I have a managed package deployed that is calling web services supplied by another party.
On Tuesday last week all calls to this web service started throwing a:
System.CalloutException: IO Exception: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 503 Service Unavailable"
It turns out the domain registration used by the web service expired and was subsequently renewed after the fact. Part of this process required the DNS entry to be updated. I'm not sure, but I believe this changed the IP address the domain was registered against.
I could successfully call the web service using SOAP UI, but calls from Salesforce were failing with the message above. I checked with the other party, they have no IP restrictions on the web services.
By Friday evening the web service calls started working again. I assume the DNS records had been refreshed. This may have been the result of the support cases that were raised or it may have been a general refresh by Salesforce of the DNS records.
In general, how often would Salesforce pickup on DNS changes?
There are a lot of assumptions about what was happening above as I didn't have access to the Salesforce support cases or the web service hosting details. I'll clear them up if I can get some more concrete details.