Straight rsync with intermediate SSH host
I assume the rsync (Deltacopy) server on C listens on the TCP port 873 and you can reach it from B. If so, this becomes the general problem of reaching a TCP port of C from A, when A cannot reach C directly, but can ssh to B which can reach C.
Use local port forwarding:
# on A
ssh -NL 8873:C_seen_from_B:873 userB@B_seen_from_A
Here 8873 is the local port number, quite arbitrary; ssh will listen on this port. I did not use 873 for two reasons:
- there may be a local rsync server that has already taken the port;
- you most likely are not allowed to use a port with number lower than 1024 anyway.
Now if you connect from A to localhost:8873 then you will reach the port 873 of C. C will see traffic coming in from B. So far there is nothing specific to rsync.
Next invoke rsync on A and wherever in the command you need to specify a LOCATION on C, use the syntax rsync://[USER@]HOST[:PORT]/LOCATION (see "access via rsync daemon" in man 1 rsync), where HOST is localhost and PORT is 8873.
E.g. if on B you would use rsync://C_seen_from_B:873/foo/bar, now on A you should use rsync://localhost:8873/foo/bar.
This way rsync on A will connect to localhost:8873 of A where ssh listens. The connection will be tunneled via SSH and the SSH server on B will connect to C_seen_from_B:873 for you. Ultimately the rsync invoked on A will talk to the rsync (Deltacopy) server running on C.