sendmail and sm-client services are very slow to start the Linux RHEL/CentOS

                                         

After changing the IP address of a server, at the time of booting up sendmail and sm-client took a very long time (about 4 minutes each) to start. Usually when sendmail is sluggish starting - it's because it can't figure out hostname details.

# cat /etc/hosts

127.0.0.1  localhost.localdomain localhost

above needs to be the very first line of /etc/hosts

if you are accepting email for a domain then it would help if you have

the fully qualified domain name set in /etc/sysconfig/network and that

name resolves via dns or at least is listed with an ip address

in /etc/hosts...

i.e.

# hostname

linuxserver.mydomain.com


 # cat /etc/sysconfig/network

NETWORKING=yes

HOSTNAME=linuxserver.mydomain.com


 # cat /etc/hosts

# Do not remove the following line, or various programs

# that require network functionality will fail.

127.0.0.1 localhost.localdomain localhost

192.168.1.2 linuxserver.mydomain.com linuxserver

# host linuxserver.mydomain.com

linuxserver.mydomain.com has address 192.168.1.2


# cat /etc/mail/local-host-names

mydomain.com

linuxserver.mydomain.com

www.mydomain.com

localhost

localhost.localdomain

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           IP forwarding also known as Internet routing is a process used to determine which path a packet or datagram can be sent. The process uses routing information to make decisions and is designed to send a packet over multiple networks.
                     Defaultly any UNIX/Linux distributions will have IP Forwarding is disabled.  As most of the peoples will not need IP Forwarding, but if we are setting up a Linux router/gateway, when you want your system to act as VPN server, and for sharing the internet connection. See the below steps for enable or disable IP forwarding in linux.

Step1: Check the IP forwarding is enable or disable
   
               In this step we have to check the IP forwarding using sysctl command

 [root@bsrtech ~]# sysctl net.ipv4.ip_forward

 
  Sample Output:
  

             net.ipv4.ip_forward = 0

Step2: To Enable IP Forwarding Temporarily
    
     To enable IP Forwarding temporarily,you can use any of the following commands

   [root@bsrtech ~]# echo 1 > /proc/sys/net/ipv4/ip_forward

     (or)
   
   [root@bsrtech ~]# sysctl -w net.ipv4.ip_forward=1 

  To check the IP Forwarding is enabled or not , execute the following command

   [root@bsrtech ~]# sysctl net.ipv4.ip_forward

Step3: To Enable IP Forwarding Permanently
  To enable IP forwarding permanently, you can change value in configuration file /etc/sysctl.conf

  [root@bsrtech ~]# vim /etc/sysctl.conf

   you can find the line "net.ipv4.ip_forward" and Set net.ipv4.ip_forward to 1
  
      

    net.ipv4.ip_forward = 0
    
    to

    net.ipv4.ip_forward = 1

  :wq (Save and close the file)

                                                          

Step4:  Reload the changes 
  To load the settings that we made to /etc/sysctl.conf file, run the following command

 [root@bsrtech ~]# sysctl -p

  Sample Output:
                                   

 Step5: To Disable IP forwarding Permanently

     you can find the line in /etc/sysctl.conf "net.ipv4.ip_forward" and Set net.ipv4.ip_forward to 0 

    net.ipv4.ip_forward = 1
    
    to

    net.ipv4.ip_forward = 0
  
   :wq (save and close the file)

 
  Reload the changes using sysctl command

  [root@bsrtech ~]# sysctl -p

  Sample Output:
                                         

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Linux Administrators  normally login to the linux servers either 
supplying a password, or using keybased authentication. sshpass 
command is a tool which allows us to automatically supply 
password to the command prompt so that automated scripts can 
be run as desired by users.




Features of sshpass :

 1.Taking backups to a remote server
 2.Executing commands on systems at a specified time.

Step1:
Setup the RPMforge  (or)  EPEL repository

Then run root 

   #yum install sshpass -y

Getting Help :

  #sshpass -h

Usage: sshpass [-f|-d|-p|-e] [-hV] command parameters
   -f filename      Take password to use from file
   -d number       Use number as file descriptor for getting password
   -p password    Provide password as argument (security unwise)
   -e                    Password is passed as env-var "SSHPASS"
  
 With no parameters - password will be taken from stdin

   -h                 Show help (this screen)
   -V                Print version information
At most one of -f, -d, -p or -e should be used

How to use sshpass command:
Login to ssh server called server.example.com with password called trDR34@r%$#:

$ sshpass -p 'trDR34@r%$#' ssh username@server.example.com

How to backup data using sshpass:

$ sshpass -p 'password' scp  -r  username@server.example.com:/var/www/*   /backupdata

(where backup data from remote server to local server)

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