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Example: Configuring a Filter to Block TFTP Access

Requirements

No special configuration beyond device initialization is required before configuring this example.

Overview

By default, to decrease vulnerability to denial-of-service (DoS) attacks, the Junos OS filters and discards Dynamic Host Configuration Protocol (DHCP) or Bootstrap Protocol (BOOTP) packets that have a source address of 0.0.0.0 and a destination address of 255.255.255.255. This default filter is known as a unicast RPF check. However, some vendors’ equipment automatically accepts these packets.

To interoperate with other vendors' equipment, you can configure a filter that checks for both of these addresses and overrides the default RPF-check filter by accepting these packets. In this example, you block Trivial File Transfer Protocol (TFTP) access, logging any attempts to establish TFTP connections.

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.

To configure this example, perform the following tasks:

• Configure the Stateless Firewall Filter
• Apply the Firewall Filter to the Loopback Interface
• Confirm and Commit Your Candidate Configuration

CLI Quick Configuration

Configure the Stateless Firewall Filter

Step-by-Step Procedure

1. Create the stateless firewall filter tftp_access_control.

   [edit]user@host# edit firewall family inet filter tftp_access_control

2. Specify a match on packets received on UDP port 69.

   [edit firewall family inet filter tftp_access_control]user@host# set term one from protocol udp user@host# set term one from port tftp

3. Specify that matched packets be logged to the buffer on the Packet Forwarding Engine and then discarded.

   [edit firewall family inet filter tftp_access_control]user@host# set term one then log user@host# set term one then discard

Apply the Firewall Filter to the Loopback Interface

Step-by-Step Procedure

• [edit]user@host# set interfaces lo0 unit 0 family inet filter input tftp_access_controluser@host# set interfaces lo0 unit 0 family inet address 127.0.0.1/32

Confirm and Commit Your Candidate Configuration

Step-by-Step Procedure

1. Confirm the configuration of the stateless firewall filter by entering 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.

   [edit]user@host# show firewall

   family inet {filter tftp_access_control {term one {from {protocol udp;port tftp;}then {log;discard;}}}}

2. Confirm the configuration of the interface by entering 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.

   [edit]user@host# show interfaces

   lo0 {unit 0 {family inet {filter {input tftp_access_control; }address 127.0.0.1/32;}}}

3. If you are done configuring the device, commit your candidate configuration.

   [edit]user@host# commit

Verification

Confirm that the configuration is operating properly:

• Verifying Logged and Discarded Packets

Verifying Logged and Discarded Packets

Purpose

Verify that the actions of the firewall filter terms are taken.

Action

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

   user@myhost> clear firewall log
2. From another host, send a packet to UDP port 69 on this router or switch.