How can I use a custom DNS server in Azure App Services with Linux? - linux

Question

I have a .NET Core API App running in Azure App Services using Windows, but now I want to test it using Linux. The app uses a custom DNS Zone just to map a custom hostname to a specific IP address.
To implement that in my Windows App Service I used the following Application Settings as stated in this page: WEBSITE_DNS_SERVER and WEBSITE_ALT_DNS_SERVER. Although it looks like a workaround, it works pretty well with Windows App Services. But in Linux it seems to have no effect and my app is not resolving the hostname as needed.
How can I use a custom DNS server in Azure App Service with Linux or add a custom hostname like editing the hosts file?

Answer 1

Unlike our App Service Windows, nameresolver.exe is not available and the Application settings for "WEBSITES_ALT_DNS" and "WEBSITES_DNS" do not populate the configuration files within the container. Below are steps are troubleshooting issues for "Alpine" based images since you don’t mention if you are using a Linux container or the default Linux OS.
Install Bind-tools-
apk update
apk add bind-tools
Run Nslookup-
Once bind-tools is installed, you'll see the server that is being used. If bind-tools are not installed, the DNS server will not be shown. Example provided below.
9031977be93a:~# nslookup google.com
Server: 127.0.0.11
Address: 127.0.0.11#53
Non-authoritative answer:
Name: google.com
Address: 216.58.194.174
Name: google.com
Address: 2607:f8b0:4005:801::200e
Update Config file-
To use a different DNS server for testing, update the /etc/resolv.conf file and update the "nameserver" to use a different DNS server. In this example, we're using Googles DNS.
vi /etc/resolv.conf
By default, the following will be in the resolv.conf file.
search reddog.microsoft.com
nameserver 127.0.0.11
options timeout:1 attempts:5 ndots:0cd /etc
If you're not familiar with "vi", press "i" on your keyboard to begin editing the file.
Once you're done, press "esc" and ":wq!" then enter.
search reddog.microsoft.com
nameserver 8.8.8.8
options timeout:1 attempts:5 ndots:0
Running Nslookup again, you'll see the new nameserver being used.
9031977be93a:~# nslookup google.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
Name: google.com
Address: 216.58.194.174
Name: google.com
Address: 2607:f8b0:4005:804::200e
Hardcoding Hostname-
You can also hardcode the IP address for the hostname in question for testing. To do so, you'll need to update the following file.
vi /etc/hosts
Add the IP address that you would like the DNS to point to. In this example, I'm changing the IP for my custom domain.
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
172.20.0.2 9031977be93a
10.10.10.10 www.polytechniks.com
Save the changes and use "ping" or "traceroute" to see the IP being used. NOTE: Nslookup does NOT use the /etc/hosts file so you will still see the IP address associated with the DNS.
9031977be93a:~# ping www.polytechniks.com
PING www.polytechniks.com (10.10.10.10): 56 data bytes
--- www.polytechniks.com ping statistics ---
8 packets transmitted, 0 packets received, 100% packet loss
9031977be93a:~# traceroute www.polytechniks.com
traceroute to www.polytechniks.com (10.10.10.10), 30 hops max, 46 byte packets
1 172.20.0.1 (172.20.0.1) 0.005 ms 0.004 ms 0.004 ms