Activating an iPad with your MDM platform is usually a straight forward process.Your hardware vendor associates your device enrollment program (DEP) code with the device, your DEP platform tells your MDM that the device belongs to you and when you turn the device on for the first time, it phones home, gets directed to MDM, your configuration profiles are pushed and the iPad gets activated, enrolled and managed. Simple right?
Turns out, there is an odd thing that Apple does that may cause problems if you use a proxy or content gateway for accessing the internet when you do the initial activation.
Enter courier.push.apple.com. This is a placeholder DNS record, which is designed to assist with load balancing the activation process. As we’ll find out, the way this DNS record is used is not standard and it helps to understand how it works when troubleshooting activating a device behind a proxy.
First, some lookups:
PS C:\> Resolve-DnsName -Name courier.push.apple.com -DnsOnly
Name Type TTL Section NameHost
---- ---- --- ------- --------
courier.push.apple.com CNAME 279 Answer courier-push-apple.com.akadns.net
So courier.push.apple.com is a CNAME for courier-push-apple.com.akadns.net . Let’s see where courier-push-apple.com.akadns.net goes.
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 180 Authority internal.akadns.net hostmaster.akamai.com 1560251729
As we can see courier-push-apple.com.akadns.net does not resolve. It just returns the SOA record for the domain.
We can also confirm that this is not a geographical DNS inconsistency by querying alternate DNS servers in different parts of the world.
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly -Server 8.8.8.8
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 163 Authority internal.akadns.net hostmaster.akamai.com 1560251729
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly -Server 1.1.1.1
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 133 Authority internal.akadns.net hostmaster.akamai.com 1560251729
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly -Server 211.11.195.114
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 180 Authority internal.akadns.net hostmaster.akamai.com 1560251729
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly -Server 212.234.34.121
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 145 Authority internal.akadns.net hostmaster.akamai.com 1560251729
PS C:\> Resolve-DnsName -Name courier-push-apple.com.akadns.net -DnsOnly -Server 167.233.5.204
Name Type TTL Section PrimaryServer NameAdministrator SerialNumber
---- ---- --- ------- ------------- ----------------- ------------
akadns.net SOA 14 Authority internal.akadns.net hostmaster.akamai.com 1560251729
Google, Cloudflare as well as name servers in France, Germany and Korea all return an SOA for courier-push-apple.com.akadns.net
A quick google of this strange behavior reveals people resolving load balanced endpoints for courier-push-apple.com.akadns.net, namely 'x'.courier-push-apple.com.akadns.net, eg. 1.courier-push-apple.com.akadns.net , 2.courier-push-apple.com.akadns.net , 3.courier-push-apple.com.akadns.net etc.
After working with an extremely helpful engineer at Apple, it was determined that the load balancing for activating iPad’s is done on the device its self. Rather than DNS telling the device which endpoint to hit, the device prepends a number like '1' to the start of courier-push-apple.com.akadns.net to make it 1.courier-push-apple.com.akadns.net. When we look up these names, we get a much different result.
PS C:\> Resolve-DnsName -Name 1.courier-push-apple.com.akadns.net -DnsOnly
Name Type TTL Section NameHost
---- ---- --- ------- --------
1.courier-push-apple.com.akadn CNAME 48 Answer apac-au-courier-4.push-apple.com.akadns.net
s.net
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 15
Section : Answer
IP4Address : 17.57.145.37
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 15
Section : Answer
IP4Address : 17.57.145.36
PS C:\> Resolve-DnsName -Name 2.courier-push-apple.com.akadns.net -DnsOnly
Name Type TTL Section NameHost
---- ---- --- ------- --------
2.courier-push-apple.com.akadn CNAME 60 Answer apac-au-courier-4.push-apple.com.akadns.net
s.net
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 60
Section : Answer
IP4Address : 17.57.145.36
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 60
Section : Answer
IP4Address : 17.57.145.37
PS C:\> Resolve-DnsName -Name 3.courier-push-apple.com.akadns.net -DnsOnly
Name Type TTL Section NameHost
---- ---- --- ------- --------
3.courier-push-apple.com.akadn CNAME 19 Answer apac-au-courier-4.push-apple.com.akadns.net
s.net
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 55
Section : Answer
IP4Address : 17.57.145.36
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 55
Section : Answer
IP4Address : 17.57.145.37
PS C:\> Resolve-DnsName -Name 10.courier-push-apple.com.akadns.net -DnsOnly
Name Type TTL Section NameHost
---- ---- --- ------- --------
10.courier-push-apple.com.akad CNAME 55 Answer apac-au-courier-4.push-apple.com.akadns.net
ns.net
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 55
Section : Answer
IP4Address : 17.57.145.37
Name : apac-au-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 55
Section : Answer
IP4Address : 17.57.145.36
Here in Australia, the 'x'.courier-push-apple.com.akadns.net endpoints are a CNAME for an A record of apac-au-courier-4.push-apple.com.akadns.net which resolves to the same two load balanced IPs.
If however I query a name server in Germany, i get much different results.
PS C:\> Resolve-DnsName -Name 10.courier-push-apple.com.akadns.net -DnsOnly -Server 167.233.5.204
Name Type TTL Section NameHost
---- ---- --- ------- --------
10.courier-push-apple.com.akad CNAME 5 Answer eu-central-courier-4.push-apple.com.akadns.net
ns.net
Name : eu-central-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 52
Section : Answer
IP4Address : 17.57.146.165
Name : eu-central-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 52
Section : Answer
IP4Address : 17.57.146.169
Name : eu-central-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 52
Section : Answer
IP4Address : 17.57.146.164
Name : eu-central-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 52
Section : Answer
IP4Address : 17.57.146.167
Name : eu-central-courier-4.push-apple.com.akadns.net
QueryType : A
TTL : 52
Section : Answer
IP4Address : 17.57.146.166
This time we get 5 load balanced IPs in the Central Europe region. None of this is unusual of course and we expect this type of behavior from modern CDN’s. The slightly unusual thing here is that courier-push-apple.com.akadns.net resolves to nothing and this is by design. What is supposed to happen during device activation is that the device queries courier.push.apple.com, which returns courier-push-apple.com.akadns.net. The very act of receiving this name host as a response on the device tells the code that is running during activation to prepend a number to courier-push-apple.com.akadns.net, making it a valid DNS query. The device now hits 'x'.courier-push-apple.com.akadns.net and receives a response from a server which allows the normal activation process to occur.
Now to the point of the article. What happens if your device requests courier.push.apple.com via your proxy server and the proxy looks up courier.push.apple.com to make sure its valid before returning the response. The proxy sees that the resultant response of courier-push-apple.com.akadns.net resolves to nothing and squashes the response since it goes nowhere. The device now, sitting and waiting for a response, never gets it and decides that it wont proceed with activation, because no server is responding to it.
How do you fix this issue? Depends on your proxy. Depends on your environment. I don’t currently have an answer but I’m working on it and may do another blog post depending on the outcome. Hopefully if your stuck on this issue this clarifies things for you in regards to how iPads interact with courier.push.apple.com.
Side note: This article is helpful to review when looking at push notification firewall requirements.
Thanks for reading – Jesse
Jesse's SysAdmin Blog 