Setting the hostname: FQDN or short name?
Solution 1:
I would choose a consistent approach across the entire environment. Both solutions work fine and will remain compatible with most applications. There is a difference in manageability, though.
I go with the short name as the HOSTNAME setting, and set the FQDN as the first column in /etc/hosts for the server's IP, followed by the short name.
I have not encountered many software packages that enforce or display a preference between the two. I find the short name to be cleaner for some applications, specifically logging. Maybe I've been unlucky in seeing internal domains like server.northside.chicago.rizzomanufacturing.com. Who wants to see that in the logs or a shell prompt?
Sometimes, I'm involved in company acquisitions or restructuring where internal domains and/or subdomains change. I like using the short hostname in these cases because logging, kickstarts, printing, systems monitoring, etc. do not need full reconfiguration to account for the new domain names.
A typical RHEL/CentOS server setup for a server named "rizzo" with internal domain "ifp.com", would look like:
/etc/sysconfig/network:
HOSTNAME=rizzo
...
-
/etc/hosts:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
172.16.100.13 rizzo.ifp.com rizzo
-
[root@rizzo ~]# hostname
rizzo
-
/var/log/messages snippet:
Dec 15 10:10:13 rizzo proftpd[19675]: 172.16.100.13 (::ffff:206.15.236.182[::ffff:206.15.236.182]) - Preparing to
chroot to directory '/app/upload/GREEK'
Dec 15 10:10:51 rizzo proftpd[20660]: 172.16.100.13 (::ffff:12.28.170.2[::ffff:12.28.170.2]) - FTP session opened.
Dec 15 10:10:51 rizzo proftpd[20660]: 172.16.100.13 (::ffff:12.28.170.2[::ffff:12.28.170.2]) - Preparing to chroot
to directory '/app/upload/ftp/SRRID'
Solution 2:
Pretty much all software is sensitive to correctly setting the hostname. While I was working at Digg I once brought the entire site down for 2 hours due to making a seemingly innocent change in /etc/hosts that affected the system's notion of hostname. Tread lightly. That said, you may be slightly confused here. I don't think the HOSTNAME= setting is directly equivalent to how Debian-based distributions use /etc/hostname.
What works for me in a heterogeneous environment is:
- Set the hostname in the vendor-recommended manner, using a conditional in your config management software.
- Use the
hostnamecommand to set the hostname used by the kernel, etc. In
/etc/hosts:127.0.0.1 localhost 10.0.0.1 hostname.example.com hostname
This configuration has not failed me yet.
Solution 3:
You will certainly have no problem finding references online which will tell you to definitely do it one way or another. It seems to me however that having a short name as the hostname, and have the fully qualified name in /etc/hosts is certainly much more prevalent. It seems like the more sensible way, as then services which need a fully qualified name can be adapted to call hostname --fqdn instead.
I've only come across one piece of software recently which rigidly requires a fqdn to be returned by hostname, which was ganeti. They document this here. I don't see any reason they cannot adapt to hostname --fqdn, however.
Solution 4:
Somewhat tangentially, while researching this question, I have been going crazy enough to check the source code of "hostname" and write a script to print investigative results (Fedora 19). What is missing is a look at "/etc/hosts", which in my humble opinion should be kept the hell out of all of this in the first place.
#!/bin/bash
function pad {
if [[ $1 == '?' ]]; then
printf "%-23s" "?"
else
printf "%-23s" "'$1'"
fi
}
# ----- Kernel -----
# Two ways to configure the kernel values:
# 1) Put FQDN into "kernel.hostname" and nothing into "kernel.domainname"
# 2) Put machine name into "kernel.hostname" and DNS domain name into "kernel.domainname" (makes more sense)
echo "== Kernel values =="
echo
H=`/sbin/sysctl -n kernel.hostname`
D=`/sbin/sysctl -n kernel.domainname`
echo "Kernel hostname: '$H'"
echo "Kernel domainname: '$D'"
# ----- What does bash say -----
echo
echo "== According to bash =="
echo
echo "HOSTNAME = '$HOSTNAME'"
# ----- Hostname config file ------
echo
echo "== Hostname config file =="
echo
ETCH="/etc/hostname"
if [[ -f $ETCH ]]; then
CONTENTS=`cat $ETCH`
echo "File '$ETCH' contains: '$CONTENTS'"
else
echo "File '$ETCH' does not exist"
fi
# ----- Network config file ------
echo
echo "== Network config file =="
echo
SYSN="/etc/sysconfig/network"
if [[ -f $SYSN ]]; then
LINE=`grep -e "^HOSTNAME=" $SYSN`
if [[ -n $LINE ]]; then
echo "File '$SYSN' contains: '$LINE'"
else
echo "File '$SYSN' exists but does not contain a line for 'HOSTNAME'"
fi
else
echo "File '$SYSN' does not exist"
fi
# ----- Nodename -------
echo
echo "== Nodename =="
echo
UNAME=`uname --nodename` # On Linux, this is the hostname
echo "The 'nodename' given by 'uname --nodename' is: '$UNAME'"
# ----- The 'hostname' mess ------
THE_HOSTNAME=`hostname`
SHORT_NAME=`hostname --short`
NIS_DNAME=`domainname`
YP_DNAME=`hostname --yp` # Same as `nisdomainname` ; this may fail with "hostname: Local domain name not set"
if [[ $? != 0 ]]; then
YP_DNAME="?"
fi
echo
echo "== 'hostname' directly obtained values =="
echo
echo "The result of gethostname();"
echo "...obtained by running 'hostname'"
echo "Hostname: $(pad $THE_HOSTNAME)"
echo
echo "The part before the first '.' of the value returned by gethostname();"
echo "...obtained by running 'hostname --short'"
echo "Short name: $(pad $SHORT_NAME)"
echo
echo "The result of getdomainname(); the code of 'hostname' seems to call this the 'NIS domain name';"
echo "...on Linux, this is the kernel-configured domainname;"
echo "...obtained by running 'domainname'"
echo "NIS domain name: $(pad $NIS_DNAME)"
echo
echo "The result of yp_get_default_domain(), which may fail;"
echo "...obtained by running 'ĥostname --yp'"
echo "YP default domain: $(pad $YP_DNAME)"
DNS_DNAME=`hostname --domain` # Same as `dnsdomainname`'
FQDN_NAME=`hostname --fqdn`
ALIAS_NAME=`hostname --alias`
echo
echo "== 'hostname' values obtained via DNS =="
echo
echo "The part after the first '.' of the 'canonical name' value returned by getaddrinfo(gethostname());"
echo "...obtained by running 'hostname --domain'"
echo "DNS domain name: $(pad $DNS_DNAME)"
echo
echo "The 'canonical name' value returned by getaddrinfo(gethostname());"
echo "...obtained by running 'hostname --fqdn'"
echo "Fully qualified hostname: $(pad $FQDN_NAME)"
echo
echo "Alias obtained by gethostbyname(gethostname());"
echo "...obtained by running 'hostname --alias'"
echo "Hostname alias: $(pad $ALIAS_NAME)"
BY_IP_ADDR=`hostname --ip-address`
ALL_IP_ADDR=`hostname --all-ip-addresses`
ALL_FQDN_NAMES=`hostname --all-fqdn`
echo
echo "== 'hostname' values obtained by collecting configured network addresses =="
echo
echo "Collect the IP addresses from getaddrinfo(gethostname()), apply getnameinfo(ip) to all those addresses;"
echo "...obtained by running 'hostname --ip-address'"
echo "By IP address: $BY_IP_ADDR"
echo
echo "Call getnameinfo(NI_NUMERICHOST) on all addresses snarfed from active interfaces;"
echo "...obtained by running 'hostname --all-ip-addresses'"
echo "All IP addresses: $ALL_IP_ADDR"
echo
echo "Call getnameinfo(NI_NAMEREQD) on all addresses snarfed from active interfaces (involves lookup in /etc/hosts);"
echo "...obtained by running 'hostname --all-fqdn'"
echo "All fully qualified hostnames: $ALL_FQDN_NAMES"
The output on an Amazon EC2 VM running Fedora 19, after manually setting the kernel values and filling /etc/hostname, but with no changes to /etc/hosts might then be like this:
== Kernel values ==
Kernel hostname: 'kyubee'
Kernel domainname: 'homelinux.org'
== According to bash ==
HOSTNAME = 'ip-172-31-24-249.localdomain'
== Hostname config file ==
File '/etc/hostname' contains: 'kyubee.homelinux.org'
== Network config file ==
File '/etc/sysconfig/network' exists but does not contain a line for 'HOSTNAME'
== Nodename ==
The 'nodename' given by 'uname --nodename' is: 'kyubee'
== 'hostname' directly obtained values ==
The result of gethostname();
...obtained by running 'hostname'
Hostname: 'kyubee'
The part before the first '.' of the value returned by gethostname();
...obtained by running 'hostname --short'
Short name: 'kyubee'
The result of getdomainname(); the code of 'hostname' seems to call this the 'NIS domain name';
...on Linux, this is the kernel-configured domainname;
...obtained by running 'domainname'
NIS domain name: 'homelinux.org'
The result of yp_get_default_domain(), which may fail;
...obtained by running 'ĥostname --yp'
YP default domain: 'homelinux.org'
== 'hostname' values obtained via DNS ==
The part after the first '.' of the 'canonical name' value returned by getaddrinfo(gethostname());
...obtained by running 'hostname --domain'
DNS domain name: ''
The 'canonical name' value returned by getaddrinfo(gethostname());
...obtained by running 'hostname --fqdn'
Fully qualified hostname: 'kyubee'
Alias obtained by gethostbyname(gethostname());
...obtained by running 'hostname --alias'
Hostname alias: ''
== 'hostname' values obtained by collecting configured network addresses ==
Collect the IP addresses from getaddrinfo(gethostname()), apply getnameinfo(ip) to all those addresses;
...obtained by running 'hostname --ip-address'
By IP address: fe80::8f6:8eff:fe49:9e21%eth0 172.31.24.249
Call getnameinfo(NI_NUMERICHOST) on all addresses snarfed from active interfaces;
...obtained by running 'hostname --all-ip-addresses'
All IP addresses: 172.31.24.249
Call getnameinfo(NI_NAMEREQD) on all addresses snarfed from active interfaces (involves lookup in /etc/hosts);
...obtained by running 'hostname --all-fqdn'
All fully qualified hostnames: ip-172-31-24-249.eu-west-1.compute.internal
The resilient way to get the fully qualified hostname in perl would then be:
sub getHostname {
my $hostname_short = `/bin/hostname --short`;
if ($? != 0) { print STDERR "Could not execute 'hostname --short' -- exiting\n"; exit 1 }
chomp $hostname_short;
my $hostname_long = `/bin/hostname`;
if ($? != 0) { print STDERR "Could not execute 'hostname' -- exiting\n"; exit 1 }
chomp $hostname_long;
if ($hostname_long =~ /^${hostname_short}\..+$/) {
# "hostname_long" is a qualified version of "hostname_short"
return $hostname_long
}
else {
# both hostnames are "short" (and are equal)
die unless ($hostname_long eq $hostname_short);
my $domainname = `/bin/domainname`;
if ($? != 0) { print STDERR "Could not execute 'domainname' -- exiting\n"; exit 1 }
chomp $domainname;
if ($domainname eq "(none)") {
# Change according to taste
return "${hostname_short}.localdomain"
}
else {
return "${hostname_short}.${domainname}"
}
}
}
and in bash it would be:
function getHostname {
local hostname_short=`/bin/hostname --short`
if [ $? -ne 0 ]; then
echo "Could not execute 'hostname --short' -- exiting" >&2; exit 1
fi
local hostname_long=`/bin/hostname`
if [ $? -ne 0 ]; then
echo "Could not execute 'hostname' -- exiting" >&2; exit 1
fi
if [[ $hostname_long =~ ^"$hostname_short"\..+$ ]]; then
# "hostname_long" is a qualified version of "hostname_short"
echo $hostname_long
else
# both hostnames are "short" (and are equal)
if [[ $hostname_long != $hostname_short ]]; then
echo "Cannot happen: '$hostname_long' <> '$hostname_short' -- exiting" >&2; exit 1
fi
local domainname=`/bin/domainname`
if [ $? -ne 0 ]; then
echo "Could not execute 'domainname' -- exiting" >&2; exit 1
fi
if [[ domainname == '(none)' ]]; then
# Change according to taste
echo "${hostname_short}.localdomain"
else
echo "${hostname_short}.${domainname}"
fi
fi
}
Notes
Note 1: HOSTNAME is a shell variable that bash provides ("Automatically set to the name of the current host.") but there is no indication about bash arrives at that value.
Note 2: Never forget /etc/hostname in /boot/initrams-FOO.img ...