XrdCl currently has a serious bug when used on dual-stack hosts.

The code for detecting network stack support currently looks at the node's hostname and resolves all available addresses for that hostname (which is incorrect: there's no guarantee the hostname is in any way connected to DNS or network address resolution).

If XrdNetUtils doesn't detect a IPv6 address for the hostname, XrdCl will claim to not be dual-stack when logging into a redirector. If it connects to the Xrootd server via IPv6, then the redirector will (incorrectly) identify the worker node as IPv6-only. This results in errors for CMS as the dual-stack host won't be redirected to IPv4-only servers.

I would suggest a twofold fix:

• If the client believes it is IPv4-only but is communicating with the redirector via a IPv6 socket, login as a dual-stack host.
• Determine the network stack support by enumerating the IP addresses on the host via getifaddrs instead of relying on the configuration of the worker node's hostname.

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