Naming etc. (Was: DHCP Help)

Josip Loncaric josip at icase.edu
Thu Apr 11 12:00:04 EDT 2002


"Robert G. Brown" wrote:
> 
> [...] my hosts are all on a private
> internal network anyway and not in nameservice.

Good policy!  Private hostnames/addresses should remain private because
they are not guaranteed to be unique across the entire Internet.  The
DNS server should contain only registered hostnames/addresses.

The head node of a cluster is typically multi-homed and its public
interface should be DNS registered, but the internal private interface
(and the client nodes on the internal private network) are best resolved
via /etc/hosts, where internal domain name is determined from the FQDN
form of the name.  If /etc/hosts on client1 contains:

192.168.1.1     client1.internal.domain   client1

then 'dnsdomainname' on client1 returns 'internal.domain' (clearly not
found in any Internet registry).  This would work fine internally, but
NOT outside the cluster (e.g. sendmail may have problems, etc.).

The /etc/hosts tables should be consistent across the cluster, even if
there are reasons to play tricks.  For example, one typically has all
machines on a fast ethernet (FE) subnet (say 192.168.1.x) but a few may
also have gigabit ethernet (GE) interfaces (say 192.168.2.x).  Using IP
level routing can result in complicated routing tables, because only
specific FE hosts can also be reached via the GE interface.  What about
name level "routing"?  While /etc/hosts can be used to make hostnames of
GE machines resolve to GE addresses on GE machines but to their FE
addresses on the FE-only machines, this can lead to problems with
software packages which assume globally consistent hostname/address
mapping.

For example, grid software (Globus) needs a globally consistent FQDN/IP
mapping.  The grid machine name is the fully-qualified domain name or
Internet name of a grid machine. It should be the name returned by the
"gethostbyname()" function (from libc) and the primary name retrieved
from DNS via nslookup.  The primary name should correspond to the host's
primary interface (if there is more than one) and be fully accessible
across the grid.  The grid could involve private addresses, but those
are visible only WITHIN an organization because private addresses must
not be routable outside an organization.  This is a serious limitation
-- so it is probably best to limit grids to publicly registered hosts
only.  Proxy processes on the head nodes to access internal machines may
be needed.

Most clusters are built around a private subnet, sometimes with IP
masquerading enabled on the head node so that the internal clients can
'call out'.  This still means that internal clients are not visible
externally, i.e. one cannot 'call in' from the outside.  As a
consequence, parallel jobs which assume global TCP connectivity of all
participating machines (e.g. MPICH-G2) will have problems in using two
clusters (each with its own private internal subnet).  At the moment,
every node (that you wish to use in a MPICH-G2 job) must
have a public IP address and must be fully accessible.  To run jobs
across several clusters with internal private networks, the MPI
programmer would need to provide a proxy process on the head node to
overcome this difficulty.

In summary, naming is a simple concept but just under the surface is a
can of worms created by established programming practices based on
diverse assumptions.  Multiply connected machines and/or public/private
network mixtures need to be set up with great care.  Tricky setups are
fragile; simplicity and transparency works better.

Sincerely,
Josip

-- 
Dr. Josip Loncaric, Research Fellow               mailto:josip at icase.edu
ICASE, Mail Stop 132C           PGP key at http://www.icase.edu./~josip/
NASA Langley Research Center             mailto:j.loncaric at larc.nasa.gov
Hampton, VA 23681-2199, USA    Tel. +1 757 864-2192  Fax +1 757 864-6134
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