Q: Why do I get "Bad configuration option gssapikeyexchange" after upgrading to macOS Sierra?

• After upgrading to macOS Sierra (10.12) and attempting to run ssh, I see an error message like:

  dhcp-18-111-116-122:~ username$ ssh username@athena.dialup.mit.edu
  /Users/username/.ssh/config: line 2: Bad configuration option: gssapikeyexchange
  /Users/username/.ssh/config: terminating, 1 bad configuration options
  

  or

  /etc/ssh/ssh_config: line 20: Bad configuration option: gssapikeyexchange
  /etc/ssh/ssh_config: terminating, 1 bad configuration options
  

  How do I fix this?

Context

• You are running macOS Sierra (10.12)
• You have a pre-existing .ssh/config or /etc/ssh/ssh_config file, possibly with a GSSAPIKeyExchange no setting that was previously required for El Capitan (Mac OS 10.11)

Answer

• The ssh program in macOS Sierra no longer supports the GSSAPIKeyExchange option
• Remove or comment out lines containing GSSAPIKeyExchange