I’ve got a Fiber connexion to the Internet (at Numéricable, one of the main ISP in France) which is forcing a dynamic IP address on me (no fixed IP address, even with a premium; They’ve settled in the XXth century). This would not be very serious except that I host a RAID server from Synology (a DS413j DiskStation with 4 hard drives in RAID-5 redundant mode) which provides several services I would like to share with the Internet (while I’m travelling with my iPhone or my laptop, I’d like to get access to my files through FTP, or my email server).

So, I decided to set things right in order to appropriately locate the Synology DiskStation in a sub-domain of my own (e.g. mail.roumazeilles.net).

The solution I opted for:

Dynamic IP addess:

Since my IP address is dynamic, there’s no way I can progress until I solved this single issue. I chose to use the DDNS service DDNS from Synology. Since I use DSM 5.0 (the most recent software version from Synology), I can reach the appropriate option through the control panel and the “External access” menu. I just added a DDNS, selecting Synology as a service supplier, and I recorded the name I wanted (let’s say ds). From this point, despite the IP address changes, my DiskStation is always accessible at ds.synology.me.

DNS subdomain:

After that, I need to point mail.roumazeilles.net onto ds.synology.me. This is slightly more complex because my roumazeilles.net domain nam is reserved atz Gandi and it is pointed toward a server hosted by OVH. It’s the hosted server (rented from OVH) which includes all the information relating to roumazeilles.net. So, I went to my server control panel and, in its DNS configuration, I modified (in your case, you may need to add) a CNAME record.

mail 10800 IN CNAME ds.synology.me. (Don’t forget the final dot/period in your CNAME entry, or it won’t work!)
Since I am lucky, my control panel at OVH includes an easy way to get the correct syntax (it’s safer): I tell it the mail subdomain is described by a CNAME record pointing to ds.synology.me (a bit terse, but not difficult).

Then, I only had to wait for the DNS information to “propagate” (from a few minutes to a few hours, sometimes up to 24-48 hours). And a little check using ping mail.roumazeilles.net confirmed that it answered from my Numéricable IP address.