Ich steh auf´m Schlauch, habt ihr einen Tipp? Ich krieg keine 
TLS-Verbindung zu meinem FTP-Server (proftpd) zustande.

Auf der Maschine selbst sieht es gut aus:

$ openssl s_client -connect localhost:21 -starttls ftp
CONNECTED(00000003)
depth=0 CN = line
verify error:num=18:self signed certificate
verify return:1
depth=0 CN = line
verify return:1
---
Certificate chain
  0 s:/CN=line

[.... AUSLASSUNG ....]

No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: ECDH, P-256, 256 bits
---
SSL handshake has read 1281 bytes and written 312 bytes
Verification error: self signed certificate
---
New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
     Protocol  : TLSv1.2
     Cipher    : [ ..... ENTFERNT .....]
     Session-ID:  [ ..... ENTFERNT .....]
     Session-ID-ctx:
     Master-Key:  [ ..... ENTFERNT .....]
     PSK identity: None
     PSK identity hint: None
     SRP username: None
     Start Time: 1515708844
     Timeout   : 7200 (sec)
     Verify return code: 18 (self signed certificate)
     Extended master secret: no
---
220 ProFTPD 1.3.5b Server (Debian) [::1]


Doch von "Außerhalb" ...


$ openssl s_client -connect 185.162.250.29:21 -starttls ftp
CONNECTED(00000003)
140246464558736:error:140770FC:SSL 
routines:SSL23_GET_SERVER_HELLO:unknown protocol:s23_clnt.c:797:
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 96 bytes and written 285 bytes
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
     Protocol  : TLSv1.2
     Cipher    : 0000
     Session-ID:
     Session-ID-ctx:
     Master-Key:
     Key-Arg   : None
     PSK identity: None
     PSK identity hint: None
     SRP username: None
     Start Time: 1515708954
     Timeout   : 300 (sec)
     Verify return code: 0 (ok)

Warum "unknown protocol" - wenn es doch auf localhost funktioniert?

Viele Grüße
Ralph
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