What Is DNS, and Why Does a Phone Need It?
DNS is the "phone book" of the internet: it translates an easy-to-remember name, such as a server name, into an IP address that the computer knows how to reach. A phone set up with a PBX server name needs working DNS to find it. When DNS doesn't respond, the phone can't register.
The phone book of the internet
Every device on the internet is identified by an IP address, a string of numbers such as 192.0.2.25. Computers do fine with numbers, but people find them hard to remember. That is why websites and servers have names, such as example.com.
DNS (Domain Name System) is what connects the two. When you type a website name, the computer first asks a DNS server: "What is the address of this name?" The server answers with a number, and only then does the computer contact the address itself.
It is like calling directory assistance to find a business's number: you know the name, you look up the number, and then you dial. The difference is that with DNS the lookup takes a few milliseconds and happens thousands of times a day without you noticing.
How it works behind the scenes
When a device needs an address, it asks the DNS server it was set up with, usually the office router, which passes the question on to the internet provider's server. If that server doesn't know the answer, it asks other servers in the chain, until it reaches the server that is responsible for that name and knows for certain.
The answer is kept in memory (cache) for a while, on the device, in the router and on the provider's server. That way, the next time, the answer arrives immediately without asking the whole chain again. Every answer comes with an "expiration date" (TTL), which sets how long it may be kept before asking again.
Because of this memory, a change of server address does not reach everyone at the same moment. Devices that remember the old address keep using it until the stored answer expires.
Why an IP phone needs DNS
An IP phone is really a small computer. To work, it has to connect to the PBX server and register with it, in effect saying "I'm extension 104, I'm here, send calls to me." The phone's settings contain the server address, and it is usually written as a name and not as a number.
So before every registration, the phone asks DNS for the address of the server's name. If DNS doesn't respond, or gives a wrong answer, the phone doesn't know where to connect. The screen shows something like "Not registered," "No Service" or an icon of a disconnected line, even though the internet itself is working.
The same is true for a softphone on a computer or mobile, and for a phone that receives its settings automatically from the server: it too has to translate a name into an address before it downloads the settings file.
Why a name and not a number
You could enter the server's IP address directly in the phone, and some systems do. But a name has a big advantage: flexibility.
When the server moves elsewhere, or when there are several servers backing each other up, it is enough to change the answer in DNS. All the phones find the new address on their own, without anyone having to go to each phone and change a setting. In an office with dozens of phones, that is the difference between a minute and a full workday.
In addition, DNS can return several addresses for the same name, or point to a different server by order of priority. This is one of the tools that lets a phone switch to a backup server when the main server is unavailable.
Record types worth knowing
DNS has different types of answers, called records. Most of them don't matter to an office manager, but a few are directly related to telephony:
| Record | What it means | Relevance to telephony |
|---|---|---|
| A | This name = a specific IPv4 address | The basics: this is how the phone finds the server |
| AAAA | This name = an IPv6 address | In systems that support IPv6 |
| CNAME | This name is an alias for another name | Lets you replace a server without changing the name in the phones |
| SRV | For a specific service (for example SIP): which servers, on which port and in what order | Lets the phone find a main server and a backup server by order of priority |
| MX | Where to send email for this name | Important for voicemail messages and faxes sent to email |
There is no need to memorize the table. What is worth remembering is that when something "can't find the server," one of the first questions is whether DNS is answering correctly.
DNS servers: your provider's, your router's, or public ones
Every device needs to know whom to ask. In most offices, the router's DHCP hands out this setting to all devices together with their address, and it usually points them to the router itself. The router passes the questions on to the internet provider's DNS servers.
You can also set large public DNS servers, whose addresses are well known. Some offices do this because the provider's servers are slow or unstable.
For users on a filtered connection there is an important point here: in some filtering services, DNS is part of the filtering itself. Changing the DNS servers on your own could break the filtering or cause websites and services to be blocked. Before making any such change, consult your filtering provider.
Common DNS problems and how they look
- "The internet works, but the phones aren't registered": one of the classic signs. Browsing familiar websites works from memory, and the phone can't translate the server name.
- After replacing a router: the new router hands out a wrong DNS address, or its DNS isn't working well.
- A fixed DNS address in the phone: someone previously set a manual DNS in the phone, and that server no longer exists.
- Slow DNS: every registration is delayed by a second or two. This is usually noticed as phones that connect slowly after a power outage.
- An old answer in memory: after an address change, one device keeps using the previous address until its memory is cleared. Restarting it solves the problem.
How to check DNS yourself
On a Windows computer, open a command prompt and type nslookup followed by the name of a website or server. If you get an address, DNS is responding. If you get a message such as "can't find" or "timed out," there is a problem.
Another simple check: ping the name of a familiar website, and then ping a familiar IP address. If the address responds and the name doesn't, the internet is working and DNS is not. This tells apart "no internet" from "no DNS."
On an IP phone you can usually see on the information screen (Status) which DNS address it received. If an unfamiliar address appears there, or 0.0.0.0, check the phone's network settings and the DHCP in the router.
A real-world example: the morning after replacing the router
On Sunday morning, the staff of a bookstore (a fictional name: "Sifrei Hadar") arrived and found that all the phones showed "Not registered." The internet worked, and emails were coming in. On Friday afternoon, a technician from the internet provider had replaced their router.
A check of the information screen on one of the phones showed that it had received a DNS address that doesn't respond. The new router had been set up with an old DNS server that the technician typed in manually. After fixing the DNS address in the router and restarting the phones, all the extensions registered within a minute.
The lesson: after replacing a router, make a test call from every type of phone in the office, and don't settle for checking that the internet works.
DNS, privacy and security
DNS queries usually travel unencrypted, and anyone along the way can see which names the device is looking up. In recent years, methods for encrypting the queries (DNS over HTTPS and DNS over TLS) have been added, and some browsers turn them on by themselves.
In a filtered office, it is important to know that this kind of encryption in the browser can bypass the DNS servers the office has set up, so many filtering providers recommend leaving this setting as they instruct.
On the protection side: only the network administrator should be able to change the DNS settings in the router, and the password for the router's management interface should not remain the default password. A DNS change in the router affects every device in the office.
DNS when working from home and on a softphone
An employee who connects from home gets the DNS server from their home router, not from the office's. In most cases this makes no difference. But if the home router is old, or if software that changes DNS is installed at home, the softphone may fail to find the server while everything works in the office.
When one employee complains "it doesn't work at my house" and everything is fine at the office, check nslookup for the server name on their computer, and try for a moment from another network, for example a mobile hotspot. If it works there, the problem is in the home network.
A VPN also changes the DNS server: while the VPN is open, queries usually go to the VPN network's server. If that server doesn't know the name, the softphone will stop connecting only while the VPN is on.
What to do after a power outage
After a power outage, everything starts up at once: the router, the switch and the phones. Sometimes the phones start faster than the router, ask DNS before it is ready, and fail. Most phones try again on their own after a short time, but sometimes it takes a few minutes.
If after ten minutes some phones are still not registered, restarting the phone (unplugging it from power or from the cable for a few seconds) almost always solves it. A UPS (uninterruptible power supply) for the router and the switch prevents this situation in advance during short outages.
How it works with us at Kesher
The Kesher phone system runs on servers in several countries, with backup between them, so a failure in one place does not stop the system. The phone only needs a working internet connection and DNS to find the server.
The equipment we supply (phones, adapters and cordless phones) arrives preconfigured, including the server address. The customer doesn't have to type addresses or names.
The system is fully adapted to NetFree and other filters, so there is no need to change the filter's DNS settings for the phones to work.
If the phones show "Not registered," a person here answers and goes through the checks with the customer, including DNS. It is not an automated answer or a queue for a representative.
FAQ
What is the difference between DNS and an IP address?
An IP address is a device's number on the network. DNS is the service that translates a name into that address.
The internet works, but the phone says "Not registered." Could it be DNS?
Yes, this is one of the common causes. Check on the phone's information screen which DNS address it received, and try nslookup from a computer on the same network.
Should I switch to a public DNS?
Sometimes it improves speed, but in a filtered office it can break the filtering. Consult the filtering provider first.
Is DNS needed even if the server is set up with an IP address?
Less so for registration itself, but the phone still uses DNS for other things, such as clock synchronization and updates. It is better to have it working.
What is nslookup?
A command-line diagnostic tool that asks DNS about a particular name and shows the answer. It quickly shows whether DNS is responding.
We changed a server address and only some of the devices switched. Why?
Because of the cache. Devices remember the previous answer until it expires. Restarting the device speeds up the switch.
What is an SRV record?
A record that tells the phone which servers provide a particular service and in what order, which allows switching to a backup server.
Who sets the DNS on the devices in the office?
Usually the DHCP in the router, which hands it out automatically to every device together with its address.
Does DNS affect call voice quality?
No. DNS is needed to find the server and register, but once the call has started, the audio travels directly without asking DNS. Choppy audio is usually caused by latency, jitter or packet loss.