ssh agent forwarding fails with "Could not open a connection to your authentication agent"

I receive a Could not open a connection to your authentication agent error message when I attempt to connect from an intermediate server to a third server using the agent forwarding option (-A) of an OpenSSH client. The first connection to the intermediate server goes smoothly using a key loaded into ssh-agent. The error message is displayed when attempting a connection to the final server.

My OpenSSH client is set to allow forwarding with ForwardAgent yes in ~/.ssh/config, and the intermediate server has AllowAgentForwarding yes in the daemon's configuration file. The client config is not overridden by a system level file.

I'm not using a terminal multiplexer in order to avoid an error stemming from environment variables not being set. To run the agent, I use exec ssh-agent zsh and verify that both SSH_AUTH_SOCK and SSH_AGENT_PID are present in the local environment. I use ssh-add to add the private keys for the intermediate and final server, respectively; I verify they are added with ssh-add -l.

All servers are of a recent version (OpenSSH 5.3) and the client is OpenSSH 6.2.

I'm posting this here because I spent a lot of time trying to find a solution using Google, reading man pages, and consulting a popular book on SSH, all to no avail.

The key to finding the problem was poring over the debugging output.

debug1: Remote: Agent forwarding disabled: mkdtemp() failed: Permission denied

The intermediate machine is a virtual server (RHEL 6.4) hosted by a cloud provider that uses an AWS stack. For reasons I can't explain, this is what permissions on the /tmp directory were set to:

drwxr-x--- 19  727  727  4096 Nov 28 05:30 tmp

Grep'ing through /etc/passwd I couldn't find a user with an ID of 727.

Correcting the permissions like so solved my woes:

sudo chown 0:0 /tmp
sudo chmod 1777 /tmp

Can anyone speak to the peculiar ownership of the /tmp directory?