Running OSX 10.10.4 as the host, Debian 8 as the guest. Virtualbox 5.0.0r101573. Using bridged networking to a wired ethernet interface. Attempting to ping to the (routable) IPv6 address of the guest from the host works. Attempting to telnet to port 22 (ssh) fails to connect. Attempting to telnet to port 22 using an IPv4 address works. Attempting to telnet to port 22 using the IPv6 address from a completely different machine works. Attempting to telnet to the IPv6 address of the host from the guest on port 22 fails (where v4 works). Tcpdump on both the guest and host can see the traffic in all cases. A VMware VM on the same host does not exhibit the problem. iptables and ip6tables were checked to ensure that there wasn't a firewall at play (OS X's firewall is similarly disabled). Netstat does show that ssh is bound to a v6 address (as also evidenced by the unrelated machine succeeding to connect). The guest is obtaining an IPv6 address via router advertisements.
This appears to be an issue with IPv6 communications between the host and the guest. Thoughts?