Golang SSH-Server: How to handle file transfer with scp? - ssh

Question

I have written a small SSH-Server in golang with the crypto/ssh package.
It supports returning an interactive shell and immediate command execution.
Here is a minimal example of the server:
package main
import (
"fmt"
"io/ioutil"
"log"
"net"
"os/exec"
"golang.org/x/crypto/ssh"
)
func main() {
c := &ssh.ServerConfig{
PasswordCallback: func(c ssh.ConnMetadata, pass []byte) (*ssh.Permissions, error) {
if c.User() == "foo" && string(pass) == "bar" {
return nil, nil
}
return nil, fmt.Errorf("password rejected for %q", c.User())
},
}
keyBytes, _ := ioutil.ReadFile("key")
key, _ := ssh.ParsePrivateKey(keyBytes)
c.AddHostKey(key)
listener, _ := net.Listen("tcp", "0.0.0.0:2200")
for {
tcpConn, _ := listener.Accept()
_, chans, reqs, _ := ssh.NewServerConn(tcpConn, c)
go ssh.DiscardRequests(reqs)
go handleChannels(chans)
}
}
func handleChannels(chans <-chan ssh.NewChannel) {
for newChannel := range chans {
go handleChannel(newChannel)
}
}
func handleChannel(newChannel ssh.NewChannel) {
channel, requests, _ := newChannel.Accept()
for req := range requests {
switch req.Type {
case "shell":
go handleShell(channel)
case "exec":
go handleExec(channel, req)
}
}
}
func handleShell(c ssh.Channel) {}
func handleExec(c ssh.Channel, r *ssh.Request) {
cmdString, args, _ := parseCommand(r.Payload)
log.Printf("exec: %s\n", cmdString)
for i := range args {
log.Printf("arg %d: %s\n", i, args[i])
}
cmd := exec.Command(cmdString, args...)
cmd.Run()
}
func parseCommand(b []byte) (string, []string, error) {
cmdString := strings.TrimSpace(string(b))
cmdArray := strings.Split(cmdString, " ")
cmd := strings.Trim(cmdArray[0], " ")
args := cmdArray[1:]
return cmd, args, nil
}
If I run the server and execute scp as follows:
scp -P 2200 test.file foo#localhost:~/
the handleExec function is called.
The output of the cmdString shows:
2015/11/22 17:49:14 exec: scp
2015/11/22 17:49:14 arg 0: -t
2015/11/22 17:49:14 arg 1: ~/
But how can I implement the handleExec function to actually save the file/dir I passed via scp?

Answer 1

I just ran into the problem of executing scp and custom commands over my ssh server and as it is undocumented how to do this I pieced together some code from the tests in crypto.ssh (https://github.com/golang/crypto/blob/master/ssh/session.go and https://github.com/golang/crypto/blob/master/ssh/session_test.go) It works with OpenSSH and the crypto.ssh client. You can for instance call session.Run() on your client and handle e.g. scp or custom commands with it.
type exitStatusMsg struct {
Status uint32
}
// RFC 4254 Section 6.5.
type execMsg struct {
Command string
}
go func(in <-chan *ssh.Request, channel ssh.Channel) {
for req := range in {
if req.Type == "exec" {
var msg execMsg
if err := ssh.Unmarshal(req.Payload, &msg); err != nil {
log.Printf("error parsing ssh execMsg: %s\n", err)
req.Reply(false, nil)
return
}
go func(msg execMsg, ch ssh.Channel) {
// ch can be used as a ReadWriteCloser if there should be interactivity
runYourCommand(msg.Command, ch)
ex := exitStatusMsg{
Status: 0,
}
// return the status code
if _, err := ch.SendRequest("exit-status", false, ssh.Marshal(&ex)); err != nil {
log.Printf("unable to send status: %v", err)
}
ch.Close()
}(msg, channel)
req.Reply(true, nil) // tell the other end that we can run the request
} else {
req.Reply(req.Type == "shell", nil)
}
}
}(requests, channel)
You need to replace runYourCommand with whatever function then executes your command and set the exit code to whatever your command/process returns.