JNCIA-SEC Network Address Translation — Timed 10 Questions

Question 1

Which TWO statements are true regarding Junos NAT rule-sets? (Choose two)

Accepted Answer

Rule-sets define the security zones (from zone and to zone) where the NAT logic applies.. NAT rule-sets group NAT rules and require source and destination security zones.

Answer

Rule-sets are limited to exactly one rule per zone.

Answer

Rule-sets can only be configured under the routing-options hierarchy.

Question 2

You have configured Destination NAT on a SRX Series device to forward web traffic to an internal server. External clients attempt to connect, but the security logs show sessions are being dropped by the security policy. Which security zone and IP address combination must the security policy permit?

Accepted Answer

From the untrusted zone to the trusted zone, using the internal private IP address of the server.. Junos OS security policies for Destination NAT evaluate traffic based on the *post-translation* destination IP address (the internal server's private IP) and the zone where the internal server resides.

Answer

From the untrusted zone to the untrusted zone, using the public destination IP address.

Answer

From the untrusted zone to the trusted zone, using the pre-translation public IP address.

Answer

From the trusted zone to the untrusted zone, using the public destination IP address.

Question 3

You manage an SRX device where multiple internal subnets require Source NAT out to the internet using a shared pool of public IP addresses. However, specific internal servers must always map to a dedicated, unique public IP address to maintain whitelisting on external partner firewalls. How should you configure this requirement?

Accepted Answer

Configure a specific source NAT rule matching the server's IP address placed above the general rule in the rule-set evaluation order.. To ensure specific hosts map to a dedicated public IP while general traffic uses a pool, you can create a specific Source NAT rule with a higher match priority (higher in rule order) specifying the server's IP, translating to a dedicated pool or address, followed by a general rule for all other traffic.

Answer

Use interface-based NAT for the servers and pool-based NAT for the remaining internal subnets.

Answer

Configure Static NAT for the entire subnet and override individual host entries using Source NAT pools.

Answer

Enable persistent-nat on the general source NAT pool and bind the server's MAC address to the public IP.

Question 4

An administrator is troubleshooting a NAT issue on an SRX Series device and wants to view active NAT sessions to verify whether source translation is happening. Which operational command should be used?

Accepted Answer

show security flow session. To view active sessions including NAT translations, the command 'show security flow session' is used, often filtered with 'match' parameters.

Answer

show security nat translation summary

Answer

show system nat active-translations

Answer

show route forwarding-table destination nat

Question 5

When configuring NAT rule sets on a Junos OS device, in what order are different types of NAT evaluated?

Accepted Answer

Destination NAT is evaluated first, followed by Source NAT, and then Static NAT.. Junos OS evaluates NAT rule sets in a specific order: Destination NAT is evaluated first, followed by Source NAT, and finally Static NAT.

Answer

All NAT types are evaluated simultaneously using parallel hardware lookups.

Answer

Source NAT is evaluated first, followed by Destination NAT, and then Static NAT.

Answer

Static NAT is evaluated first, followed by Destination NAT, and then Source NAT.

Question 6

An administrator needs to configure Destination NAT to map incoming traffic destined for a public IP address (203.0.113.50) on port 443 to an internal web server at private IP address (192.168.1.100) on port 8443. Which configuration hierarchy is used to define the pool for this destination NAT mapping?

Accepted Answer

set security nat destination pool web-pool address 192.168.1.100 port 8443. Destination NAT pools are defined under the 'set security nat destination pool address port ' hierarchy in Junos OS.

Answer

set security nat destination rule-set rs1 rule r1 then translation pool web-pool

Answer

set security nat source pool web-pool address 192.168.1.100 port 8443

Answer

set security nat static rule-set static-rs rule r1 then destination-nat pool web-pool

Question 7

A network engineer implements pool-based Source NAT with Port Address Translation (PAT) on a Junos OS device using a pool containing a single public IP address. After deployment, users in the trust zone experience intermittent failures when opening numerous concurrent browser tabs to various internet sites. 'show security flow session' reveals many sessions in a closing state. What is the root cause?

Accepted Answer

Port exhaustion has occurred because all available source ports for the single translation IP are in use.. Port exhaustion occurs when a single IP address runs out of available source ports (totaling 65,535, though ephemeral ports are usually a subset like 1024-65535). Heavy concurrent browser usage quickly exhausts these ports.

Answer

The security policy action is set to permit instead of translating, causing port reuse collisions.

Answer

The source NAT pool lacks the 'port-overloading' statement, causing the device to drop excess traffic.

Answer

Junos OS requires an explicit maximum-sessions limit per pool address to prevent CPU overload.

Question 8

An administrator deploys Static NAT on a Junos OS device to provide a one-to-one mapping between a public IP address (198.51.100.10) and an internal server IP address (10.1.1.10). When external clients attempt to ping the public IP address, the SRX does not respond. Other services work correctly. What must be configured to allow the SRX to respond to ICMP requests for the Static NAT IP?

Accepted Answer

Configure Proxy ARP on the external egress interface for the static NAT public IP address.. Static NAT on Junos OS requires Proxy ARP to be enabled on the external interface so that the SRX answers ARP requests for the static NAT public IP address originating from the upstream router.

Answer

Configure a Source NAT rule matching the ICMP traffic from the internal server.

Answer

Add an explicit static route for the internal server pointing to the Null0 interface.

Answer

Enable ICMP inspection within the security ALG configuration under security algorithms.

Question 9

An administrator configures interface-based Source NAT on the outgoing external interface of a SRX Series device. However, internal hosts report being unable to reach external web servers. Upon checking the security flow session, the administrator notices that packets are egressing the interface with the actual internal private IP address rather than the interface IP. What is the most likely cause?

Accepted Answer

The source NAT rule is missing the 'then source-nat interface' action statement.. Interface-based Source NAT requires the configuration under the security nat source rule-set to reference the correct 'from zone' and 'to zone', and the rule must contain 'then source-nat interface'. If the rule is missing or not applied to the traffic flow direction, translation will not occur.

Answer

The security policy is evaluating the post-translation source IP instead of the pre-translation IP.

Answer

Proxy ARP must be manually enabled on the internal interface for interface-based NAT to function.

Answer

Interface-based Source NAT requires an explicit routing instance to bypass the master routing table.

Question 10

An administrator configures Destination NAT so that traffic arriving on the external interface destined for IP 198.51.100.25 is translated to 10.0.0.5. After applying the configuration, internal hosts on the trust zone attempt to access 198.51.100.25 and fail. What feature is required to allow internal hosts to access a server hosted inside their own network via its public IP address?

Accepted Answer

Configure source NAT for traffic originating from the internal zone destined back to the internal zone (Hairpin NAT).. Accessing a local server via its public IP address from the internal network requires 'twice NAT' or configuring source NAT alongside destination NAT (often referred to as DNS re-writing or hairpin/NAT loopback configuration) so the server replies back to the SRX rather than directly to the internal client.

Answer

Change the destination NAT rule to evaluate source zones as 'any' instead of 'untrust'.

Answer

Enable proxy ARP on the internal interface for the public destination IP address.

Answer

Configure a static route on the internal hosts pointing directly to the internal server IP.