documentation/docs/user-guide/ssh_to_legacy_linux.md

SSH To Legacy Linux Systems

On RHEL8 and newer systems Redhat started to remove the support for certain SSL algorithms that are considered unsafe to use. Therefore connections to (very) old Linux systems do not work out of the box without specifying certain exceptions/parameters

The most common options that might be needed are the following:

ssh -v \
    -o KexAlgorithms=diffie-hellman-group1-sha1 \
    -o Ciphers=3des-cbc \
    -o HostkeyAlgorithms=ssh-rsa \
    <yoursystem>

The algorithms (i.e. the part after the = need to be adapted towards the system you try to connect to)

Allow Legacy Crypto Policies

On RHEL9 or newer the system administrator even might need to explicitly allow legacy crypto policies.

A usual error message pointing to this is:

debug1: Host 'x02da-gfctrl-2' is known and matches the RSA host key.
debug1: Found key in /home/ebner/.ssh/known_hosts:1
debug1: openssl error C0BDF464547F0000:error:03000098:digital envelope routines:do_sigver_init:invalid digest:crypto/evp/m_sigver.c:340:

ssh_dispatch_run_fatal: Connection to 129.129.99.82 port 22: error in libcrypto

The easiest way is to allow SHA1 generally by setting the systems crypto policy accordingly. With Hiera/Pupppet this is

base::crypto_policy: 'DEFAULT:SHA1'

(see Cryptographic Policy)

Alternatively you might allow these outdated cryptographic algorithms not system wide but individually for the required connections:

Create a file openssl_legacy.cnf with the following content:

.include /etc/ssl/openssl.cnf
[openssl_init]
alg_section = evp_properties
[evp_properties]
rh-allow-sha1-signatures = yes

SSH to the legacy system with the following command:

OPENSSL_CONF=openssl_legacy.cnf ssh -oHostKeyAlgorithms=+ssh-rsa -oPubkeyAcceptedAlgorithms=+ssh-rsa -o KexAlgorithms=diffie-hellman-group1-sha1 <your_legacy_system>

To avoid creating a file and simplifying the process we recommend to create a helper script script like this ssh_legacy.sh to avoid leaving legacy settings/files on the system:

#!/bin/bash

tmpfile=$(mktemp)
echo ${tmpfile}
cat <<EOF > "$tmpfile"
.include /etc/ssl/openssl.cnf
[openssl_init]
alg_section = evp_properties
[evp_properties]
rh-allow-sha1-signatures = yes
EOF

# Set up a trap to delete the temp file on exit (success, error, or interrupt)tr
trap 'rm -f "$tmpfile"' EXIT

OPENSSL_CONF="$tmpfile" ssh -oHostKeyAlgorithms=+ssh-rsa -oPubkeyAcceptedAlgorithms=+ssh-rsa -o KexAlgorithms=diffie-hellman-group1-sha1 $@

Only use the above workaround if you really have to connect to legacy systems! Legacy algorithms are considered insecure and should be avoided whenever possible.