Many times when working with docker containers I feel the need of assigning a known beforehand IP address to a container. This is a huge advantage if you want to control the network access to and from a container with a tool like iptables. However, current docker version (1.11.1) does not allow this operation out of the box, but there is an official way of achieving this. Thanks to docker network command a user may create a fully customizable network and connect a container to it. You may find full information at the official Docker site, here.
I will start with a clean docker installation on a test vagrant machine (Ubuntu Trusty). After installing docker, as usual, you may see docker0 network interface. This a default bridging interface. I will follow the documentation and create an isolated network using the same subnet, addresses and names.
So the first step is to create a new network:
This network will allow me to use a 172.25.0.1 - 172.25.255.254 address range. If I run ifconfig now I will see that a new interface is created. In my case, docker calls it br-98446a2a4f1f. Just to be sure I reboot my machine to see if this network persists across reboots and it does.
Now I want to start nginx container with 172.25.0.2 address, I can do it with the following command:
If I get inside the container and run ip addr command I will see that the assigned IP address is, in fact, the requested one:
Now, I will start another container just to check the connectivity between them:
So, if I get inside a second container I'm able to perform ping and telnet with the first one:
Time to check that linking between containers also works, I will start my containers this way:
Then, if I connect to my_nginx_02 container I will be able to ping and telnet my_nginx_01 host.
As you can see my_nginx_01 resolves to the IP address assigned during the startup. With this configuration, you may be able to control your security perimeter using FORWARD chain in your iptables configuration.
At the time being communication through email is less important than 10 years ago. Facebook, Whatsapp, Telegram, Slack, etc there is a feeling like old good email has less importance now for a meaning of communication. However, there is a field where this way of communication still being very important, when not the only way of communication. I'm talking about servers here.
For example, when ssh server detects break-in attempt it will try to send an email notification (or at least it could be configured in this way), or when some crontab-job fails, it also will try to send an email. But there is a problem, sometimes installing full-fledged MTA is counter-productive. Think about Raspberry Pi, if you install Postfix there it will be eating precious (and limited) resources of the machine itself. And we are not talking about the effort you will need to invest in an administration of the email server.
For myself, I found a much simpler solution called msmtp. According to their website:
msmtp is an SMTP client.
In the default mode, it transmits a mail to an SMTP server (for example at a free mail provider) which takes care of further delivery.
To use this program with your mail user agent (MUA), create a configuration file with your mail account(s) and tell your MUA to call msmtp instead of /usr/sbin/sendmail.
In other words, you may connect to any SMTP server and send messages to any directions as if they were users on localhost. I will be using gmail.com for this.
First of all, you will need to install it. Surprisingly for me, the version contained within Ubuntu repository is pretty old. However, you may build the latest version, this is not covered in this article.
Then I created a new email address at Gmail just to receive notifications from servers.
Configuration is pretty straightforward.
Please note the /etc/aliases file. We will need to use it later.
I'm not sure about the user which is going to execute msmtp command, so the log file should be created beforehand with proper permissions.
Now you can test the whole setup with the following command:
If everything is OK you should receive an email at firstname.lastname@example.org. If not, try to see the debug output and figure out what went wrong.
Fixing mail command
For mail command to work you will need to put the following in /etc/mail.rc. Please note that in order to have mail utility you will need to install mailutils package which, unfortunately, on Ubuntu also includes Postfix server.
You can test that mail works with following command:
When crontab fails to execute some command it sends an email message. In order to perform the operation, it uses sendmail executable. If you remember, in the beginning, I also installed msmtp-mta package. Doing so also creates a link /usr/sbin/sendmail to /usr/bin/msmtp. This way crontab does not need any additional configuration.
The problem here is that crontab always sends a message to a local user. I mean, if I log in as admin user and create a crontab job for this user the email will be sent to admin at localhost. It can be fixed using the following approaches.
This file defines email aliases for user. For example:
Given this configuration, all emails which go to root@localhost will also be sent to email@example.com and firstname.lastname@example.org. Also, all messages to unknown (non-existent) users will be sent to email@example.com.
Crontab offers a more elegant way of sending emails. In the beginning of the crontab file, you can edit it with crontab -e, declare MAILTO variable. Like this: