In-depth understanding of Linux to modify hostname

When I felt that I was very familiar with modifying hostname under Linux system, I encountered a few problems today. These problems taught me a good lesson, many knowledge points. When you think you have mastered it, you are actually Only the fur is understood. Technology is live, don't try it before you stop!

Experimental environment: Red Hat Enterprise Linux Server release 5.7 (Tikanga), other versions of Linux may be different. Please refer to the actual environment.

In fact, I have modified the hostname many times, generally only need to modify the relevant configuration under the two files /etc/hosts and /etc/sysconfig/network. However, today I ran into two problems:

Question 1: Why is the HOSTNAME in the /etc/sysconfig/network configuration file localhost.localdomain, but the displayed hostname is po132345806-a, where is the configuration value of the hostname?

 
  1:
   [root@po132345806-a ~] 
  # more /etc/hosts 
   2:  
  # Do not remove the following line, or various programs 
  3:  
  # that require network functionality will fail. 
   4:
   127.0.0.1 localhost.localdomain localhost 
   5:
   ::1 localhost6.localdomain6 localhost6 
   6:
   [root@po132345806-a ~] 
  # more /etc/sysconfig/network 
   7:
  NETWORKING=yes 
   8:
   NETWORKING_IPV6=yes 
   9:
   HOSTNAME=localhost.localdomain 
 

There are pictures and facts, lest people not believe this phenomenon. When I first encountered this special situation, I was very puzzled. Googled some information and my own practice to figure it out

Question 2: After modifying the hostname, how to make it take effect immediately without restarting the operating system.

Question 3: How many ways to modify hostname?

Question 4: Does hostname have anything to do with the configuration under /etc/hosts?

Question 5: How to check the value of hostname, which one shall prevail?

Answer to question 1 : I always thought that the value of hostname is configured in /etc/sysconfig/network, what is the HOSTNAME configuration in this file, and what is the hostname value. But why does the above situation occur? Did /etc/sysconfig/network

It is not the configuration file of hostname, is there any other configuration file? So I experimented and modified the HOSTNAME in the /etc/sysconfig/network file to DB-Server, and found that

The value of hostname still does not change, so restart the computer

 
"/etc/sysconfig/network" 3L, 66C written 
132345806-a ~]# hostname 
806-a.gfg1.esquel.com 
132345806-a ~]# more /proc/sys/kernel/hostname 
806-a.gfg1.esquel.com 
132345806-a ~]# sysctl kernel.hostname 
ostname = po132345806-a.gfg1.esquel.com 
132345806-a ~]# 
132345806-a ~]# reboot 
 

After restarting, I found that the hostname has changed to DB-Server, which means that the HOSTNAME under the configuration file /etc/sysconfig/network has been modified to take effect. So that means someone should have modified kernel.hostname, please see the following experiment

 
[root@DB-Server ~]# more /etc/sysconfig/network 
   
NETWORKING=yes 
   
NETWORKING_IPV6=yes 
   
HOSTNAME=DB-Server.localdomain 
   
[root@DB-Server ~]# echo Test > /proc/sys/kernel/hostname 
   
[root@DB-Server ~]# more /etc/proc/sys/kernel/hostname 
   
/etc/proc/sys/kernel/hostname: No such file or directory 
   
[root@DB-Server ~]# more /proc/sys/kernel/hostname 
   
Test 
   
[root@DB-Server ~]# /etc/init.d/network restart 
   
Shutting down interface eth0: [ OK ] 
   
Shutting down loopback interface: [ OK ] 
   
Bringing up loopback interface: [ OK ] 
   
Bringing up interface eth0: 
   
Determining IP information for eth0... done. 
   
[ OK ] 
   
[root@DB-Server ~]# hostname 
   
Test 
   
[root@DB-Server ~]#  
   
 

注意：其实 /etc/init.d/network restart 没有什么用。只是当时实验时以为必须重启网络服务。

在SecureCRT新建克隆一个会话发现hostanme已经从DB-Server变为Test了，但是/etc/sysconfig/network的值还是DB-Server.localdomain，并没有变为Test。

 
[root@Test ~]# more /etc/sysconfig/network 
   
NETWORKING=yes 
   
NETWORKING_IPV6=yes 
   
HOSTNAME=DB-Server.localdomain 
   
[root@Test ~]# hostname 
   
Test 
   
[root@Test ~]# more /etc/hosts 
   
# Do not remove the following line, or various programs 
   
# that require network functionality will fail. 
   
127.0.0.1 localhost.localdomain localhost 
   
::1 localhost6.localdomain6 localhost6 
   
[root@Test ~]# more /proc/sys/kernel/hostname 
   
Test 
   
[root@Test ~]#  
   
 

但是如果重启系统后hostname会变为DB-Server，Google了一些英文文档资料才知道，hostname是Linux系统下的一个内核参数，它保存在/proc/sys/kernel/hostname下，但是它的值是Linux启动时从rc.sysinit读取的。

hostname is a kernel parameter which stores hostname of the system. Its location is"/proc/sys/kernel/hostname"

The value for this parameter is loaded to kernel by rc.sysinit file during the boot process.

而/etc/rc.d/rc.sysinit中HOSTNAME的取值来自与/etc/sysconfig/network下的HOSTNAME，代码如下所示，至此，我们可以彻底明白了。

HOSTNAME=`/bin/hostname`

HOSTTYPE=`uname -m`

unamer=`uname -r`

set -m

if [ -f /etc/sysconfig/network ]; then

. /etc/sysconfig/network

if [ -z "$HOSTNAME" -o "$HOSTNAME" = "(none)" ]; then

HOSTNAME=localhost

结论：/etc/sysconfig/network 确实是hostname的配置文件，hostname的值跟该配置文件中的HOSTNAME有一定的关联关系，但是没有必然关系，hostname的值来自内核参数/proc/sys/kernel/hostname，如果我通过命令sysctl kernel.hostname=Test修改了内核参数，那么hostname就变为了Test了。

问题2： 修改了hostname后，如何使其立即生效而不用重启操作系统。

方法1：修改了/etc/sysconfig/network下的HOSTNAME后，然后使用echo servername > /proc/sys/kernel/hostname。

[root@DB-Server ~]# echo Test >/proc/sys/kernel/hostname

注意当前会话还是不会变化，但是后续新建会话则会生效。

方法2：修改了/etc/sysconfig/network下的HOSTNAME后，然后使用sysctl kernel.hostname命令使其立即生效

[root@DB-Server ~]# sysctl kernel.hostname=Test2

kernel.hostname = Test2

注意当前会话还是不会变化，但是后续新建会话会生效。

方法3：修改了/etc/sysconfig/network下的HOSTNAME后，然后使用hostname命令使其生效

[root@Test ~]# hostname DB-Server

注意当前会话还是不会变化，但是后续新建会话会生效。

其实呢，这几种方式只是结合永久性修改和临时性修改hostname，使其不必重启Linux服务器，哈哈，不知道你明白没。

问题3： 修改hostname有几种方式？

1： hostname DB-Server --运行后立即生效（新会话生效），但是在系统重启后会丢失所做的修改

2： echo DB-Server > /proc/sys/kernel/hostname --运行后立即生效（新会话生效），但是在系统重启后会丢失所做的修改

3： sysctl kernel.hostname=DB-Server --运行后立即生效（新会话生效），但是在系统重启后会丢失所做的修改

4: 修改/etc/sysconfig/network下的HOSTNAME变量 --需要重启生效，永久性修改。

问题4： hostname跟/etc/hosts 下配置有关系吗？

如果从我上面的实验来看，其实hostname跟/etc/hosts下的配置是没有关系的。hostname的修改、变更完全不依赖hosts文件。其实hosts文件的作用相当如DNS，提供IP地址到hostname的对应。早期的互联网计算机数量少，单机hosts文件里足够存放所有联网计算机。不过随着互联网的发展，这就远远不够了。于是就出现了分布式的DNS系统。由DNS服务器来提供类似的IP地址到域名的对应。具体可以man hosts查看相关信息。

Linux系统在向DNS服务器发出域名解析请求之前会查询/etc/hosts文件，如果里面有相应的记录，就会使用hosts里面的记录。/etc/hosts文件通常里面包含这一条记录

127.0.0.1 localhost.localdomain localhost

hosts文件格式是一行一条记录，分别是IP地址、hostname、 aliases，三者用空白字符分隔，aliases可选。

127.0.0.1到localhost这一条建议不要修改，因为很多应用程序会用到这个，比如sendmail，修改之后这些程序可能就无法正常运行。

但是呢，其实hostname也不是说跟/etc/hosts一点关系都没有。在/etc/rc.d/rc.sysinit中，有如下逻辑判断，当hostname为localhost后localhost.localdomain时，将会使用接口IP地址对应的hostname来重新设置系统的hostname。

# In theory there should be no more than one network interface active

# this early in the boot process -- the one we're booting from.

# Use the network address to set the hostname of the client. This

# must be done even if we have local storage.

ipaddr=

if [ "$HOSTNAME" = "localhost" -o "$HOSTNAME" = "localhost.localdomain" ]; then

ipaddr=$(ip addr show to 0/0 scope global | awk '/[[:space:]]inet / { print gensub("/.*","","g",$2) }')

if [ -n "$ipaddr" ]; then

eval $(ipcalc -h $ipaddr 2>/dev/null)

hostname ${HOSTNAME}

我们来实验一下吧，修改hosts、network文件，修改后的值如下所示：

[root@localhost ~]# more /etc/hosts

# Do not remove the following line, or various programs

# that require network functionality will fail.

::1 localhost.localdomain localhost

127.0.0.1 localhost.localdomain localhost

192.168.244.128 DB-Server.localdomain DB-Server

[root@localhost ~]# more /etc/sysconfig/network

NETWORKING=yes

NETWORKING_IPV6=yes

HOSTNAME=localhost.localdomain

重启系统后，我们再截图看看情况：

所以这也是有时候人们以为hostname的值跟hosts文件有关系的缘故。

问题5： 如何查看hostname的值，以那个为准？

[root@DB-Server ~]# hostname

DB-Server

[root@DB-Server ~]# more /proc/sys/kernel/hostname

DB-Server

[root@DB-Server ~]# more /etc/sysconfig/network

NETWORKING=yes

NETWORKING_IPV6=yes

HOSTNAME=localhost.localdomain

Whichever shall prevail, if you understand the first 4 questions, then understanding this question is very simple.

References:

http://jblevins.org/log/hostname

http://www.ducea.com/2006/08/07/how-to-change-the-hostname-of-a-linux-system/

https://www.kernel.org/doc/Documentation/sysctl/kernel.txt

http://soft.chinabyte.com/os/281/11563281.shtml

In-depth understanding of Linux to modify hostname

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