AW: [SUPPORT REQUEST] Configuration of libssh host key algos on client

[Sebastian Kraust <sebastian.kraust@xxxxxxxxxx>]

Hello Jakub,

thanks for your reply.

It is exactly the other way round. The client does seem to accept only the save algorithms while the server only allows ssh-rsa. To my knowledge, this is due to the server using an older version of libssh (0.7.x) and the client being the newest version.

I am not sure if this impacts your previous suggestion. If not, could you tell me which command I do have to append to which file, please?

Also, I have appended the complete configuration file oft he client (/etc/ssh/sshd_config), if this helps.

Thanks again!

Best regards,
Sebastian

-----Ursprüngliche Nachricht-----
Von: Jakub Jelen <jjelen@xxxxxxxxxx> 
Gesendet: Montag, 25. Mai 2020 18:42
An: libssh@xxxxxxxxxx
Betreff: Re: [SUPPORT REQUEST] Configuration of libssh host key algos on client

NOTICE: This message originated from outside of the company. Please exercise caution when replying or opening links and attachments.




On Mon, 2020-05-25 at 15:52 +0000, Sebastian Kraust wrote:
> Hello libssh-team,
>
> I am currently working on a project using libssh under the hood, but 
> have problems to get it to work. I hope you can provide some help.
>
> Task
> Write a client for an existing server which cannot be 
> changed/configured by me.
>
> Approach
> Connect to the server using the function `ssh_connect`.
>
> Error
> kex error : no match for method server host key algo: server [ssh- 
> rsa], client [ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-
> nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256]
>
> Problem
> Due to the restriction that I can only change the client side, I have 
> to change the client so that it accepts the ssh-rsa algo.
> According to the docs, it should be capable of doing so.
>
> Troubleshooting so far
> Added
> PubkeyAcceptedKeyTypes ssh-ed25519*,ssh-rsa*,ssh-dss*,ecdsa-sha2
> to /etc/ssh/sshd_config to allow every algo on the client side.
>
> I still get the same error. I do believe that the config might not be 
> the correct file to configure libssh.
>
> Can you give me some direction where I have to configure libssh so 
> that the client also accepts the ssh-rsa algorithm? If you need more 
> information, please let me know.

The server is configured to accept only secure algorithms (eddsa, ecdsa and rsa with sha2 -- rsa-sha2-512,rsa-sha2-256). You probably configured your client to use only the old (ssh-rsa), which is not compatible with the new ones (and not considered secure anymore).

If you need some backward compatibility with old server, append the
SHA2 (rsa-sha2-512,rsa-sha2-256) algorithms, otherwise use only them.

Regards,
Jakub

> Thanks for your help in advance.
>
>
> Mit freundlichen Grüßen / Best regards
>
> i.A. Sebastian Kraust
> Forschungsingenieur / Research Engineer
>
> b-plus GmbH
> Osterhofener Str. 13 | 93055 Regensburg Tel +49 941 46624 208 | Fax 
> +49 991 270302 99 sebastian.kraust@xxxxxxxxxx
>
> Besucheradresse / Visitor address:
> b-plus automotive GmbH
> Osterhofener Str. 13, 93055 Regensburg, Germany
>
> Website<http://www.b-plus.com/> | XING< 
> https://www.xing.com/companies/b-plusgmbh> | FACEBOOK< 
> https://www.facebook.com/bplusGmbH/> | LinkedIn< 
> https://www.linkedin.com/company/b-plus-gmbh/>
> [cid:image003.jpg@01D632BD.3948FA20]<
> https://www.b-plus.com/de/news-events/newsansicht/article/b-plus-gehoe
> rt-zu-bayerns-best-50.html
> >
>
> b-plus GmbH
> Geschäftsführer / Managing Director: Dipl.-Ing.(FH) Michael Sieg 
> Gerichtsstand /Handelsregister / Place of jurisdiction / Commercial
> register: HRB 1753 Deggendorf / Germany Diese E-Mail enthält 
> vertrauliche und/oder rechtlich geschützte Informationen. Wenn Sie 
> nicht der richtige Adressat sind oder diese E-Mail irrtümlich erhalten 
> haben, informieren Sie bitte sofort den Absender und löschen Sie diese 
> Mail. Das unerlaubte Kopieren sowie die unbefugte Weitergabe dieser 
> Mail ist nicht gestattet.
> This e-mail may contain confidential and/or privileged information.
> If you are not the intended recipient (or have received this e-mail in 
> error) please notify the sender immediately and delete this e- mail. 
> Any unauthorized copying, disclosure or distribution of the contents 
> in this e-mail is strictly forbidden.
>
--
Jakub Jelen
Senior Software Engineer
Security Technologies
Red Hat, Inc.


#	$OpenBSD: sshd_config,v 1.101 2017/03/14 07:19:07 djm Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# Logging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
PermitRootLogin yes
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#PubkeyAuthentication yes

# Expect .ssh/authorized_keys2 to be disregarded by default in future.
AuthorizedKeysFile	.ssh/authorized_keys .ssh/authorized_keys2

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
PrintMotd no
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*

# override default of no subsystems
Subsystem	sftp	/usr/lib/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#	X11Forwarding no
#	AllowTcpForwarding no
#	PermitTTY no
#	ForceCommand cvs server
PubkeyAcceptedKeyTypes ssh-ed25519*,ssh-rsa*,ssh-dss*,ecdsa-sha2*