This article reproduced the HTTP 499 error causes and workarounds for ArcGIS Server 10.1
Today, using ArcGIS Server 10.5.1 to publish the WMS Service, everything is fine during the publishing process, the rest service is found to be properly accessible after publishing, and the WMS Service is unavailable with the following error message:
Error hints
Because the server is installed in the intranet, through mapping and other ways to access the external network, initially thought it was an ArcGIS server installation problem, and then search online and through the attempt to discover the problem:
This is an issue with the permissions mechanism for ArcGIS Server 10.5.1, where access to all services is set to private, externally inaccessible (oddly enough, why rest is not restricted at this time), log on to the manager and reset the service permissions back to the public state, and the service is accessible. As shown in the following illustration:
How to set up in Manager in ArcGIS Server 10.2
How to set up in Manager in ArcGIS Server 10.5.1
After the setup is complete, the service will be restarted automatically, and then the WMTS service can be accessed to find everything is OK.