Fedora 41: SSH to RHEL6 - error in libcrypto

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1

Hello, I have an ancient server running RHEL6 which cannot be upgraded (and is not connected to the internet). However, with Fedora 40 I was able to ssh in with this config:

Host bohemia
    User root
    HostName bohemia.local
    PubkeyAcceptedKeyTypes ssh-rsa
    HostKeyAlgorithms +ssh-rsa

After upgrade to Fedora 41 I am getting:

ssh_dispatch_run_fatal: Connection to 192.168.200.83 port 22: error in libcrypto

Any ideas? Here is the full ssh log:

$ ssh -vvv bohemia
OpenSSH_9.8p1, OpenSSL 3.2.2 4 Jun 2024
debug1: Reading configuration data /home/lzap/.ssh/config
debug1: /home/lzap/.ssh/config line 1: Applying options for *
debug1: /home/lzap/.ssh/config line 174: Applying options for bohemia
debug1: Reading configuration data /etc/ssh/ssh_config
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/30-libvirt-ssh-proxy.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/30-libvirt-ssh-proxy.conf
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/50-redhat.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug2: checking match for 'final all' host bohemia.nuc.local originally bohemia
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 3: not matched 'final'
debug2: match not found
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 5: Including file /etc/crypto-policies/back-ends/openssh.config depth 1 (parse only)
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug3: gss kex names ok: [gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-]
debug3: kex names ok: [curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512]
debug1: configuration requests final Match pass
debug1: re-parsing configuration
debug1: Reading configuration data /home/lzap/.ssh/config
debug1: /home/lzap/.ssh/config line 1: Applying options for *
debug2: add_identity_file: ignoring duplicate key ~/.ssh/id_ed25519
debug2: add_identity_file: ignoring duplicate key ~/.ssh/id_rsa
debug2: add_identity_file: ignoring duplicate key ~/.ssh/fedorapeople_rsa
debug1: /home/lzap/.ssh/config line 27: Applying options for *.local
debug1: Reading configuration data /etc/ssh/ssh_config
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/30-libvirt-ssh-proxy.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/30-libvirt-ssh-proxy.conf
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/50-redhat.conf depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug2: checking match for 'final all' host bohemia.nuc.local originally bohemia
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 3: matched 'final'
debug2: match found
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 5: Including file /etc/crypto-policies/back-ends/openssh.config depth 1
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug3: gss kex names ok: [gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-]
debug3: kex names ok: [curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512]
debug2: resolving "bohemia.nuc.local" port 22
debug3: resolve_host: lookup bohemia.nuc.local:22
debug3: channel_clear_timeouts: clearing
debug3: ssh_connect_direct: entering
debug1: Connecting to bohemia.nuc.local [192.168.200.83] port 22.
debug3: set_sock_tos: set socket 3 IP_TOS 0x48
debug2: fd 3 setting O_NONBLOCK
debug1: fd 3 clearing O_NONBLOCK
debug1: Connection established.
debug3: timeout: 5000 ms remain after connect
debug1: identity file /home/lzap/.ssh/id_ed25519 type 3
debug1: identity file /home/lzap/.ssh/id_ed25519-cert type -1
debug1: identity file /home/lzap/.ssh/id_rsa type 0
debug1: identity file /home/lzap/.ssh/id_rsa-cert type -1
debug1: identity file /home/lzap/.ssh/fedorapeople_rsa type 0
debug1: identity file /home/lzap/.ssh/fedorapeople_rsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_9.8
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: compat_banner: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000002
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to bohemia.nuc.local:22 as 'root'
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug3: order_hostkeyalgs: no algorithms matched; accept original
debug3: send packet: type 20
debug1: SSH2_MSG_KEXINIT sent
debug3: receive packet: type 20
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ext-info-c,kex-strict-c-v00@openssh.com
debug2: host key algorithms: ssh-rsa
debug2: ciphers ctos: aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes128-gcm@openssh.com,aes128-ctr
debug2: ciphers stoc: aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes128-gcm@openssh.com,aes128-ctr
debug2: MACs ctos: hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256,hmac-sha1,umac-128@openssh.com,hmac-sha2-512
debug2: MACs stoc: hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256,hmac-sha1,umac-128@openssh.com,hmac-sha2-512
debug2: compression ctos: zlib@openssh.com,zlib,none
debug2: compression stoc: zlib@openssh.com,zlib,none
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: host key algorithms: ssh-rsa,ssh-dss
debug2: ciphers ctos: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: ciphers stoc: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: MACs ctos: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: MACs stoc: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: compression ctos: none,zlib@openssh.com
debug2: compression stoc: none,zlib@openssh.com
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha2-256 compression: zlib@openssh.com
debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha2-256 compression: zlib@openssh.com
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug3: send packet: type 34
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(2048<8192<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug3: receive packet: type 31
debug1: SSH2_MSG_KEX_DH_GEX_GROUP received
debug3: send packet: type 32
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug3: receive packet: type 33
debug1: SSH2_MSG_KEX_DH_GEX_REPLY received
debug1: Server host key: ssh-rsa SHA256:pMc05oO7DXNhkvz1X9Em1tuyXRqQFtKU8OUn/mskFvo
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
Warning: Permanently added 'bohemia.nuc.local' (RSA) to the list of known hosts.
debug2: bits set: 4046/8192
ssh_dispatch_run_fatal: Connection to 192.168.200.83 port 22: error in libcrypto
2

Added security, ssh

3

You need to enable the weak ssh algorithms to talk to very old OS like RHEL6. try adding this to you .ssh/config for the RHEL6 hosts:

Host *
    PubkeyAcceptedKeyTypes +ssh-rsa
    HostKeyAlgorithms +ssh-rsa
4

Thanks, I already have that. No change.

5

Consider using containers or virtualization, e.g. Podman, Distrobox, Toolbx, LXC, etc.

1 Like
6

Yes, I usually use distrobox with i.e. ubuntu 16 in order to connect to old system.

1 Like
7

If you look though the debug output from ssh you should see the algorithems and ciphers that are offer from the RHEL system.

Also what is the key type you are using? It will need to be RSA,
the newer (recommended) ed25519 ones are not supported on old systems.

8

I do have RSA and ED, have been using that for quite a while now. Even if I choose explicitly my RSA key it does not work:

ssh -i .ssh/id_rsa bohemia
ssh_dispatch_run_fatal: Connection to 192.168.200.83 port 22: error in libcrypto

The full verbose output is in the OP, I can hardly understand what is going on.

9

You can also try this:

  • Downgrade the crypto policies.
  • Regenerate the keys to 2048+ bit, both auth and hostid.
  • Regenerate the cert with the SHA-256 signature algorithm.

If nothing helps, they likely dropped the relevant piece of code.

See also: Changes/OpenSSLDistrustSHA1SigVer - Fedora Project Wiki

1 Like
10

Ah updating policy did work. Thanks!

sudo update-crypto-policies --set LEGACY
ssh xxx
1 Like
11

The following policy might be better if Fedora 40 policies were working since it’s more secure than LEGACY.

sudo update-crypto-policies --set FEDORA40

I had to reboot after making this change before my SSH command worked like before.

1 Like