NFSv3) does it on TCP and am too lazy to look it up, but the RFCs are
there to be read.

    rgb

>
> Cheers
>
> Carsten
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