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Shivani ChaudhariShivani Chaudhari 

Unable to tunnel through proxy. Proxy returns "HTTP/1.0 503 Service Unavailable

Hello Everyone,

We are making an Outbound call from Salesforce to the external system and getting below error.

Error: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 503 Service Unavailable

When we try to send the same request from Postman it is working as expected and getting a response. - From the same Org, they were other Outbound calls that are working fine.

We have tested with salesforce, they are saying to whitelist IP ranges, which contains 1,52,000. This is not possible to whitelist this much IPs at third parties end. Please suggest and help to resolve the issue. We have tested it woking fine in Spring 20 version but Summer 20 its not working.
AbhishekAbhishek (Salesforce Developers) 
Hi Shivani,

Can you check with your network team to whitelist the salesforce IPs.


Hope above information was helpful.

Please mark as Best Answer so that it can help others in the future.

Shivani ChaudhariShivani Chaudhari
I Abhishek,

We are using our personal network. So there is no issue with the firewall. I hit the API request from Winter 20 Org of salesforce and i received the response and I hit the same request using same system and internet through Summer 20 Org of Salesforce, then i m getting this error.
AbhishekAbhishek (Salesforce Developers) 
If you suspect that this a release issue.

Please log a case with salesforce support they can give you the exact reason.
Shivani ChaudhariShivani Chaudhari
Hi Abhishek,
I already created case in salesforce. but they are not able to provide a solution. We are facing this issue from 1 month and the Salesforce team is only saying Whitelist IPs at third party end. Third-party is not ready to whitelist this much Ips.
AbhishekAbhishek (Salesforce Developers) 
Apologies then we can't assist here.

Because we have already had a solution to whitelist Third-party IP's but you have mentioned they are not ready.

Then again sorry to say this is not a salesforce issue.