"No kex alg" during Git client operations after Bitbucket Server and Data Center upgrade
Platform Notice: Data Center Only - This article only applies to Atlassian products on the Data Center platform.
Note that this KB was created for the Data Center version of the product. Data Center KBs for non-Data-Center-specific features may also work for Server versions of the product, however they have not been tested. Support for Server* products ended on February 15th 2024. If you are running a Server product, you can visit the Atlassian Server end of support announcement to review your migration options.
*Except Fisheye and Crucible
Summary
Bitbucket 7.17.0 includes a newer version of Apache SSHD. The newer version of SSHD has different defaults with the SHA1 keys being removed. Any SSH client that only uses SHA1 keys will fail with a "no kex alg" error. This has been seen on RedHat 5 systems where only SHA1 is default.
Environment
Bitbucket 7.17.0
Diagnosis
Client git command sees error.
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$ git pull
no kex alg
fatal: The remote end hung up unexpectedlyTo check which key algorithms are being used on each system the ssh -Q kex can be used.
kex alg on RHEL 5
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| kex_algorithms (3)
| diffie-hellman-group-exchange-sha1
| diffie-hellman-group14-sha1
| diffie-hellman-group1-sha1kex alg on the Bitbucket server
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| kex_algorithms (9)
| ecdh-sha2-nistp521
| ecdh-sha2-nistp384
| ecdh-sha2-nistp256
| diffie-hellman-group-exchange-sha256
| diffie-hellman-group18-sha512
| diffie-hellman-group17-sha512
| diffie-hellman-group16-sha512
| diffie-hellman-group15-sha512
| diffie-hellman-group14-sha256Cause
Bitbucket 7.17.0 has its own SSHD based on Apache SSHD 2.7.0. Previous versions of Bitbucket were using Apache SSHD 2.4.0.
The SHA-1 key exchange was removed by default in Apache SSHD 2.6.0. This means that clients that only support SHA-1 keys will fail to negotiate an exchange algorithm.
Solution
To allow the older client and the SHA1 algorithms to be recognised you would need to explicitly allow SHA1 key exchange on the Bitbucket server.
Adding the SHA1 key to the /etc/ssh/sshd_config on the Bitbucket server will allow the SSHD to recognise the SHA1 keys. You will need to restart Bitbucket after the change for this to take effect.
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KexAlgorithms +diffie-hellman-group1-sha1Was this helpful?