I have two web servers each with a public IP that make outbound SFTP connections to various SFTP servers to update content. I would like to setup a Forward proxy for SFTP outbound connections. Does such a proxy exist?
You can proxy SSH connections (and thus also SFTP which is encapsulated within a SSH connection) by using a generic SOCKS proxy. I am not aware of any protocol-specific proxies for SSH/SFTP.
(It would be easier to answer if you gave a hint about your ultimate objective. What is the purpose of wanting to have the proxy?)
Using SSH to tunnel SSH through a proxy:
Alternative method with Dante:
Alternatively you could install dante-server (a SOCKS proxy server software) on the proxy machine and configure your /etc/tsocks.conf to connect there instead of 127.0.0.1. Then just run your SFTP operations with the tsocks wrapper as in the step 4 above. If you do this, you need to make sure that the dante server is configured so that it does not allow connections from unauthorized users.
Hope this helps!