Bug 1364 defines the following:

1. Configure cherokee with a reverse proxy handler to a given backend and a 5 second timeout
2. In that backend, just sleep for 10 s before responding
3. Do two POST one after the other
4. In the error log you'll find :

-----
[11/06/2012 14:26:49.791] (error) socket.c:708 - Could not write to socket:
    write(18, ..): 'Bad file descriptor' | It looks like you've hit a bug in the server.
    Please, do not hesitate to report it at http://bugs.cherokee-project.com/
    so  the developer team can fix it.
-----

Issue #1236 defines the use of a DNS entry that results both an IPv4 and an IPv6 address.
Test if the selection of the proxy is using all addresses, in all protocols.
