Tcl

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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Tcl


load ./chilkat.dll

set success 0

#  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.

set tunnel [new_CkSshTunnel]

#  List the allowed algorithms for each category, in order of preference.
set json [new_CkJsonObject]

CkJsonObject_UpdateString $json "kex" "curve25519-sha256@libssh.org,ecdh-sha2-nistp256"
CkJsonObject_UpdateString $json "hostKey" "ssh-ed25519,ecdsa-sha2-nistp256"
CkJsonObject_UpdateString $json "cipher" "chacha20-poly1305@openssh.com,aes256-ctr"
CkJsonObject_UpdateString $json "mac" "hmac-sha2-256,hmac-sha2-512"

#  Apply the allow-list before connecting.
set success [CkSshTunnel_SetAllowedAlgorithms $tunnel $json]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    delete_CkJsonObject $json
    exit
}

CkSshTunnel_put_DestHostname $tunnel "db.internal.example.com"
CkSshTunnel_put_DestPort $tunnel 5432

set sshPort 22
set success [CkSshTunnel_Connect $tunnel "ssh.example.com" $sshPort]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    delete_CkJsonObject $json
    exit
}

puts "Connected."

set waitForThreadExit 1
set success [CkSshTunnel_CloseTunnel $tunnel $waitForThreadExit]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    delete_CkJsonObject $json
    exit
}


delete_CkSshTunnel $tunnel
delete_CkJsonObject $json