Network Configuration & Troubleshooting Flashcards

Which command tests whether TCP port 80 on host 203.0.113.5 is open without using nmap?

Answer: nc -zv 203.0.113.5 80

'nc -zv host port' attempts a TCP connection without sending data (-z) and reports the result verbosely (-v).

A Linux host with IP 192.168.10.5 needs to masquerade traffic to the internet through eth0. Which iptables command achieves this?

Answer: iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

MASQUERADE in the nat table's POSTROUTING chain dynamically replaces the source IP with the outgoing interface's address.

What does the 'ip link set eth0 mtu 9000' command do?

Answer: Configures the MTU of eth0 to 9000 bytes for jumbo frames

Setting MTU to 9000 enables jumbo frames, which can improve throughput on high-speed LAN segments that support it.

Which configuration directive in /etc/resolv.conf allows the resolver to try multiple domain suffixes when resolving a short hostname?

Answer: search

The 'search' directive specifies a list of domain suffixes appended when a hostname without dots is queried.

You observe high retransmission rates in 'ss -s' output. What does this most likely indicate?

Answer: Network congestion or packet loss causing TCP to resend unacknowledged segments

TCP retransmissions occur when acknowledgments are not received in time, typically due to packet loss or congestion.

Which file persists custom hostname-to-IP mappings on Linux and is consulted before DNS by default?

Answer: /etc/hosts

The /etc/hosts file provides static hostname-to-IP mappings and is typically checked before DNS per /etc/nsswitch.conf order.

What is the function of the 'bonding' kernel module in Linux networking?

Answer: It aggregates multiple network interfaces into a single logical interface for redundancy or throughput

Bonding (Link Aggregation) combines multiple NICs into one bond interface, supporting modes like active-backup and LACP.