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Re: TrustKeysFrom .. a host netgroup?

From: Marion Hakanson
Subject: Re: TrustKeysFrom .. a host netgroup?
Date: Fri, 10 Jan 2003 16:10:52 -0800

> Why would you want to trust DNS ?
> M


Luke spoke (typed) my mind before I got a chance to.  I trust DNS here
because it makes sense to do so.  There are certainly circumstances
where it does _not_ make sense to trust DNS, but there are other things
to weigh when making that decision, and having cfengine's author make
the decision for me is suboptimal, in my opinion.

For us, it's convenient, it's one source of information, and the consequences
of DNS being compromised are not so terrible when it comes to cfengine, as it
is used here.  For example, we do not run cfengine continually, nor
automatically.  We use it at OS install time, and manually when configuration
changes are made.  Maybe someday we'll trust cfengine and our own cfengine
programming enough that we'll turn it loose on its own, but not just yet.

What irks me is when I have to make my code more complex in order to avoid
keeping the same piece of information in multiple places, a situation which
experience has shown causes problems far more often than we've ever had DNS
compromises to deal with.  And if I don't want to trust DNS, I can enter
IP addresses everywhere that hostnames are accepted -- most network utilities
work this way.

For those of you who want to use host names in this particular situation,
here's what I've come up with.  Note that the ${policyhost} macro and the
"policyhost" class get setup in a globally shared file that is imported
right before this bit of code gets imported into cfservd.conf:


    policyhostip = ( ExecResult(/bin/sh -c "host ${policyhost} | grep address | 
awk '{print $4}'") )
    policyhostip = ( ExecResult(/bin/ksh -c "nslookup ${policyhost} | egrep '^Ad
dress:' | tail -1 | awk '{print $2}'") )

    TrustKeysFrom = ( "$(policyhostip)" )
    AllowUsers = ( root )
    cfrunCommand = ( "$(cf_workdir)/bin/cfagent" )

    AllowConnectionsFrom = ( 192.168.10-20.* )

    AllowConnectionsFrom = ( "$(policyhostip)" )
. . .

Above, we are trusting a shell and three utilities along with DNS.
I'll leave it to the reader to decide if that mess of code is more or
less error-prone than it would be to simply trust DNS alone.

Why do I do this?  Because I use ${policyhost} (a _name_) in other
clauses, e.g. in update.conf and all over cfagent.conf to tell the client
where to find the latest config files.  Cfengine trusts DNS and accepts
a hostname _there_ (in "server=$(policyhost)" options of "copy" clauses),
why not in cfservd.conf?

Representing the same piece of information in two different locations,
in two different ways, is just asking for trouble.  They _will_ get out
of sync and lead to problems.  Way more often than DNS is compromised.
Give me a simple and consistent user (programmer) interface any day.

Thanks and regards,

Marion Hakanson <address@hidden>
CSE Computing Facilities

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