1. Introduction
As the golden standard of secure remote access, the Secure Shell (SSH) protocol has several layers of protection. One of them involves recording and keeping track of the known servers on the client side.
known_hosts
By default, the known_hosts file for a given user is located at:
cat /home/user_name/.ssh/known_hosts
github.com ssh-rsa
***
github.com ecdsa-sha2-nistp256 ***
github.com ssh-ed25519 ***
Basically, the file contains a list with several columns, separated by whitespace:
- Identifying host data
- Host key type
- Host key value
- Optional comment The first column can be hashed or cleartext, depending on the setting of HashKnownHosts in /etc/ssh/ssh_config. When hashed, the first field of each line starts with |1|, a HASH_MAGIC marker. After the latter, the field continues with a random 160-bit string, otherwise known as a salt, followed by a 160-bit SHA1 hash. Each of these is encoded in base64. The main idea is to hide the IP address or hostname data, which would otherwise be directly visible Either way, known_hosts contains a mapping between a server as identified by its characteristics and its key. ## Known Hosts Checking When connecting to a remote host, SSH checks the known_hosts file of the client to confirm the address or hostname for the server match the key we get from it. If there is a match, the session setup can continue. Otherwise, we get an error. The entry for 192.168.6.66 in the known_hosts file doesn’t match the (Elliptic Curve Digital Signature Algorithm, ECDSA) key we got back from the server at that address. Critically, if we don’t know what caused the error, we should heed the text in capital letters: something nasty can indeed be happening. On the other hand, the reasons for such an issue can be valid and trivial:
- dynamic IP address
- changed hostname
- reinstalled system
- reinstalled SSH
- Docker container
- misconfigured DHCP
- relocated client In fact, there can be many more. ## Bypass Known Hosts The error text when connecting to a misidentified host tells us a few remedies for the situation. ### Correct the Row Since we already know which row of the known_hosts file doesn’t match (the suffix :1 of /home/erickquinteros/.ssh/known_hosts:1), we can correct the host data, key type, and value. By default, there are several host keys:
- /etc/ssh/ssh_host_rsa_key
- /etc/ssh/ssh_host_ecdsa_key
- /etc/ssh/ssh_host_ed25519_key ### Remove the Row If we trust the host and don’t want to bother correcting the line by hand, we can simply remove the entry with the supplied command:
ssh-keygen -f "/home/user_name/.ssh/known_hosts" -R "github.com"
# Host github.com found: line 1
# Host github.com found: line 2
# Host github.com found: line 3
/home/user_name/.ssh/known_hosts updated.
Original contents retained as /home/user_name/.ssh/known_hosts.old
Permanently Ignore
Another way to bypass the host checks is by adding a Host statement for the offending server in our ssh_config:
We can disable several checks:
- StrictHostKeyChecking no means we won’t need a match to connect to a server
- UserKnownHostsFile /dev/null_ overrides our default known_hosts path with the empty /dev/null
- GlobalKnownHostsFile /dev/null overrides the default global known hosts file path again with the empty /dev/null Essentially, this combination of three options strips the security of hosts checking and prevents additions to the known_hosts files for a given machine. ### Temporarily Ignore We may want to ignore the known hosts only temporarily:
$ ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o GlobalKnownHostsFile=/dev/null 192.168.6.66
Directly pass each of the options via -o flags when connecting to the misidentified server. Doing so enables easier debugging without global changes to the configuration.
Bibliography
What Is the SSH known_hosts File and How to Temporarily Ignore It
Further Reading
Check out the other articles in this series:
- ssh-agent:
-
ssh-keygen:
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known_hosts:
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