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Re: Free TCP/IP port numbers?




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On Sun, Oct 01, 2017 at 07:43:47AM -0400, Gene Heskett wrote:
> On Sunday 01 October 2017 03:34:19 tomas@xxxxxxxxxx wrote:
> 
> > On Sun, Oct 01, 2017 at 01:28:39AM -0400, Gene Heskett wrote:
> >
> > [...]
> >
> > > > > Assuring that my port is not in this IANA list is not enough to
> > > > > ensure that my port number will not clash with a port number
> > > > > used by a Debian package (by default).
> > > > >
> > > > > So your answer to my question is wrong.
> > >
> > > In which case debian should publish the unlisted ports they do use,
> > > if for no other reason than to "stake a claim".
> >
> > "Debian" "should". Gene, you "should" know better ;-)
> >
> > Want to start with it? Write a script which scans the /etc files in
> > all Debian packages for network configurations.
> >
> That might be possible IF you wanted to use a tool like grep, but in 30 
> years I've not found a way to silence the "binary file matches" messages 
> from grep. That apparently un-muffle-able noise without chaining two or 
> more invocations of grep makes it worthless for 95% of the searches I 
> might do. The best I can do finds 460 instances of " port " in my 
> own /etc tree, but from looking at that output, less than 100 actually 
> assign a number, most use the output of some other function to assign 
> the port.

Out of grep's fine manual:

       --binary-files=TYPE
              If the first few bytes of a file indicate that the
              file contains binary data, assume that the file is
              of type TYPE.  By default, TYPE is binary, and grep
              normally outputs either a one-line message saying
              that a binary file matches, or no message if there is
              no match.  If TYPE is without-match, grep assumes that
              a binary file does not match;  this  is  equivalent  to
              the  -I option.   If  TYPE  is text, grep processes
              a binary file as if it were text; this is equivalent
              to the -a option.  Warning: grep --binary-files=text
              might output binary garbage, which can have nasty side
              effects if the output is a terminal and if the terminal
              driver interprets some of it as commands.

So I'd try --binary-file=without-match (or its shorter cousin -I).

> So opening up every deb in /var/cache/apt/archives to search thru each 
> ones /etc files might take this machine a week or more, and you would 
> still have less than 25% of the numerical values. One things for sure, 
> it would take a more imaginative approach than mine because so much of 
> it appears to be dynamic assignments. One would have to emulate how each 
> goes about it, and then its only valid for that machine at that box of 
> time, however long it took.
> 
> However, since it seems so much of that is dynamic, one could possibly 
> use the dynamic method to find a currently unused server port when the 
> client requests a connection, and the client can check the number 
> assigned against its own list of ports, and accept or reject, wash rinse 
> repeat until one is usable by both.  Correctly done, I see at least 
> 20,000 possibilities in the /etc/services list. The OP just needs to 
> find a coder who can write such a critter.

Problem is, there are several such "dynamic" approaches. The older one
is Sun RPC's portmapper, which does have its downsides.

So my advice would be "relax". If there's a sysadmin around, just make
debugging of a clash easier, if not, look into all of those container/
orchestration frameworks, which have to tackle the problem more
systematically.

Cheers
- -- tomás
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