Let’s simulate a more complicated scenario. The ability to set up a jump server without letting direct SSH access onto it is an essential component of safe and proper SSH setup. This way, we do not run ssh on sshd connects directly to and gives control of that connection to our local client.Īs an added benefit, the jump server cannot see any traffic traveling through it due to it being encrypted within the SSH tunnel. Instead of forwarding the key-challenge response via agent, ProxyJump forwards the stdin and stdout of our local client to the destination host. (Read more: “ SSH configuration: ssh_config.”) Host myserver HostName User virag IdentityFile /users/virag/keys/ed25519 ProxyJump jump Host jump HostName User default
A modern approach is to use ProxyJump or its command line equivalent -J. To navigate through jump servers, we actually don’t need agent forwarding. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
Users with the ability to bypass file permissions on the remote host (for the agent’s Unix-domain socket) can access the local agent through the forwarded connection. In fact, the man page regarding ForwardAgent reads:Īgent forwarding should be enabled with caution. In essence, forwarding using an agent is the same as sharing a private key with anyone that has root on a machine throughout the chain. Even though socket files are well protected by the OS, a root user can impersonate another user and point the SSH client to their own malicious agent. The problem is that anyone with the root permissions anywhere in the chain can use the created socket to hijack our local ssh-agent. (Read more: “ SSH handshake explained.”)īehind the scenes, ssh-agent binds to a Unix domain socket to communicate with other programs ( $SSH_AUTH_SOCK environment variable). Instead, the agent’s forwarding feature allows our local agent to reach through an existing SSH connection and authenticate on a remote server through an environment variable.īasically, as client-side SSH receives key challenges, the agent will forward these challenges upstream to our local machine, where the challenge response will be constructed via a locally stored private key and forwarded back downstream to the destination server for authentication. It does not write to disk or export keys.
#Ssh proxy config how to
(Read more: “ How to manage SSH keys.”) It holds private keys and certificates used for authentication in memory. Ssh-agent is a key manager that exists as a separate program from SSH. To understand why, let’s dig a bit deeper. Given how common this method is, it may surprise you to learn this is not recommended. A solution to this security threat is enabling agent forwarding. A jump server is usually a multi-user environment, meaning any single party with elevated privileges could compromise any private key. One way to configure a multi-hop setup is by storing a private key for the destination server on your jump server. (Read more: “ How to set up an SSH jump server.”) Jump hosts minimize threats by forcing all SSH traffic to go through a single hardened location and minimizing an individual node’s SSH endpoints to the outside world. An SSH jump server is a proxy standing between clients and the rest of the SSH fleet.
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