launch-node: get sshfp entries from the host

It turns out bionic ssh-keygen doesn't have the "-D" to produce the
sshfp records; switch to logging in and getting these via "ssh-keygen
-r" on the host.

Change-Id: Icb6efd7c4fd9623af24e58c69f8a188a4c1fb4c9

launch/sshfp.py

@@ -3,19 +3,32 @@
 import argparse
 import subprocess
 
-def generate_sshfp_records(hostname, ip):
+def generate_sshfp_records(hostname, ip, local):
     '''Given a hostname and and IP address, scan the IP address (hostname
     not in dns yet) and return a bind string with sshfp records'''
 
-    s = subprocess.run(['ssh-keyscan', '-D', ip],
+    if local:
+        p = ['ssh-keyscan', '-D', ip]
+    else:
+        # Handle being run via sudo which is the usual way
+        # this is run.
+        p = ['ssh', '-o', 'StrictHostKeyChecking=no',
+             '-i', '/root/.ssh/id_rsa',
+             'root@%s' % ip, 'ssh-keygen', '-r', ip]
+
+    s = subprocess.run(p,
                        stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE).stdout.decode('utf-8')
-
     fingerprints = []
     for line in s.split('\n'):
         if not line:
             continue
         _, _, _, algo, key_type, fingerprint = line.split(' ')
+        # ssh-keygen on the host seems to return DSS/DSA keys, which
+        # aren't valid to log in and not shown by ssh-keyscan -D
+        # ... prune it.
+        if algo == '2':
+            continue
         fingerprints.append(
             (algo, key_type, fingerprint))
 
@@ -32,17 +45,19 @@ def generate_sshfp_records(hostname, ip):
     return ret
 
 
-def sshfp_print_records(hostname, ip):
-    print(generate_sshfp_records(hostname, ip))
+def sshfp_print_records(hostname, ip, local=False):
+    print(generate_sshfp_records(hostname, ip, local))
 
 
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument("hostname", help="hostname")
     parser.add_argument("ip", help="address to scan")
+    parser.add_argument("--local", action='store_true',
+                        help="Run keyscan locally, rather than via ssh")
     args = parser.parse_args()
 
-    sshfp_print_records(args.hostname, args.ip)
+    sshfp_print_records(args.hostname, args.ip, args.local)
 
 if __name__ == '__main__':
     main()