curl SFTP transfer from Cygwin on Win10 to Ubuntu 18.04 fails with Unknown host key type: 1835008

[Voris, Ben]

curl issue https://github.com/curl/curl/issues/7057 was closed with:

"This seems to be purely a libssh2 issue and not a curl one."

Curl reports "libssh2/1.7.0"

On the same system, ssh reports " OpenSSH_8.5p1, OpenSSL 1.1.1f  31 Mar 2020"

The curl code in https://github.com/curl/curl/blob/master/lib/vssh/libssh2.c has a number of defines to control what type of host keys it will accept, including LIBSSH2_KNOWNHOST_KEY_ED25519

Was the curl built with this set?

Details are in the curl issue, but here they are again.

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Here is the curl failure:

: curl -vvv -s -T t.cpp sftp://bvoris@nucnuc/tmp/t2.cpp
* STATE: INIT => CONNECT handle 0x800085338; line 1634 (connection #-5000)
* Added connection 0. The cache now contains 1 members
* STATE: CONNECT => RESOLVING handle 0x800085338; line 1680 (connection #0)
* family0 == v4, family1 == v6
*   Trying 192.168.1.5:22...
* STATE: RESOLVING => CONNECTING handle 0x800085338; line 1762 (connection #0)
* Connected to nucnuc (192.168.1.5) port 22 (#0)
* STATE: CONNECTING => PROTOCONNECT handle 0x800085338; line 1825 (connection #0)
* SFTP 0x8000847c8 state change from SSH_STOP to SSH_INIT
* Found host nucnuc in /home/BVoris/.ssh/known_hosts
* Unknown host key type: 1835008
* SFTP 0x8000847c8 state change from SSH_INIT to SSH_SESSION_FREE
* SFTP 0x8000847c8 state change from SSH_SESSION_FREE to SSH_STOP
* multi_done
* The cache now contains 0 members
* SSH DISCONNECT starts now
* SSH DISCONNECT is done
* Closing connection 0

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The curl/libcurl version:

curl 7.76.1 (x86_64-pc-cygwin) libcurl/7.76.1 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.9 zstd/1.4.9 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.0.4) libssh2/1.7.0 nghttp2/1.37.0
Release-Date: 2021-04-14
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli Debug GSS-API HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz Metalink NTLM NTLM_WB PSL SPNEGO SSL TLS-SRP TrackMemory UnixSockets zstd


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The known_hosts entry from the client:

nucnuc ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICmjvQ5jehz5Jwt1PDGJBSgcXVhoMRnbn/E2p3srSK+c

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curl is run on CYGWIN_NT-10.0 3.2.0(0.340/5/3) 2021-03-29 08:42 x86_64 Cygwin

The target system has:

OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017