TLDR:

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# Must specified `id: "ssh"` in order to override the default forwarded SSH port.
config.vm.network :forwarded_port, guest: 22, host: 2322, id: "ssh"

What’s the problem? If running multiple virtual machines managed by Vagrant, SSH port collision will happen:

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Vagrant cannot forward the specified ports on this VM, since they
would collide with some other application that is already listening
on these ports. The forwarded port to 2222 is already in use
on the host machine.
To fix this, modify your current projects Vagrantfile to use another
port. Example, where '1234' would be replaced by a unique host port:
  config.vm.network :forwarded_port, guest: 22, host: 1234
Sometimes, Vagrant will attempt to auto-correct this for you. In this
case, Vagrant was unable to. This is usually because the guest machine
is in a state which doesn't allow modifying port forwarding.

The example is correct but incomplete. By default the SSH port (22) on the guest machine is forwarded to port 2222 on the host machine:

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# Source: https://github.com/mitchellh/vagrant/blob/v1.7.2/plugins/kernel_v2/config/vm.rb
if !@__networks["forwarded_port-ssh"]
  network :forwarded_port,
    guest: 22,
    host: 2222,
    host_ip: "127.0.0.1",
    id: "ssh",
    auto_correct: true
end

Changing to another port in Vagrantfile does not solve the problem. For example, switching from 2222 to 2322:

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# Create a forwarded port mapping which allows access to a specific port
# within the machine from a port on the host machine. In the example below,
# accessing "localhost:8080" will access port 80 on the guest machine.
config.vm.network "forwarded_port", guest: 22, host: 2322

In this situation, you are forwarding both ports 2222 and 2322 to the SSH port on the guest machine, as shown in VirtualBox port forwarding settings:

When bringing up the machine, there will be multiple host machine ports forwarding to the same SSH port.

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$ vagrant up
==> default: Forwarding ports...
    default: 22 => 2322 (adapter 1)
    default: 22 => 2222 (adapter 1)

The SSH configuration shows the same outcome:

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$ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User chao
  Port 2222

Of course, you can do:

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$ ssh -p 2322 chao@localhost

This works, but we like the simple vagrant ssh command, and we just want to retain a single port 2322.

The solution is appending id: "ssh", and it must be specified in order to override the default forwarded SSH port. See < https://github.com/mitchellh/vagrant/issues/3232#issuecomment-39354659>:

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# Must specified `id: "ssh"` in order to override the default forwarded SSH port.
config.vm.network :forwarded_port, guest: 22, host: 2322, id: "ssh"

After appending id: "ssh", the SSH configuration has also been updated:

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$ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User chao
  Port 2322

Now, you are good to go:

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$ vagrant ssh

I can use SSH directly instead of Vagrant SSH command for interactive programs such as top or running tmux sessions. But I frequently just want to run Tmux or Byobu upon login. I can do:

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$ ssh -F ssh-config vagrant -t byobu

But still too much trouble to go through all SSH configuration steps. So, I ended up with two-step process:

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$ vagrant ssh

Now inside the guest machine, I immediately type:

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$ byobu

To bring up new Tmux session or attach existing sessions.

Vagrant command vagrant ssh connects to the running virtual machine via SSH. SSH arguments can also be added:

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$ vagrant ssh -h
Usage: vagrant ssh [options] [name] [-- extra ssh args]
Options:
    -c, --command COMMAND            Execute an SSH command directly
    -p, --plain                      Plain mode, leaves authentication up to user
    -h, --help                       Print this help

For example, execute a single command:

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$ vagrant ssh -c date
Wed Dec 10 12:00:00 UTC 2014
Connection to 127.0.0.1 closed.

or:

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$ vagrant ssh -- date
Wed Dec 10 12:00:00 UTC 2014

which prints no connection closed message.

Another example:

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$ vagrant ssh -- 'cat /etc/hosts'
127.0.0.1 localhost
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

However, interactive applications cannot be used in this way:

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$ vagrant ssh -- top
TERM environment variable not set.

Well, we already have a virtual machine running, we can just use SSH directly.

I’ve got a new laptop, and I need to install the SSH public key of the new machine to all my AWS EC2 instances in order to enable keyless access. I can use ssh-copy-id to install the public key one instance at a time, but I can also do it all at once:

$ aws ec2 describe-instances --output text \
  --query 'Reservations[*].Instances[*].{IP:PublicIpAddress}' \
  while read host; do \
  ssh-copy-id -i /path/to/key.pub $USER@$host; done

Somehow if using PublicIpAddress, some IP addresses in the response were cluttered in a single line. So, I use {IP:PublicIpAddress} instead.

For non-standard port:

$ ssh-copy-id -i /path/to/key.pub "$USER@$host -p $port"

The only problem is that it might install a duplicate key in ~/.ssh/authorized_keys file of the remote instance, if the key has already been installed. One way to solve this problem is to test the login from the new machine and generate only the IP addresses that the new machine does not have access to:

$ aws ec2 describe-instances --output text \
  --query 'Reservations[*].Instances[*].{IP:PublicIpAddress}' \
  | while read host; do ssh -q $USER@$host exit \
  || echo $host; done > instances.txt

Now back to the old machine, install the public key at once:

$ cat instances.txt | while read host; \
  do ssh-copy-id -i /path/to/key.pub $USER@$host; done