[Beowulf] automount on high ports

Bogdan Costescu Bogdan.Costescu at iwr.uni-heidelberg.de
Wed Jul 2 08:53:31 PDT 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 :-))

Bogdan Costescu

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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