# 503 responses
The 503 repsonse code means "Backend unavailable" (opens new window). Causes and solutions are varied, depending on which server answers with it, and to which request. It can mean the server is overloaded, the request is unsupported or blocked.
If a curl request with one of Fastly's IP addresses returns a 503:
$ curl -I https://151.101.130.207 -H "Host:www.example.com" -k
HTTP/1.1 503 Service unavailable
...
This article is for you.
# Before MarfeelCDN Activation
The possible causes listed here usually manifest before MarfeelCDN activation.
# Domain IP address
Fastly only supports domains available at IPv4 addresses. Domains which remote address is IPv6 are not compatible with MarfeelCDN.
Check the remote IP address in the browser:
The IP is IPv4 in this example.
# Fastly blacklisted
Even though a 403 response code is expected for blacklisted requests, a 503 can indicate the same thing. It is likely the case if you can access the domain from the browser, but Fastly can't.
Read the guide about the 403 response code to know how to proceed.
# After MarfeelCDN Activation
If the tenant's domain answers with a 503 after MarfeelCDN activation, and you have excluded all potential configuration issues, this section is for you.
- Server tolerance: Marfeel can reduce the number of requests sent to the tenant's origin server with different strategies. Read about how to reduce requests.