When you need to establish connection with a hostname, that has a dynamic IP address (most typical situation – a hostname, that has an unstable Internet connection), Dynamic DNS becomes a simple yet stable solution.

When a hostname obtains an IP address, it is automatically sent to Dynamic DNS system, but users are able to connect using a symbolic name, that remains unchanged (for example, hostname.com).  In order to prevent IP caching, Dynamic DNS records have low TTL values (e.g. 60 seconds).

We’re going to cover the process of using such a configuration with a pppd connection, this package is widely used in Linux with almost all types of Internet connections.

We will take dyndns.org as a Dynamic DNS provider (you’re welcome to select another one: there is no real difference between them). We won’t describe the process of signing up and creating DNS records, but you’re always welcome to follow this guide, and ask us if anything is unclear, though it should be.

We will use inadyn for our connections. It gives the possibility to have your own fixed hostname registered on the internet, although your IP might be changing. It checks periodically whether the IP address stored by the DNS server is the real current IP address of the machine that is running INADYN. Though there are some limitations, that are described here, it is a simple solution, that doesn’t require any specific knowledge. It is a free software, that allows to connect with many Dynamic DNS providers, and you can obtain it by following this link.

You will need to download it, unpack and compile using make.

We will use it in the following way:

inadyn is started when the connection is being established (pppd acquires an IP address and creates an interface). Immediately after that it tries to perform an update and then remains in memory to periodically check for updates. If inadyn is already present in memory, it will be terminated before running a new instance.

In order to do that, we need to touch /etc/ppp/ip-up.local file (create a new one if it doesn’t exist) with the following contents:

#!/bin/sh
kill `ps -A |grep inadyn |sed ‘s/[ |\t].*$//’`
sleep 1
/etc/ppp/inadyn -u login -p xxxxx -a lampdocs.dyndns.org –background –update_period_sec 3600

Replace xxxxx with your password, login with your username, obtained at dyndns.org and lampdocs.dyndns.org with a symbolic name you’ve set for your host with dynamic IP.

You can change kill `ps -A |grep inadyn |sed ‘s/[ |\t].*$//’` if you don’t like it. Here is another variant:

#!/bin/sh
pkill inadyn
sleep 1
/etc/ppp/inadyn -u login -p xxxxx -a lampdocs.dyndns.org –background –update_period_sec 3600

That’s enough for configuration. Inadyn will be automatically started when a new pppd connection is established. You might need to enable inadyn requests in firewall, so it could connect using http without any problems. Inadyn requires just 2 addresses to connect, and both of them have fixed IPs:

checkip.dyndns.org (IP 208.78.70.70)
members.dyndns.org (IP 204.13.248.112)

You can enable these requests by issuing the following commands:

iptables -t filter -I OUTPUT -p tcp -d 208.78.70.70 –destination-port 80 -j ACCEPT
iptables -t filter -I OUTPUT -p tcp -d 204.13.248.112 –destination -port 80 -j ACCEPT

Then you should input

iptables -t filter -I INPUT -p tcp -s 208.78.70.70 –source-port 80 -j ACCEPT
iptables -t filter -I INPUT -p tcp -s 204.13.248.112 –source-port 80 -j ACCEPT

Or

iptables -t filter -I INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT

- they have the same functionality for our needs. That’s all, Dynamic DNS should be working for you. Note, that you will have to configure inadyn if you choose another Dynamic DNS provider, the configuration is not complicated and you can find it in inadyn documentation.

Please, let us know if you have any problems using Dynamic DNS.

Dynamic update represents the idea of exchanging data between two computers with known names both visiting an unknown network where we don’t know, care or trust the underlying address.

It is now possible to point your neighbor at the IETF conference to a web page on your laptop by pointing at the URL on your business card using this technology.

When this feature is enabled, your computer has a potential vulnerability, if your bind configuration is secured using TSIG keys or SIG(0). I will create a post on this subject a bit later, but now I’m going to tell you how to disable Dynamic Updates, if you are unsure whether they are secure.

We need to edit named.conf using any editor available. For each domain zone, that should be secured, we need to add a string. Let me show you how to do it. Here is my zone config before:

zone “lampdocs.com” { type master; file “/var/named/lampdocs.com.db”; };

We’re adding a line to disable dynamic DNS updates:

zone “lampdocs.com” { type master; file “/var/named/lampdocs.com.db”;
allow-update { none; };
};

That’s all. Now Dynamic DNS updates are disabled for your domain. Don’t forget to restart bind to activate your input.

You’ve registered a new domain and pointed to correct DNS servers. Everything seems to be nice: an account at your hosting is created too. You’re sitting and waiting for DNS to start resolving…

But after two or three days your fresh domain is still not accessible. You’re trying to contact hosting support and domain registrar support, but both are telling you that everything is set up and you just have to wait. But you may wait as long as you want; if your domain does not resolve within 2-3 days, it won’t resolve at all. And you have to do a simple trick that worked for me.

You need to have a hosting service to create an account for your lost domain. You won’t need it more than several hours, so you may just ask one of your friends who have an account with unlimited domains to host your domain for 3 hours.

All you need is to create an account for your domain and change DNS to another server, then you should wait until your domain resolves. Once it starts responging, you should change your DNS back to previous values. I can almost guarantee you it will be resolving to new DNS in just some minutes.

Don’t ask me why does this work as I really have no idea. Just like Windows reboot – often helps but you cannot explain why.