MAC OSX – SSH Error ‘No Matching Exchange Method Found’

KB ID 0001245 Dtd 13/10/16

Problem

I thought my RoyalTSX had broken today, I upgraded it a couple of weeks ago, and I upgraded to macOS Sierra version 10.12 the other day. After this, all my SSH sessions refused to connect with this error;

Mac OSX SSH Error no matching key exchange

 

Unable to negotiate with x.x.x.x port 22: no matching key exchange found. Their offer diffie-hellman-group1-sha1

Solution

This is not Apple’s fault, it’s OpenSSH version 7. SHA1 is weak, so support for it has been removed. Which is fine, but all my clients Cisco Firewalls/Routers/Switches are probably all using  RSA/SHA1. So until they re all updated I’m going to need to re-enable SHA1.

Open a terminal windows and execute the following;

sudo nano /etc/ssh/ssh_config
ENTER YOUR PASSWORD

Locate the line ‘ #   MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160′ and remove the Hash/Pound sight from the beginning. Then paste the following on the end;

HostkeyAlgorithms ssh-dss,ssh-rsa

KexAlgorithms +diffie-hellman-group1-sha1

 

MacOSX Allow SHA1

 Theres no reason to reboot, it should work straight away.

Related Articles, References, Credits, or External Links

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Author: PeteLong

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35 Comments

  1. I tried your work around, but it only gives me a different error. Is it possible that something more needs to be changed? Thanks

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    • What Error did you get?

      P

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    • Fixed the issue immediately. Thanks

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  2. Thanks a lot for this. Helped me straight away!!!

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    • No Probs – Glad to help 🙂 P

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  3. Still perfect.
    Thanks For all.

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  4. Thanks for this.Work just as described!

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    • Worked like a charm – Thanks!

      Post a Reply
  5. It didn’t like me allowing that line. It kept throwing up a new error, though to be fair, my line was longer and looked different.

    However, putting the pound sign back and just adding that bit to the bottom worked straight away.

    Thanks for the help.

    Post a Reply
  6. Using all 3 changes will invalidate all host-keys in ‘known_hosts’.
    Only the last line was actually needed for me: KexAlgorithms diffie-hellman-group1-sha1

    With the caveat that this will force all ssh negotiations down to this less secure protocol.

    A better option is to leave /etc/ssh/ssh_config alone alltogether, and create ~/.ssh/config in your home-dir (alongside the known_hosts file)
    In ~/.ssh/config create an entry as follows for the equipment that use this key-exchange. Use as identification the name or ip you actually use on your commandline. (i.e. use ‘192.168.0.1’ or ‘firewall’ if you use ‘ssh 192.168.0.1’ or ‘ssh firewall’)

    #force key exchange:
    host 192.168.0.1 firewall.local firewall
    KexAlgorithms diffie-hellman-group1-sha1

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  7. Thanks a lot, this worked a treat for me. : )

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  8. Thanks a lot.. IT worked for me..

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  9. Muchas Gracias, Work

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  10. Fabulous. Best Cisco resource on the Internet.

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  11. Worked PERFECTLY!!!! Thank you.

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  12. Thanks for the pointers. I only needed the last line in order to SSH onto my legacy Cisco switches. Also, it’s more secure to use a “+” which appends SHA1 to the usable set of algorithms, rather than using SHA1 as the default algorithm.

    KexAlgorithms +diffie-hellman-group1-sha1

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    • Hi, Great response! I’ve update the article accordingly ThanQ

      P

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  13. Fantastic! Helped when I needed it!

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  14. It only happens if the Cisco device is using IOS version 12 or older. Version 15 works fine. If you have smartnet contract with Cisco, just upgrade to the recommended software. Instead of messing around with ssh file, I just ssh to an unix server and then ssh to Cisco device.

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    • Thats fine, but what about Cisco ASA? The problem is inherently a client one, some people don’t have modern code on their devices.

      Pete

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  15. We’re going to secure ourselves right out of being able to work!

    Thanks for this – you were the first result when I Googled the problem!

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  16. Thanks a lot ! It’s work for me.

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    • Just adding the KexAlgorithms +diffie-hellman-group1-sha1 lines to the end of my ssh_config file worked on Mac OSX 10.12.5 (16F73)

      Thanks !

      Dan

      Post a Reply
  17. Hi,

    I had to do this to make it work:

    Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
    MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160
    KexAlgorithms +diffie-hellman-group1-sha1
    HostkeyAlgorithms ssh-dss,ssh-rsa

    Post a Reply
  18. Thanks so much… this works PERFECTLY.

    Post a Reply
    • No problem glad to help
      P

      Post a Reply
  19. Big ups to my dawg. This worked like a charm. And I’m computer illiterate.

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    • No worries – Glad to help!

      Pete

      Post a Reply

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