The SSH server feature enables an SSH client to make a secure, encrypted connection to a Cisco router. This connection provides functionality that is similar to that of an inbound Telnet connection. Before SSH, security was limited to Telnet security. SSH allows a strong encryption to be used with the Cisco IOS XR software authentication. The SSH server in Cisco IOS XR software works with publicly and commercially available SSH clients.

The SSH client feature is an application running over the SSH protocol to provide device authentication and encryption. The SSH client enables a Cisco router to make a secure, encrypted connection to another Cisco router or to any other device running the SSH server. This connection provides functionality that is similar to that of an outbound Telnet connection except that the connection is encrypted. With authentication and encryption, the SSH client allows for a secure communication over an insecure network.

The SSH client in the Cisco IOS XR software worked with publicly and commercially available SSH servers. The SSH client supported the ciphers of AES, 3DES, message digest algorithm 5 (MD5), SHA1, and password authentication. User authentication was performed in the Telnet session to the router. The user authentication mechanisms supported for SSH were RADIUS, TACACS+, and the use of locally stored usernames and passwords.

The SSH client supports setting DSCP value in the outgoing packets.

ssh client dscp <value from 0 – 63>

If not configured, the default DSCP value set in packets is 16 (for both client and server).

The SSH client supports the following options:

RP/0/5/CPU0:router#ssh ?
  A.B.C.D  IPv4 (A.B.C.D) address
  WORD     Hostname of the remote node
  X:X::X   IPv6 (A:B:C:D...:D) address
  vrf      vrf table for the route lookup
RP/0/5/CPU0:router#ssh 10.1.1.1 ?
  cipher            Accept cipher type
  command           Specify remote command (non-interactive)
  source-interface  Specify source interface
  username          Accept userid for authentication
  <cr>              
RP/0/5/CPU0:router#ssh 192.68.46.6 username admin command "show redundancy sum" 
Password: 


Wed Jan  9 07:05:27.997 PST
    Active Node    Standby Node
    -----------    ------------
       0/4/CPU0        0/5/CPU0 (Node Ready, NSR: Not Configured)

RP/0/5/CPU0:router#

SSH includes support for standard file transfer protocol (SFTP) , a new standard file transfer protocol introduced in SSHv2. This feature provides a secure and authenticated method for copying router configuration or router image files.

The SFTP client functionality is provided as part of the SSH component and is always enabled on the router. Therefore, a user with the appropriate level can copy files to and from the router. Like the copy command, the sftp command can be used only in EXEC mode.

The SFTP client is VRF-aware, and you may configure the secure FTP client to use the VRF associated with a particular source interface during connections attempts. The SFTP client also supports interactive mode, where the user can log on to the server to perform specific tasks via the Unix server.

The SFTP Server is a sub-system of the SSH server. In other words, when an SSH server receives an SFTP server request, the SFTP API creates the SFTP server as a child process to the SSH server. A new SFTP server instance is created with each new request.

The SFTP requests for a new SFTP server in the following steps:

• The user runs the sftp command with the required arguments

• The SFTP API internally creates a child session that interacts with the SSH server

• The SSH server creates the SFTP server child process

• The SFTP server and client interact with each other in an encrypted format

• The SFTP transfer is subject to LPTS policer "SSH-Known". Low policer values will affect SFTP transfer speeds

Note	In IOS-XR SW release 4.3.1 onwards the default policer value for SSH-Known has been reset from 2500pps to 300pps. Slower transfers are expected due to this change. You can adjust the lpts policer value for this punt cause to higher values that will allow faster transfers

When the SSH server establishes a new connection with the SSH client, the server daemon creates a new SSH server child process. The child server process builds a secure communications channel between the SSH client and server via key exchange and user authentication processes. If the SSH server receives a request for the sub-system to be an SFTP server, the SSH server daemon creates the SFTP server child process. For each incoming SFTP server subsystem request, a new SSH server child and a SFTP server instance is created. The SFTP server authenticates the user session and initiates a connection. It sets the environment for the client and the default directory for the user.

Once the initialization occurs, the SFTP server waits for the SSH_FXP_INIT message from the client, which is essential to start the file communication session. This message may then be followed by any message based on the client request. Here, the protocol adopts a 'request-response' model, where the client sends a request to the server; the server processes this request and sends a response.

The SFTP server displays the following responses:

• Status Response

• Handle Response

• Data Response

• Name Response

Note	The server must be running in order to accept incoming SFTP connections.

Verifying the authenticity of a server is the first step to a secure SSH connection. This process is called the host authentication, and is conducted to ensure that a client connects to a valid server.

The host authentication is performed using the public key of a server. The server, during the key-exchange phase, provides its public key to the client. The client checks its database for known hosts of this server and the corresponding public-key. If the client fails to find the server's IP address, it displays a warning message to the user, offering an option to either save the public key or discard it. If the server’s IP address is found, but the public-key does not match, the client closes the connection. If the public key is valid, the server is verified and a secure SSH connection is established.

The IOS XR SSH server and client had support for DSA based host authentication. But for compatibility with other products, like IOS, RSA based host authentication support is also added.

One of the method for authenticating the user in SSH protocol is RSA public-key based user authentication. The possession of a private key serves as the authentication of the user. This method works by sending a signature created with a private key of the user. Each user has a RSA keypair on the client machine. The private key of the RSA keypair remains on the client machine.

The user generates an RSA public-private key pair on a unix client using a standard key generation mechanism such as ssh-keygen. The max length of the keys supported is 4096 bits, and the minimum length is 512 bits. The following example displays a typical key generation activity:

bash-2.05b$ ssh-keygen –b 1024 –t rsa
Generating RSA private key, 1024 bit long modulus

The public key must be in base64 encoded (binary) format for it to be imported correctly into the box. You can use third party tools available on the Internet to convert the key to the binary format.

Once the public key is imported to the router, the SSH client can choose to use the public key authentication method by specifying the request using the “-o” option in the SSH client. For example:

client$ ssh -o PreferredAuthentications=publickey 1.2.3.4

If a public key is not imported to a router using the RSA method, the SSH server initiates the password based authentication. If a public key is imported, the server proposes the use of both the methods. The SSH client then chooses to use either method to establish the connection. The system allows only 10 outgoing SSH client connections.

Currently, only SSH version 2 and SFTP server support the RSA based authentication. For more information on how to import the public key to the router, see the Implementing Certification Authority Interoperability on the Cisco ASR 9000 Series Router chapter in this guide.

Note	The preferred method of authentication would be as stated in the SSH RFC. The RSA based authentication support is only for local authentication, and not for TACACS/RADIUS servers.

Authentication, Authorization, and Accounting (AAA) is a suite of network security services that provide the primary framework through which access control can be set up on your Cisco router or access server. For more information on AAA, see the Authentication, Authorization, and Accounting Commands on the Cisco ASR 9000 Series RouterSoftware module in the System Security Command Reference for Cisco ASR 9000 Series Routers publication and the Configuring AAA Services on the Cisco ASR 9000 Series Router module in the System Security Configuration Guide for Cisco ASR 9000 Series Routers publication.

An authentication method in which the authentication information is entered using a keyboard is known as keyboard-interactive authentication. This method is an interactive authentication method in the SSH protocol. This type of authentication allows the SSH client to support different methods of authentication without having to be aware of their underlying mechanisms.

Currently, the SSHv2 client supports the keyboard-interactive authentication. This type of authentication works only for interactive applications.

Note	The password authentication is the default authentication method. The keyboard-interactive authentication method is selected if the server is configured to support only the keyboard-interactive authentication.

This feature brings in the functionality of automatically generating the SSH host-key pairs for the DSA, ECDSA (such as ecdsa-nistp256 , ecdsa-nistp384 , and ecdsa-nistp521 ), ED25519 and RSA algorithms. This in turn eliminates the need for explicitly generating each SSH host-key pair after the router boots up. Because the keys are already present in the system, the SSH client can establish connection with the SSH server soon after the router boots up with the basic SSH configuration. This is useful especially during zero touch provisioning (ZTP) and Golden ISO boot up scenarios.

Before the introduction of this feature, you had to execute the crypto key generate command in EXEC mode to generate the required SSH host-key pairs.

Although the host-key pairs are auto-generated with the introduction of this feature, you still have the flexibility to select only the required algorithms on the SSH server. You can use the ssh server algorithms host-key command in Global Configuration mode to achieve the same. Alternatively, you can also use the existing crypto key zeroize command in EXEC mode to remove the algorithms that are not required.

Note	In a system upgrade scenario from version 1 to version 2, the system does not generate the SSH host-key pairs automatically if they were already generated in version 1. The host-key pairs are generated automatically only if they were not generated in version 1.

This feature introduces the support for Ed25519 public-key algorithm, when establishing SSH sessions, on Cisco IOS XR 64-bit platforms. This algorithm offers better security with faster performance when compared to DSA or ECDSA signature algorithms.

The order of priority of public-key algorithms during SSH negotiation between the client and the server is:

• ecdsa-sha2-nistp256

• ecdsa-sha2-nistp384

• ecdsa-sha2-nistp521

• ssh-ed25519

• ssh-rsa

• ssh-dsa

The default order of authentication methods for SSH clients on Cisco IOS XR routers is as follows:

• On routers running Cisco IOS XR SSH:

  • public-key , password and keyboard-interactive

• On routers running CiscoSSH (open source-based SSH):

  • public-key , keyboard-interactive and password

• Do not use client multiplexing for heavy transfer of data as the data transfer speed is limited by the TCP speed limit. Hence, for a heavy data transfer it is advised that you run multiple SSH sessions, as the TCP speed limit is per connection.
• Client multiplexing must not be used for more than 15 concurrent channels per session simultaneously.
• You must ensure that the first channel created at the time of establishing the session is always kept alive in order for other channels to remain open.

• The line template default session-limit command is not supported for SSH.

The figure below provides an illustration of a client-server interaction over a SSH multichannel connection.

As depicted in the illustration,

• The client multiplexes the collection of channels into a single connection. This allows different operations to be performed on different channels simultaneously. The dotted lines indicate the different channels that are open for a single session.
• After receiving a request from the client to open up a channel, the server processes the request. Each request to open up a channel represents the processing of a single service.

Note	The Cisco IOX software supports server-side multiplexing only.

You can use the ssh server max-auth-limit command to specify the maximum number of authentication attempts allowed for SSH connection.

Router#configure
Router(config)#ssh server max-auth-limit 5
Router(config)#commit

Router#show running-configuration ssh
ssh server max-auth-limit 5
ssh server v2
!

RP/0/RP0/CPU0:Oct 6 10:03:58.029 UTC: SSHD_[68125]: %SECURITY-SSHD-3-ERR_GENERAL : Max authentication tries reached-exiting

To enable X.509v3 certificate-based authentication for SSH, these tasks for server and user authentication:

Server Authentication:

• Configure the list of host key algorithms—With this configuration, the SSH server decides the list of host keys to be offered to the client. In the absence of this configuration, the SSH server sends all available algorithms to the user as host key algorithms. The SSH server sends these algorithms based on the availability of the key or the certificate.

• Configure the SSH trust point for server authentication—With this configuration, the SSH server uses the given trust point certificate for server authentication. In the absence of this configuration, the SSH server does not send x509v3-ssh-rsa as a method for server verification. This configuration is not VRF-specific; it is applicable to SSH running in all VRFs.

  The above two tasks are for server authentication and the following ones are for user authentication.

User Authentication:

• Configure the trust points for user authentication—With this configuration, the SSH server uses the given trust point for user authentication. This configuration is not user-specific; the configured trust points are used for all users. In the absence of this configuration, the SSH server does not authenticate using certificates. This configuration is not specific to a VRF; it is applicable to SSH running in all VRFs.

  You can configure up to ten user trust points.

• Specify the username to be picked up from the certificate—This configuration specifies which field in the certificate is to be considered as the username . The common-name from the subject name or the user-principle-name(othername) from the subject alternate name , or both can be configured.

• Specify the maximum number of authentication attempts allowed by the SSH server—The value ranges from 4 to 20. The default value is 20. The server closes the connection if the number of user attempts exceed the configured value.

• AAA authentication configuration—The AAA configuration for public key is the same as that for the regular or keyboard-interactive authentication, except that it mandates local method in the authentication method list.

/* Configure the lits of host key algorithms */
Router#configure
Router(config)#ssh server algorithms host-key x509v3-ssh-rsa
Router(config)#commit

/* Configure the SSH trustpoint for server authentication */
Router#configure
Router(config)#ssh server certificate trustpoint host tp1
Router(config)#commit

/* Configure the trustpoints to be used for user authentication */
Router#configure
Router(config)#ssh server trustpoint user tp1
Router(config)#ssh server trustpoint user tp2
Router(config)#commit

/* Specifies the username to be picked up from the certificate. 
In this example, it specifies the user common name to be picked up from the subject name field */
Router#configure
Router(config)#ssh server certificate username common-name
Router(config)#commit

/* Specifies the maximum authentication limit for the SSH server */
Router#configure
Router(config)#ssh server max-auth-limit 5
Router(config)#commit

/* AAA configuration for local authentication with certificate and 
remote authorization with TACACS+ or RADIUS */
Router#configure
Router(config)#aaa authentication login default group tacacs+ local
Router(config)#aaa authorization exec default group radius group tacacs+
Router(config)#commit

ssh server algorithms host-key x509v3-ssh-rsa
!
ssh server certificate trustpoint host tp1
!
ssh server trustpoint user tp1
ssh server trustpoint user tp2
!
ssh server certificate username common-name
!
ssh server max-auth-limit 5
!

Router#show ssh server
Wed Feb 19 15:23:38.752 IST
---------------------
SSH Server Parameters
---------------------

Current supported versions := v2
                  SSH port := 22
                  SSH vrfs := vrfname:=default(v4-acl:=, v6-acl:=)  
              Netconf Port := 830
              Netconf Vrfs := vrfname:=default(v4-acl:=, v6-acl:=)  
 
 Algorithms 
---------------
        Hostkey Algorithms := x509v3-ssh-rsa, ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,ssh-rsa,ssh-dsa
   Key-Exchange Algorithms := ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group14-sha1
     Encryption Algorithms := aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
            Mac Algorithms := hmac-sha2-512,hmac-sha2-256,hmac-sha1
 
 Authetication Method Supported 
------------------------------------
                 PublicKey := Yes
                  Password := Yes
      Keyboard-Interactive := Yes
         Certificate Based := Yes
 
 Others  
------------
                     DSCP  := 16
                Ratelimit  := 60
             Sessionlimit  := 100
                Rekeytime  := 60
       Server rekeyvolume  := 1024
  TCP window scale factor  := 1
            Backup Server  := Enabled, vrf:=default, port:=11000
Host Trustpoint            := tp1
User Trustpoints           := tp1 tp2

Router#show ssh session details
Wed Feb 19 15:33:00.405 IST
SSH version : Cisco-2.0 

id      key-exchange           pubkey               incipher    outcipher   inmac         outmac   
---------------------------------------------------------------------------------------------------- 
Incoming Sessions 
1       ecdh-sha2-nistp256     x509v3-ssh-rsa       aes128-ctr  aes128-ctr  hmac-sha2-256 hmac-sha2-256 

Router#show ssh
Sun Sep 20 18:14:04.122 UTC
SSH version : Cisco-2.0

id  chan pty  location   state         userid    host           ver authentication connection type
----------------------------------------------------------------------------------------------------------------------------
Incoming sessions
4   1    vty0 0/RP0/CPU0 SESSION_OPEN  9chainuser 10.105.230.198 v2  x509-rsa-pubkey Command-Line-Interface

Outgoing sessions

• OpenSSH certificates use the Certificate Authority (CA) infrastructure to act as a trusted entity while signing the host or user certificates.

• OpenSSH certificates contain a public and private key pair, including identity and validity information. These are signed using a standard SSH public key using the ssh-keygen utility.

• The router certificate includes information such as the host public key, public key of the signing CA, type (host), certificate validity, Key ID, serial number of the certificate, and so on.

• The user certificate contains the user's public key, the public key of the signing CA, Key ID, type (user), serial number, certificate validity, principals matched against the login username, and so forth.

• The CA is just another SSH key created using the ssh-keygen utility. However, rather than utilizing it for authenticating the router or user directly, it's used to sign and validate the other keys that are used for authenticating the router and the user.

• You can view the router and user certificate properties using the ssh-keygen.

• The OpenSSH certificates support the following encryptions:

  • RSA

  • DSA

  • ECDSA

  • ED25519

• You must have a client machine which has OpenSSH feature with the ssh-keygen utility to act as CA.

The following high-level steps help you set up OpenSSH based Authentication:

1. Create a trustpoint in the router and configure the router to use this trustpoint for the host and user authentication.

2. Creating CA, the CA here is a dedicated system with OpenSSH feature that provides a certificate signing infrastructure using the ssh-keygen utility.

3. Host authentication, the host here is the Cisco IOS XR router.

4. User authentication, a user is any entity attempting to access the router. Generally refers to system to access the router CLI remotely. User is also referred to as client.

5. Access the router in the client using the OpenSSH authentication

This section contains the detailed procedure to enable this feature in your router.

1. Create a trustpoint in the router and configure the router to use this trustpoint for the host and user authentication.

   1. [Router Config mode] Create a trustpoint in the router.

      Router# config
      Router(config)# crypto ca openssh trustpoint test
      Router(config)# commit
      

   2. [Router Config mode] Configure the trustpoint for host authentication.

      Router# config
      Router(config)# ssh server openssh trustpoint host test
      Router(config)# commit
      

   3. [Router Config mode] Configure the trustpoint for user authentication

      Router# config
      Router(config)# ssh server openssh trustpoint user test
      Router(config)# commit
      

2. Creating CA

   1. [CA Server] In the dedicated machine with OpenSSH feature to act as CA, generate a certificate using the ssh-keygen utility:

      [root@CAServer test]# ssh-keygen -t rsa -f cacert
      Generating public/private rsa key pair.
      Enter passphrase (empty for no passphrase): 
      Enter same passphrase again: 
      Your identification has been saved in cacert.
      Your public key has been saved in cacert.pub.
      The key fingerprint is:
      SHA256:/B2b8V7jKXwGphf75fkO74U/mpuHgDHmvF4okexdKhY root@CAServer
      The key's randomart image is:
      +---[RSA 2048]----+
      |                 |
      |                 |
      |                 |
      |      ...+       |
      |       ES +.o    |
      |      . +=+o X . |
      |       = +o.O O.+|
      |      . o... B+@*|
      |        ..  .=XBX|
      +----[SHA256]-----+
      		  
      [root@CAServer test]# ls
      cacert  cacert.pub
      

      Note	Leave the passphrase empty.

3. Host (Router) authentication

   1. [CA Server] Open the CA public key from CA server and copy it contents.

      [root@CAServer test]# cat cacert.pub
      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCigl/zhyjuGOBYz5bu+GL76
      HBaROV0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE7sZIJ0anU9vYSJZW8zrl8z06G
      qzmnJqRRaXa9vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0IEMhlVN0hCqE4
      cIFgLxgHpYAaqyl2hISaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf/tbF
      IBPWpuuuA3LvpZIiTaztevQaWYSyK22h3tp3K62IOBX3gUd4Yr+Gvo4PNA26e
      21cUE2aVJsl6J9MeFITR2NzY1cmZ44KWi6bglkPlE4KBiRsbHCvs4wlaUaO5q
      hNj1BdH3/Hha4x root@CAServer
      

   2. [Router EXEC mode] Add the contents of the CA public key to router trustpoint.

      Router#crypto ca openssh authenticate test
      Enter the CA pubkey.
      End with a blank line or the word "quit" on a line by itself
      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCigl/zhyjuGOBYz5bu+GL7
      6HBaROV0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE7sZIJ0anU9vYSJZW8zrl8z0
      6GqzmnJqRRaXa9vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0IEMhlVN0hC
      qE4cIFgLxgHpYAaqyl2hISaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf
      /tbFIBPWpuuuA3LvpZIiTaztevQaWYSyK22h3tp3K62IOBX3gUd4Yr+Gvo4P
      NA26e21cUE2aVJsl6J9MeFITR2NzY1cmZ44KWi6bglkPlE4KBiRsbHCvs4wl
      aUaO5qhNj1BdH3/Hha4x root@CAServer
      Do you accept this certificate? [yes/no]: yes
      

   3. [Router EXEC mode] Validate the copied CA public key by viewing the OpenSSH certificates in the CA trustpoint configured in the router.

      Router#show crypto ca openssh certificates 
      Fri Sep 16 06:59:38.347 UTC
      
      Trustpoint       : test
      ===============================================================
      CA certificate 
      ===============================================================
      
      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCigl/zhyjuGOBYz5bu+GL76HBa
      ROV0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE7sZIJ0anU9vYSJZW8zrl8z06GqzmnJq
      RRaXa9vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0IEMhlVN0hCqE4cIFgLxgHp
      YAaqyl2hISaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf/tbFIBPWpuuuA3Lv
      pZIiTaztevQaWYSyK22h3tp3K62IOBX3gUd4Yr+Gvvcjdvjwevfo4PNA26e21cUE
      2aVJsl6J9eFITR2NzY1cmZ44KWi6bglkPlE4KBiRsbHCvs4wlaUaO5qhNj1BdH3/
      Hha4x root@CAServer
      

   4. [Router EXEC mode] Generate a CSR for the CA public key in the router.

      Router#crypto ca openssh enroll test
      Fri Sep 16 06:34:41.230 UTC
      Display Certificate Request to terminal? [yes/no]: yes
      ---Hostkey follows---
      
      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCaXqjc45LohfiHJ1iq8sSpaJmdR
      QQJo6bRMhkdxY1pbjEYrwjPTn5SnC1NZYwsTPSHlbYBxQRLBHLv80Gbb0v+uJ1T0T
      4tAmLgSYPXaHqYIyepCeMKSkSKLgZ0Pf+oGBMtf3uUuLqCgnFAwjrzDBXJYfF+bd/
      ieXMwKKNH3YiceLOqe4BAYRU6m+wiuZ8is+bIfy32Eq7gWuPUz8XpxaCt3icpqfrj
      7/vm7amKf1GpiheaRJH0Cg4JAmJpAQkuPjx+Y9SZw2yTJP+IKr9tSoSWyiHo2B/Yg
      3yERd7M8dQEsvrGy5KIf92x+eLPlGl5gB9ykEPDUpXeaYTu5wtDR/Jd
      
      ---End - This line not part of hostkey---
      Redisplay enrollment request? [yes/no]: n
      

   5. [Router EXEC mode] Select the hostkey contents of the CSR file and copy the hostkey of the CSR.

   6. [CA server] Create a .pub file in the CA server for the CSR hostkey and paste the copied hostkey contents in this file.

      [root@CAServer test]# vim host.pub
      /* Here we are using the vim text editor to create the host.pub file */
      /* You can use any text editor of your choice  */
      

   7. [CA server] Execute the following block to sign the CSR file using the CA certificate

      [root@CAServer test]# ssh-keygen -h -s cacert -I "server" -V +10w -z 10 host.pub
      Signed host key host-cert.pub: id "server" serial 10 valid from 2022-09-16T12:26:00 to 2022-11-25T12:27:17
      

      Note	Use the following command to sign the CSR file using the CA certificate: ssh-keygen -h -s <CACert> -I <IdentityOfCSRSys> -V <CertValidity> -z <CertSerialNo> <CopiedCSRFile>

      Parameter	Description
      CACert	Specify the filename of the CA Server private key
      CertValidity	Specify the validity period for the certificate.
      CertSerialNo	Specify a serial number for the certificate.
      CopiedCSRFile	Specify the name of the file created to copy the contents of CSR in the router.

   8. [CA server] Open the signed host certificate and copy the contents.

      [root@CAServer test]# cat host-cert.pub
      ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1yc2EtY2VydC12MDFAb3BlbnNza
      C5jb20AAAAgzv0OXl42NNK9C4PtLZniRwBk5jbeS8quNhzVKsRpO7UAAAADAQABAAA
      BAQCaXqjc45LohfiHJ1iq8sSpaJmdRQQJo6bRMhkdxY1pbjEYrwjPTn5SnC1NZYwsT
      PSHlbYBxQRLBHLv80Gbb0v+uJ1T0T4tAmLgSYPXaHqYIyepCeMKSkSKLgZ0Pf+oGBM
      tf3uUuLqCgnFAwjrzDBXJYfF+bd/ieXMwKKNH3YiceLOqe4BAYRU6m+wiuZ8is+bIf
      y32Eq7gWuPUz8XpxaCt3icpqfrj7/vm7amKf1GpiheaRJH0Cg4JAmJpAQkuPjx+Y9S
      Zw2yTJP+IKr9tSoSWyiHo2B/Yg3yERd7M8dQEsvrGy5KIf92x+eLPlGl5gB9ykEPDU
      pXeaYTu5wtDR/JdAAAAAAAAAAoAAAACAAAABnNlcnZlcgAAAAAAAAAAYyQeAAAAAAB
      jgGdNAAAAAAAAAAAAAAAAAAABFwAAAAdzc2gtcnNhAAAAAwEAAQAAAQEAooJf84co7
      hjgWM+W7vhi++hwWkTldKVUuC8d6X9Y3I6xZFYmzyipFXl/5RO7GSCdGp1Pb2EiWVv
      M65fM9Ohqs5pyakUWl2vb38DZo7zXUcMbgQN1JPxtbG5nLrDFwV6GOs9CBDIZVTdIQ
      qhOHCBYC8YB6WAGqspdoSEmqJkwjYZGw++9DrfM28kY5ehtD7NIIGB1pny2X/7WxSA
      T1qbrrgNy76WSIk2s7Xr0GlmEsittod7adyutiDgV94FHeGK/hr6ODzQNunttXFBNm
      lSbJeifTHhSE0djc2NXJmeOCloum4JZD5ROCgYkbGxwr7OMJWlGjuaoTY9QXR9/x4W
      uMQAAAQ8AAAAHc3NoLXJzYQAAAQAIywc9o2OWzFq32MnE9IZVVRRiItdXaMVE1EvYu
      G92JK7wnMJd50M6QDyfkNmGF4ramF90/bVQpl3UYJzVxCJSEodAq6OmlG3zx/MVayT
      unMwV2Fq75PpaoZVpyEKx4kLKA6rNU5Tmbht2OfMQKFvIWyxTDmeLFMvnpt8R0Yrz4
      sG5EP1+4E3WthfzZr42Mq2LQJt6aBeYHZDZSp++j7RpA7+T/6n1aGtAjtDIKprOQuE
      1higCZmdI+kUZDOXjMJlPmJAnV8fdtnnEpYCyzYeD+rSSF7dlDVrTaiFdqrfCXh+uY
      jR1E621sP7UEJOWeiBqSDTJxSRdRBNZq9TLmgJH host.pub
      

   9. [Router EXEC mode] Import the signed host certificate to the router.

      Router# crypto ca openssh import test certificate 
      /* This command opens the CA trustpoint and you must paste the contents of signed certificate copied from the CA server */
      Fri Sep 16 07:00:27.573 UTC
      
      
      Enter the OpenSSH certificate.
      End with a blank line
      
      ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1yc2EtY2VydC12MDFAb3BlbnNzaC
      5jb20AAAAgzv0OXl42NNK9C4PtLZniRwBk5jbeS8quNhzVKsRpO7UAAAADAQABAAABA
      QCaXqjc45LohfiHJ1iq8sSpaJmdRQQJo6bRMhkdxY1pbjEYrwjPTn5SnC1NZYwsTPSH
      lbYBxQRLBHLv80Gbb0v+uJ1T0T4tAmLgSYPXaHqYIyepCeMKSkSKLgZ0Pf+oGBMtf3u
      UuLqCgnFAwjrzDBXJYfF+bd/ieXMwKKNH3YiceLOqe4BAYRU6m+wiuZ8is+bIfy32Eq
      7gWuPUz8XpxaCt3icpqfrj7/vm7amKf1GpiheaRJH0Cg4JAmJpAQkuPjx+Y9SZw2yTJ
      P+IKr9tSoSWyiHo2B/Yg3yERd7M8dQEsvrGy5KIf92x+eLPlGl5gB9ykEPDUpXeaYTu
      5wtDR/JdAAAAAAAAAAoAAAACAAAABnNlcnZlcgAAAAAAAAAAYyQeAAAAAABjgGdNAAA
      AAAAAAAAAAAAAAAABFwAAAAdzc2gtcnNhAAAAAwEAAQAAAQEAooJf84co7hjgWM+W7v
      hi++hwWkTldKVUuC8d6X9Y3I6xZFYmzyipFXl/5RO7GSCdGp1Pb2EiWVvM65fM9Ohqs
      5pyakUWl2vb38DZo7zXUcMbgQN1JPxtbG5nLrDFwV6GOs9CBDIZVTdIQqhOHCBYC8YB
      6WAGqspdoSEmqJkwjYZGw++9DrfM28kY5ehtD7NIIGB1pny2X/7WxSAT1qbrrgNy76W
      SIk2s7Xr0GlmEsittod7adyutiDgV94FHeGK/hr6ODzQNunttXFBNmlSbJeifTHhSE0
      djc2NXJmeOCloum4JZD5ROCgYkbGxwr7OMJWlGjuaoTY9QXR9/x4WuMQAAAQ8AAAAHc
      3NoLXJzYQAAAQAIywc9o2OWzFq32MnE9IZVVRRiItdXaMVE1EvYuG92JK7wnMJd50M6
      QDyfkNmGF4ramF90/bVQpl3UYJzVxCJSEodAq6OmlG3zx/MVayTunMwV2Fq75PpaoZV
      pyEKx4kLKA6rNU5Tmbht2OfMQKFvIWyxTDmeLFMvnpt8R0Yrz4sG5EP1+4E3WthfzZr
      42Mq2LQJt6aBeYHZDZSp++j7RpA7+T/6n1aGtAjtDIKprOQuE1higCZmdI+kUZDOXjM
      JlPmJAnV8fdtnnEpYCyzYeD+rSSF7dlDVrTaiFdqrfCXh+uYjR1E621sP7UEJOWeiBq
      SDTJxSRdRBNZq9TLmgJH host.pub
      

   10. [Router EXEC mode] Verify the host certificate import in the router.

       Router#show crypto ca openssh certificates       
       Fri Sep 16 07:00:49.488 UTC
       
       Trustpoint       : test
       ===================================================================
       CA certificate 
       ===================================================================
       ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCigl/zhyjuGOBYz5bu+GL76HBaROV
       0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE7sZIJ0anU9vYSJZW8zrl8z06GqzmnJqRRaXa9
       vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0IEMhlVN0hCqE4cIFgLxgHpYAaqyl2hI
       SaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf/tbFIBPWpuuuA3LvpZIiTaztevQa
       WYSyK22h3tp3K62IOBX3gUd4Yr+Gvo4PNA26e21cUE2aVJsl6J9MeFITR2NzY1cmZ44
       KWi6bglkPlE4KBiRsbHCvs4wlaUaO5qhNj1BdH3/Hha4x root@CAServer
        
       Router certificate 
       ====================================================================
       Type         : Host Certificate
       Key ID       : server
       Serial       : 10
       Valid        : from Fri Sep 16 06:56:00 2022 to Fri Nov 25 06:57:17 2022
       

4. User authentication

   1. [Client machine] Generate an SSH key pair in the client system using the ssh-keygen utility for the user.

      [root@userclient test]# ssh-keygen -t rsa 
      Generating public/private rsa key pair.
      Enter file in which to save the key (/root/.ssh/id_rsa): /root/openssh_client/test/user
      Enter passphrase (empty for no passphrase): 
      Enter same passphrase again: 
      Your identification has been saved in /root/openssh_client/test/user.
      Your public key has been saved in /root/openssh_client/test/user.pub.
      The key fingerprint is:
      SHA256:rNmS7P0u6l1pm75Kb4KhMxZThwaJ/AMnA9C//Z1GVEY root@userclient.cisco.com
      The key's randomart image is:
      +---[RSA 2048]----+
      |++ . .     .E    |
      |  B +       o    |
      |   B . .   o     |
      |    + +.. .      |
      |     * .S.       |
      |    +.o=  ..     |
      |     +*+oo+.     |
      |    =..=++=o     |
      |   . ++.+XO.     |
      +----[SHA256]-----+
      [root@userclient test]# ls
      user  user.pub
      

   2. [Client machine] Open the SSH public key file.

      Note	Copy the public key content for the user certificate.

      [root@userclient test]# cat user.pub
      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCspUNwiwlEy0VXQ1Ruh2peRnAP12LSICNe9
      H76xyBiCIXFLLXHTUZZM+W/Pa97pg3fObxaqyNYaeojfwmGeNyPLS9Ha0mqRuLmVCT/1got5I
      Rn1AZhufZz7iz1AdW8DMC//KUnUS/T+cEwGrZ//sbIPTMsQZhhaQVk9xqFp9ghPMxwar3vaHa
      t9NL6ThrR+viue9IOY5LKMeRnqrf2GFX3L6gHfcgYv9fQOKxI11WjTA645rQyB+NumVlrG6KI
      as/xmBCEFHpChGZ1/GSB/atrKeVEWqzsJkpQHXEtE7hwK8gMrL+ad38mbV2Zz6Cc7KHJFEWaZ
      sfjFscCP0kzU1gX root@userclient.cisco.com
      

   3. [CA server] Create a .pub file in the CA server for the user certificate public key and paste the public key contents from the previous step in this file.

      [root@CAServer test]# vim user.pub
      /* Here we are using the vim text editor to create the user.pub file */
      /* You can use any text editor of your choice  */
      

   4. [CA server] Sign the user public key using the CA certificate private key.

      [root@CAServer test]# ssh-keygen -s cacert -I "user" -V +10w -n testuser -z 20 user.pub 
      Signed user key user-cert.pub: id "user" serial 20 valid from 2022-09-16T12:42:00 to 2022-11-25T12:43:24
      	
      

      Note	The command to sign the CSR file using the CA certificate: ssh-keygen -s <CACert> -I <IdentityOfSysReqCert> -V <CertValidity> -n <Username> -z <CertSerialNo> <CopiedUserCertName>

      Note	In addition to the mandatory fields specified for the user certificate, you can also configure critical options and extensions for the user certificate. For detailed information on the critical options and extensions, refer ssh-keygen.

      Parameter	Description
      CACert	Specify the filename of the CA Server private key/
      IdentityOfSysReqCert	Specify the identity of the certificate as User
      CertValidity	Specify the validity period for the certificate.
      <Username>	Specify the principals that you want to add to the certificate. Note   During authentication to the router, the principal in the user certificate is matched against the login username and requests with matching principal and username are permitted for further communication. Note   You can have multiple principals that are associated with the same certificate. The principals must be separated by commas in the IdentityOfSysReqCert field in command to sign the user certificate file using CA certificate.
      CertSerialNo	Specify a serial number for the certificate.
      CopiedUserCertName	Specify the name of the file created to copy the contents of the user certificate file in the client machine.

   5. [CA server] Open the signed user certificate in the CA server and copy the contents.

      [root@CAServer test]# cat user-cert.pub
      ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1yc2EtY2VydC12MDFAb3BlbnNzaC5jb20AA
      AAg6xlcZNQTKmUO27dHFcUCk7UzVCPWFMCep7Ldb4lBF6MAAAADAQABAAABAQCspUNwiwlEy0V
      XQ1Ruh2peRnAP12LSICNe9H76xyBiCIXFLLXHTUZZM+W/Pa97pg3fObxaqyNYaeojfwmGeNyPL
      S9Ha0mqRuLmVCT/1got5IRn1AZhufZz7iz1AdW8DMC//KUnUS/T+cEwGrZ//sbIPTMsQZhhaQV
      k9xqFp9ghPMxwar3vaHat9NL6ThrR+viue9IOY5LKMeRnqrf2GFX3L6gHfcgYv9fQOKxI11WjT
      A645rQyB+NumVlrG6KIas/xmBCEFHpChGZ1/GSB/atrKeVEWqzsJkpQHXEtE7hwK8gMrL+ad38
      mbV2Zz6Cc7KHJFEWaZsfjFscCP0kzU1gXAAAAAAAAABQAAAABAAAABHVzZXIAAAAAAAAAAGMkI
      cAAAAAAY4BrFAAAAAAAAACCAAAAFXBlcm1pdC1YMTEtZm9yd2FyZGluZwAAAAAAAAAXcGVybWl
      0LWFnZW50LWZvcndhcmRpbmcAAAAAAAAAFnBlcm1pdC1wb3J0LWZvcndhcmRpbmcAAAAAAAAAC
      nBlcm1pdC1wdHkAAAAAAAAADnBlcm1pdC11c2VyLXJjAAAAAAAAAAAAAAEXAAAAB3NzaC1yc2E
      AAAADAQABAAABAQCigl/zhyjuGOBYz5bu+GL76HBaROV0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE
      7sZIJ0anU9vYSJZW8zrl8z06GqzmnJqRRaXa9vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0I
      EMhlVN0hCqE4cIFgLxgHpYAaqyl2hISaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf/tbFI
      BPWpuuuA3LvpZIiTaztevQaWYSyK22h3tp3K62IOBX3gUd4Yr+Gvo4PNA26e21cUE2aVJsl6J9
      MeFITR2NzY1cmZ44KWi6bglkPlE4KBiRsbHCvs4wlaUaO5qhNj1BdH3/Hha4xAAABDwAAAAdzc
      2gtcnNhAAABABKOHeuTo9OMg6K+HjASpRXD7rQgiiOdljKdkpw4FZlwCOdBegQwPQkFYTNHmrH
      frQYY72ZINCAjseq+ZSUCkCqJjyXbvY+ZdmRyy76pQvjitgolZjppJqX38nz3uqz/81A/ZuJiF
      811sgJF0Loj7XDN9wjF/zBtsxsXPp7R5c775dmmFgZWQHbSWDlNmnPd9vLZMyBwId//+HV/bCF
      LjbqI/nr/amLVjcI0liOZXzsH7bcLFBSDZ3Epd6IAqFEe+URqvscjaaghcvnshvcafdgfaruO0
      wedsZX53/pEBKhlGacsachFa+S2QuYqTafqnEtkvJoNKVe7UDq/R4kEXM1s9CclIMOficYJm5L
      as+ALR4= root@CAServer.cisco.com
      

   6. [CA server] Create a .pub file in the client machine fo the CA signed user certificate and past the signed certificate contents in this file.

      [root@CAServer test]# vim user-cert.pub
      /* Here we are using the vim text editor to create the user-cert.pub file */
      /* You can use any text editor of your choice  */
      

   7. [Client machine] View the user certificate in the client machine.

      [root@userclient test]# ssh-keygen -Lf user-cert.pub
      user-cert.pub:
              Type: ssh-rsa-cert-v01@openssh.com user certificate
              Public key: RSA-CERT SHA256:rNmS7P0u6l1pm75Kb4KhMxZThwaJ/AMnA9C//Z1GVEY
              Signing CA: RSA SHA256:/B2b8V7jKXwGphf75fkO74U/mpuHgDHmvF4okexdKhY
              Key ID: "user"
              Serial: 20
              Valid: from 2022-09-16T12:44:00 to 2022-11-25T12:45:51
              Principals: 
                      testuser
              Critical Options: (none)
              Extensions: 
                      permit-X11-forwarding
                      permit-agent-forwarding
                      permit-port-forwarding
                      permit-pty
                      permit-user-rc
      

   8. [Client machine] Open the known hosts file in the client system and add the public key of the CA to this file.

      Note	Add the CA public key to the known hosts file in the following format: @cert-authority <hostname> <CA Public Key>

      cat testuser@192.0.2.2 /root/.ssh/known_hosts
      @cert-authority ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCigl/zhyjuGOBYz5bu
      +GL76HBaROV0pVS4Lx3pf1jcjrFkVibPKKkVeX/lE7sZIJ0anU9vYSJZW8zrl8z06GqzmnJq
      RRaXa9vfwNmjvNdRwxuBA3Uk/G1sbmcusMXBXoY6z0IEMhlVN0hCqE4cIFgLxgHpYAaqyl2h
      ISaomTCNhkbD770Ot8zbyRjl6G0Ps0ggYHWmfLZf/tbFIBPWpuuuA3LvpZIiTaztevQaWYSy
      K22h3tp3K62IOBX3gUd4Yr+Gvo4PNA26e21cUE2aVJsl6J9MeFITR2NzY1cmZ44KWi6bglkP
      lE4KBiRsbHCvs4wlaUaO5qhNj1BdH3/Hha4x root@CAServer.cisco.com

   9. [Router Config mode] Configure the username in the router

      Router# config
      Router(config)# username testuser
      Router(config-un)# group root-lr 
      Router(config-un)# commit
      

5. [Client machine] Access the router in the client using the OpenSSH certificate.

   [root@userclient test]# ssh -o CertificateFile=user-cert.pub -i user testuser@192.0.2.2 -o StrictHostKeyChecking=yes 
   Router#

   Note	The command to access the router in the client machine remotely: ssh -o CertificateFile=<CA_Signed_User_Certificate_Name> -i <User_Certificate_Private_Key> <Username >@<Router_IP> -o StrictHostKeyChecking=yes

Use the disable auth-methods command in ssh server configuration mode to disable the specific authentication method for the SSH server.

Public-key authentication includes certificate-based authentication as well. Hence, disabling public-key authentication automatically disables the certificate-based authentication.

Router#configure
Router(config)# ssh server disable auth-methods keyboard-interactive
Router(config-ssh)# commit

!
ssh server 
 disable auth-methods keyboard-interactive
!

Router#show ssh server 

Wed Feb 23 10:38:37.716 UTC
Authentication Method Supported 
------------------------------------
                 PublicKey := Yes
                  Password := Yes
      Keyboard-Interactive := No
         Certificate Based := Yes