From Network Security Wiki
Jump to navigation Jump to search

Interview Preparation

Linux Interviews

Python Interviews

Shell Interviews

AWS Prep

Interview Questions


  • What does Directory Execute (x) permission means? ability to 'cd' into that dir
  • Which cmd sets max size of core dumps? ulimit
  • Which cmd used to inspect system call made by running proces?
ptrace ?
lsof ?
  • In bash which cmd used to execute a shel script in current shel context?
all of the above ?
  • Print top 5 lines
cat foo.txt | head -5
head -5 foo.txt
  • Redirect error stream to file errors? foo2>errors?
  • Command used to create a new process? fork()
  • Command to continue process after closing terminal? nohup
  • init was replace by which default service manager in RHEL? systemd
  • Check env variables of a running process?
/proc FS
all of the abpve?
  • Web Server can do what?
LB ?
URL Rewrite
  • HTTP/2 was inspired by what? SPDY
  • Most commonly used compression in response from server to client in HTTP? gzip

AWS Networking

Round 1

  • BGP Route manupulation
  • Anycast
  • Linux check WSF
  • SSL speed up at LB level:
Session Ticket
OSPC Stapling
  • Cookie Uses
  • Google BBR - congestion control algorithm
  • DNS check if reply is spoofed or not?
  • How does client knows when to switch to TCP?

Round 2

  • Experiennce with diffcult customer
  • Learnt from feedback
  • How to remotely access Windows server?
RDP, VNC (Teamviewer, Webex for PC)
  • RDP Port no
TCP port 3389 and UDP port 3389.
  • Page file
  • Service
  • Service fails, how to check logs? (Event logs viewer)
  • Kernel
  • Inode
  • Check CPU, Memory, HDD info?
  • Check Load Average? What is the output?(1,5,15 min avrg), what does it mean?
  • Check Clients we are connected to? netstat -ant -> Foreign Address
  • ifconfig -a - eth0:1 - What does :1 mean? (vlan)
  • Check file in use by which process? lsof
  • Runlevels
  • Check DNS config? /etc/resolv.conf & interface config
  • Iptables?
  • NAT
  • How does Traceroute works?
  • Request & response headers
curl -v
  • Commands used in daily troubleshooting
telnet nc ipconfig netstat traceroute top ps free df dh
1st attempt - 404
2nd attempt - 200
3rd attempt - 404
4th attempt - 200
LB having 2 servers, one is not having app installed, check logs & disable it.


1st Round Online Challenge

   Paste.png     This section is under construction.

Telephonic Round

  • Tell me something where you did something extra during a case/your job
  • Tell me something where you learned something during a case/your job
  • Where do you store your code(Github)? How can some one other edit code?
  • How DNS works? Explain in detail.
  • DNS traffic Flow? Search local cache > hosts file > DNS server
  • What is VoIP? How SIP works?
  • Scenario:
Srv3 is slow, latency, how to troubleshoot? top, free -h, df -h
How to check traffic requests coming per second? netstat -s, ss -s, tcpdump (filter GET,POST), Server -> Filter access.log file for GET

Google Challenge

   Paste.png     This section is under construction.


  • OOM Killer
  • Page Fault
A page fault (sometimes called #PF, PF or hard fault) is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process.
  • Swap Memory
  • DNS Rate Limiting Troubleshoot
Create Static entry
Increase Cache Time
  • DNS latency in Server or Network Troubleshoot:
Check RTT for other traffic & DNS Specific traffic.
  • Print below logs:
startTime : 12332121324
endTime : 21342313222
serviceName : abc
status : 200
startTime : 12332121324
endTime : 21342313222
serviceName : abc
status : 200
startTime : 12332121324
endTime : 21342313222
serviceName : abc
status : 200
startTime : 12332121324
endTime : 21342313222
serviceName : abc
status : 200
startTime : 12332121324
endTime : 21342313222
serviceName : abc
status : 200


for i in l:
    if ":" in i:

for i in range(len(a)):

for i in b:
for i in range(len(a)):
    if "startTime " in a[i]:
       print('\n', end="")

AWS Sydney

Round 2

Load Balancer
  • Explain LB usage in detail
  • How it distributes connections from TCP prospective?
  • How is connection initiated & how it reaches Server?
  • Can LB use VIP to connect to Server?

*When in Citrix Netscaler it uses VIP as source IP for connecting to Server?

  • Scenario for eBGP:
[A]-----[R1]              [R4]----[B]
  • How is maintenance done in R2?(ASPath Prepend and Local_Pref)
Shutdown command? not preferred, disruptive way
Local Pref? Nope
Med?  Nope
IS local Pref used in eBGP?
Where is MED set on? (Med cannot be used in eBGP)
 AS-Path prepend:   Way to add AS number to the list of subnet u want to advertise. 
                    This is a way to route poisoning. 
                    Tell the outside world not to follow the path.
                    Applied outwardly. 
                    Impacts incoming path.
 Local preference:  Applied while the traffic coming inside.  
                    Impacts traffic while going out.  
                    Non transitive. 
                    Propagates within the AS-Path.
                    Higher is preferred
 MED:               When your router has connection with two other routers with same AS. 
                    Let's say you have 2 subnets behind your router.  
                    You can use MED value to mention which networks should be accessed through which links. 
                    It is advertised outwards. 
                    Impacts the incoming traffic. 
                    Semi transitive. 
                    Propagates to one AS.
                    Lower is preferred
                    Should be used carefully as it reduces network resiliency.

  • Can we do Load balancing across R2 & R3? (ECMP?)
  • Loop Prevention in eBPG?
  • Loop prevention in iBGP? For above Scenario? (Split Horizon, do not advertize to other iBGP peers, but learn directly from neighbor)
  • Need for a direct connection b/w R1 & R4? What if not possible? (Run IGP)
  • How Router Reflector prevents loops (using RR, RR Clients, Non-Client Peers)
  • When will a router received be discarded:
Originator-ID same as own Router-id(the route is discarded in this case)
Originator-ID different from Router-ID
  • Skill you learned in past 12 months? Python
  • Describe a project you worked on in Python?
  • How did you learned Python?
Scenario 1
[PC]-------[Branch India]-------------------------------[HQ US]------------[Update Server]
  • How does PC reach Server for updates? Explain all steps - ARP, DHCP,DNS, TCP, HTTP(GET) , etc
  • If Download is Slow, how will you troubleshoot?
  • Where will you take captures? (All devices)
  • Which device's captures will you see Retransmissions? (All)
  • TCP Slow Start?
  • What happens if TCP Packet is lost?
  • Will Latency cause slowness?
  • If 1 server is in US & Another is in India with same BW, will the download time change?
  • If so, how much?
Scenario 2
[PC]------[SW]-------[Router]------[DNS Server]
  • Captures on Switch, what will be Source & Dest IP as well as MAC?

Round 1

  • OSPF States
  • Type 4 LSA Details
  • Type 3 LSA Details
  • Route Selection Hierarchy - E1, E2, N1, N2
  • Looping prevention in OSPF 1) Only best route selected in SPF; 2) All Areas must connect to Backbone Area.
  • ABR has 2 DB Tables for 2 interfaces each in different areas, each learning same route with same cost, how will best path be selected.
  • How is route removed in OSPF, What is there in LSU?
  • Loop prevention method for eBGP(AS Path list own AS Number, route removed)
  • Loop prevention method for iBGP
To avoid routing information loops within the same AS, all IBGP routers must peer with every router within AS
Only TCP sessions required, not physical connections) – this is called full mesh IBGP. 
IBGP uses split horizon rule (covered in other article), which states that routes learned from one IBGP neighbor is never sent to the rest of IBGP neighbors. 
That way, when you have full mesh IBGP, routing information loops will not occur, because all routers have the same routing information.
  • Attributes for affecting Outbound traffic? MED & Local Preference?
  • Attributes for affecting Inbound traffic?
  • High CPU Troubleshooting
  • Commands: GREP, AWK, SED
  • Case/Implementation you worked - NSM repro, SIP Asterisk repro
  • Scripting/Automation example - Top VS list


  • Is UDP Stateful
  • How is UDP connectionless
  • FTP types
  • What is Frag Offset
  • IKE v1 vs IKE v2


  • Point to Point Network stuck at 2-way state in OSPF
  • OSPF loop prevent mechanism
  • why Bgp is used in enterprise
  • Famous BGP communities
  • STP replacement => switch fabric in nexus sw


Telephonic Screening
  • DHCP, which packets are unicast, why?
  • 3-way Handshake parameters exchanged
  • How Traceroute works
  • VLAN, types of ports, importance of Trunk interface
  • Router on a stick
  • Importance of TTL
  • IP header fields
  • How certificates verify URL? Can it be Tempered?
  • What is SACK
  • What parameters change in a packet when crossing a Router? Does it recalculate Checksums?
2nd 3rd Round
  • HTTP 1.0 vs 1.1
  • What is Hyper-visor?
  • How Virtual Network Works?
  • Difference between Bridge & Switch?
  • Example of L4 Service in Netscaler?
  • How does a HTTP Request looks like?
  • Explain a case you worked recently
  • How Content Switching Works?
  • Python script logic to find duplicates in list(using arrays)
x = [1,2,4,6,8,2,0,3,4,1,9]
for i in range(len(x)):
  for j in range(i+1, len(x)):
   if x[i] == x[j]:
     print x[i]
  • WanEM usage in Repro
  • Jitter vs Latency
  • DNS ALG Working
  • DNS Doctoring, DNS across NAT
  • Does DNS use TCP or UDP? Who decides it?
  • Active vs Passive FTP, ALG working
  • 3-way Handshake
  • What parameters are exchanged in 3-way Handshake
  • What is Window Scaling Factor?
  • How to calculate MTU?
  • What is HTTP Option Connect used for?
  • What is use of SACK?
  • OSPF States? How is Master Elected(Highest Router ID becomes Master)? At which Stage?
  • What is 2 Way State?
  • Is OSPF Reliable protocol?
  • What are DBD, LSU, LSAck? Which ones are Acknowledged?
  • What does IP Packet looks like?
  • What is DF Bit?
  • What ICMP Message is sent when Packet is dropped due to larger packet than MTU with DF set?
  • Can MTU be larger than 1500?
  • What is Elastic IP address?
4th Round
  • TCP 3way Handshake in Depth
  • What value Sequence number represents?
  • TCP Slow Start
  • How does Receiver controls flow so that it is not overwhelmed by Data? (Delayed Ack & Windows = 0)
  • Explain flow in below scenarios:
  • Explain Source & Dest MAC & IP values in above scenarios at each stage.
  • How does Switches in above populate MAC Table?
  • How is the ARP a Broadcast with Destination MAC as 00:00:00:00:00:00? (This is at ARP header, BC uses ff:ff:ff.. in Ethernet header)
  • What is DNS Iterative & Recursive Query?
  • OSPF: what parameters need to be matching in hello?
  • Does Stub Flags need to be matched?
  • What can be the issue if it gets stuck at Init state?
  • What can be the issue if it gets stuck at Exstart state? MTU Mismatch.
  • Scenario based Internet is not reachable in depth Troubleshooting for: [PC]----[Router]----[Internet]
  • How Traceroute works? Which protocol it uses? (ICMP & UDP)
  • SSL handshake? when Server cert or Client cert asked?
  • What does certificate looks like? What fields are there?
  • How are SSL Keys Calculated?
  • How Virtualization works? XenServer? What is Hypervisor?
5th Round Consulting
  • What parameters need to be configure in a Firewall initial setup? Screening, NAT, Routing, Policies, MIP, VIP?
  • Can a ScreenOS FW work on L7? yes with ALG, AV, DPI.
  • Explain OSPF case you worked?
  • Have you created any Scripts?
  • Netscaler case you worked recently
  • How to find out a session among 224K users, when customer dont know IP address? (Can use Curl custom User Agent & Filter it in traces)
  • Difference bw HTTP 1.0 & 1.1
  • How to TS Latency in wireshark? Enable Timestamp - Preference > Protocol > TCP > Calc TS.
  • How does HTTP Request look like? Which fields are mandatory?
  • What is the importance of Host Header? (used when server is having multiple virtual hosts bound to same IP address, used to send request to correct vhost)
  • How to configure Apache Server? What are the various directives? What is root directory?
  • How does HTTPS figure out the correct Server certificate to be sent if multiple vHosts bound to same IP address? At what stage in SSL?
There is an TLS Extension called SNI, used to MAP correct Server Name to Certificate.
Sent in Client Hello?? [Need to verify]
  • Linux Knowledge? Explain any case resolved with that?
  • App is not accessible in below scenario, How to troubleshoot:
IPtables, CPU, Memory, HDD, etc
LSOF, open file limit in old Unix very less ~256, need to extend it, in linux Socket is same as files, so server stop responding after 256 limit.
  • LB 2 commomnly used methods (Source IP & Cookie Insert), How they work?
  • Which one is more scalable/Efficient?
Stateless Session Persistence: Cookie inserted by ADC is more efficient because no need to create a table, NS will insert cookie & forget, with reply, it will read cookie value, decrypt it & fwd request.
State-full Session Persistence: Server will insert cookie, NS will hash it & fwd based on Hash value but will need to keep a table in memory with all hashes & IP Addresses.
Same is true for Source IP based Persistence, Also inefficient behind NAT
Using Set-cookie-header = by Server - insert Name & Value Fields
Client sends cookie in Cookie Header
Who ever generates cookie, will be able to read it
  • Explain Repro efforts you have done in details.
10th Round
  • Explain SIP Flow
  • Does it uses TCP? (no it uses UDP for both SIP & RTP)


  • FTP active vs passive - policy required
  • ARP vs GARP
  • ARP packet vs GARP packet differences
  • PUSH vs URG. Which one will be process earlier in receiving side?
  • VPN Scenario
-2 VPN Tunnel 
-One using Internet & other MPLS
-Source is same
-Destination has 2 servers
- = secure app
- = general app
-Secure app should use MPLS
-General app should use Internet
  • SA?
  • SA vs Proxy id
  • Route based VPN with non juniper device possible? what differences?
  • Fragmented traffic - whoo will fragment? How will dest know if it is a last fragment?
  • MTU vs MSS?
  • Max MTU size for ethernet?
  • Max MTU value for fiber optic possible?
  • Window?
  • Max Window size
  • Will receiver send Ack immediately after receiving a segment is received
  • SSL Handshake
  • HTTP Status Codes
  • HTTP Request and Response methods
  • SRX flow
  • SRX Why SNAT & DNAT before route lookup
  • What was the issue with ScreenOS NAT lookup method
  • Types of NAT supported in SRX
  • Logs which plane generates - data/routing plane

Convergys (SRX TAC)

  • 3-way handshake
  • Why 2nd syn
  • Parameters exchanged in 3 way handshake
  • TCP flags
  • Push vs URG flag
  • MSS vs Window size
Receive window size is the maximum amount of received data, in bytes, 
that can be buffered at one time on the receiving side of a connection. 
The sending host can send only that amount of data before waiting for an 
acknowledgment and window update from the receiving host
  • PC connected in LAN. What will happen - GARP, DHCP, etc
  • GARP - src & dst MAC
  • GARP - Ethernet header MAC
  • DHCP - DORA process
  • User unable to access Internet - Troubleshooting Approach
  • User traffic blocked in FW - Troubleshooting Approach
  • Latency in FW troubleshooting - Troubleshooting Approach
  • Main vs Aggressive Mode difference? Which one is faster? Which one is secure?


  • IP Header Fields, Flags & length
  • TCP Header Fields, Flags & length
  • How ICMP will recognize reply message? Identification field
  • Where Segmentation occurs
  • What is MTU
  • URG vs Push Flag
  • Data Offset
  • NAT-T
  • DORA
  • DHCP Relay
  • Use of GARP
  • Port no of Telnet, SMTP, FTP
  • Why FTP uses 2 ports
  • Phase 1 parameters
  • Phase 1 errors
  • If Proxy ID mismatch occurs, will VPN come up or just data will not transfer?
IKE Phase 1 successful, Phase 2 fails due to proxy-id mismatch
The Proxy ID on the local and remote VPN device must match for phase 2 to complete the VPN negotiations
  • Use of Inverse ARP(Frame Relay)
  • Scenario: Ping from PC1 to PC2 - Explain ARP, IP, MAC table, etc


  • Traceroute
  • ICMP though PAT {I think FW will identify each session based on Identification field}
  • Bandwidth vs throughput
  • DPD
  • FW down, how long VPN will stay up
  • Traceroute
  • AnyCache Route
  • Need for NAT-T
  • Congestion Control
  • Active vs Passive FTP
  • WAN Optimization


  • OSPF + VPN + FTP; FTP is slow
  • Dailup VPN
  • RTT
  • Hub & Spoke VPN
  • FTP across Network is slow
  • What is Acknowledgement
  • What is MSS
  • Aggressive Mode vs Main Mode
  • FTP Slow across Switch, Router, FW; How to Troubleshoot.


  • vpn support multicat,broadcst
  • dhcp across vpn, firewall interfaces in diff subnets
  • traffic shapping
  • fragmentation
  • ssl


  • Reno
  • Difference b/w Http/1.0 and Http/1.1 - Only Persistent connections & more methods? yes methods as well
  • Cookies, Caching and proxy - not in much detail types and little explanation is fine
  • Time Stamping : [1]
  • RTT calculate = Time to segment send + ack received without retransmission
  • How MSS value is decided = decided by server & client & sent in 1st & 2nd Packet? send informs its MSS ,exchanged in 1st and 2nd packets of 3 way handshake
  • Relationship between MSS and MTU = MTU - [IP & TCP Headers] = MSS
  • Difference b/w them IPSec and SSL? [2]
  • Why IPSec uses two phases and ssl one phase? main purpose of 2nd phse of IPSEC is to provide PFS as it is normally used for long duration connections (eg VPN ) while SSL is used for comapartively short durations additinal steps are required to enable PFS in SSL,Refer link[3]
  • Nat-t hash calc = RFC file's HASH? Yes - [4]
The HASH is calculated as follows:
HASH = HASH(CKY-I | CKY-R | IP | Port)
  • Nat which side, marker?
  • How query resolved? Meaning Recursive & Iterative? clients send recursive queries ,ISP DNS servers do iterative queries likewise flow can be explained
  • Reverse DNS query? Where is this used? in IPv6? mails ,nslookup etc.
  • DHCP Flags - Which Flags? all are important - DHCPOFFER: 0x8000 for broadcast, 0x0000 for unicast

7 Header Fields
7.1 General Headers

7.1.1 Cache-Control Request Response
7.1.2 Connection
7.1.6 Transfer-Encoding
7.1.7 Upgrade
7.1.8 Via

7.2 Client Request Headers

7.2.6 Cookie
7.2.10 If-Match
7.2.11 If-Modified-Since
7.2.12 If-None-Match
7.2.13 If-Range
7.2.14 If-Unmodified-Since
7.2.18 Referer
7.2.19 TE

7.3 Server Response Headers

7.3.3 ETag
7.3.5 Proxy-Authenticate
7.3.8 Set-Cookie

7.4 Entity Headers

7.4.10 Last-Modified

8 Caching (overview is fine if not able to read completely )

8.1 Request Directives
8.2 Response Directives

10 Security

10.3 DNS Spoofing
10.6 Proxies and Caching
  • Http status code
  • Http headers
  • Cookies, Caching and proxy
  • Types of ARP and explanation & usage
  • TCP and UDP difference
  • 3 Way Hand shake
  • TCP flags
  • Difference b/w PSH and URG
  • TCP options
  • MSS, SACK,
  • Widow scaling
  • Slow start and fast re-transmission
  • TCP well known ports
  • Fragmentation
  • SSL Handshake
  • DNS Types of records, Zones, How query resolved, Reverse DNS query
  • DHCP - DORA Process, Flags, Relay


  • Detailed data transfer(http/ftp) steps
  • EC Window, CWR
  • 3-way handshake (Sequence number)
  • Acknowledgment field value in a Syn Packet
  • Window size=0; what conditions
Server is overloaded
Window scaling is not supported or configured
Some old OS in use by server
  • Fields in a TCP & IP Header

Tech M

  • TCP in juniper srx flow


  • TCP vs UDP - what is a reliable service?
  • TCP Flags - names & roles
  • RST vs FIN Flags
  • 2 types of FW technologies - Stateful vs Proxy
  • User is sending confidential data in gmail. How will you capture it? HTTPS/SSL.
  • Debug vs Snoop? Which is more CPU intensive? Debug
  • Asymmetric Routing? ICMP redirection in context of Asymmetric routing
  • What is NAT64? NAT 64 for traffic coming through a VPN?
It is a simple NAT from ipv6 to ipv4.
  • JSRP vs NSRP
  • Backup Control plane possible or not?
Not possible. Only active firewall will have a Control Plane.
  • How to resolve Split Brain? Immediately resolve it using temporary fix
Make one FW inoperable
  • Cold sync
  • Does Management Interface belongs to a VR?
Yes: Its assigned to Default VR - trust-vr
  • NSM manage through management interface
Yes, in SRX it is managed through management interface.
  • Logs transfer from NSM using management port
Logs will transfer as they are self traffic.
  • Can we connect NSM to FW using Mgmt interface. How will it send logs from Mgmt Interface.

SOC Profile

  • OSPF states
  • If Frag causes slowness. You lower the MTU. It resolves the issue. What change does it make?
Ans: It bring MSS lower than MTU. otherwise every packet will defrag.. (i think)
  • Does DHCP traffic needs policy? If yes what port numbers, What command is used?
  • DHCP packet flow? What port numbers involved? Which packet is Broadcast or unicast?
  • DHCP Relay agent packet flow? Commands?
  • SRX: How to enable SSH access? what commands required?
  • SRX: If HTTPS is enabled on Interface & SSH,Telnet on Zone, What access will work?
  • NAT device installed by ISP in a VPN, What port, protocol numbers need to be opened?
  • What changes are required in the Firewalls for NAT-T?
  • Screening options? Name of 3 Attacks? SYN Flood attack limit? UDP attack limit?

TechM (Chd)

  • Ph1 Configuration steps
  • Ph1 Troubleshooting
  • Traffic not reaching Destination roubleshooting
  • NAT in ScreenOS
  • VIP
  • Nat DST
  • DIP
  • Troubleshooting VPN
  • Commands in ScreenOS

Convergys (JTAC L2)

*VPN traffic is not reaching other gateway device? How to prove? 
 Ans:Take snoop simultaneusly at both sites.

* What filters to use for VPN Traffic?
 Ans: 4 filters- two for & to the gateways, two to & for the PCs

*4 VPNs between 2 devices, how to identify which packet is for which VPN?
 Ans:SPI will be unique & remains same for a single VPN

*ESP has Port no?

*IMP:What is NAT-T? Why packet will drop without NAT-T? at which packet exchange will it drop?
 Ans:At which packet level does it start using UDP port 4500?

*Scenario 1:

3 firewalls are in full mesh VPN. 2 firewalls A & B have overlapping subnets.
 Why will the ping from A to B private IPs fail?
 Will VPN come UP?
 What is the solution?
   Ans: to use MIP in Tunnel
 Ping from A to C private IP will fail?
 Ping from C to A private IP will fail?

*Scenario 2:

Client is conected to a L2 switch. Switch to Firewall. Firewall to Internet. Client does an active FTP connection to FTP server.
 How will traffic pass through firewall?
 What is the role of ALG?
 Which command of FTP is used?
 IMP:Which 2 parameters of FTP Command are useful?
    Ans: 1. PORT Command; 2._________

*Troubleshoot Latency
*Fragmentation in detail(ID Field, MF flag, Frag Offset)
*TCP WSF? is it Unidirectional or bidirectional?  Max value for WSF?   Max Window size?
*You have two PCAP files for same firewall. One on Ingress port & one for Egress port.
 How to find out latency? How to find packet drops? How to trace packets(ID Field)

ZScalar (Chd)

  • DNS? need for DNS
  • Traffic troubleshooting
  • HTTP is in which layer?
  • DNS uses protocol? UDP & TCP
  • Why not use UDP for all DNS traffic? (Ans:Huge Overhead)
  • Reverse of DNS possible?
  • ScreenOS CPU utilization check command
  • Port no of DNS, SSH, HTTP
  • SSH access is there but WebUI not opening?

2nd Level

  • 302,403,401 error HTTP
  • SSL Handshake steps - 4 phases
  • Hashing vs Encryption, examples of protocols
  • Port Numbers - 80, 443
  • Proxy Server functions
  • ALG - Active vs Passive FTP
  • Destination NAT - Proxy ARP, Server issues, Debugging
  • If the Server does not have a reverse route to reach firewall,it will drop reply packet. Workaround is to create a Source NAT rule in firewall
  • Aggressive vs Main Mode
  • Is Preshared Key, ID shared in clear text in Aggressive mode? (no, its Hashed)
  • Dynamic IP in Site, Which mode is used? (Aggressive)
  • What is a Digital Signature?

TechM (Chd)

  • SRX fxp1 interface

For SRX240B:

ge-0/0/0 interface will be mapped to fxp0 (out-of-band management) 
ge-0/0/1 interface will be mapped to fxp1 (control).

The interfaces that are mapped to fxp0 and fxp1 are device specific.

  • IPS blocked webmail. Where to get the alerts?
Analysis & Reporting > IPS > Intrusion Events
  • Upgrade process of Sourcefire Sensors:


Reboot Sensor
Select System Restore
Set IP config
Select SCP, Enter Server IP, Credentials & ISO File name
Download & Mount ISO
  • Screenos flow
  • What is Sanity Checking?
  • VPN
  • NSRP Preempt behavior
  • Proxy Server Experience


  • packt filter fw
  • stateful fw
  • scenario: reverse traffic
  • tcp 3way -flags
  • What is flow control?
  • ospf vs rip differences
  • port no of http,dns,https,etc
  • what is dns?
  • chown
  • chmod
  • Disk usage - df -h
  • System resources - top, pstree
  • SMTP Test with Telnet:
dig mx
nslookup -query=mx
telnet smtp
MAIL from: <>
RCPT to: <>
Subject: Test message
This is a test message.
  • HTTP test using telnet:

To retrieve the document as well as the headers, use GET instead of HEAD. If you want the root document, use GET / HTTP/1.1 (or HEAD / HTTP/1.1).

telnet http
HEAD /example/example.shtml HTTP/1.1
Connection: close

BT Interview

  • vip vs nat-dst diff?
Ans: Vip needs same subnet IP as interface IP
  • diff policy vpn vs route vpn?
  • encrytion at which packet during vpn Aggr /main mode?
  • diff bw main and aggr mode?
  • More secure which 1 ?
Ans: IKE ID (name) is sent in plain text in Aggressive mode & encrypted in main mode.
  • Intrazone block option?
  • Mip has no port translation
  • Route based VPN : 1) has tunnel 2) NAT Dest supported.
  • What are the IKE v2 benefits?
  • NSM Services ==> HASrv, DevSrv, GuiSrv
  • If all devices seems to be down in NSM ==> DevSrv service is down
  • Requirements for NSRP Active-Active? VSI interfaces config
  • Change timeout value of a Predefined service - Yes, Possible by editing the service.
  • Life Size, key change after 100Mb data transfer? Cisco 1 GB? Juniper?
  • Life Size is in which phase? Phase 2 only. Life time is in both Phase 1 & 2.
  • Vsys config
  • SSG 550M ==> what is meaning of M? = can be converted to Junos J-series Services Route
  • Enable ALG on a custom service? Create policy, select custom Service & select Application to select the ALG.
  • Can you create a VPN with overlapping subnets using Policy based VPN?
  • What is Rommon/Bios in ScreenOS called?
Ans: Bootloader
  • ScreenOS vs SRX
  • What is difference between 8,2 & 8.3 Cisco ASA firmware?
8.2 packet arrives - check ACL before un-natting ==> create ACL with translated IP not real
2 nats> static or GLobal
8.3 object Natting ==> instead of calling, calling object in ACL ==> ASA will do NAT first & ACL should be added for real IP.
Object or Twice NAT.
  • IPv6 is supported in ScreenOS or not?

Tech M

  • V,E,B bit in OSPF
  • LSA area 2,0,1
  • LSA Type 3 -6 routers in area 2
  • Type 3 LSA area 1-0-2 changes?
  • L2TP VPN Flow?
  • No of Junos Candidate configs?


  • VR vs VSYS
  • What are UTM Features?
  • Can you assign 4 zones to 4 VRs? What routes need to be configured?
Ans: Trust-vr to Untrust-vr default route already exists by default, need to create a Reverse route.
  • Migration plan for ScreenOS to SRX?
  • VPN Migration
  • OSPF areas, type 7
  • BGP route selection
  • BGP route reflectors
  • What is PFS?
  • What is DH protocol? How it works? WHich versions are supported in ScreenOS?
  • What are phase 1 stages
  • Tunnel interface
  • Flow in ScreenOS
  • What is ALG?

2nd level

  • What is a VPN?
  • ESP vs AH?
  • How NSRP Failover occurs?
  • What is a Null Route?
  • What is a Cookie? How is Cookie Calculated?
  • What is a Control & Data Link?
  • What is NSRP Lite?
  • NSRP Virtual MAC Calculation?
  • Proxy-ID? Which Phase?
  • What is VSI? VSD? How is VSI assigned? (eth0/0:1)
  • Can we assign IP to HA Interface?
  • What is a Management Port? Can it have User/Passthrough Data?
  • How do you create a Zone?
Select Layer 2 or 3
Select VR
  • Priority order of VIP, Source Nat, Dest NAT, Policy NAT?
  • Default NSRP Priority?
  • What is RTO?
  • What is FSTAB? min space for booting a device?
  • Functions of DevSrv?
  • NSM Distributed Architecture?
  • What is a VLAN? Duplex? Collision? Trunk?
  • What is rsync? Where is it used in NSM?
  • Difference between rsync & SCP?
Scp basically reads the source file and writes it to the destination. 
It performs a plain linear copy, locally, or over a network. Use scp for your day to day tasks.
Commands that you type once in a while on your interactive shell. 
Its simpler to use, and in those cases rsync optimizations won't help much. 
Scp has only a few switches.
Rsync also copies files locally or over a network. 
But it employs a special delta transfer algorithm and a few optimizations to make the operation a lot faster. 
Rsync has a plethora of command line options, allowing the user to fine tune its behavior. 
It supports complex filter rules, runs in batch mode, daemon mode, etc. 
For recurring tasks, like cron jobs, use rsync. 
It will take advantage of data already transferred, performing very quickly and saving on resources. 
It is an excellent tool to keep two directories synchronized over a network.
  • NSRP supported in Transparent Mode?

When operating in Transparent mode, the backup in an Active/Passive pair can only be managed from one Layer 2 zone at a time. By default, this zone is V1-Trust. If management is needed from V1-Untrust, V1-DMZ, or a custom layer 2 zone, use the command:

set interface vlan1 nsrp manage zone <zone name>

Note: This command must be enabled on both devices in the NSRP cluster. Verify:

get interface vlan1

In backup mode, only traffic from V1-TRUST can manage the box Note: Ensure that the manage-ip addresses are different for both the primary and backup firewalls.

  • NSRP States? What is Inoperable state?
  • OSPF Areas? Type 7 LSA? In backbone converts to what?
  • Difference between init 0 & init 6?


  • When Next-hop lookup take place in ScreenOS flow? After policy lookup or at Arp lookup?
  • Silly window protocol
  • URG vs PUSH Flags
  • Urgent pointer
  • ARP in ipv6
  • IKE v2
  • Active vs Passive FTP - ALG
  • Overlapping subnets without NAT
  • ospf v6
  • ipv6
  • Netscreen session analyzer
  • AH vs ESP in Wireshark
  • DHCP
  • DHCP Relay agent
  • 2 DHCP servers in a subnet/Network
  • Hub and Spoke VPN - Policies
  • Tracert in Windows vs Linux - Tracert using UDP
On Unix-like operating systems, the traceroute utility uses User Datagram Protocol (UDP) datagrams by default, with destination port numbers ranging from 33434 to 33534. 
The traceroute utility usually has an option to instead use ICMP Echo Request (type 8) packets, like the Windows tracert utility does, or to use TCP SYN packets.
  • Tracert 3 values
  • 4 way handshake- Benefits
  • ALG opens:
 Pasv  - in passive mode opens port
 Port - in active mode opens port
  • Diff bw ACL & Poloicy? ACL is at Interface level;Policy at Zone;;ACL= network/transport layer;Pol=ALG=Application layer
  • Session layer handles Dialog control.
  • How to block P2P?


  • TFTP ALG? Yes TFTP requires ALG
  • DPI = Deep Packet Inspection
  • Bastion Host : A bastion host is a special purpose computer on a network specifically designed and configured to withstand attacks. The computer generally hosts a single application, for example a proxy server, and all other services are removed or limited to reduce the threat to the computer. It is hardened in this manner primarily due to its location and purpose, which is either on the outside of the firewall or in the DMZ and usually involves access from untrusted networks or computers.

A bastion host is a computer that is fully exposed to attack. The system is on the public side of the demilitarized zone (DMZ), unprotected by a firewall or filtering router. Indeed the firewalls and routers can be considered bastion hosts. Due to their exposure a great deal of effort must be put into designing and configuring bastion hosts to minimize the chances of penetration. Other types of bastion hosts include web, mail, DNS, and FTP servers. There are two common network configurations that include bastion hosts and their placement. The first requires two firewalls, with bastion hosts sitting between the first "outside world" firewall, and an inside firewall, in a demilitarized zone (DMZ). Often smaller networks do not have multiple firewalls, so if only one firewall exists in a network, bastion hosts are commonly placed outside the firewall

These are several examples of bastion host systems/services:

   DNS (Domain Name System) server
   Email server
   FTP (File Transfer Protocol) server
   Proxy server
   VPN (Virtual Private Network) server
   Web server

Best Practices

Because bastion hosts are particularly vulnerable to attack, due to the level of required access with the outside world to make them useful, there are several best practice suggestions to follow:

   Disable or remove any unneeded services or daemons on the host.
   Disable or remove any unneeded user accounts.
   Disable or remove any unneeded network protocols.
   Configure logging and check the logs for any possible attacks.
   Run an intrusion detection system on the host.
   Patching the operating system with the latest security updates.
   Lock down user accounts as much as possible, especially root or administrator accounts.
   Close all ports that are not needed or not used.
   Use encryption for logging in to the server.

Will the firewall work without a default gateway? What if the device is in a LAN only?
What should be the position of the IDP and IPS?
What is HIPS?
Architecture of checkpoint firewall?


•	Window scale, SACK , mss, mtu , TCP off-loading complete details about each topic and troubleshooting scenarios on the same.
•	How does TCP headers looks with SACK in action ?? like does it still uses ACK feild or not? if it uses then what info does ACK and SACK feild contains?
•	Fragmentation scenarios: where in we will be asked about packet headers, after fragmentation at various hops.
•	How does ICMP path discovery works.How does it work with presence of vpn/tunnel?
•	How do you truobleshoot using wireshark packet capture?? I was shown a wireshark capture of HTTP access to various site/urls!I was asked to short-list all the urls accessed during the capture time; use filter "". likewise they might ask different filters!!
•	Troubleshooting approach on some traffic not working...

•	CISCO thinks ourteam is strong at this topic:-)!! so they ask in and out of VPNs, different troubleshooting scenarios, with NAT, how VPN works with NAT device in between n all.
•	Other than VPNs and TCP/IP they will ask questions on topic that we mention in our resume!! Unfortunately i mentioned about SNMP ;-) so they asked some basic questions: Like why is it used for? whats the diff bw v1 & V3 of SNMP?

•	complete working:messages:
•	Whats SIP re-invite ?
•	how does DHCP server recongnises different SIP phone vendors?

•	I explained basic working and diff modes i knew.I told them i dont have in depth knowledge.
•	Is it necessary to have (S,G) entry ??something like that..dont remember properly
•	Tom(ESC engineer) will ask one of best case experience!!! be prepared to answer with troubleshooting steps u followed,upto packet level details.

1.	What is alg. And also they asked about specific examples about how ALG functions. (Typically expecting FTP)
2.	Difference between active and passive FTP (In detail including the PASSV command)
3.	When there is a control channel established in FTP, and we open a data channel, do we see a three way handshake happening again on wireshark captures? Ans is yes.
4.	What is the use of ALG in case of passive FTP.

1.	Explain the procedure of SIP message exchange starting from a new phone booting up and registration? – Read from CnE given nicely along with headers. They expect headers.
2.	How does ALG help in SIP << Here they expect the function of ALG looking into the SDP headers and open pin holes based on Connection and media identifiers in SDP.
3.	Difference between DHCP discover sent by IP Phone and Computer? Ans: Options field vary in both (No reqd in detail)

Then they asked me about the most difficult case I handled.

1.	Then they discussed about how the SACK and WSF help in flow control and congestion control
2.	How can we achieve the same functionality of trace-route using IP packets assuming that trace route is blocked in the network? Ans: IP header there is option of source route which helps us in doing the same.
3.	Also asked about fast transmit, slow start and congestion avoidance along with TCP chimney and TCP offloading engine
4.	Screening all the options in detail. 
5.	How does SMTP work
6.	What is the use of reverse DNS lookup
7.	How do u ensure that the clear text traffic is getting encrypted or not, if you have captures at internal and external interface of the firewall? << By looking at size and time field in the captures>>
8.	What is protocol anomaly.
9.	DHCP relay and dhcp message exchanges. 
10.	Scenario:  PC-1 is in vlan10 and dhcp server is in vlan20. Now PC-1 boots up how will it get the IP address and flow associated with the same. << Explain the concept of dhcp relay>>

RFC round was TCP—rfc1323. And cross Qn as per the presentation.

•	How do you Troubleshoot High CPU.
•	About NSRP. (concept)
•	Types of NAT and its uses.
•	Few general questions on URL filtering.
•	How will you filter a HTTPS URL.
•	Why do we use GRE over IPSec?
•	If IPSec has encryption then why we need GRE?
•	Basic of SNMP regarding community an all

Q: + 3 web servers behind the firewall. All are accessed from the internet through MIPs.1 out of 3 web servers is not reachable. Troubleshoot
check traffic from the client to firewall through internet. OK
Communication between server and firewall. NO
Sniffers on firewall and server. OK
Firewall traffic sent but nothing on the server.
Check any devices between them?
only a switch.
narrow down the problem on the switch.
Possibly an ARP issue.
when check the switch the MAC of the server was mapped to wrong port.

Q: Remote connect VPN. When connected internet does not work.
Internet VIA local network or through VPN? Through VPN
When VPN connected only Internet is down or not able access internal resources also? Only internet.
Flow issue
check flow
correct route? Yes
then policy
correct policy? No need policy as it is between same security zone.
Is intra zone block on? It is OFF
At last it was like there is some option in CISCO when the traffic come from same interface and goes out through the same we need to set some command to allow.

Q: Site to site VPN. Site A has a Web server and we are trying to access from site B but it does not happen.
VPN UP? Yes.
Only this server or other local resources? Only this server
Check flow?
Both sides packet sent to tunnel.
Sniffer on ingress site A? Yes we see packet.
Sniffer on egress site B? NO we don’t see anything.
What do we see?
3 way handshake, http get() but data sent from source but not receive by site B.
Ans. DF bit set. How to handle this. Adjust MSS.



•	ISAKMP headers,payloads
•	If u r using other than IPSEC does ISAKMP is supported nd how?
•	Headers and how does it change with the propogation
•	TCP/ip and ethernet " " " " " " " " 
•	PC-->SWITCH--->ROUTER--->SWITCH--->PC(Changes in ethernet,arp,tcp,ip header)
•	Checksum calculation of TCP header
•	mss,mtu
•	incase of latency what do u see in wireshark.What all fields, u chk for ?
•	GRE over ipsec headers 
•	GRATITIOUS ARP and ARP header diff?
•	Scenario of assymetric routing..Ping was working but TCP connection was not working
•	How does trace route works
•	Sliding window ,Window size,scaling window factor
•	ICMP, DIFF between MSL and TTL?
•	path mtu discovery
•	Wireshark in detail
•	Scenario of FTP ALG.Control session is being formed but data not flowing

•	Attacks
•	Screening options
•	Vulnerability Tools 
•	How do u write a signature
•	Linux

Study from:
•	Basics of TCP/IP from TCP/IP guide
•	C&E:Screening options ,attack and defence mecahnism

•	TCP/IP complete
•	What is slow start,why we use it ?Is it compulsory to use slow start?
•	What is congestion control?
•	How will you troubleshoot if you have congestion in the network?
•	Components of mss?
•	Complete DNS header 
•	BGP troubleshooting
•	ISAKMP header
•	Difference between AH/ESP ?OR tunnel mode/tranport  mode?
•	Multicast 
•	Sparse mode flow
•	PIM pruning
•	Source specific multicast
•	VOIP (SIP ALG---flow etc)
•	TCP chimney
•	TCP tickle

Juniper Networks

What is TCP Sack?
DNS Doctoring
Types of firewall, with details
Tcp header
Window scaling
sliding window
ip header
IP Padding
UDP header
Diff between TCP and UDP, which is faster and why?
3 way close
TCP handshake and tear down
FTP Active/passive
Active in details
Dail up VPN with multiple networks on the firewall, requirement is to have only one policy on the NSR.
Which is secure spilt tunneling or default tunneling
Scenario:- Web access is slow, few sites working fast few are little slow:- (answer he expected was with UTM feature the browsing will be little slow and without that it will be faster)
Types of wireless and their uses. 

Ø  General introduction and enquiry about experience.
Ø  Customer calls up and says throughput it low. How will you deal?
Ø  Why will an interface go in half duplex mode.
Ø  If reboot resolves the issue, what could it be related to?
Ø  Only TCP is slow. UDP is not slow.
Ø  Examples of UDP.
Ø  What is TCP MSS and MTU?
Ø  Why do we need two settings – MTU and MSS?
Ø  Why do you want to set MSS when we can reduce MTU?
Ø  Where and when is MSS value exchanged?
Ø  How do you decide what MSS value to set?
Ø  Difference between TCP and UDP?
Ø  What is windowsize? Can we modify it on firewall?
Ø  On the firewall, can you see window size being exchanged without using Snoop/Sniffers?
Ø  What information do you see in “get session”?
Ø  What are the 2 parameters which can bring Phase 2 down?
Ø  What is phase 1 and phase 2 timeout?
Ø  What timeout do you see in get sa?
Ø  Why does the timeout between 2 Juniper firewalls show as unlimited?
Ø  What is SPI and how it is calculated?
Ø  When is SPI value exchanged?
Ø  How do you verify RTO is getting Synced on backup firewall in NSRP?
Ø  When you see a get session output, how do you verify if the traffic is DIP or MIP?
Ø  Main difference between OSPF and BGP configuration on the firewall?
Ø  Why do we need to define neighbours in BGP but not in OSPF?
Ø  In transparent mode, how is the decision making and forwarding done?
Ø  Packet flow in transparent mode? Flood and Arp/Traceroute modes?
Ø  Switch------(F/W in Transparent mode)-----Switch------Router
You have 50-100 VLANs configured. How will you make sure that the traffic would pass through the firewall?
Ø  In NSRP, both firewalls show as (M). How will you troubleshoot?
Ø  HA is directly connected, and HA is UP?
Ø  Task CPU is high. How will you troubleshoot?
Ø  In get OS task, how will you see which task is high.
Ø  You see Session Scan is high. What does that mean?

what is a session
how does it look and what are its components
how do you identify a session in a sniffer capture
what is a tcp syn check 
when do we receive a rst and fin in a tcp connection
what do you mean by disabling syn check
what is a sequence check in tcp
what is tcp mss
why is mss not used in udp
what is path mtu 
what does traffic failover happen in NSRP failover 
what is arp, what is garp
what Is mac table, what is cam table

Screening :  
1.       What are DOS Attacks
2.       Screening Options : SYN FLOOD ( Because I gave him example of this)
3.       Why it is an attack
4.       Where its configured

Multicast : Almost everything
5.       Multicast : Configuration
6.       How IGMP and PIM works
7.       Basics of Multicast
8.       Command to check mroute
9.       How IGMP proxy is configured and where, all options in that

NSRP: Explain
1.       Set nsrp ageout-ack
2.       Set nsrp rto mirror non-vsi
3.       Set nsrp vsd master always exists
4.       Default vsd group
5.       How will you check session is on master or backup : Flag, timeout
6.       How will you check if master session is on backup : Src-ip,dst-ip,src-port,dst-port

1.What is debug apppry
2. use of application proxy ( he answered to me : it checks if its http,ftp,imap,smtp,pop3 and send it to AV engine because AV scans only these traffic and not like DNS etc.)

1.ARP and G-arp
2.OSI layers
3.How does packet flow through the OSI when One PC telnets to another PC
4.How does MIP work
5.scenarios of G-ARP,where is it used
6.One tough case which you have dealt with.
7.NSRP no brain,split brain
8.How to avoid the above no brain and split brain problems
9.In NSRP what extra command is needed for backup to be manageable from internet,set flow mac cache mgt
10.what does that command do
11.If track-ip does not work,what debugs would you take
12.What will debug track-ip output look like
13.what would debug ip ip output look like
14.what would debug icmp icmp look like
15.basic connection to internet not working,how will you troubleshoot does MIP work,give a scenario.
17.In NSRP A/P.Sessions get synced,how would you show the customer it is the same session no both the firewalls,Ans—using session flags,,,they would remain d same on both
18.VPN troubleshooting
19.hub and spoke,redundant,over lapping VPN.
20.How would you go about troubleshooting all of them.
21.Why do we need hub and spoke VPN
22.How does SIP work.
23.Messages of SIP.What would debug sip all show
24.Troubleshooting SIP,I can hear voice on one side of the phone,,I am not able to hear on the other end
Ans—you need to do debug sip all and check for in the SIP message whether the port on which the data is flowing through is blocked,or does it need to be opened.
25.What is Autoconnect VPN
26.How is your approach to an issue you know and an issue you do not know

1. What should I have to have internet access from a PC 
2. What is arp 
3. What is difference between ARP and GARP
4. when is garp used .. is g arp a request or a response 
5. what is osi layer 
6. protocols in layer 4 other than tcp/udp 
7. difference in tcp and udp , in which scenarios are tcp and udp preferred. 
8. concept of convergence in UDP
9  how does mac address help in communication 
10. what is sequence number 
11. what is stateful inspection 
12. why is syn checking , why is it required on the firewall
13. data flow on the firewall
14. what is sanity check, reverse route lookup 
15. what is a session, explain get session output.. 
16. How Src natted session looklike
17. Session existing for tunnel traffic
18. Site to site VPN and ping not going through VPN how you will troubleshoot.
19. bad spi ... what is it.. 
20. what config changes are required if u have a cisco router as a peer for VPN
21. packet flow in transparent mode. 
22. Dynamic Routing.
23. OSPF and BGP difference.
24. Internal and External:- Why Internal and External
25. What is ALG? Explain ALG with Active FTP Example. What is ACtive FTP?
26. Fragmentation capture on Wireshark. How you will prove that this is fragmented is happened on firewall?
27. What is Path-MTU?
28. How to aaoid fragmentation?
29. scenario for PAT.. how is it different from NAT- DST
30. scenario for mip, its advantages. 
31. what is NSRP 
32. why is master always exist used . 
33. why is preempt used. whyis nsrp session ageout command used. 
34. WEb sense / url filtering  trouble-shooting 
35. troubleshooting AV updates 
36. if ftp is not working, what are the debugs that u will do
37. what is policy based routing... 
38. how do u troubleshoot any problem that u dont understand...

1.       What is Arp and G-Arp
2.       Layer 2 Forwarding mechanism
3.       How does firewall forward the frames in the transparent mode?
4.       How are the broadcasts processed in the transparent mode?
5.       What is the Command to check the forwarding table in transparent mode?
6.       Explain OSI layers.
7.       What is the difference between MSS and MTU?
8.       What is Path-MTU ?
9.       Difference between OSPF and BGP
10.   How does TCP three way handshake Work ?
11.   What is Fragmentation ? How will you check the Fragmentation from the captures done at Trust and Untrust side .
12.   What is difference between UDP and TCP ? How does UDP verify the data has been  lost?
13.   What is Area 0 and why so we use it ?
14.   If we have two devices in different area will they form neighborship ?
15.   How to troubleshoot an issue in which ospf neighbor is not coming up ?
16.   How does BGP work ? and If BGP is in the same As then is it required to configure the peers for establishing  the neighbors ?
17.   How does Poison reverse work in RIP ?
18.   Explain about multicasting
19.   Why do we use the command “Set nsrp master-always-exist “ ? and in what situation we use that command.
20.   What is difference between aggressive and main mode in VPN
21.   You have route based vpn in between juniper and cisco. Juniper has 3 networks behind it and cisco has 1 network. How to setup the vpn ?
22.   What is PBR and at which level does it work ?
23.   What is ALG ? Explain with an example.
24.   How does Active FTP work ?
25.   How to check the translation from the session and what does the output of the “get session “ show when you have translation.
26.   What is MIP ?
27.   What to check if AV is not working?
28.   How to check if AV is not getting updated?
29.   What is application proxy ?
30.   Why do we use “debug httpfx all” ?

1.       TCP/IP Basics & Advanced:
a.       OSI Layers- detailed understanding,
b.       Diff between TCP and UDP,
c.       Knowledge of other protocols other than TCP/UDP (atleast names like ICMP, GRE, ESP, AH etc)
d.      Flow of a packet between 2 network devices (Layer3, Layer-2)
e.      Structure of a packet (with respect to headers being added and removed while crossing every layer)
f.        TCP/IP headers (not directly asked but requires to answer the structure of a packet in detail)

2.       General:
a.       Knowledge of applications like FTP,HTTP, telnet and how they work.
b.      How ALG is being used on the firewall and why.
c.       Basic packet flow through the firewall.

3.       Switching:
a.       Basic knowledge of Layer-2 Switching and VLAN.
b.      Understanding of a Broadcast domain.
c.       Difference between Layer-2 switching and Layer-3 switching.
d.      Understanding of ARP and Gratituous ARP.
e.      Communication between 2 devices in a single broadcast domain.

4.       Layer-3:
a.       Deep understanding of routing.
b.      Complete flow between 2 layer-3 devices.
c.       Changes happen to a packet while it traverses through various layer-3 devices.
d.      How does a firewall different from other Layer-3 devices.
e.      Routing Protocols – RIP, OSPF, BGP (only if u say u know the basics but only the basics)

5.       Firewall Concepts:
a.       NAT – all types and how it works (detailed understanding and difference between each)
b.      Firewall usage (Policies, screening etc)
c.       VPN – Everything about VPN.
d.      UTM - Understanding of how firewall behaves, decides and handles the packet with respect to the UTM features

6.       Others:
a.       NSRP (complete understanding and troubleshooting)
b.      Transparent mode.
c.       Traffic Shaping
d.      VOIP (troubleshooting approach)
e.      Logical thinking and troubleshooting approach with respect to an issue.
f.        Confidence
g.       Communication.

Interview Questionnaire


  • Benefit of using Transparent mode?
  • Basic difference between IDS & IPS?
  • Phase 1,2 no of packets? encryption starts at which packet in VPN?
  • PFS
  • VPN Troubleshooting
  • Stateful Inspection
  • Aggressive mode / Main mode more secure?
  • Task/Flow CPU High
  • Reasons for High Memory?


  • Flow in Netsreen firewall
  • MIP vs VIP
  • Multi-Cell policy
  • Traffic Shapping is config where?
  • 100 VPNs terminating on the Juniper firewall? Filter
  • Pseudo session ?
  • Snoop Detail?
  • Debug flow basic
  • Precautions while debugging
  • sync NSRP devices manually
  • NSRP non-propagating parameters
  • How to avoid Split-Brain scenario?
  • How to avoid No Brain scenario?
  • Secondary path in NSRP