CkPython

SSH Tunnel Set Allowed SSH Algorithms

Demonstrates the Chilkat SshTunnel.SetAllowedAlgorithms method, which restricts the SSH connection to a specific set of algorithms. A JsonObject supplies comma-separated allow-lists for the kex, hostKey, cipher, and mac categories. It must be called before Connect.

Important: You typically should not set allowed algorithms explicitly. By default Chilkat orders algorithms according to best practices and accounts for known vulnerabilities.

Background: SSH negotiates a mutually supported algorithm from each category at connection time, and pinning that choice makes an application brittle: if the server later drops a weak algorithm, the two sides may fail to agree and the tunnel stops working. The legitimate reasons to override are a security policy mandating specific algorithms or a compatibility problem with an old server; otherwise let the library's defaults evolve with current guidance.

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import sys
import chilkat

success = False

#  Demonstrates the SshTunnel.SetAllowedAlgorithms method, which restricts the SSH connection to a
#  specific set of algorithms.  The only argument is a JsonObject.  It must be called before
#  Connect.
#  
#  Note: You typically should NOT set allowed algorithms explicitly.  By default Chilkat orders
#  algorithms according to best practices and accounts for known vulnerabilities.  Hard-coding
#  them can make an application brittle if a server later changes its allowed algorithms.

tunnel = chilkat.CkSshTunnel()

#  List the allowed algorithms for each category, in order of preference.
json = chilkat.CkJsonObject()
json.UpdateString("kex","curve25519-sha256@libssh.org,ecdh-sha2-nistp256")
json.UpdateString("hostKey","ssh-ed25519,ecdsa-sha2-nistp256")
json.UpdateString("cipher","chacha20-poly1305@openssh.com,aes256-ctr")
json.UpdateString("mac","hmac-sha2-256,hmac-sha2-512")

#  Apply the allow-list before connecting.
success = tunnel.SetAllowedAlgorithms(json)
if (success == False):
    print(tunnel.lastErrorText())
    sys.exit()

tunnel.put_DestHostname("db.internal.example.com")
tunnel.put_DestPort(5432)

sshPort = 22
success = tunnel.Connect("ssh.example.com",sshPort)
if (success == False):
    print(tunnel.lastErrorText())
    sys.exit()

print("Connected.")

waitForThreadExit = True
success = tunnel.CloseTunnel(waitForThreadExit)
if (success == False):
    print(tunnel.lastErrorText())
    sys.exit()