Negotiation failed. The connection was closed by the server.

Question

FileZilla reports "Unknown Host Key" | "Hostkey algorithm: ecdsa-sha2-nistp521 521" then allows me to connect.

var ftp = new Sftp();
ftp.Settings.SshParameters.PreferredHostKeyAlgorithm = SshHostKeyAlgorithm.ECDsaNistP521;
ftp.Connect("host", serverPort: 22);
ftp.Login("user", "password");
var validated = ftp.PutFile(uploadFileName, $"/inbound/{Parse.FileNamePart(uploadFileName)}") == new FileInfo(uploadFileName).Length;
ftp.Disconnect();

Note that I've purchased your latest Sftp version today (I've been a client since 2012), but still can't connect. Thanks.

Lukas Pokorny · Answer 1 · Jun 5, 2020

answered Jun 5, 2020 by Lukas Pokorny (126,870 points)

Try adding the following line to your code to enable ECDSA host key algorithm with NIST P-521 curve:

ftp.Settings.HostKeyAlgorithms |= SshHostKeyAlgorithm.ECDsaNistP521;

commented Jun 5, 2020 by jlongenderfer (120 points)

Thanks Lukas.
I'm still getting the same error. Here is my current code and verbose log.

                var ftp = new Sftp();
                if (ftpConsoleLogging) ftp.LogWriter = new RebexConsoleLogWriter(LogLevel.Verbose);
                ftp.Settings.UseLargeBuffers = true;
                ftp.Settings.SshParameters.HostKeyAlgorithms |= SshHostKeyAlgorithm.ECDsaNistP521;
                ftp.Settings.SshParameters.PreferredHostKeyAlgorithm = SshHostKeyAlgorithm.ECDsaNistP521;
                ftp.Connect("sftp.4myrebate.com", serverPort: 22);
                ftp.Login("login", "password");
                var validated = ftp.PutFile(uploadFileName, $"/inbound/{Parse.FileNamePart(uploadFileName)}") == new FileInfo(uploadFileName).Length;
                ftp.Disconnect();
-------
Info: Assembly: Rebex.Sftp 2020 R2 for .NET 4.0-4.8
Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
Info: Culture: en; Windows-1252
Proxy: Resolving 'sftp.4myrebate.com'.
Proxy: Connecting to 3.224.150.201:22 (no proxy).
Proxy: Connection established.
SSH: Sending data:
0000 |53-53-48-2D-32-2E-30-2D 52-65-62-65-78-53-53-48| SSH-2.0-RebexSSH
0010 |5F-35-2E-30-2E-37-34-35 30-2E-30-0D-0A         | _5.0.7450.0..
SSH: Received data:
0000 |53-53-48-2D-32-2E-30-2D 53-79-6E-63-70-6C-69-66| SSH-2.0-Syncplif
0010 |79-5F-4D-65-5F-53-65-72 76-65-72-0D-0A         | y_Me_Server..
SSH: Server is 'SSH-2.0-Syncplify_Me_Server'.
SSH: Negotiation started.
SSH: Sending packet SSH_MSG_KEXINIT (1263 bytes).
0000 |14-A6-65-5B-D0-5F-9B-77 1B-EB-E0-98-48-7A-A3-48| ..e[._.w....Hz.H
0010 |2E-00-00-01-0A-63-75-72 76-65-32-35-35-31-39-2D| .....curve25519-
0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
0050 |73-74-70-32-35-36-2C-64 69-66-66-69-65-2D-68-65| stp256,diffie-he
0060 |6C-6C-6D-61-6E-2D-67-72 6F-75-70-2D-65-78-63-68| llman-group-exch
0070 |61-6E-67-65-2D-73-68-61 32-35-36-2C-64-69-66-66| ange-sha256,diff
0080 |69-65-2D-68-65-6C-6C-6D 61-6E-2D-67-72-6F-75-70| ie-hellman-group
0090 |31-34-2D-73-68-61-32-35 36-2C-64-69-66-66-69-65| 14-sha256,diffie
00A0 |2D-68-65-6C-6C-6D-61-6E 2D-67-72-6F-75-70-31-35| -hellman-group15
00B0 |2D-73-68-61-35-31-32-2C 64-69-66-66-69-65-2D-68| -sha512,diffie-h
00C0 |65-6C-6C-6D-61-6E-2D-67 72-6F-75-70-31-36-2D-73| ellman-group16-s
00D0 |68-61-35-31-32-2C-64-69 66-66-69-65-2D-68-65-6C| ha512,diffie-hel
00E0 |6C-6D-61-6E-2D-67-72-6F 75-70-2D-65-78-63-68-61| lman-group-excha
00F0 |6E-67-65-2D-73-68-61-31 2C-64-69-66-66-69-65-2D| nge-sha1,diffie-
0100 |68-65-6C-6C-6D-61-6E-2D 67-72-6F-75-70-31-34-2D| hellman-group14-
0110 |73-68-61-31-2C-65-78-74 2D-69-6E-66-6F-2D-63-00| sha1,ext-info-c.
0120 |00-00-A7-65-63-64-73-61 2D-73-68-61-32-2D-6E-69| ...ecdsa-sha2-ni
0130 |73-74-70-35-32-31-2C-73 73-68-2D-64-73-73-2C-72| stp521,ssh-dss,r
0140 |73-61-2D-73-68-61-32-2D 32-35-36-2C-73-73-68-2D| sa-sha2-256,ssh-
0150 |72-73-61-2D-73-68-61-32 35-36-40-73-73-68-2E-63| rsa-sha256@ssh.c
0160 |6F-6D-2C-72-73-61-2D-73 68-61-32-2D-35-31-32-2C| om,rsa-sha2-512,
0170 |73-73-68-2D-72-73-61-2C 78-35-30-39-76-33-2D-73| ssh-rsa,x509v3-s
0180 |69-67-6E-2D-72-73-61-2D 73-68-61-32-35-36-40-73| ign-rsa-sha256@s
0190 |73-68-2E-63-6F-6D-2C-78 35-30-39-76-33-2D-73-69| sh.com,x509v3-si
01A0 |67-6E-2D-72-73-61-2C-78 35-30-39-76-33-2D-73-69| gn-rsa,x509v3-si
01B0 |67-6E-2D-64-73-73-2C-65 63-64-73-61-2D-73-68-61| gn-dss,ecdsa-sha
01C0 |32-2D-6E-69-73-74-70-32 35-36-00-00-01-05-61-65| 2-nistp256....ae
01D0 |73-32-35-36-2D-67-63-6D 40-6F-70-65-6E-73-73-68| s256-gcm@openssh
01E0 |2E-63-6F-6D-2C-61-65-73 31-32-38-2D-67-63-6D-40| .com,aes128-gcm@
01F0 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-61-65-73-32| openssh.com,aes2
0200 |35-36-2D-63-74-72-2C-61 65-73-31-39-32-2D-63-74| 56-ctr,aes192-ct
0210 |72-2C-61-65-73-31-32-38 2D-63-74-72-2C-33-64-65| r,aes128-ctr,3de
0220 |73-2D-63-74-72-2C-74-77 6F-66-69-73-68-32-35-36| s-ctr,twofish256
0230 |2D-63-74-72-2C-74-77-6F 66-69-73-68-31-39-32-2D| -ctr,twofish192-
0240 |63-74-72-2C-74-77-6F-66 69-73-68-31-32-38-2D-63| ctr,twofish128-c
0250 |74-72-2C-61-65-73-32-35 36-2D-63-62-63-2C-61-65| tr,aes256-cbc,ae
0260 |73-31-39-32-2D-63-62-63 2C-61-65-73-31-32-38-2D| s192-cbc,aes128-
0270 |63-62-63-2C-33-64-65-73 2D-63-62-63-2C-74-77-6F| cbc,3des-cbc,two
0280 |66-69-73-68-32-35-36-2D 63-62-63-2C-74-77-6F-66| fish256-cbc,twof
0290 |69-73-68-31-39-32-2D-63 62-63-2C-74-77-6F-66-69| ish192-cbc,twofi
02A0 |73-68-31-32-38-2D-63-62 63-2C-74-77-6F-66-69-73| sh128-cbc,twofis
02B0 |68-2D-63-62-63-2C-63-68 61-63-68-61-32-30-2D-70| h-cbc,chacha20-p
02C0 |6F-6C-79-31-33-30-35-40 6F-70-65-6E-73-73-68-2E| oly1305@openssh.
02D0 |63-6F-6D-00-00-01-05-61 65-73-32-35-36-2D-67-63| com....aes256-gc
02E0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-61-65| m@openssh.com,ae
02F0 |73-31-32-38-2D-67-63-6D 40-6F-70-65-6E-73-73-68| s128-gcm@openssh
0300 |2E-63-6F-6D-2C-61-65-73 32-35-36-2D-63-74-72-2C| .com,aes256-ctr,
0310 |61-65-73-31-39-32-2D-63 74-72-2C-61-65-73-31-32| aes192-ctr,aes12
0320 |38-2D-63-74-72-2C-33-64 65-73-2D-63-74-72-2C-74| 8-ctr,3des-ctr,t
0330 |77-6F-66-69-73-68-32-35 36-2D-63-74-72-2C-74-77| wofish256-ctr,tw
0340 |6F-66-69-73-68-31-39-32 2D-63-74-72-2C-74-77-6F| ofish192-ctr,two
0350 |66-69-73-68-31-32-38-2D 63-74-72-2C-61-65-73-32| fish128-ctr,aes2
0360 |35-36-2D-63-62-63-2C-61 65-73-31-39-32-2D-63-62| 56-cbc,aes192-cb
0370 |63-2C-61-65-73-31-32-38 2D-63-62-63-2C-33-64-65| c,aes128-cbc,3de
0380 |73-2D-63-62-63-2C-74-77 6F-66-69-73-68-32-35-36| s-cbc,twofish256
0390 |2D-63-62-63-2C-74-77-6F 66-69-73-68-31-39-32-2D| -cbc,twofish192-
03A0 |63-62-63-2C-74-77-6F-66 69-73-68-31-32-38-2D-63| cbc,twofish128-c
03B0 |62-63-2C-74-77-6F-66-69 73-68-2D-63-62-63-2C-63| bc,twofish-cbc,c
03C0 |68-61-63-68-61-32-30-2D 70-6F-6C-79-31-33-30-35| hacha20-poly1305
03D0 |40-6F-70-65-6E-73-73-68 2E-63-6F-6D-00-00-00-61| @openssh.com...a
03E0 |68-6D-61-63-2D-73-68-61 32-2D-32-35-36-2D-65-74| hmac-sha2-256-et
03F0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-68-6D| m@openssh.com,hm
0400 |61-63-2D-73-68-61-32-2D 32-35-36-2C-68-6D-61-63| ac-sha2-256,hmac
0410 |2D-73-68-61-32-2D-35-31 32-2D-65-74-6D-40-6F-70| -sha2-512-etm@op
0420 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
0430 |68-61-32-2D-35-31-32-2C 68-6D-61-63-2D-73-68-61| ha2-512,hmac-sha
0440 |31-00-00-00-61-68-6D-61 63-2D-73-68-61-32-2D-32| 1...ahmac-sha2-2
0450 |35-36-2D-65-74-6D-40-6F 70-65-6E-73-73-68-2E-63| 56-etm@openssh.c
0460 |6F-6D-2C-68-6D-61-63-2D 73-68-61-32-2D-32-35-36| om,hmac-sha2-256
0470 |2C-68-6D-61-63-2D-73-68 61-32-2D-35-31-32-2D-65| ,hmac-sha2-512-e
0480 |74-6D-40-6F-70-65-6E-73 73-68-2E-63-6F-6D-2C-68| tm@openssh.com,h
0490 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2C-68-6D-61| mac-sha2-512,hma
04A0 |63-2D-73-68-61-31-00-00 00-1A-6E-6F-6E-65-2C-7A| c-sha1....none,z
04B0 |6C-69-62-2C-7A-6C-69-62 40-6F-70-65-6E-73-73-68| lib,zlib@openssh
04C0 |2E-63-6F-6D-00-00-00-1A 6E-6F-6E-65-2C-7A-6C-69| .com....none,zli
04D0 |62-2C-7A-6C-69-62-40-6F 70-65-6E-73-73-68-2E-63| b,zlib@openssh.c
04E0 |6F-6D-00-00-00-00-00-00 00-00-00-00-00-00-00   | om.............
SSH: Received packet SSH_MSG_KEXINIT (650 bytes).

-- sending two parts due to site char limit --

commented Jun 5, 2020 by jlongenderfer (120 points)

-- part two --

0000 |14-92-E7-1E-E7-E2-F8-56 42-33-9D-5C-84-2B-BE-91| .......VB3.\.+..
0010 |97-00-00-00-59-64-69-66 66-69-65-2D-68-65-6C-6C| ....Ydiffie-hell
0020 |6D-61-6E-2D-67-72-6F-75 70-2D-65-78-63-68-61-6E| man-group-exchan
0030 |67-65-2D-73-68-61-31-2C 64-69-66-66-69-65-2D-68| ge-sha1,diffie-h
0040 |65-6C-6C-6D-61-6E-2D-67 72-6F-75-70-31-2D-73-68| ellman-group1-sh
0050 |61-31-2C-64-69-66-66-69 65-2D-68-65-6C-6C-6D-61| a1,diffie-hellma
0060 |6E-2D-67-72-6F-75-70-31 34-2D-73-68-61-31-00-00| n-group14-sha1..
0070 |00-1B-73-73-68-2D-72-73 61-2C-65-63-64-73-61-2D| ..ssh-rsa,ecdsa-
0080 |73-68-61-32-2D-6E-69-73 74-70-35-32-31-00-00-00| sha2-nistp521...
0090 |5A-62-6C-6F-77-66-69-73 68-2D-63-62-63-2C-61-65| Zblowfish-cbc,ae
00A0 |73-31-39-32-2D-63-62-63 2C-61-65-73-31-32-38-2D| s192-cbc,aes128-
00B0 |63-62-63-2C-63-61-73-74 31-32-38-2D-63-62-63-2C| cbc,cast128-cbc,
00C0 |61-65-73-31-32-38-2D-63 74-72-2C-61-65-73-31-39| aes128-ctr,aes19
00D0 |32-2D-63-74-72-2C-61-65 73-32-35-36-2D-63-74-72| 2-ctr,aes256-ctr
00E0 |2C-61-72-63-66-6F-75-72 31-32-38-00-00-00-5A-62| ,arcfour128...Zb
00F0 |6C-6F-77-66-69-73-68-2D 63-62-63-2C-61-65-73-31| lowfish-cbc,aes1
0100 |39-32-2D-63-62-63-2C-61 65-73-31-32-38-2D-63-62| 92-cbc,aes128-cb
0110 |63-2C-63-61-73-74-31-32 38-2D-63-62-63-2C-61-65| c,cast128-cbc,ae
0120 |73-31-32-38-2D-63-74-72 2C-61-65-73-31-39-32-2D| s128-ctr,aes192-
0130 |63-74-72-2C-61-65-73-32 35-36-2D-63-74-72-2C-61| ctr,aes256-ctr,a
0140 |72-63-66-6F-75-72-31-32 38-00-00-00-78-68-6D-61| rcfour128...xhma
0150 |63-2D-73-68-61-31-2C-68 6D-61-63-2D-72-69-70-65| c-sha1,hmac-ripe
0160 |6D-64-31-36-30-2C-68-6D 61-63-2D-72-69-70-65-6D| md160,hmac-ripem
0170 |64-31-36-30-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| d160@openssh.com
0180 |2C-75-6D-61-63-2D-36-34 40-6F-70-65-6E-73-73-68| ,umac-64@openssh
0190 |2E-63-6F-6D-2C-75-6D-61 63-2D-31-32-38-40-6F-70| .com,umac-128@op
01A0 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
01B0 |68-61-32-2D-32-35-36-2C 68-6D-61-63-2D-73-68-61| ha2-256,hmac-sha
01C0 |32-2D-35-31-32-00-00-00 78-68-6D-61-63-2D-73-68| 2-512...xhmac-sh
01D0 |61-31-2C-68-6D-61-63-2D 72-69-70-65-6D-64-31-36| a1,hmac-ripemd16
01E0 |30-2C-68-6D-61-63-2D-72 69-70-65-6D-64-31-36-30| 0,hmac-ripemd160
01F0 |40-6F-70-65-6E-73-73-68 2E-63-6F-6D-2C-75-6D-61| @openssh.com,uma
0200 |63-2D-36-34-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| c-64@openssh.com
0210 |2C-75-6D-61-63-2D-31-32 38-40-6F-70-65-6E-73-73| ,umac-128@openss
0220 |68-2E-63-6F-6D-2C-68-6D 61-63-2D-73-68-61-32-2D| h.com,hmac-sha2-
0230 |32-35-36-2C-68-6D-61-63 2D-73-68-61-32-2D-35-31| 256,hmac-sha2-51
0240 |32-00-00-00-1A-6E-6F-6E 65-2C-7A-6C-69-62-2C-7A| 2....none,zlib,z
0250 |6C-69-62-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-00| lib@openssh.com.
0260 |00-00-1A-6E-6F-6E-65-2C 7A-6C-69-62-2C-7A-6C-69| ...none,zlib,zli
0270 |62-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-00| b@openssh.com...
0280 |00-00-00-00-00-00-00-00 00-00                  | ..........
SSH: Group exchange (legacy form).
SSH: Sending packet SSH_MSG_KEX_30 (5 bytes).
0000 |1E-00-00-04-00                                 | .....
SSH: SSH connection closed.
SSH: Negotiation failed. The connection was closed by the server.
Info: Rebex.Net.SshException: The connection was closed by the server.
   at Rebex.Net.SshSession.cfmh[g,h](njnf`2 avj, Int32 avk, njng avl, h avm, g avn, g avo)
   at Rebex.Net.SshSession.cfmo(afjp avx)
   at afjt.ssjc(SshSession gdx, Byte[] gdy, Byte[] gdz, Byte[] gea, Byte[] geb, afji& gec, Byte[]& ged, SshPublicKey& gee)
   at Rebex.Net.SshSession.cfmq(Byte[] avy)
   at Rebex.Net.SshSession.Negotiate()
   at Rebex.Net.Sftp.emin.fnhk(ftxd amk, Boolean aml)
   at Rebex.Net.Sftp.pqan(String po, Int32 pp, SshParameters pq, ftxd pr)a

commented Jun 5, 2020 by Lukas Pokorny (126,870 points)

Thanks for the log! It looks Rebex SFTP detected this server to be a "legacy" SSH server due to its lack of support for any key exchange algorithms based on SHA-2, and therefore it tried to negotiate using SSH in legacy group exchange mode, which was rejected by the server. Please try forcing modern group exchange mode using this setting:

ftp.Settings.SshParameters.UseLegacyGroupExchange = false;

commented Jun 5, 2020 by jlongenderfer (120 points)

This worked! Thank you.

commented Jun 5, 2020 by Lukas Pokorny (126,870 points)

Thanks for letting us know! We will enhance the detection routine in the next release to make this unnecessary.

Negotiation failed. The connection was closed by the server.

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Negotiation failed. The connection was closed by the server.

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