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Aitch
Asked: 2011-03-29 00:50:46 +0800 CST

Hash function in shell that could be used for an ssh port from (say) hwaddr of eth0

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I have a growing number of remote machines that ssh home and setup a connection to allow subsequent access via tunnel so I can get into them for maintenance.

Currently, I must manually configure these machines with a unique forward ssh port prior to install at the remote location by editing a script. The rest of the install is automated (PXE). It becomes tiresome (and risks error) setting this port manually, and prevents me confidently handing the full process off to a tech.

Question> Given a clean, debian install is it feasible to write a numeric hash function of (say) the mac address on eth0 that would be deterministic and fall in some range (say 30000-60000) and you could reasonably expect to be unique(++). I guess I've got bash, awk etc to play with. I would prefer to stick to shell related tools if possible but could use python if pushed.

(++) I would amend the tunnelling script to increment the port if it had problems on the first port tried.

Example Suggested Input: ifconfig eth0 | grep HWaddr | awk '{ print $5 }' == 08:00:27:aa:bb:cc

Example Required Output: 34567

hash ssh mac-address tunneling

3 Answers

  1. This should also do what is required.

    port=$(/sbin/ifconfig eth0 | grep HWaddr | awk '{ print $5 }' | md5sum | cut -d ' ' -f1)
port=${port:25}
let "port = 0x$port"
let "port %= 30000"
let port="$port + 30000"
echo $port
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  2. I really like your thinking and I'm curious to see your tunneling script but the cynic in me says someone will mention SSH keys :-)

    We just use cssh for lots of small shell windows.

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  3. Best Answer

    Your hwaddr is just a big number, so you could always just take that mod whatever your range is and add the offset.

    #!/usr/bin/perl

$iface = $ARGV[0] || "eth0";
open(IFCONFIG, "-|") || exec "/sbin/ifconfig", $iface;
while (<IFCONFIG>) {
    if (/HWaddr ([0-9a-f:]{17})/i) {
        ($hwaddr = $1) =~ s/://g;
        $hwint = hex($hwaddr);
        print $hwint % 30000 + 30000, "\n";
    }
}
close(IFCONFIG);

    Here's the Math::BigInt version:

    #!/usr/bin/perl -w

use Math::BigInt;

$iface = $ARGV[0] || "eth0";
open(IFCONFIG, "-|") || exec "/sbin/ifconfig", $iface;
while (<IFCONFIG>) {
    if (/HWaddr ([0-9a-f:]{17})/i) {
        ($hwaddr = $1) =~ s/://g;
        $hwint = Math::BigInt->new("0x" . $hwaddr);
        print $hwint % 30000 + 30000, "\n";
    }
}
close(IFCONFIG);
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