Solution : https://service.sap.com/sap/support/notes/155954 (SAP Service marketplace login required)
Key words : 
customer-specific proxy server, provide dedicated ip adresses, sap service marketplace - release, access sap service marketplace, restrict access based, external service providers, note concerns problems, terms proxy reason, restrict internet access, default dns server
Related Notes : 
       
| 1505354 | SAP Download Manager fails with SSL terminating Proxy | 
| 846399 | Software Download Center - Performance Analysis | 
| 422409 | Access to SAP Service Marketplace (Collective Note) |