Configurando o nome do host: FQDN ou nome curto?

Eu escolheria uma abordagem consistente em todo o ambiente. Ambas as soluções funcionam bem e permanecerão compatíveis com a maioria dos aplicativos. Há uma diferença na capacidade de gerenciamento, no entanto.

Eu vou com o nome abreviado como a configuração HOSTNAME e defino o FQDN como a primeira coluna /etc/hostsdo IP do servidor, seguido pelo nome abreviado.

Não encontrei muitos pacotes de software que imponham ou exibam uma preferência entre os dois. Acho que o nome curto é mais limpo para alguns aplicativos, especificamente o registro. Talvez eu tenha tido azar em ver domínios internos como server.northside.chicago.rizzomanufacturing.com. Quem quer ver isso nos logs ou em um prompt de shell ?

Às vezes, estou envolvido em aquisições ou reestruturações de empresas onde os domínios internos e/ou subdomínios mudam. Eu gosto de usar o nome de host curto nesses casos porque log, kickstarts, impressão, monitoramento de sistemas, etc. não precisam de reconfiguração completa para contabilizar os novos nomes de domínio.

Uma configuração típica de servidor RHEL/CentOS para um servidor chamado "rizzo" com domínio interno "ifp.com" seria semelhante a:

/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'

Praticamente todos os softwares são sensíveis à configuração correta do nome do host. Enquanto eu estava trabalhando no Digg, uma vez eu desativei o site inteiro por 2 horas devido a uma mudança aparentemente inocente /etc/hostsque afetou a noção de nome de host do sistema. Pise levemente. Dito isto, você pode estar um pouco confuso aqui. Eu não acho que a HOSTNAME=configuração seja diretamente equivalente a como as distribuições baseadas em Debian usam /etc/hostname.

O que funciona para mim em um ambiente heterogêneo é:

1. Defina o nome do host da maneira recomendada pelo fornecedor, usando uma condicional em seu software de gerenciamento de configuração.
2. Use o hostnamecomando para definir o nome do host usado pelo kernel, etc.

3. Em /etc/hosts:

   127.0.0.1    localhost
   10.0.0.1     hostname.example.com     hostname
   

Esta configuração ainda não me falhou.

Você certamente não terá problemas para encontrar referências on-line que lhe dirão para fazer isso de uma maneira ou de outra. Parece-me, no entanto, que ter um nome curto como o nome do host e ter o nome totalmente qualificado em /etc/hosts certamente é muito mais prevalente. Parece ser a maneira mais sensata, pois os serviços que precisam de um nome totalmente qualificado podem ser adaptados para chamar hostname --fqdn.

Eu só encontrei um software recentemente que exige rigidamente que um fqdn seja retornado por hostname, que foi ganetti. Eles documentam isso aqui . Não vejo nenhuma razão para que eles não possam se adaptar hostname --fqdn, no entanto.

Um pouco tangencialmente, enquanto pesquisava essa questão, fiquei louco o suficiente para verificar o código-fonte de "hostname" e escrever um script para imprimir os resultados da investigação (Fedora 19). O que está faltando é uma olhada em "/etc/hosts", que na minha humilde opinião deve ser mantido fora de tudo isso em primeiro lugar.

#!/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"

A saída em uma VM do Amazon EC2 executando o Fedora 19 , após definir manualmente os valores do kernel e preenchimento /etc/hostname, mas sem alterações /etc/hostspode ser assim:

== 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

A maneira resiliente de obter o nome do host totalmente qualificado em perl seria:

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}"
      }
   }
}

e no bash seria:

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
}

Notas

Nota 1: HOSTNAME é uma variável de shell que o bash fornece ("Definido automaticamente com o nome do host atual"), mas não há indicação de que o bash chega a esse valor.

Nota 2: Nunca esqueça /etc/hostname em /boot/initrams-FOO.img...

Short answer: I generally use the FQDN as the hostname (as recommended on RH6/7 docs). However, the more righteous approach would be to use a single-label name as the hostname, setting the FQDN via /etc/hosts. So, pick an approach and stick with it whenever possible.

Long answer: The main advantage of using the FQDN as the hostname is that the machine name intrinsically embeds domain information. This is very useful when receiving email alert and/or logs for multiple customers/domains, as it avoid duplicate hostnames (ie: hostnames are more-or-less guaranteed to be unique across multiple sites/customers/domains). For example, SNMP sysName show the hostname by default and, by using the FQDN, it simply conveys a more useful information. The same is true for zabbix-agent (and other monitor tools) or for bash $HOSTNAME. While you can configure such tools for using FQDNs even on machine with single-label hostname, or configure them in an hierarchical model which clearly shows the domain containing the machine, this is additional work.

Some application even require using a FQDN as the hostname, but they are quite the exception. Still, when the exception hits, the homogeneous use of single-label hostname is lost. This probably is the main reason behind RedHat's recommendation to use a FQDN as the hostname on RH6/7. More recent docs are more vague. On "Performing a standard RHEL 8 installation" one can read:

The host name can be either a fully-qualified domain name (FQDN) in the format hostname.domainname, or a short host name with no domain name. Many networks have a Dynamic Host Configuration Protocol (DHCP) service that automatically supplies connected systems with a domain name. To allow the DHCP service to assign the domain name to this machine, specify only the short host name

while "Performing an advanced RHEL 8 installation" states:

If your network does not provide a DHCP service, always use the FQDN as the system’s host name

In the years between the original question, user applications have learned to treat FQDN hostnames without the issue described in the previous answers. For example, bash PS1 prompt uses \h (the hostname up to the first '.') by default, and rsyslog does not preserve FQDN in logfiles by default. In other words, the usability drawback of a FQDN hostname are no more for many common tools. For these reasons, I generally set the system hostname as a FQDN, leaving the shortname on /etc/hosts as required.

The reason to not use a FQDN hostname is that, frankly, this is The Right Thing To Do. As respectively stated in the debian docs and hostname man page:

This file should contain only the system hostname, not a fully qualified domain name

and

It is recommended that this name contains only a single label, i.e. without any dots

So, while I do feel uneasy to use a FQDN hostname, it is easier using it in my environments.

Please note this is not an appeal against single-label hostname. If they work for you, continue using it. Otherwise, give the FQDN hostname a try.

As opções /etc/hosts funcionam bem.

Mas você quer ter certeza de que todos os arquivos apropriados estão atualizados, execute a ferramenta de configuração

Hm... Nos hosts linux, se você deseja alterar HOSTNAME e FQDN, deve resolver 3 etapas (por exemplo, novo host é rizzo.ifp.com ):

Etapa #1 Altere o valor de HOST na configuração de propriedades de REDE:

sudo vi /etc/sysconfig/network

Altere ou adicione string:

HOSTNAME=rizzo.ifp.com

Etapa #2 Edite sua configuração de hosts

sudo vim /etc/hosts
#IPv4
127.0.0.1       localhost localhost.localdomain localhost4 localhost4.localdomain4
[External IP]   rizzo rizzo.ifp.com

Passo #3 Reinicie seu host Bem feito, apenas verifique a nova configuração

[rizzo@rizzo]# hostname -f
rizzo.ifp.com

A ordem não está certa. Tem que ser:

1.2.3.4 full.example.com full

Então o exemplo poderia ser assim:

[External IP] rizzo.example.com rizzo