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curl sftp connection problem
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From: toby via curl-users <curl-users_at_lists.haxx.se>
Date: Tue, 10 Mar 2026 17:05:40 -0600
Hi
i have latest libssh2, curl, openssh and oroftpd installed from source
curl -V
curl 8.18.0 (x86_64-pc-linux-gnu) libcurl/8.18.0 mbedTLS/3.6.5 zlib/1.2.12 libssh2/1.11.1 librtmp/2.3
Release-Date: 2026-01-07
Protocols: dict file ftp ftps http https imap imaps ipfs ipns mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps tftp ws wss
Features: alt-svc AsynchDNS HSTS HTTPS-proxy Largefile libz NTLM SSL threadsafe UnixSockets
the sftp program from openssh works fine connecting to both proftpd (set up for sftp) and sshd
/usr/local/bin/sftp -v -oPort=212 user_at_server.net
curl to the proftpd also set uo for ftps works fine (proftpd ftps : 21 and sfto : 212)
/usr/local/bin/curl -v -k -u uwer:pass --ssl-reqd ftp://myserver.net/
i really want to use curl with sftp to connect to either sshd or proftpd instead of using the sftp command
i get the same error using curl to either sshd or proftpd with this line
/usr/local/bin/curl -v -k -u user sftp://myserver.net:212
* libssh2 cryptography backend: openssl compatible
* User: 'user'
* Failure establishing ssh session: -8, Unable to exchange encryption keys
* closing connection #0
curl: (2) Failure establishing ssh session: -8, Unable to exchange encryption keys
this is from th proftpd sftp.log file
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: client (xxx.xxx.xx.xxx:52884) connected to server (xxx.xxx.xx.xxx:212)
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: sent server version 'SSH-2.0-mod_sftp'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: received client version 'SSH-2.0-libssh2_1.11.1'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: handling connection from SSH2 client 'libssh2_1.11.1'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session key exchange: ecdh-sha2-nistp256
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session host key algorithm: rsa-sha2-512
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server hostkey: rsa-sha2-512
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session client-to-server encryption: chacha20-poly1305_at_openssh.com
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server-to-client encryption: chacha20-poly1305_at_openssh.com
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session client-to-server compression: none
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server-to-client compression: none
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: client at xxx.xxx.xx.xxx sent SSH_DISCONNECT message: Shutdown (Application disconnected)
i had a curl encryption problem a long time ago and had to add some new cipher line to a ssh/sshd config file but openssh isn't involved here
just proftpd
amy place to start would be appreciated
Date: Tue, 10 Mar 2026 17:05:40 -0600
Hi
i have latest libssh2, curl, openssh and oroftpd installed from source
curl -V
curl 8.18.0 (x86_64-pc-linux-gnu) libcurl/8.18.0 mbedTLS/3.6.5 zlib/1.2.12 libssh2/1.11.1 librtmp/2.3
Release-Date: 2026-01-07
Protocols: dict file ftp ftps http https imap imaps ipfs ipns mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps tftp ws wss
Features: alt-svc AsynchDNS HSTS HTTPS-proxy Largefile libz NTLM SSL threadsafe UnixSockets
the sftp program from openssh works fine connecting to both proftpd (set up for sftp) and sshd
/usr/local/bin/sftp -v -oPort=212 user_at_server.net
curl to the proftpd also set uo for ftps works fine (proftpd ftps : 21 and sfto : 212)
/usr/local/bin/curl -v -k -u uwer:pass --ssl-reqd ftp://myserver.net/
i really want to use curl with sftp to connect to either sshd or proftpd instead of using the sftp command
i get the same error using curl to either sshd or proftpd with this line
/usr/local/bin/curl -v -k -u user sftp://myserver.net:212
* libssh2 cryptography backend: openssl compatible
* User: 'user'
* Failure establishing ssh session: -8, Unable to exchange encryption keys
* closing connection #0
curl: (2) Failure establishing ssh session: -8, Unable to exchange encryption keys
this is from th proftpd sftp.log file
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: client (xxx.xxx.xx.xxx:52884) connected to server (xxx.xxx.xx.xxx:212)
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: sent server version 'SSH-2.0-mod_sftp'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: received client version 'SSH-2.0-libssh2_1.11.1'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: handling connection from SSH2 client 'libssh2_1.11.1'
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session key exchange: ecdh-sha2-nistp256
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session host key algorithm: rsa-sha2-512
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server hostkey: rsa-sha2-512
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session client-to-server encryption: chacha20-poly1305_at_openssh.com
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server-to-client encryption: chacha20-poly1305_at_openssh.com
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session client-to-server compression: none
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: + Session server-to-client compression: none
2026-03-10 16:54:44,027 mod_sftp/1.2.0[7982]: client at xxx.xxx.xx.xxx sent SSH_DISCONNECT message: Shutdown (Application disconnected)
i had a curl encryption problem a long time ago and had to add some new cipher line to a ssh/sshd config file but openssh isn't involved here
just proftpd
amy place to start would be appreciated
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