2010-02-10
Beware of using Linux's hostname -s switch
The hostname program has a common switch, -s, which is documented
(in the Linux version) as:
-s, --short- Display the short host name. This is the host name cut at the first dot.
Although you would not expect it from this description, running
'hostname -s' will do a gethostbyname() and thus often a DNS lookup
in most Linux versions of hostname. This can of course fail if your
DNS is not working, which winds up with the very peculiar result of
hostname failing. And all of this because you innocently decided to
trim out any dots that might be present using the most obvious and
easiest approach.
(Most scripts don't cope very well with this, partly because the Bourne
shell makes it annoyingly difficult to deal with programs failing in
command substitutions and partly because come on, who expects hostname
to fail?)
Red Hat Enterprise 5, Fedora 8, Ubuntu 6.06 and Ubuntu 8.04 have
versions of hostname that behave this way. Fedora 11 has a
version that does not, because someone filed a bug about it; unfortunately I
can't tell if this has been fixed upstream or if an upstream bug has
been filed (or if it would be useful to do so).
The sad conclusion is that for the next several years, if you need the local hostname without any dots on it you should write something like:
hostname | sed 's/\..*//'
instead of using the shorter, nicer hostname -s.
(We found this out the hard way last night, when we had some sort of network issue that made our DNS servers unreachable to some machines while some of our status check scripts were running.)