SSH直连正常但ss/config文件失效，多跳登录故障求助

Hey there, sorry to hear your multi-hop SSH setup started acting up out of nowhere—let’s break down the possible culprits since you mentioned it worked before without obvious config changes on your end:

1. SSH Config Changes on the Bastion (Jump) Host #

Even if you didn’t touch your local setup, the bastion host’s admin might have adjusted SSH server settings:

• Check /etc/ssh/sshd_config on the bastion for AllowTcpForwarding, GatewayPorts, or PermitOpen—if any of these were set to no or restricted to specific ports, it would block your jump traffic.
• Low values for MaxSessions or MaxStartups could also cause new connections to be throttled or timed out.
• Did the bastion get an OpenSSH version update recently? Some updates introduce changes to ProxyJump/ProxyCommand handling that might break compatibility.

2. Hidden Network Layer Changes #

Network rules or connectivity often shift without obvious notice:

• Firewall/Routing Rules: The bastion’s outbound rules might now block traffic to your final host’s SSH port (default 22), or your local network/ISP added deep packet inspection that flags SSH multi-hop traffic. Also, check if the final host’s inbound rules only allow specific IPs—if the bastion’s public IP changed (e.g., dynamic IP renewal), that could be the issue.
• Latency/Packet Loss: Run ping final_host and mtr final_host from the bastion to test connectivity. Severe packet loss or latency can cause SSH handshake timeouts.

3. Local SSH Client Issues (Hidden State/Cache) #

Your local client might have silent issues even without config edits:

• Clear your SSH known hosts cache (or just the entries for the bastion and final host): rm ~/.ssh/known_hosts (or edit the file to remove specific lines). Stale host key entries can cause unexpected connection hangs.
• Did your local OpenSSH auto-update? Newer versions sometimes tighten default encryption/MAC algorithms, which might clash with older configurations on the bastion or final host.
• Enable debug mode to pinpoint the hang: ssh -vvv -J user@bastion user@final_host. Look for the last few lines of output—this will tell you if it’s stuck after connecting to the bastion, during the jump to the final host, or at authentication.

4. Authentication/Key Permissions Problems #

Even working keys can break if permissions change:

• Verify your local private key has strict permissions: chmod 600 ~/.ssh/id_rsa (replace with your key filename). SSH rejects keys with overly open permissions.
• Check the bastion’s ~/.ssh/authorized_keys file—ensure your public key is still present, and the file permissions are set to 600 (SSH will ignore it if permissions are too loose, like 777).
• Test password authentication (if allowed) to rule out key issues: ssh -J user@bastion user@final_host -o PreferredAuthentications=password.

5. Final Host SSH Config/State Changes #

The target machine might be the source of the problem:

• Check sshd_config on the final host for AllowUsers or AllowGroups—if these were updated to restrict access from the bastion’s user, that would block the jump.
• Is the final host’s SSH service overloaded? Too many active connections can delay new sessions until they time out.
• Verify the final host is resolvable from the bastion: Run nslookup final_host or cat /etc/hosts on the bastion to make sure the hostname maps to the correct IP.

Quick Troubleshooting Checklist #

1. First, confirm you can still log into the bastion directly: ssh user@bastion. If this fails, the issue is between your local machine and the bastion.
2. From the bastion, try logging into the final host directly: ssh user@final_host. If this hangs, the problem is between the bastion and final host (or the final host itself).
3. Use the debug command mentioned earlier to get granular details on where the connection is stalling.

内容的提问来源于stack exchange，提问作者elyase