Web Hosting Journal
Sometimes, you may want to run a screen command in a remote server. That makes it necessary to run the command inside the screen session while starting it.
# screen -d -m sh -c "yourcommand"
From the man page of Screen:
-d -m : Start screen in “detached” mode. This creates a new session but doesn’t attach to it. This is useful for system startup scripts.
sh -c: Starts a shell and runs a command for you.
Exim provides a quick way to check the total number of mails in the queue. This is done using the exim -bpc Although, this is not the same for postfix. Postfix doesn’t come with an easy way to do that.
A quick tip on what I use to check the postfix queue number is the following command:
# mailq | tail -n 1
-- 6899 Kbytes in 1518 Requests.
Basically, postfix returns the queue statistics at the end of the queue listing command. We are simply tailing that to find the number.
CentOS 7 doesn’t come with ifconfig tools. It encourages users to use ‘ip’ tool for network administration. Although, it is still possible to use ifconfig with CentOS 7. ifconfig is a part of net-tools package. All you have to do is to install the net-tools package using yum.
Run the following command to install net-tools package in CentOS 7, this will install ifconfig as well:
# yum install net-tools -y
# ifconfig
In CentOS 6, timezone files are located under /usr/share/zoneinfo. So, if your zone is for example, America/Chicago (UTC -6), it would be /usr/share/zoneinfo/America/Chicago and so on.
CentOS 6, uses a file called ‘localtime’ located under /etc to determine it’s currently set timezone.
# ls -la /etc/localtime
This file, is either the actual time zone file moved to this location or a symlink to the timezone under zoneinfo directory. So if you want to change the timezone, first you need to determine which timezone to use and then symlink it to localtime. You can do that using the following:
# rm -f /etc/localtime
# ln -s /usr/share/zoneinfo/Asia/Dhaka /etc/localtime
# date
This would set the current timezone to GMT +6 BDT or Asia/Dhaka timezone, zone I belong to.
CentOS 7, comes with a tool called ‘timedatectl’. This can be used to find and set the symlink for you instead of doing the work that were required in CentOS 6.
To list available time zones, run:
# timedatectl list-timezones
You can find your desired timezone, as following:
# timedatectl list-timezones | grep Chicago
Now, to set a time zone, use the command set-timezone with timedatectl command. For example, if I want to set the time zone to America/Chicago, I would run the following:
# timedatectl set-timezone America/Chicago
# date
This also should create the symlink of locatime file to the zoneinfo directory. You can see that with the following:
# ls -l /etc/localtime
If you are using Centos 7 or RHEL 7 or any of it’s variant, you are probably using ‘Firewalld’ by default. Although, if you are a iptables fan like me, who likes it’s simplicity and manipulative nature instead of a full form firewall, then you probably have disabled firewalld from your CentOS 7 instance and using iptables. There are couple of servers, where I use runtime iptables rules for postrouting and masquerading. These rules are dynamically generated by my scripts instead of the sysconfig file under:
/etc/sysconfig/iptables
This file is generated upon running the iptables save command:
service iptables save
which I rarely do so.
Error Details
Which is why, I don’t have a /etc/sysconfig/iptables file in those servers and a common error I see while restarting iptables in those system is the following:
# systemctl restart iptables.service
Assertion failed on job for iptables.service.
How to Fix The Error
The error appears because you don’t have any rule in /etc/sysconfig/iptables or the file doesn’t exist either. You can ignore the error as iptables would still run. To eradicate the error, simply make sure you have some iptables rules loaded on your system using the status command:
iptables -S
And then, run:
service iptables save
Once done, restarting iptables shouldn’t show the error any longer.