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Remote Access Overview

You (the network administrator) can access a router, switch, or security device remotely using services such as DHCP, Finger, FTP, rlogin, SSH, and Telnet services. This topic shows you how to configure remote access using Telnet, SSH, FTP, and Finger services.

System Services Overview

For security reasons, remote access to the router is disabled by default. You must configure the router explicitly so that users on remote systems can access it. Users can access the router from a remote system by means of the DHCP, finger, FTP, rlogin, SSH, and Telnet services. In addition, Junos XML protocol client applications can use Secure Sockets Layer (SSL) or the Junos XML protocol-specific clear-text service, among other services.

Note:

To protect system resources, you can limit the number of simultaneous connections that a service accepts and the number of processes owned by a single user. If either limit is exceeded, connection attempts fail.

Configure Telnet Service for Remote Access to a Router or Switch

To configure the router or switch to accept Telnet as an access service, include the telnet statement at the [edit system services] hierarchy level:

By default, the router or switch supports a limited number of simultaneous Telnet sessions and connection attempts per minute.

Optionally, you can include either or both of the following statements to change the defaults:

  • connection-limit limit—Maximum number of simultaneous connections per protocol (IPV4 and IPv6). The range is from 1 through 250. The default is 75. When you configure a connection limit, the limit is applicable to the number of telnet sessions per protocol (IPv4 and IPv6). For example, a connection limit of 10 allows 10 IPv6 telnet sessions and 10 IPv4 telnet sessions.

  • rate-limit limit—Maximum number of connection attempts accepted per minute (from 1 through 250). The default is 150. When you configure a rate limit, the limit is applicable to the number of connection attempts per protocol (IPv4 and IPv6). For example, a rate limit of 10 allows 10 IPv6 telnet session connection attempts per minute and 10 IPv4 telnet session connection attempts per minute.

You cannot include the telnet statement on devices that run the Junos-FIPS software. We recommend that you do not use Telnet in a Common Criteria environment.

Configure FTP Service for Remote Access to the Router or Switch

To configure the device to accept FTP as an access service, include the ftp statement at the [edit system services] hierarchy level:

By default, the router or switch supports a limited number of simultaneous FTP sessions and connection attempts per minute. You can include either or both of the following statements to change the defaults:

  • connection-limit limit—Maximum number of simultaneous connections per protocol (IPV4 and IPv6). The range is a value from 1 through 250. The default is 75. When you configure a connection limit, the limit is applicable to the number of sessions per protocol (IPv4 and IPv6). For example, a connection limit of 10 allows 10 IPv6 FTP sessions and 10 IPv4 FTP sessions.

  • rate-limit limit—Maximum number of connection attempts accepted per minute (a value from 1 through 250). The default is 150. When you configure a rate limit, the limit is applicable to the number of connection attempts per protocol (IPv4 and IPv6). For example, a rate limit of 10 allows 10 IPv6 FTP session connection attempts and 10 IPv4 FTP session connection attempts.

You can use passive FTP to access devices that accept only passive FTP services. All commands and statements that use FTP also accept passive FTP. Include the ftp statement at the [edit system services] hierarchy level to use either active FTP or passive FTP.

To start a passive FTP session, use pasvftp (instead of ftp ) in the standard FTP format (ftp://destination). For example:

You cannot include the ftp statement on routers or switches that run the Junos-FIPS software. We recommend that you not use the FTP service in a Common Criteria environment.

Configure Finger Service for Remote Access to the Router

To configure the router to accept finger as an access service, include the finger statement at the [edit system services] hierarchy level:

By default, the router supports a limited number of simultaneous finger sessions and connection attempts per minute. Optionally, you can include either or both of the following statements to change the defaults:

  • connection-limit limit—Maximum number of simultaneous connections per protocol (IPv4 and IPv6). The range is a value from 1 through 250. The default is 75. When you configure a connection limit, the limit is applicable to the number of sessions per protocol (IPv4 and IPv6). For example, a connection limit of 10 allows 10 IPv6 clear-text service sessions and 10 IPv4 clear-text service sessions

  • rate-limit limit—Maximum number of connection attempts accepted per minute (a value from 1 through 250). The default is 150. When you configure a rate limit, the limit is applicable to the number of connection attempts per protocol (IPv4 and IPv6). For example, a rate limit of 10 allows 10 IPv6 session connection attempts per minute and 10 IPv4 session connection attempts per minute.

You cannot include the finger statement on routers that run the Junos-FIPS software. We recommend that you not use the finger service in a Common Criteria environment.

Configure SSH Service for Remote Access to the Router or Switch

To configure the router or switch to accept SSH as an access service, include the ssh statement at the [edit system services] hierarchy level:

By default, the router or switch supports a limited number of simultaneous SSH sessions and connection attempts per minute. Use the following statements to change the defaults:

  • connection-limit limit—Maximum number of simultaneous connections per protocol (IPv4 and IPv6). The range is a value from 1 through 250. The default is 75. When you configure a connection limit, the limit is applicable to the number of SSH sessions per protocol (IPv4 and IPv6). For example, a connection limit of 10 allows 10 IPv6 SSH sessions and 10 IPv4 SSH sessions.

  • max-sessions-per-connection number—Include this statement to specify the maximum number of SSH sessions allowed per single SSH connection. This allows you to limit the number of cloned sessions tunneled within a single SSH connection. The default value is 10.

  • rate-limit limit—Maximum number of connection attempts accepted per minute (a value from 1 through 250). The default is 150. When you configure a rate limit, the limit is applicable to the number of connection attempts per protocol (IPv4 and IPv6). For example, a rate limit of 10 allows 10 IPv6 SSH session connection attempts per minute and 10 IPv4 SSH session connection attempts per minute.

  • data-limit—Data limit before renegotiating session keys (bytes)

  • time-limit—Time limit before renegotiating session keys (minutes)

Starting in Junos OS Release 19.4R1 and Junos OS Release 17.4R3, you can disable either the SSH login password or the challenge-response authentication using the no-password-authentication and no-challenge-response options at the [edit system services ssh] hierarchy level.

By default, a user can create an SSH tunnel over a CLI session to a router running Junos OS via SSH. This type of tunnel can be used to forward TCP traffic, bypassing any firewall filters or access control lists. By bypassing firewall filters or access control lists, this type of tunnel allows access to resources beyond the router. Use the no-tcp-forwarding option to prevent a user from creating an SSH tunnel to a router via SSH.

For information about other configuration settings, see the following topics:

Configure the Root Login Through SSH

By default, users are allowed to log in to the router or switch as root through SSH when the authentication method does not require a password. To control user access through SSH, include the root-login statement at the [edit systems services ssh] hierarchy level:

allow—Allows users to log in to the router or switch as root through SSH.

deny—Disables users from logging in to the router or switch as root through SSH.

deny-password—Allows users to log in to the router or switch as root through SSH when the authentication method (for example, RSA) does not require a password.

The default is deny-password.

Configure Incoming SFTP Connections

SSH File Transfer Protocol (SFTP) is a network protocol that provides file access, file transfer, and file management over any reliable data stream. Starting in Junos OS Release 19.1R1, we have globally disabled the incoming SFTP connections by default. If desired, you can globally enable incoming SFTP connections by configuring the statement sftp-server at the [edit system services ssh] hierarchy level. Prior to Junos OS Release 19.1R1, incoming SFTP connections were globally enabled by default.

Note:

Only the incoming SFTP connections are disabled by default. For example, given devices A and B (where device A is running 19.1R1), you cannot connect through SFTP from B to A by default. However, you can connect through SFTP from device B to device A if you configure sftp-server on device A.

The incoming SFTP connections are disabled by default. To enable incoming SFTP connections:

  1. Configure the sftp-server statement at the [edit system services ssh] hierarchy level:
  2. Commit the configuration.

    The sftp-server statement is now active. Therefore, the incoming SFTP connections are enabled.

Configure the SSH Protocol Version

By default, only version 2 of the SSH protocol is enabled.

To configure the router or switch to use version 2 of the SSH protocol, include the protocol-version statement and specify v2 at the [edit system services ssh] hierarchy level:

Systems in FIPS mode always use SSH protocol version v2.

Configure the Client Alive Mechanism

The client alive mechanism is valuable when the client or server depends on knowing when a connection has become inactive. It differs from the standard keepalive mechanism because the client alive messages are sent through the encrypted channel. The client alive mechanism is not enabled by default. To enable it, configure the client-alive-count-max and client-alive-interval statements. This option applies to SSH protocol version 2 only.

In the following example, unresponsive SSH clients will be disconnected after approximately 100 seconds (20 x 5):

Configure the SSH Fingerprint Hash Algorithm

To configure the hash algorithm used by the SSH server when it displays key fingerprints, include the fingerprint-hash statement and specify md5 or sha2-256 at the [edit system services ssh] hierarchy level:

The md5 hash algorithm is unavailable on systems in FIPS mode.

The telnet Command

You can use the CLI telnet command to open a Telnet session to a remote device:

Note:

On SRX100, SRX210, SRX220, SRX240, SRX300, SRX320, SRX340, SRX345, and SRX1500 devices, the maximum number of concurrent Telnet sessions is indicated in the following table. Platform support depends on the Junos OS release in your installation.

SRX100

SRX210SRX220

SRX240

SRX300SRX320SRX340

SRX345

SRX1500

3

3

5

3

5

5

To exit the Telnet session and return to the Telnet command prompt, press Ctrl-].

To exit the Telnet session and return to the CLI command prompt, enter quit.

Table 1: CLI telnet Command Options

Option

Description

8bit

Use an 8-bit data path.

bypass-routing

Bypass the routing tables and open a Telnet session only to hosts on directly attached interfaces. If the host is not on a directly attached interface, an error message is returned.

host

Open a Telnet session to the specified hostname or IP address.

inet

Force the Telnet session to an IPv4 destination.

interface source-interface

Open a Telnet session to a host on the specified interface. If you do not include this option, all interfaces are used.

no-resolve

Suppress the display of symbolic names.

port port

Specify the port number or service name on the host.

routing-instance routing-instance-name

Use the specified routing instance for the Telnet session.

source address

Use the specified source address for the Telnet session.

The ssh Command

You can use the CLI ssh command to use the secure shell (SSH) program to open a connection to a remote device:

Note:

On SRX100, SRX210, SRX220, SRX240, SRX300, SRX320, SRX340, SRX345, and SRX1500 devices, the maximum number of concurrent SSH sessions is indicated in the following table. Platform support depends on the Junos OS release in your installation.

SRX100

SRX210SRX220

SRX240

SRX300SRX320SRX340

SRX345

SRX1500

3

3

5

3

5

5

Table 2 describes the ssh command options.

Table 2: CLI ssh Command Options

Option

Description

bypass-routing

Bypass the routing tables and open an SSH connection only to hosts on directly attached interfaces. If the host is not on a directly attached interface, an error message is returned.

host

Open an SSH connection to the specified hostname or IP address.

inet

Force the SSH connection to an IPv4 destination.

interface source-interface

Open an SSH connection to a host on the specified interface. If you do not include this option, all interfaces are used.

routing-instance routing-instance-name

Use the specified routing instance for the SSH connection.

source address

Use the specified source address for the SSH connection.

v1

Force SSH to use version 1 for the connection.

v2

Force SSH to use version 2 for the connection.

Configure SSH Host Keys for Secure Copying of Data

Secure Shell (SSH) uses encryption algorithms to generate a host, server, and session key system that ensures secure data transfer. You can configure SSH host keys to support secure copy (SCP) as an alternative to FTP for the background transfer of data such as configuration archives and event logs. To configure SSH support for SCP, you must complete the following tasks:

  • Specify SSH known hosts by including hostnames and host key information in the Routing Engine configuration hierarchy.

  • Set an SCP URL to specify the host from which to receive data. Setting this attribute automatically retrieves SSH host key information from the SCP server.

  • Verify that the host key is authentic.

  • Accept the secure connection. Accepting this connection automatically stores host key information in the local host key database. Storing host key information in the configuration hierarchy automates the secure handshake and allows background data transfer using SCP.

Tasks to configure SSH host keys for secure copying of data are:

Configure SSH Known Hosts

To configure SSH known hosts, include the host statement, and specify hostname and host key options for trusted servers at the [edit security ssh-known-hosts] hierarchy level:

Host keys are one of the following:

  • dsa-key key—Base64 encoded Digital Signature Algorithm (DSA) key for SSH version 2.

  • ecdsa-sha2-nistp256-key key—Base64 encoded ECDSA-SHA2-NIST256 key.

  • ecdsa-sha2-nistp384-key key—Base64 encoded ECDSA-SHA2-NIST384 key.

  • ecdsa-sha2-nistp521-key key—Base64 encoded ECDSA-SHA2-NIST521 key.

  • ed25519-key key—Base64 encoded ED25519 key.

  • rsa-key key—Base64 encoded public key algorithm that supports encryption and digital signatures for SSH version 1 and SSH version 2.

  • rsa1-key key—Base64 encoded RSA public key algorithm, which supports encryption and digital signatures for SSH version 1.

Configure Support for SCP File Transfer

To configure a known host to support background SCP file transfers, include the archive-sites statement at the [edit system archival configuration] hierarchy level.

Note:

When specifying a URL in a Junos OS statement using an IPv6 host address, you must enclose the entire URL in quotation marks (" ") and enclose the IPv6 host address in brackets ([ ]). For example, “scp://username<:password>@[host]<:port>/url-path”;

Setting the archive-sites statement to point to an SCP URL triggers automatic host key retrieval. At this point, Junos OS connects to the SCP host to fetch the SSH public key, displays the host key message digest or fingerprint as output to the console, and terminates the connection to the server.

To verify that the host key is authentic, compare this fingerprint with a fingerprint that you obtain from the same host using a trusted source. If the fingerprints are identical, accept the host key by entering yes at the prompt. The host key information is then stored in the Routing Engine configuration and supports background data transfers using SCP.

Update SSH Host Key Information

Typically, SSH host key information is automatically retrieved when you set a URL attribute for SCP using the archival configuration archive-sites statement at the [edit system] hierarchy level. However, if you need to manually update the host key database, use one of the following methods.

Retrieve Host Key Information Manually

To manually retrieve SSH public host key information, configure the fetch-from-server option at the [edit security ssh-known-hosts] hierarchy level. You must to specify the host from which to retrieve the SSH public key.

Import Host Key Information from a File

To manually import SSH host key information from a known_hosts file, include the load-key-file option at the [edit security ssh-known-hosts] hierarchy level. You must specify the path to the file from which to import host key information.

Configure the SSH Service to Support Legacy Cryptography

The SSH server in Junos OS is based on OpenSSH 7 and defaults to a more secure set of ciphers and key-exchange algorithms. OpenSSH 7 omits some legacy cryptography.

Note:

Lack of support for legacy cryptography in devices causes Junos Space device discovery to fail. To work around this issue, configure the device to support the 3des-cbc or blowfish-cbc cipher, or both, and the dh-group1-sha1 key-exchange method. This issue does not affect devices running Junos OS with upgraded FreeBSD.

Note:

See the OpenSSH 7 documentation at https://www.openssh.com/ for more information about these extensions.

Junos OS supports the following set of ciphers by default:

  • chacha20-poly1305@openssh.com

  • aes128-ctr

  • aes192-ctr

  • aes256-ctr

  • aes128-gcm@openssh.com

  • aes256-gcm@openssh.com

In Junos OS, the following ciphers are not supported by default, but you can configure your device to support them. They are listed from the most secure to the least secure:

  • aes256-cbc

  • aes192-cbc

  • aes128-cbc

  • 3des-cbc

  • blowfish-cbc

  • cast128-cbc

  • arcfour256

  • arcfour128

  • arcfour

Junos OS supports the following set of key-exchange methods by default:

  • curve25519-sha256

  • ecdh-sha2-nistp256

  • ecdh-sha2-nistp384

  • ecdh-sha2-nistp521

  • group-exchange-sha2

  • dh-group14-sha1

In Junos OS, the following key-exchange methods are not supported by default, but you can configure your device to support them:

  • group-exchange-sha1

  • dh-group1-sha1

To configure the SSH service to support legacy cryptography:

Note:

By configuring an ordered set of ciphers, key-exchange methods, or message authentication codes (MACs), the newly defined set is applied to both server and client commands. Changes to the defaults affect the file copy command when you use Secure Copy Protocol (SCP).

  1. Add support for ciphers by using the set system services ssh ciphers [ cipher 1 cipher 2 ... ] command. We recommend that you add the ciphers to the end of the configuration list so that they are among the last options used. In the following example, the arcfour and blowfish-cbc ciphers are added to the default set:
  2. Add support for key-exchange methods by using the set system services ssh key-exchange [ method 1 method 2 ... ] command. We recommend that you add the key-exchange methods to the end of the configuration list so that they are among the last options used. In the following example, the dh-group1-sha1 key-exchange method is added to the default set:
  3. Commit the configuration:

See Also

Configure Outbound SSH Service

You can configure a device running Junos OS to initiate a TCP/IP connection with a client management application. If the management application does not reach a Juniper Networks device, for example, the device being a firewall. In such cases, outbound-ssh can be configured on the Juniper Networks device. An outbound-ssh configuration initiates a reverse SSH connection from server to client to the management application. This outbound SSH connection is closed only after the configuration are removed from the device.

Note:

There is no initiation command with outbound SSH. After you configure and commit outbound SSH, the device begins to initiate an outbound SSH connection based on the committed configuration. The device repeatedly attempts to create this connection until successful. If the connection between the device and the client management application is dropped, the device again attempts to create a new outbound SSH connection until successful. This connection is maintained until the outbound SSH stanza is removed from the configuration.

To configure the device for outbound SSH connections, include the outbound-ssh statement at the [edit system services] hierarchy level:

[edit system services outbound-ssh]

The following topics describe the tasks for configuring the outbound SSH service.

Send the Public SSH Host Key to the Outbound SSH Client

Each time the router or switch establishes an outbound SSH connection, it first sends an initiation sequence to the management client. This sequence identifies the router or switch to the management client. Within this transmission is the value of device-id.

To configure the device identifier of the router or switch, include the device-id statement at the [edit system services outbound-ssh client client-id] hierarchy level:

The initiation sequence when secret is not configured:

During the initialization of an SSH connection, the client authenticates the identity of the device using the public SSH host key of the device. Therefore, before the client can initiate the SSH sequence, the client needs the public SSH key of the device. When you configure the secret statement, the device passes its public SSH key as part of the outbound SSH connection initiation sequence.

When the secret statement is set and the device establishes an outbound SSH connection, the device communicates its device ID, its public SSH key, and an SHA1 hash derived in part from the secret statement. The value of the secret statement is shared between the device and the management client. The client uses the shared secret to authenticate the public SSH host key it is receiving to determine whether the public key is from the device identified by the device-id statement.

Using the secret statement to transport the public SSH host key is optional. You can manually transport and install the public key onto the client system.

Note:

Including the secret statement means that the device sends its public SSH host key every time it establishes a connection to the client. It is then up to the client to decide what to do with the SSH host key if the client already has an SSH key for that device. We recommend that you replace the client’s copy of the SSH host key with the new key. Host keys can change for various reasons. By replacing the key each time a connection is established, you ensure that the client has the latest key.

To send the router’s or switch’s public SSH host key when the device connects to the client, include the secret statement at the [edit system services outbound-ssh client client-id] hierarchy level:

The following message is sent by the device when the secret attribute is configured:

Configure Keepalive Messages for Outbound SSH Connections

After the client application has the router’s or switch’s public SSH host key, it can initiate the SSH sequence as if it had created the TCP/IP connection. The client can then authenticate the device using its copy of the router’s or switch’s public host SSH key as part of that sequence. The device authenticates the client user through the mechanisms supported in Junos OS (RSA/DSA public string or password authentication).

To enable the device to send SSH protocol keepalive messages to the client application, configure the keep-alive statement at the [edit system services outbound-ssh client client-id] hierarchy level:

Configure a New Outbound SSH Connection

When disconnected, the device begins to initiate a new outbound SSH connection. To specify how the device reconnects to the server after a connection is dropped, include the reconnect-strategy statement at the [edit system services outbound-ssh client client-id] hierarchy level:

You can also specify the number of retry attempts and set the amount of time before the reconnection attempts stop. See Configure Keepalive Messages for Outbound SSH Connections.

Configure the Outbound SSH Client to Accept NETCONF as an Available Service

To configure the application to accept NETCONF as an available service, include the services netconf statement at the [edit system services outbound-ssh client client-id] hierarchy level:

Configure Outbound SSH Clients

To configure the clients available for this outbound SSH connection, list each client with a separate address statement at the [edit system services outbound-ssh client client-id] hierarchy level:

Note:

Outbound SSH connections support IPv4 and IPv6 address formats.

Configure Routing Instances for Outbound SSH Clients

To use the management routing instance, first enable the mgmt_junos routing instance using the set system management-instance command.

To use any other routing instance, first configure the routing instance at the [edit routing-instances] hierarchy.

If you do not specify a routing instance, your device will establish the outbound SSH connection using the default routing table.

Configure NETCONF-Over-SSH Connections on a Specified TCP Port

Junos OS enables you to restrict incoming NETCONF connections to a specified TCP port without configuring a firewall. To configure the TCP port used for NETCONF-over-SSH connections, include the port statement at the [edit system services netconf ssh] hierarchy level. The configured port accepts only NETCONF-over-SSH sessions. Regular SSH session requests for this port are rejected.

You can either configure the default port 830 for NETCONF connections over SSH, as specified in RFC 4742, Using the NETCONF Configuration Protocol over Secure Shell (SSH), or configure any port from 1 through 65535.

Note:

  • The default SSH port (22) continues to accept NETCONF sessions even with a configured NETCONF server port. To disable the SSH port from accepting NETCONF sessions, specify this in the login event script.

  • We do not recommend configuring the default ports for FTP (21) and Telnet (23) services for configuring NETCONF-over-SSH connections.

Configure Password Retry Limits for Telnet and SSH Access

To prevent brute force and dictionary attacks, a device performs the following actions for Telnet or SSH sessions by default:

  • Disconnects a session after a maximum of 10 consecutive password retries.

  • After the second password retry, introduces a delay in multiples of 5 seconds between subsequent password retries.

    For example, the device introduces a delay of 5 seconds between the third and fourth password retry, a delay of 10 seconds between the fourth and fifth password retry, and so on.

  • Enforces a minimum session time of 20 seconds, during which a session cannot be disconnected. Configuring the minimum session time prevents malicious users from disconnecting sessions before the password retry delay goes into effect. Configuring the minimum session time also prevents them from attempting brute force and dictionary attacks with multiple logins.

You can configure the password retry limits for Telnet and SSH access. In this example, you configure the device to take the following actions for Telnet and SSH sessions:

  • Allow a maximum of four consecutive password retries before disconnecting a session.

  • Introduce a delay in multiples of 5 seconds between password retries that occur after the second password retry.

  • Enforce a minimum session time of 40 seconds, during which a session cannot be disconnected.

To configure password retry limits for Telnet and SSH access:

  1. Set the maximum number of consecutive password retries before a Telnet or SSH or telnet session is disconnected. The default number is 10, but you can set a number from 1 through 10.
  2. Set the threshold number of password retries after which a delay is introduced between two consecutive password retries. The default number is 2, but you can specify a value from 1 through 3.
  3. Set the delay (in seconds) between consecutive password retries after the threshold number of password retries. The default delay is in multiples of 5 seconds, but you can specify a value from 5 through 10 seconds.
  4. Set the minimum length of time (in seconds) during which a Telnet or SSH session cannot be disconnected. The default is 20 seconds, but you can specify an interval from 20 through 60 seconds.
  5. After you configure the device, enter commit in configuration mode.

Example: Configure a Filter to Block Telnet and SSH Access

Requirements

You need two devices running Junos OS with a shared network link. No special configuration beyond basic device initialization (management interface, remote access, user login accounts, etc.) is required before configuring this example. While not a strict requirement, console access to the R2 device is recommended.

Note:

Our content testing team has validated and updated this example.

Overview and Topology

In this example, you create an IPv4 stateless firewall filter that logs and rejects Telnet or SSH packets sent to the local Routing Engine, unless the packet originates from the 192.168.1.0/30 subnet. The filter is applied to the loopback interface to ensure that only traffic destined to the local device is affected. You apply the filter in the input direction. An output filter is not used. As a result all locally generated traffic is allowed.

  • To match packets originating from a specific subnet or IP prefix, you use the source-address IPv4 match condition applied in the input direction.

  • To match packets destined for the Telnet port and SSH ports, you use the protocol tcp match condition combined with a port telnet and port ssh IPv4 match conditions applied in the input direction.

Example Topology

Figure 1 shows the test topology for this example. The firewall filter is applied to the R2 device, making it the device under test (DUT). The R1 and the R2 devices share a link that is assigned a subnet of 192.168.1.0/30. Both devices have loopback addresses assigned from the 192.168.255.0/30 prefix using a /32 subnet mask. Static routes provide reachability between loopback addresses because an interior gateway protocol is not configured in this basic example.

Figure 1: Example Topology Example Topology

Configuration

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode.

CAUTION:

By design the sample filter restricts Telnet and SSH access to R2 unless it originates from the shared subnet at R1. If you use SSH or Telnet to access the R2 device directly, you will lose connectivity when the filter is applied. We recommend that you have console access when configuring this example. If needed you can use the R1 device as a jump host to launch an SSH session to R2 after the filter is applied. Alternatively, consider modifying the sample filter to also permit the IP subnet assigned to the machine you use to access the R2 device.

Perform the following tasks to configure this example:

CLI Quick Configuration

Quick Configuration for the R1 Device

To quickly configure the R1 device, edit the following commands as needed and paste them into the CLI at the [edit] hierarchy level. Be sure to issue a commitin configuration mode to activate the changes.

Quick Configuration for the R2 Device

To quickly configure the R2 device, edit the following commands as needed and paste them into the CLI at the [edit] hierarchy level. Be sure to issue a commit in configuration mode to activate the changes.

Tip:

Consider using commit-confirmed when making changes that might affect remote access to your device. Activating a Junos OS Configuration but Requiring Confirmation

Configure the R1 Device

Step-by-Step Procedure

Follow these steps to configure the R1 device:

  1. Configure the interfaces:

  2. Configure the host name and static route to the R2 device’s loopback address. You also configure Telnet and SSH access:

Verify and Commit the Configuration at the R1 Device

Step-by-Step Procedure

Complete the following steps to verify and commit your candidate configuration at the R1 device:

  1. Confirm interface configuration with the show interfaces configuration mode command. If the command output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

  2. Verify the static route used to reach the R2 device’s loopback address and that SSH and Telnet access are enabled. Use the show routing-options and show system services configuration mode commands. If the command output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

  3. When satisfied with the configuration on the R1 device, commit your candidate configuration.

Configure the R2 Device

Step-by-Step Procedure

Complete the following steps to configure the R2 device. You begin by defining the stateless firewall filter that selectively blocks Telnet and SSH access:

  1. Position yourself at the edit firewall family inet filter local_acl hierarchy:

  2. Define the filter term terminal_access. This term permits Telnet and SSH from the specified source prefix(s):

  3. Define the filter term terminal_access_denied. This term rejects SSH and Telnet from all other source addresses. This term is configured to log matches to the term and to generate an explicit Internet Control Message Protocol (ICMP) destination unreachable response back to the packet’s source. See Firewall Filter Logging Actions for details on filter logging options.

    Tip:

    You can use the discard action to suppress generation of ICMP error messages back to the source. See Firewall Filter Terminating Actions for details.
  4. Optional.

    Define the filter term tcp-estab. This term permits outbound access to the Internet to support connections to the Juniper Mist cloud (tcp-established is a bit-field match condition, tcp-flags "(ack | rst)", which indicates an established TCP session, but not the first packet of a TCP connection):

  5. Define the filter term default-term. This term accepts all other traffic. Recall that Junos OS stateless filters have an implicit deny term at their end. The default-term overrides this behavior by terminating the filter with an explicit accept action. The termination of the filter results in all other traffic being accepted by the filer.

    Note:

    For this example we are allowing all other traffic, but for your network you might want to secure the routing engine. See protecting the routing engine for more information.

  6. Configure the loopback interface, and apply the filter in the input direction:

  7. Configure the host name, the ge-0/0/0 interface, the static route to the R1 device’s loopback address, and enable remote access through SSH and Telnet:

Verify and Commit the Configuration at Device R2

Step-by-Step Procedure

Complete the following steps to verify and commit your candidate configuration at the R2 device:

  1. Confirm the configuration of the stateless firewall filter with the show firewall configuration mode command. If the command output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

  2. Confirm interface configuration and filter application with the show interfaces configuration mode command. If the command output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

  3. Verify the static route used to reach the loopback address of the R1 device, and verify that Telnet and SSH access are enabled. Use the show routing-options and show system services configuration mode commands. If the command output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

  4. When satisfied with the configuration on the R2 device, commit your candidate configuration.

    Tip:

    Consider using commit-confirmed when making changes that might affect remote access to your device.

Verify the Stateless Firewall Filter

Confirm that the firewall filter to limit Telnet and SSH access is working properly.

Verify Accepted Packets

Purpose

Verify that the firewall filter correctly allows SSH and Telnet when the traffic is sourced from the 192.168.1.0/30 subnet.

Action

  1. Clear the firewall log on your router or switch.

  2. From a host at an IP address within the 192.168.1.0/30 subnet, use a ssh 192.168.255.2 command to verify that you can log in to the device using SSH from an allowed source address. This packet should be accepted, but the packet header information for this packet should not be logged in the firewall filter log buffer in the Packet Forwarding Engine. You will be prompted to save the SSH host key if this is the first SSH login as user between these devices.

    Note:

    By default the R1 device will source the SSH traffic from the egress interface used to reach the destination. As a result this traffic is sourced from the 192.168.1.1 address assigned to the R1 device’s ge-0/0/0 interface.

  3. Log out of the CLI at the R2 device to close the SSH session.

  4. From a host at an IP address within the 192.168.1.0/30 subnet, use the telnet 192.168.255.2 command to verify that you can log in to your router or switch using Telnet from an allowed source address. This packet should be accepted, but the packet header information for this packet should not be logged in the firewall filter log buffer in the Packet Forwarding Engine.

  5. Log out of the CLI to close the Telnet session to the R2 device.

  6. Use the show firewall log command to verify that the firewall log buffer on the R2 device’s Packet Forwarding Engine (PFE) does not contain any entries with a source address in the 192.168.1.0/30 subnet.

Verify Logged and Rejected Packets

Purpose

Verify that the firewall filter correctly rejects SSH and Telnet traffic that does not originate from the 192.168.1.0/30 subnet.

Action

  1. Clear the firewall log on your router or switch.

  2. Generate SSH traffic sourced from the loopback address of the R1 device. The source address of this traffic is outside of the allowed 192.168.1.0/30 subnet. Use the ssh 192.168.255.2 source 192.168.255.1 command to verify that you cannot log in to the device using SSH from this source address. This packet should be rejected, and the packet header information should be logged in the firewall filter log buffer.

    The output shows that the SSH connection is rejected. This output confirms that the filter is generating an ICMP error message and that it correctly blocks SSH traffic when sent from a disallowed source address.

  3. Generate Telnet traffic sourced from the loopback address of the R1 device. The source address of this traffic is outside of the allowed 192.168.1.0/30 subnet. Use the telnet 192.168.255.2 source 192.168.255.1 command to verify that you cannot log in to the device using Telnet from this source address. This packet should be rejected, and the packet header information for this packet should be logged in the firewall filter log buffer in the PFE.

    The output shows that the Telnet connection is rejected. This output confirms that the filter is generating an ICMP error message and that it correctly blocks Telnet traffic when sent from a disallowed source address.

  4. Use the show firewall log command to verify that the firewall log buffer on the R2 device contains entries showing that packets with a source address of 192.168.255.1 were rejected.

    The output confirms that traffic from the 192.168.255.1 source address matched the filter’s terminal_access_denied term. The Action column displays an R to indicate that these packets were rejected. The interface, transport protocol, and source and destination addresses are also listed. These results confirm that the firewall filter is working properly for this example.

Related Documentation

Release History Table
Release
Description
19.4R1
Starting in Junos OS Release 19.4R1 and Junos OS Release 17.4R3, you can disable either the SSH login password or the challenge-response authentication using the no-password-authentication and no-challenge-response options at the [edit system services ssh] hierarchy level.
19.1R1
Starting in Junos OS Release 19.1R1, we have globally disabled the incoming SFTP connections by default. If desired, you can globally enable incoming SFTP connections by configuring the statement sftp-server at the [edit system services ssh] hierarchy level
18.3R1
Starting in Junos OS Release 18.3R1, the ssh-dss and ssh-dsa hostkey algorithms are deprecated— rather than immediately removed—to provide backward compatibility and a chance to bring your configuration into compliance with the new configuration.
18.3R1
(SRX Series and MX Series only) Starting in Junos OS Release 19.3R1, you can specify the name of the routing instance on which the outbound SSH connectivity needs to be established by including the routing-instance statement at the [edit system services outbound-ssh] hierarchy level: