Intrvw

AMZN Networking

Round 1

• BGP Route manupulation

Directions?

• 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 DNS client knows when to switch to TCP?

TC Flag

Round 2

• Experience with diffcult customer
• Learnt from feedback

Windows

• 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)

Linux

• 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?

Networking

• NAT
• How does Traceroute works?
• Request & response headers

curl -v http://google.com

• Commands used in daily troubleshooting

telnet 
nc 
ipconfig
netstat
traceroute
top
ps
free
df
dh

Scenario

[PC]---------[Website]
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.

Round 4 - Architecture

• Explain case you have worked - Voip CB Repro
• Design a Ecommerse website with full redundancy, explain protocols used:

  - Avi SE scaling, use 3 node cluster for controller
  - Use 2 AZs
  - How DB will sync
  - GSLB usage - geodb Networking
  - how persistence will work
  - Storage redundancy
  - Storage slow, use SSD
  - Pics load slowly in Japan

Round 5 - Linux

• Boot process in detail
• What is kernel? How does it communicate with HW/User? System calls
• Name the various System calls
• What is initrd ? What it contains/does ? (drivers to load disk,etc)
• Runlevels
• Upstart/Systemd
• What is Swap?
• How to find swap usage?
• How to find if a file is moving from RAM to SWAP? iotop(process wise usage) & iostat(part wise usage)
• How to check processes using most CPU/How to check CPU usage per core?
• Process States:

   - Stop   (my answers)
   - Start
   - Running
   - Sleeping  ==> Interupptable waiting/ uninterruptable waiting -> Explain each
   - Killed
   - Zombie

• Type of File systems - FAT32/ExFAT, NTFS, EXT3, EXT4
• How to use a brand new disk in Linux machine

- Format
- Partition
- Mount

• What are permissions -> 640?

 6 -> user      - Read/Write
 4 -> group   - Read
 0 -> Others - None

• How to check Memory usage - top//free -m // cat /proc/meminfo

  - Buffer/Cache field?
  - Swap field?

• How to troubleshoot if a file stopped coping in between saying no disk space, but disk space is there.

 df -h => check free disk space
 df -i  => Free inodes check

• What is inode? How can they get full(mail server; large no of empty files)?

Round 6 - Networking

• Design a website for 500 Employees - use 2 AZs?
• Troubleshoot a website not reachable?

curl -I,   telnet,    nc,    ping

• How to check if DNS is working or not?

dig @42.2.2 google.com

• Check packet captures using tcpdump.
• Explain DNS resolution in Full details.
• Troubleshooting a slow website:

Top  --> CPU, Memory,  Process usages, Load avg
df -h  --> Disk usage
free -m
cat /proc/meminfo
netstat -ant
cpu ==> 0 "us" user space
top -o %CPU

• Explain TCP Window? How it can be full
• OSPF vs BGP? why not use OSPF for Internet instead of BGP?
• OSPF packet types?

   Hello
   DBD
   LSR
   LSU [LSAa]
   LSAck

• What is Area 0 in OSPF?
• BGP Attributes
• BGP getting more traffic from one ISP than other? How to balance that?
• What is DDOS attack? How to mitigate it?

Need to know about the type of attack
Syn Flood attack can be mitigate by using Syn-Cookie

• What is Syn-Cookie used for?

Round 7

• Repro Attempt
• Something not done within committed time
• Something done differently from a colleague
• Something you have done apart from regular work
• Tough Feedback received; What you did about it.

AMZN DMS

1st Round Online Challenge

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:
  [LB]------[srv1][srv2][srv3]srv4]
  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

CouchBase

• 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
------------------------

• My Solution:

f=open('text.txt','r')
l=f.readlines()

a=[]

for i in l:
if ":" in i:
a.append((i.strip('\n')).split(':'))

b=[]
for i in range(len(a)):
b.append(a[i][0])

b=set(b)

for i in b:
print(i,end='')

for i in range(len(a)):
if "startTime " in a[i]:
print(a[i][1],a[i+1][1],a[i+2][1],a[i+3][1],end='')
print('\n', end="")

Juniper

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

Dell

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

Aryaka

   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

IBM

   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

AVI

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:

                [PC]-------[SW]--------[SW]------[PC]
                [PC]-------[SW]-------[Router]----[SW]------[PC]

   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:

[PC]-----[Router]-----[Server]
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)