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JNCIA-SEC and JNCIS-SEC lab 3 of 16

Screens and ALGs

Bind a screen to the untrust zone in alarm mode, read what it catches from crafted attacks, let it drop a real flood, then see exactly why FTP stops when its ALG is off.

Run this lab

It needs Lite: its 4 nodes are set to 5 vCPU and 7,168 MB in total. That fits the free demo too.

In your workspace: Shared, then JNCIA-SEC and JNCIS-SEC SRX security guided labs, then 03 - Screens and ALGs

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Level
Intermediate
Time
About 60 minutes
Nodes need
5 vCPU, 7,168 MB
Smallest plan
Lite

The scenario

Hartwell's public site in the DMZ has started seeing odd traffic from the internet, and a supplier's FTP server has stopped working through the new firewall for someone who tried to tighten the rules. You'll switch on the SRX's screens in alarm mode first, as anyone careful would, read what they catch, and then let them drop. Then you'll see why FTP needs an ALG.

DeviceInterfaceAddressZone or gateway
SRXge-0/0/0203.0.113.1/24zone untrust
SRXge-0/0/110.10.10.1/24zone trust
SRXge-0/0/210.10.20.1/24zone dmz
LAN-PCeth010.10.10.10/24gateway 10.10.10.1
DMZ-WEBeth010.10.20.10/24gateway 10.10.20.1
OUTeth0203.0.113.10/24gateway 203.0.113.1

The start configuration already permits HTTP and ping from untrust to DMZ-WEB (policy PUBLIC-WEB), and web and FTP from trust to untrust (policy LAN-OUT, which logs closed sessions).

OUT has two tools for this lab. attack KIND TARGET sends crafted test packets, where KIND is syn, scan, land, synfin, noflag, spoof or all. It's Python using raw sockets, and it only runs inside this lab. LAN-PC has ftptest active and ftptest passive, which log in to the FTP server on OUT and list a folder.

What you’ll practise

  • Explain where screens sit in the packet path
  • Configure flood and malformed-packet screens with alarm-without-drop first
  • Read screen statistics and move to enforcement
  • Explain pinholes and prove what the FTP ALG does

The tasks

  1. Baseline
  2. A screen in alarm mode
  3. Let it drop
  4. Why FTP needs an ALG

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of Screens and ALGs4 nodes, 3 links. Every link is listed as text below the diagram.SRXLAN-PCDMZ-WEBOUTge-0/0/2eth0ge-0/0/1eth0ge-0/0/0eth0
4 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • SRX ge-0/0/0 to OUT eth0
  • SRX ge-0/0/1 to LAN-PC eth0
  • SRX ge-0/0/2 to DMZ-WEB eth0
Nodes in Screens and ALGs, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
SRXJuniper vSRX NextGen templatevsrxng-24.4R1.92 vCPU, 4,096 MB
LAN-PCEVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 1,024 MB
DMZ-WEBEVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 1,024 MB
OUTEVE-NG GUI server (Docker)eve-gui-server:latest1 vCPU, 1,024 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder JNCIA-SEC and JNCIS-SEC SRX security guided labs.

  3. Open 03 - Screens and ALGs and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Cisco and Juniper Networks. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.