DNS propagation can be thought of as the time it takes DNS records to expire on a server. For example, if you update your nameservers to point to a different hosting company, those new nameservers must propagate across the Internet. Each ISP has their own time frame on how often they update/expire their cached DNS records. Because there is no single shared standard throughout the Internet, this entire process can take from a few hours to up to 72 hours.
Additionally, DNS Propagation is a rather inaccurate term used to describe the process of DNS resolution. The word "propagation" implies that DNS changes spread from DNS server to DNS server, and that you have no control over how quickly it happens. In fact, DNS servers check in with your local DNS server as needed, and the administrator of your local DNS server controls the 'time to live' (TTL) values for all DNS records in your domain.
This page provides details on DNS Propagation, DNS Caching, TTL, Flushing DNS cache, and tools for checking propagation status.
DreamHost cannot control how fast your DNS propagates throughout the Internet.
How DNS caching works
Because of the large volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e., the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer.
Put simply, DNS caching allows any DNS server around the world to store locally the DNS records for a domain on that server. This allows the server to use those records immediately without the need for a new DNS query. Eventually, however, the DNS must be updated, which is when the time to live (TTL) value comes into play.
Time to live (TTL)
How long a resolver caches a DNS response (i.e., how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks.
For example, the TTL on DreamHost’s server is set to 4 hours by default. This is the amount of time the DNS record is cached on the server.
Changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.dreamhost.com, and then changes the IP address to which www.dreamhost.com resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called the caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified when the TTL expires.
It’s important to note that since it can take several hours for DNS records to resolve in all locations, different computers in different locations see the update at different times.
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When you change the nameserver records for your domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes are immediately available for any DNS servers that do not have them cached. And, any DNS changes on your domain other than the nameserver records and authoritative DNS server names can be nearly instantaneous, if you so choose (by lowering the TTL ahead of time, and waiting until the old TTL expires before making the change).
Adjusting Time To Live (TTL)
The Time-To-Live (TTL) value for a domain is how long a website is cached (stored in memory) on a web server. The average time is 4 hours.
If you are moving a domain from one hosting provider to another, it may be a good idea to drop the TTL as low as possible BEFORE you initiate the migration. This allows your DNS records at your current host to expire faster so you do not need to wait the full 4 hours until the records change.
After the TTL for a domain is dropped, you must continue to wait until the current TTL expires. This could be 4 hours (depending on your hosting provider's TTL). Only after the current TTL has expired will the new TTL value control DNS updates.
Once the 4 hours has expired and your new TTL is active, you should update your DNS to point to the new hosting provider.
If for some reason you need to speed up or slow down the TTL for a domain hosted at DreamHost:
- Create a ticket on the (Panel > 'Support' > 'Contact Support') page in your panel and DreamHost support will adjust it for you.
- Make sure you let support know to make the change at least four hours before the time you’d like to update your DNS, as it will take that long to enable the new TTL.
Flushing the DNS cache
If you’ve made recent changes to a domain or other DNS related services, and are not seeing changes, you might need to flush your DNS cache in order to see the new changes appear at your location.
Please visit the following article for more details on clearing DNS cache for Windows, Mac, and Linux:
Tools to help you check the DNS status
The following tools help you confirm in which locations your DNS has updated:
Viewing your site before the DNS has propagated
You can still view your site before the DNS has changed. There are two ways to do this:
- Previewing your site with a dreamhosters.com subdomain before changing DNS
- How to view your DreamHost site using your hosts file