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Linux SSH Config: Removing ciphers and MACs

Posted on 2014-01-13
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Last Modified: 2014-01-19
Hi, I need to remove CBC ciphers and the following MACs...
 - hmac-md5
 - hmac-md5-96
 - hmac-sha1-96
I edited my "/etc/ssh/ssh_config" by changing...
# Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc

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to...
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128

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and...
# MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160

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to...
MACs hmac-sha1,umac-64@openssh.com,hmac-ripemd160

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Then I restarted sshd by executing the following command...
service sshd restart

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It shows...
Stopping sshd: [ OK ]
Starting sshd: [ OK ]

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But a server scan still shows that the cipher and MACs are still supported.
How do I remove the cipher and MACs correctly?
Thanks!
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Question by:killdurst
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7 Comments
 
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Assisted Solution

by:Jan Springer
Jan Springer earned 125 total points
ID: 39780290
Supported or allowed?
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LVL 30

Expert Comment

by:serialband
ID: 39780386
In case you didn't make a typo, /etc/ssh/ssh_config is used by the ssh client.  You should be editing /etc/ssh/sshd_config and then restarting the server. (note the extra d )
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Author Comment

by:killdurst
ID: 39781251
By supported, I mean allowed.

So I can just copy and paste the two lines from ssh_config to sshd_config right?
Restarting the server is not at option at this point actually.
Can I just restart the sshd service?
Thanks!
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Accepted Solution

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serialband earned 375 total points
ID: 39782887
Yes, copy the lines to sshd_config.  You can restart just the sshd service without affecting already connected users.  Those users remain on the older configuration untill they disconnect, and new user connections use the new settings.
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Author Comment

by:killdurst
ID: 39784383
Ok, I think the changes took effect after I restarted the service cos now I can't SSH in to the server.
I'm getting a "Algorithm negotiation failed".
I'm using SSH Secure Shell to remote in.
When logging in, I tried the following encryption / MAC algorithm combinations...
AES 128 / HMAC-MD5
AES 128 / HMAC-SHA1
AES 192 / HMAC-MD5
AES 192 / HMAC-SHA1
AES 256 / HMAC-MD5
AES 256 / HMAC-SHA1
Arcfour 128 / HMAC-MD5
Arcfour 128 / HMAC-SHA1

Arcfour 256 is not listed as an option.

I can still log in using Putty though... and I can also modify the sshd_config and restart the service using the web console.

Just wondering why I can't ssh in using secure shell...

Thanks...
0
 
LVL 30

Expert Comment

by:serialband
ID: 39785581
Did you edit the client configuraton, ssh_config to include Arcfour 256?
0
 
LVL 1

Author Comment

by:killdurst
ID: 39793156
Actually I've commented back the Ciphers and the MACs lines in ssh_config.

Anyway, I've decided to stick to using Putty for the command line interface and Filezilla for FTP from now onwards.

Thanks for your help regarding the tip to edit sshd_config.
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