Q:

Why won't my SSH keys forward properly?

I'm trying to ssh from my machine to another machine and from there to a third machine. When I do it manually, it works like this:
localhost$ scp ~/.ssh/id_rsa myuser@myhost.something.something.com:.
localhost$ ssh myuser@myhost.something.something.com
myhost.something.something.com$ ssh -i id_rsa myuser@10.25.100.42
10.25.100.42$ 

But I would like to do it directly, in one step. And I want my SSH keys from localhost to forward to the third machine. I thought this would work, but it doesn't:
localhost$ ssh -A -t myuser@myhost.something.something.com ssh 10.25.100.42
Permission denied (publickey).
Connection to myhost.something.something.com closed.

Why? How can I do this in a single command from localhost?
Here is some more detail you can see when I use the verbose flag:
localhost$ ssh -v -A -t myuser@myhost.something.something.com ssh 10.25.100.42

OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /etc/ssh_config
debug1: /etc/ssh_config line 20: Applying options for *
debug1: Connecting to myhost.something.something.com [X.X.X.X] port 22.
debug1: Connection established.
debug1: identity file /home/myuser/.ssh/id_rsa type 1
debug1: identity file /home/myuser/.ssh/id_rsa-cert type -1
debug1: identity file /home/myuser/.ssh/id_dsa type -1
debug1: identity file /home/myuser/.ssh/id_dsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2 pat OpenSSH*
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5-etm@openssh.com none
debug1: kex: client->server aes128-ctr hmac-md5-etm@openssh.com none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA 62:d2:58:47:f7:c6:21:b4:a1:b0:cf:4e:44:42:e4:9a
debug1: Host 'myhost.something.something.com' is known and matches the RSA host key.
debug1: Found key in /home/myuser/.ssh/known_hosts:255
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/myuser/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: read PEM private key done: type RSA
debug1: Authentication succeeded (publickey).
Authenticated to myhost.something.something.com ([X.X.X.X]:22).
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: Requesting authentication agent forwarding.
debug1: Sending command: ssh 10.25.100.42
debug1: client_input_channel_open: ctype auth-agent@openssh.com rchan 2 win 65536 max 16384
debug1: channel 1: new [authentication agent connection]
debug1: confirm auth-agent@openssh.com
Permission denied (publickey).
debug1: channel 1: FORCE input drain
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: client_input_channel_req: channel 0 rtype eow@openssh.com reply 0
debug1: channel 0: free: client-session, nchannels 2
debug1: channel 1: free: authentication agent connection, nchannels 1
Connection to myhost.something.something.com closed.
Transferred: sent 3532, received 3312 bytes, in 4.4 seconds
Bytes per second: sent 796.1, received 746.6
debug1: Exit status 255

A:

If you want to forward connection to your authentication agent, you need to have the authentication agent running on your machine. You can check it using ssh-add -l. If it is not running, start it and add the keys:
eval `ssh-agent`
ssh-add ~/.ssh/id_rsa
# or other keys you want to use from the other server

and then after connecting to the other server with -A switch, you will be able to authenticate using this key further:
local     $ ssh -A myuser@myhost.something.something.com
something $ ssh-add -l                     # should list your key
something $ ssh 10.25.100.42

But what you are really trying to achieve is something different. You want to use first host as a jumpbox to connect to the second one and it can be made much more convenient using ProxyCommand:
ssh -oProxyCommand="ssh -W 10.25.100.42:22" myuser@myhost.something.something.com

or better with .ssh/config:
Host something
  Hostname myhost.something.something.com
  User myuser
Host second
  Hostname 10.25.100.42
  User myuser
  ProxyCommand ssh -W %h:%p something

and then you will get there just using
ssh second

