Q:

How to enable SSH X11 forwarding through additional server?

I have hosts A,B and C. From host A I can access through ssh only B. From B I can access C. 
I want to be able to run X11 programs on C and forward display to A.
I tried this:
A$ ssh -X B
B$ ssh -X C
C$ xclock
Error: Can't open display:

But it doesn't work.

A:

There are several ways to do this, the one I prefer is to forward the ssh port:
First, connect to machine B and forward [localPort] to C:22 through B
A$ ssh -L [localPort]:C:22 B

Next, connect to C from A through this newly-created tunnel using [localPort], forwarding X11
A$ ssh -X -p [localPort] localhost

Now we can run X11 programs on C and have them display on A
C$ xclock

[localPort] can be any port that you are not already listening to on A, I often use 2222 for simplicity.

A:

This can easily be accomplished using port forwarding:
A$ ssh -NL 2022:C:22 B &
A$ ssh -X -p 2022 localhost
C$ xclock

Port localhost:2022 is forwarded to C:22 via B
SSH to C via localhost:2022
Use X as normal

A:

Assuming the problem is that the middle machine doesn't have X, but it otherwise configured to allow forwarding X11, just install xauth.
on a yum-based system (fedora, redhat, centos):
B$ sudo yum install xauth

on an apt-based system (debian, ubuntu):
B$ sudo apt-get install xauth

