internal: add simple keyscanner

This is an initial implementation and not a replacement candidate
for ssh-keyscan since it does only scan the key of the algorithm
the client and server agreed upon. This agreement may change
depending on the key being used, making it useless for distributed
usages.

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/fluxcd/source-controller v0.0.1-alpha.2
 	github.com/manifoldco/promptui v0.7.0
 	github.com/spf13/cobra v1.0.0
+	golang.org/x/crypto v0.0.0-20200302210943-78000ba7a073
 	k8s.io/api v0.18.2
 	k8s.io/apimachinery v0.18.2
 	k8s.io/client-go v0.18.2

internal/keyscan/keyscan.go

@@ -0,0 +1,40 @@
+package keyscan
+
+import (
+	"encoding/base64"
+	"fmt"
+	"net"
+
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/knownhosts"
+)
+
+func ScanKeys(host string) ([]byte, error) {
+	col := &collector{}
+	config := &ssh.ClientConfig{
+		User:            "git",
+		HostKeyCallback: col.StoreKey(),
+	}
+	client, err := ssh.Dial("tcp", host, config)
+	if err == nil {
+		defer client.Close()
+	}
+	if len(col.knownKeys) > 0 {
+		return col.knownKeys, nil
+	}
+	return col.knownKeys, err
+}
+
+type collector struct {
+	knownKeys []byte
+}
+
+func (c *collector) StoreKey() ssh.HostKeyCallback {
+	return func(hostname string, remote net.Addr, key ssh.PublicKey) error {
+		c.knownKeys = append(
+			c.knownKeys,
+			fmt.Sprintf("%s %s %s\n", knownhosts.Normalize(hostname), key.Type(), base64.StdEncoding.EncodeToString(key.Marshal()))...,
+		)
+		return nil
+	}
+}