Passing ssh options to git clone

Question

The recently released git 2.3 supports a new variable "GIT_SSH_COMMAND" which can be used to define a command WITH parameters.

GIT_SSH_COMMAND="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no" git clone user@host

$GIT_SSH_COMMAND takes precedence over $GIT_SSH, and is interpreted by the shell, which allows additional arguments to be included.

Add them to your ~/.ssh/config:

Host host
    HostName host
    User user
    SshOption1 Value1
    SshOption2 Value2

The Host entry is what you’ll specify on the command line, and the HostName is the true hostname. They can be the same, or the Host entry can be an alias. The User entry is used if you do not specify user@ on the command line.

If you must configure this on the command line, set the GIT_SSH environment variable to point to a script with your options in it.

Another option made to specify different keys is git config core.sshCommand with git 2.10 + (Q3 2016).

This is an alternative to the environment variable described in Boris's answer)

See commit 3c8ede3 (26 Jun 2016) by Nguyễn Thái Ngọc Duy (pclouds).
^{(Merged by Junio C Hamano -- gitster -- in commit dc21164, 19 Jul 2016)}

A new configuration variable core.sshCommand has been added to specify what value for GIT_SSH_COMMAND to use per repository.

Similar to $GIT_ASKPASS or $GIT_PROXY_COMMAND, we also read from config file first then fall back to $GIT_SSH_COMMAND.

This is useful for selecting different private keys targetting the same host (e.g. github)

core.sshCommand:

If this variable is set, git fetch and git push will use the specified command instead of ssh when they need to connect to a remote system.
The command is in the same form as the GIT_SSH_COMMAND environment variable and is overridden when the environment variable is set.

It means the git clone can be:

cd /path/to/my/repo
git config core.sshCommand 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no' 
# later on
git clone host:repo.git

If you want to apply that for all repos, as user1300959 adds in the comments, you would use a global configuration.

git config --global core.sshCommand 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no'

Repository level configuration without impacting the system level settings

Consolidating the already available answers, I am choosing the below steps. This ensures that the configuration changes do not impact at the machine level, but just for the repository being worked on. This is needed in my case as my script needs to be executed on a shared Bamboo agent.

1.Clone the repository taking the GIT_SSH_COMMAND approach.

GIT_SSH_COMMAND="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no" git clone ssh://url

2.Once cloned, navigate into repository directory.

cd repo-dir

3.Set core.sshCommand configuration so that all future calls can be just run with git commands like usual, but internally consuming the provided git options.

git config core.sshCommand 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no'

Here is tricky example how to pass the ssh arguments by using GIT_SSH variable:

$ echo 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no $*' > ssh
$ chmod +x ssh
$ GIT_TRACE=1 GIT_SSH="$PWD/ssh" git clone user@host

_{Note: Above lines are terminal command-lines which you should paste into your terminal. It'll create a file ssh, make it executable and executes it.}

If you'd like to pass the private key option, please check How to tell git which private key to use?.