Networking Fundamentals: TCP/IP and DNS

Networking questions appear in 100% of DevOps/SRE interviews. Master these concepts and you'll handle anything they throw at you.

TCP/IP Fundamentals

The TCP Three-Way Handshake

Client                     Server
   |                          |
   |-------- SYN ------------>|
   |                          |
   |<------- SYN-ACK ---------|
   |                          |
   |-------- ACK ------------>|
   |                          |
   |  Connection Established  |

Interview question: "What happens if the final ACK is lost?"

Answer: The server retransmits SYN-ACK until it receives ACK or times out. The connection will eventually be established or fail after retries.

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	Best effort
Ordering	Ordered	No ordering
Speed	Slower (overhead)	Faster
Use cases	HTTP, SSH, databases	DNS, video, gaming

Key TCP States

# View TCP connections and states
netstat -an | grep tcp
ss -tan

# Common states:
# LISTEN      - Waiting for connections
# ESTABLISHED - Active connection
# TIME_WAIT   - Waiting after close (2x MSL)
# CLOSE_WAIT  - Received FIN, waiting for app
# FIN_WAIT_1/2 - Sent FIN, waiting for ACK/FIN

Interview question: "You see thousands of TIME_WAIT connections. Is this a problem?"

Answer: TIME_WAIT is normal—it prevents old packets from being confused with new connections. However, too many can exhaust ephemeral ports. Solutions:

• Enable tcp_tw_reuse for outbound connections
• Increase ephemeral port range: net.ipv4.ip_local_port_range
• Use connection pooling

DNS Deep Dive

DNS Resolution Process

1. Browser checks cache
2. OS checks /etc/hosts, then nsswitch.conf
3. Query local resolver (from /etc/resolv.conf)
4. Resolver checks cache
5. If not cached, resolver queries:
   Root servers → TLD servers → Authoritative servers
6. Answer cached and returned

DNS Record Types

Record	Purpose	Example
A	IPv4 address	example.com → 93.184.216.34
AAAA	IPv6 address	example.com → 2606:2800:...
CNAME	Alias	www → example.com
MX	Mail server	@ → mail.example.com
TXT	Text data	SPF, DKIM, verification
NS	Name server	@ → ns1.example.com
PTR	Reverse lookup	IP → hostname
SRV	Service location	_http._tcp → ...

DNS Troubleshooting

# Basic lookup
dig example.com

# Query specific record type
dig example.com MX

# Query specific DNS server
dig @8.8.8.8 example.com

# Trace the full resolution path
dig +trace example.com

# Check TTL
dig example.com | grep -E "^example.com"

# Reverse lookup
dig -x 93.184.216.34

Load Balancing Concepts

Layer 4 vs Layer 7

Aspect	Layer 4 (TCP/UDP)	Layer 7 (HTTP)
Speed	Faster	Slower
Intelligence	IP/Port only	Content-aware
SSL termination	No	Yes
Sticky sessions	Source IP hash	Cookies
Use case	High throughput	Web apps

Load Balancing Algorithms

Algorithm	Behavior	Best For
Round Robin	Rotate through servers	Equal capacity servers
Least Connections	Send to least busy	Varying request times
IP Hash	Same client → same server	Session persistence
Weighted	Prefer higher capacity	Mixed server specs
Random	Random selection	Simple, even distribution

Essential Network Commands

# Check connectivity
ping -c 4 host
traceroute host  # or tracepath

# DNS lookup
nslookup host
host domain
dig domain

# Port scanning and connectivity
nc -zv host port
telnet host port

# Network statistics
netstat -tulpn  # listening ports
ss -tulpn       # faster alternative

# Packet capture
tcpdump -i eth0 port 80
tcpdump -i any host 10.0.0.1

# Network interfaces
ip addr
ip route

Interview Practice

Q: "A service can't connect to the database. How do you troubleshoot?"

# 1. Check if DNS resolves
dig db.internal

# 2. Check if host is reachable
ping db.internal

# 3. Check if port is open
nc -zv db.internal 5432

# 4. Check local firewall
iptables -L -n
# or firewalld
firewall-cmd --list-all

# 5. Check if service is listening on target
ssh db.internal "ss -tlnp | grep 5432"

# 6. Check for network path issues
traceroute db.internal
mtr db.internal

Next, we'll put it all together with real troubleshooting scenarios. :::