SSH Host Key Algorithm Mismatch: How to Diagnose and Fix
This EX200 practice question tests your understanding of deploy, configure, and maintain systems. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
$ journalctl -u sshd.service -p err --since yesterday
-- Logs begin at Mon 2024-03-25 10:00:00 EDT, end at Tue 2024-03-26 09:30:00 EDT
Mar 25 14:22:31 server sshd[1234]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.1.100 user=root
Mar 25 14:22:35 server sshd[1234]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 25 14:23:01 server sshd[1235]: fatal: Unable to negotiate with 192.168.1.101 port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss
Mar 25 14:23:02 server sshd[1235]: Connection closed by 192.168.1.101
Refer to the exhibit. An administrator sees that a user from 192.168.1.101 cannot connect to the SSH server. Based on the log, what is the most probable cause?
Exhibit
$ journalctl -u sshd.service -p err --since yesterday
-- Logs begin at Mon 2024-03-25 10:00:00 EDT, end at Tue 2024-03-26 09:30:00 EDT
Mar 25 14:22:31 server sshd[1234]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.1.100 user=root
Mar 25 14:22:35 server sshd[1234]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 25 14:23:01 server sshd[1235]: fatal: Unable to negotiate with 192.168.1.101 port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss
Mar 25 14:23:02 server sshd[1235]: Connection closed by 192.168.1.101
A
The client's host key type is not supported by the server
The log shows negotiation failure due to missing host key type.
B
The server's firewall is blocking the connection
Why wrong: No firewall-related messages in the log.
C
The SSH service is not running
Why wrong: The log shows sshd activity, so the service is running.
D
The client's IP is blacklisted
Why wrong: No blacklist messages appear in the log.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The client's host key type is not supported by the server
The log shows 'no matching host key type found. Their offer: ssh-rsa'. This indicates the client offered an ssh-rsa host key, but the server's configuration (likely via the `HostKeyAlgorithms` directive in `/etc/ssh/sshd_config`) does not include ssh-rsa. In modern OpenSSH (e.g., RHEL 8/9), ssh-rsa is often disabled by default due to its reliance on SHA-1, which is considered weak. The server requires a different host key type (e.g., rsa-sha2-256, rsa-sha2-512, or ecdsa-sha2-nistp256), causing the connection to fail before authentication even begins.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The client's host key type is not supported by the server
Why this is correct
The log shows negotiation failure due to missing host key type.
Related concept
Read the scenario before looking for a memorised answer.
The log shows sshd activity, so the service is running.
✗
The client's IP is blacklisted
Why it's wrong here
No blacklist messages appear in the log.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The RHCSA exam often tests the distinction between authentication failures (e.g., wrong password or key) and key exchange failures (e.g., unsupported host key algorithm), leading candidates to mistakenly blame firewall rules or service status when the log clearly points to a cryptographic algorithm mismatch.
Trap categories for this question
Command / output trap
The log shows sshd activity, so the service is running.
Detailed technical explanation
How to think about this question
The SSH key exchange process begins with the server sending its host key to the client; the client checks if the host key type is in its allowed list. The `HostKeyAlgorithms` option in sshd_config controls which host key types the server offers. In RHEL 8/9, the default `HostKeyAlgorithms` often excludes `ssh-rsa` (which uses SHA-1) in favor of `rsa-sha2-256` and `rsa-sha2-512` (which use SHA-2). To allow ssh-rsa, an administrator would need to add `ssh-rsa` to the `HostKeyAlgorithms` line, but this is discouraged for security reasons. The client's log message 'no matching host key type found' is the exact symptom of this mismatch.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the EX200 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Quick reference
Asymmetric Encryption Algorithm Comparison
Algorithm
Key Exchange
Signatures
Equivalent Security Key
Notes
RSA-3072
Yes
Yes
128-bit
Widely deployed; slow for bulk data
ECDSA P-256
No
Yes
128-bit
Fast signatures; standard TLS certs
ECDH / ECDHE
Yes
No
128-bit
Perfect forward secrecy in TLS 1.3
DH / DHE
Yes
No
128-bit (3072-bit key)
Replaced by ECDHE in modern TLS
Ed25519
No
Yes
~128-bit
SSH keys, modern PKI
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Deploy, configure, and maintain systems — This question tests Deploy, configure, and maintain systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The client's host key type is not supported by the server — The log shows 'no matching host key type found. Their offer: ssh-rsa'. This indicates the client offered an ssh-rsa host key, but the server's configuration (likely via the `HostKeyAlgorithms` directive in `/etc/ssh/sshd_config`) does not include ssh-rsa. In modern OpenSSH (e.g., RHEL 8/9), ssh-rsa is often disabled by default due to its reliance on SHA-1, which is considered weak. The server requires a different host key type (e.g., rsa-sha2-256, rsa-sha2-512, or ecdsa-sha2-nistp256), causing the connection to fail before authentication even begins.
What should I do if I get this EX200 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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