How to run a local script in a remote server?

I want a way to replicate this command using ssh/crypto:

ssh user@host /bin/bash < dummy.sh

I can easily run binaries that's already in the host with simple

session.Run(cmd)

But I am having issues running complex command like this.

As /u/JHunz said, the < is not really part of the command line sent to the other machine. It just tells the local shell to connect dummy.sh to the stdin of the ssh command (which is why it only works if the file is locally available). It's pretty much exactly as if you typed the contents of that file into ssh, then pressed Ctrl+D to simulate EOF and close the command.

Now, as for how to replicate this in Go: the interface for crypto/ssh.Session seems to be similar to that of os/Exec.Cmd, so my guess would be something like:

file, err := os.Open("dummy.sh")
if err != null { return err; }
defer file.Close()
session.Stdin = file
session.Run(cmd)

Disclaimer: I've never had occasion to use the crypto/ssh package.

The cmd here should be something like "bash -c" for crypto/ssh, or "ssh user@host bash -c" for using os/exec, as a slice ofc. +1 on the explanation.
It's also possible to copy the originating script to the remote machine by running cat > /tmp/outfile.sh as the cmd. The file will be sent to Stdin, cat which executes on the remote host will read from it, and then write the contents into a file on the system. It might be better do so it like this, just so there's no weird tty issues or whatever (I haven't hit them myself, but some things require a tty and break if you don't have one).

The reading of the file and passing it in via the < is part of the shell, not SSH, so it's no wonder you're having trouble replicating it using just the SSH package.  I'm guessing the best approach is to read the whole script in, then pass it all as the cmd to Session.Run

You should execute it in sh -c - otherwise you'd pollute the system with temporary scripts.

Ignoring best practises (ie error handling), the basic way I do this in our server is as follows :
signer, err := ssh.ParsePrivateKey([]byte(connectionInfo.PrivateKey))
sshConfig := &ssh.ClientConfig{
    User: connectionInfo.UserID,
    Auth: []ssh.AuthMethod{
        ssh.PublicKeys(signer),
    },
    Timeout: time.Second * 5,
}
conn, err := net.DialTimeout("tcp", connectionInfo.AddressString(), sshConfig.Timeout)
..
c, chans, reqs, err := ssh.NewClientConn(conn, connectionInfo.AddressString(), sshConfig)
..
client = ssh.NewClient(c, chans, reqs)

So to run a command 
session, err := client.NewSession()
var b bytes.Buffer
session.Stdout = &b
session.Run("ls -l") // or what ever you want to execute

b contains output
Copy a script to the server for execution:
session, err := client.NewSession()
bytesReader := bytes.NewReader(bytesToWrite)
scp.Copy(int64(len(bytesToWrite)), 0644, filename, bytesReader, path, session)

I hope this helps.

Thanks, I have been trying to avoid doing that but looks like this is the only way out. Let me try it.