[Beowulf] automount on high ports
Bogdan.Costescu at iwr.uni-heidelberg.de
Wed Jul 2 11:53:31 EDT 2008
On Wed, 2 Jul 2008, Robert G. Brown wrote:
> 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.
'man 7 socket' and look up SO_REUSEADDR. I don't quite know what you
mean by 'forking a connection'; when the daemon encounters a fork()
all open file descriptors (including sockets) are being kept in both
the parent and the child. The child (usually the part of the daemon
that processes the content that comes on that connection) gets the
same 4-tuple as the parent. The parent closes its file handle so that
only the child is then active on that connection.
> You can see this by running e.g. netstat -a.
I seriously doubt that you have seen such a behaviour. Empirical
evidence which might pass easier than theoretical one: on the e-mail
server that I admin, there is an iptable rule to only allow incoming
connections to port 25 - if connections would suddenly be migrated to
different ports they would be blocked and I would not receive any
e-mails from this list. But I do, especially during the past few
days... (not that I complain :-))
IWR, University of Heidelberg, INF 368, D-69120 Heidelberg, Germany
Phone: +49 6221 54 8869/8240, Fax: +49 6221 54 8868/8850
E-mail: bogdan.costescu at iwr.uni-heidelberg.de
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