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Linux & Shell Scripting · Question Set

Networking & Package Management – Linux & Shell Scripting interview questions

Networking & Package Management interview questions for placements and exams.

Subject: Linux & Shell ScriptingDifficulty: Mixed17 questions in this set

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Linux & Shell Scripting – Networking & Package Management questions (17)

Go through each question and its explanation. Use this set as a focused practice pack for Linux & Shell Scripting.

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1. What does systemctl enable do?

Difficulty: MediumType: MCQTopic: Service Management

Systemctl enable creates symbolic links for a service unit file, configuring it to start automatically at system boot. This doesn't start the service immediately - it only sets it to auto-start on future boots. Use systemctl start to start immediately, or systemctl enable --now to both enable and start in one command. Systemctl is the command-line interface to systemd, the init system used by most modern Linux distributions. Common commands: systemctl start service (start service), systemctl stop service (stop service), systemctl restart service (restart service), systemctl reload service (reload config without restart), systemctl status service (show status and recent logs), systemctl disable service (disable auto-start). Systemctl list-units --type=service shows all services. Systemctl daemon-reload reloads systemd configuration after editing unit files. Systemctl is-enabled service checks if service is set to auto-start. Understanding systemctl is fundamental for managing services in modern Linux systems, controlling web servers, databases, monitoring agents, and custom applications.

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6. Explain firewall configuration in Linux using iptables and firewalld. How do you allow or block network traffic?

Difficulty: HardType: SubjectiveTopic: Firewall Tools

Iptables is the traditional Linux firewall managing packet filtering rules. Rules organized in chains (INPUT for incoming, OUTPUT for outgoing, FORWARD for routed packets) within tables (filter, nat, mangle). View rules with iptables -L -n -v. Rules processed sequentially - first matching rule applies, so order matters. Basic iptables rules: iptables -A INPUT -p tcp --dport 22 -j ACCEPT allows SSH, iptables -A INPUT -p tcp --dport 80 -j ACCEPT allows HTTP, iptables -A INPUT -s 192.168.1.0/24 -j ACCEPT allows traffic from specific subnet, iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT allows established connections. Set default policy: iptables -P INPUT DROP drops all not explicitly allowed. Persist iptables rules with iptables-save > /etc/iptables/rules.v4 and restore with iptables-restore < /etc/iptables/rules.v4. Install iptables-persistent package on Debian/Ubuntu for automatic persistence. Delete rule: iptables -D INPUT rule_number. Insert rule at specific position: iptables -I INPUT 1 rule inserts at top. Firewalld (default on RHEL/CentOS 7+) provides dynamic firewall management with zones concept. List zones: firewall-cmd --get-zones. Default zone: firewall-cmd --get-default-zone. Add service: firewall-cmd --add-service=http --permanent (permanent flag persists across reboots). Add port: firewall-cmd --add-port=8080/tcp --permanent. Reload: firewall-cmd --reload applies changes. Firewalld zones: public (default for public interfaces, limited access), internal (trusted internal networks), dmz (DMZ with limited external access), trusted (all traffic allowed). Rich rules provide advanced control: firewall-cmd --add-rich-rule='rule family=ipv4 source address=1.2.3.4 reject' --permanent. Understanding firewall management is essential for securing Linux servers against unauthorized access.

7. Explain DNS configuration in Linux including /etc/hosts, /etc/resolv.conf, and DNS troubleshooting commands.

Difficulty: MediumType: SubjectiveTopic: DNS Config

The /etc/hosts file provides static hostname-to-IP mappings checked before DNS queries. Format: IP_address hostname [aliases]. Example: 127.0.0.1 localhost, 192.168.1.10 server.local server. Changes take effect immediately. Use hosts file for local development, overriding DNS for specific hosts, or improving resolution speed for frequently accessed hosts. The /etc/resolv.conf file configures DNS servers for system-wide name resolution. Format: nameserver IP lists DNS servers (queries in order listed), search domain sets search domains for short hostnames, domain sets local domain. Example: nameserver 8.8.8.8, nameserver 8.8.4.4 uses Google DNS. Modern systems often have resolv.conf managed by systemd-resolved or NetworkManager - check for symlinks. Systemd-resolved manages DNS in modern systems. Configuration in /etc/systemd/resolved.conf. Check status: systemd-resolve --status or resolvectl status. Set DNS servers: resolvectl dns interface 8.8.8.8. Flush cache: resolvectl flush-caches. Resolution order: /etc/hosts → systemd-resolved cache → configured DNS servers → /etc/nsswitch.conf defines this order. DNS troubleshooting tools: nslookup domain shows DNS query result and server used. Dig (domain information groper) provides detailed DNS query information: dig domain shows A record, dig domain MX shows mail servers, dig @8.8.8.8 domain queries specific DNS server, dig +trace domain shows full DNS resolution path from root servers. Host command: host domain simpler output than dig. Common DNS issues: wrong DNS server (check resolv.conf), DNS server not responding (test with dig @server), caching issues (flush cache), local hosts file overriding DNS (check /etc/hosts), NetworkManager overwriting resolv.conf (make immutable with chattr +i or configure NetworkManager). Understanding DNS configuration is crucial for network connectivity and troubleshooting name resolution issues.

8. Compare different methods for transferring files between Linux systems: scp, rsync, and sftp. When should you use each?

Difficulty: MediumType: SubjectiveTopic: File Transfer

SCP (Secure Copy) uses SSH protocol for encrypted file transfers. Basic usage: scp source user@host:destination copies to remote, scp user@host:source destination copies from remote. Copy directories: scp -r directory/ user@host:path. Preserve timestamps and permissions: scp -p. Simple and secure but copies entire files even if destination has similar file - no delta transfer optimization. Rsync is more efficient, only transferring changed portions of files (delta transfer). Basic usage: rsync -avz source/ user@host:destination. Flags: -a (archive mode preserving permissions, timestamps, symlinks), -v (verbose), -z (compression), -P (progress and partial transfers). Rsync over SSH: rsync -avz -e ssh. Exclude files: rsync --exclude='*.tmp'. Dry run: rsync -n tests without changes. Rsync advantages: resume interrupted transfers, only transfer changed data (efficient for large files or slow connections), delete files in destination not in source (--delete), bandwidth limiting (--bw-limit), preserve hard links, ACLs, and extended attributes. Use rsync for backups, syncing large directories, or when source and destination have similar content. Rsync is ideal for incremental backups and mirroring. SFTP (SSH File Transfer Protocol) provides interactive file transfer over SSH with commands similar to FTP. Connect: sftp user@host. Commands: ls (remote list), lls (local list), cd (remote change dir), lcd (local change dir), put file (upload), get file (download), mkdir, rm. Batch mode: sftp -b batchfile user@host processes commands from file. Good for interactive transfers or when GUI-like interface needed. Choose SCP for simple one-time transfers of small files, rsync for efficient transfers of large directories or when files might change (backups, syncing), and SFTP for interactive browsing and transferring. All use SSH for security. For very large files, consider rsync with compression (--compress-level=9) and progress monitoring. Understanding these tools enables efficient secure file transfers in DevOps workflows.

9. Describe a systematic approach to troubleshooting network connectivity issues. Include commands and diagnostic steps.

Difficulty: HardType: SubjectiveTopic: Network Diagnostics

Start with basic connectivity: ping 127.0.0.1 tests loopback interface ensuring TCP/IP stack works. Ping default gateway (ip route | grep default shows gateway) tests local network connectivity. Ping external IP like 8.8.8.8 (Google DNS) tests internet connectivity without DNS. If this works but domain names don't, the issue is DNS. Check interface configuration: ip addr shows IP addresses and interface status (UP/DOWN), ip link shows physical layer status. Verify correct IP address, subnet mask, and interface is UP. Check route table with ip route or route -n ensuring default gateway exists. Incorrect IP configuration or missing gateway prevents communication beyond local network. DNS troubleshooting: cat /etc/resolv.conf shows configured DNS servers. Test DNS with nslookup domain or dig domain. If resolution fails, try different DNS server: nslookup domain 8.8.8.8. Check /etc/hosts for local hostname overrides. NetworkManager or systemd-resolved might manage DNS configuration - check their status. Port and service testing: telnet host port or nc -zv host port tests specific port connectivity. Check if service is listening with netstat -tuln or ss -tuln. Verify firewall rules with iptables -L -n or firewall-cmd --list-all. Check if remote firewall blocks connection. Use tcpdump or wireshark for packet capture: tcpdump -i eth0 port 80 captures HTTP traffic for detailed analysis. Common issues and solutions: no IP address (DHCP problem - check dhclient or NetworkManager), wrong subnet (verify netmask), no default gateway (check routing), DNS failure (check resolv.conf and DNS server), firewall blocking (check iptables/firewalld), cable unplugged (check ip link for interface state), service not listening (check netstat and service status). Document findings and solutions for future reference.

11. Explain SSH key-based authentication and security best practices for SSH configuration. How do you set up and secure SSH access?

Difficulty: HardType: SubjectiveTopic: SSH Security

Generate SSH key pair with ssh-keygen -t rsa -b 4096 -C 'comment' creating private key (~/.ssh/id_rsa) and public key (~/.ssh/id_rsa.pub). Never share private key. Use ed25519 keys for better security with smaller key size: ssh-keygen -t ed25519. Protect private key with passphrase for additional security layer. Copy public key to remote server with ssh-copy-id user@host or manually append public key to ~/.ssh/authorized_keys on remote server with correct permissions (700 for .ssh directory, 600 for authorized_keys file). Test key authentication: ssh user@host should login without password. If prompted for password, check permissions and sshd_config settings. Secure SSH configuration in /etc/ssh/sshd_config: disable root login (PermitRootLogin no), disable password authentication (PasswordAuthentication no) after setting up keys, change default port (Port 2222) for security through obscurity, allow specific users (AllowUsers user1 user2), use Protocol 2 only, set LoginGraceTime 30 to prevent connection hanging attacks, limit authentication attempts (MaxAuthTries 3). Advanced security: use fail2ban to block brute force attempts by banning IPs with multiple failed logins, configure two-factor authentication with Google Authenticator PAM module, use SSH certificates instead of keys for large deployments, implement jump hosts/bastion servers for accessing internal networks, restrict SSH access by IP with firewall rules or TCPWrappers (/etc/hosts.allow, /etc/hosts.deny). Monitoring and auditing: monitor /var/log/auth.log for failed login attempts and suspicious activity, set up alerts for successful root logins or logins from unexpected IPs, regularly audit authorized_keys files for unauthorized entries, rotate SSH keys periodically, and review SSH configuration with tools like ssh-audit. Apply updates promptly to patch security vulnerabilities. Understanding SSH security is critical for protecting server access.

12. Explain advanced package management including repository configuration, package dependencies, and resolving package conflicts.

Difficulty: HardType: SubjectiveTopic: Package Management

Repository configuration for Debian/Ubuntu in /etc/apt/sources.list and /etc/apt/sources.list.d/*.list. Format: deb http://repo.url/ubuntu focal main restricted adds repository for Ubuntu 20.04 (focal). Components: main (officially supported), restricted (proprietary drivers), universe (community maintained), multiverse (restricted by copyright). Add PPA (Personal Package Archive): add-apt-repository ppa:user/repo adds third-party repository. RHEL/CentOS repository configuration in /etc/yum.repos.d/*.repo. Format: [repo-id] name=Repository Name, baseurl=http://repo.url, enabled=1, gpgcheck=1, gpgkey=http://repo.url/RPM-GPG-KEY. Add EPEL (Extra Packages for Enterprise Linux): yum install epel-release adds additional packages not in official repos. Import GPG keys: rpm --import keyfile verifies package authenticity. Dependency management: apt/yum automatically resolve dependencies installing required packages. Check dependencies: apt-cache depends package or yum deplist package. Reverse dependencies: apt-cache rdepends package shows packages depending on this one. Broken dependencies: apt --fix-broken install or yum check resolves issues. Hold packages from upgrade: apt-mark hold package or yum versionlock add package. Package conflicts occur when packages provide same files or have incompatible dependencies. Resolve by: identifying conflicting packages (dpkg -S filename shows which package owns file), removing one conflicting package, using alternatives system (update-alternatives --config program selects default), or building from source if packaged versions conflict. Force installation with dpkg --force-overwrite or rpm --force (use cautiously - can break system). Best practices: keep systems updated (security patches), enable automatic security updates, test updates on non-production first, use stable repositories for production (avoid testing/unstable), document custom repositories and packages, backup before major upgrades, monitor package changelogs for breaking changes, and clean package cache periodically (apt clean, yum clean all). Understanding package management prevents dependency hell and maintains stable systems.

13. Explain systemd service management including creating custom service units, service dependencies, and troubleshooting service failures.

Difficulty: HardType: SubjectiveTopic: Service Management

Systemd service units defined in /etc/systemd/system/ (custom) or /lib/systemd/system/ (packaged). Unit file format has [Unit], [Service], and [Install] sections. Example: [Unit] Description=My Service, After=network.target ensures service starts after network. [Service] Type=simple, User=appuser, ExecStart=/path/to/binary, Restart=on-failure. [Install] WantedBy=multi-user.target enables with systemctl enable. Service types: simple (default, main process specified by ExecStart), forking (service forks background process), oneshot (process expected to exit, good for scripts), notify (service sends notification when ready), idle (delays execution until other jobs finish). Choose type based on application behavior. ExecStartPre and ExecStartPost run commands before/after main process. Dependencies and ordering: Wants= suggests dependencies (service starts even if dependency fails), Requires= enforces dependencies (service fails if dependency fails), After=/Before= controls ordering without enforcing dependencies. Example: After=postgresql.service Requires=postgresql.service ensures database starts first and is required. Conflicts= prevents services from running simultaneously. Troubleshooting: systemctl status service shows current state and recent logs. Journalctl -u service shows full logs, journalctl -u service -f follows logs live, journalctl -u service --since "1 hour ago" filters by time. Check service file syntax: systemd-analyze verify service.service. View dependencies: systemctl list-dependencies service. Test configuration: systemctl daemon-reload reloads after editing units. Common issues: service fails immediately (check ExecStart path and permissions), service times out starting (increase TimeoutStartSec or fix slow startup), service restarts repeatedly (check Restart= setting and logs for crash cause), service doesn't start at boot (check WantedBy= and systemctl enable status), environment variables not set (use Environment= or EnvironmentFile=). Create minimal test service to isolate issues. Understanding systemd is crucial for managing services in modern Linux environments.

14. How do you ensure package security in Linux? Discuss package verification, security updates, and vulnerability scanning.

Difficulty: MediumType: SubjectiveTopic: Package Management

Package verification uses GPG signatures to ensure packages haven't been tampered with and come from trusted sources. APT checks signatures automatically if repository provides Release.gpg file. Import repository keys: apt-key add keyfile (legacy) or place in /etc/apt/trusted.gpg.d/. For YUM/DNF, gpgcheck=1 in repo configuration enables verification, keys imported with rpm --import. Verify individual packages: rpm -K package.rpm checks signature. Security updates are critical for patching vulnerabilities. Enable automatic security updates: install unattended-upgrades on Debian/Ubuntu, configure /etc/apt/apt.conf.d/50unattended-upgrades to only install security updates. For RHEL/CentOS, yum-cron or dnf-automatic handle automatic updates. Configure notification on updates. Test updates in staging before production, but don't delay security patches excessively. Monitor security advisories: subscribe to distribution security mailing lists (ubuntu-security-announce, centos-announce), check vendor security pages, or use automated tools. Apt-get changelog package shows recent changes including security fixes. Yum updateinfo list security shows available security updates. USN (Ubuntu Security Notices) and RHSA (Red Hat Security Advisories) provide detailed vulnerability information. Vulnerability scanning: use tools like Lynis (system auditing), OpenSCAP (security compliance), or commercial scanners like Nessus. Check installed packages for known vulnerabilities: apt-cache policy shows installed versions, compare against CVE databases. Debian Security Tracker and Red Hat CVE database track vulnerabilities. Debsecan on Debian identifies packages with security issues. Best practices: minimize installed packages (smaller attack surface), keep systems updated (automate security patches), verify package sources (only use trusted repositories), audit installed packages regularly (remove unused), use configuration management to maintain consistent secure state, monitor security advisories proactively, and scan for vulnerabilities periodically. Consider container security scanning for containerized applications. Understanding package security prevents compromised packages from affecting systems.

17. Explain advanced network diagnostic tools including traceroute, mtr, tcpdump, and netcat. Provide use cases for each.

Difficulty: HardType: SubjectiveTopic: Network Diagnostics

Traceroute maps the network path to a destination showing each hop (router) along the way. Basic usage: traceroute hostname. Output shows hop number, hostname/IP, and three round-trip times. High latency at specific hop indicates bottleneck. Asterisks (*) indicate no response (router filtering ICMP). Use traceroute to identify where network delays or failures occur, troubleshoot routing issues, or understand network topology. MTR (My Traceroute) combines ping and traceroute functionality, continuously monitoring network path and providing statistics. Usage: mtr hostname displays real-time updating view. Shows loss percentage, sent/received packet counts, and latency statistics (best, average, worst, standard deviation) for each hop. More useful than traceroute for identifying intermittent issues or patterns. Use mtr to diagnose packet loss locations, identify unstable network paths, or monitor connection quality over time. Tcpdump captures and analyzes network packets at low level. Basic usage: tcpdump -i interface captures on specific interface, tcpdump port 80 captures HTTP traffic, tcpdump host 1.2.3.4 captures traffic to/from specific host, tcpdump -w file.pcap saves to file for later analysis. Filters: tcpdump 'tcp and port 443' captures HTTPS. Advanced: tcpdump -A shows packet contents in ASCII. Use tcpdump to debug protocol-level issues, verify firewall rules, analyze network problems, or capture traffic for security analysis. Netcat (nc) is networking Swiss Army knife for reading/writing network connections. Uses: nc -l -p port listens on port (server mode), nc host port connects to host:port (client mode). Test port connectivity: nc -zv host port scans port. Transfer files: nc -l -p 1234 > file (receiver), nc host 1234 < file (sender). Debug services by manually sending protocol commands: nc smtp_server 25 to interact with mail server. Create simple TCP/UDP clients and servers for testing. Combining tools: use ping for basic connectivity, traceroute to identify path issues, mtr for ongoing monitoring and statistics, tcpdump for detailed protocol analysis, and netcat for testing specific ports or protocols. Each tool provides different perspective on network behavior. Understanding these tools enables diagnosing complex network issues efficiently.

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Networking & Package Management – Linux & Shell Scripting interview questions

Set overview

Linux & Shell Scripting – Networking & Package Management questions (17)

1. What does systemctl enable do?

Continue your preparation

2. What package manager does RHEL/CentOS use?

3. What is curl primarily used for?

4. What does netstat -tuln show?

5. What does the ping command do?

6. Explain firewall configuration in Linux using iptables and firewalld. How do you allow or block network traffic?

7. Explain DNS configuration in Linux including /etc/hosts, /etc/resolv.conf, and DNS troubleshooting commands.

8. Compare different methods for transferring files between Linux systems: scp, rsync, and sftp. When should you use each?

9. Describe a systematic approach to troubleshooting network connectivity issues. Include commands and diagnostic steps.

10. Which command updates package lists on Debian/Ubuntu systems?

11. Explain SSH key-based authentication and security best practices for SSH configuration. How do you set up and secure SSH access?

12. Explain advanced package management including repository configuration, package dependencies, and resolving package conflicts.

13. Explain systemd service management including creating custom service units, service dependencies, and troubleshooting service failures.

14. How do you ensure package security in Linux? Discuss package verification, security updates, and vulnerability scanning.

15. Which command displays network interface configuration in Linux?

16. What is SSH used for in Linux?

17. Explain advanced network diagnostic tools including traceroute, mtr, tcpdump, and netcat. Provide use cases for each.

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Networking & Package Management – Linux & Shell Scripting interview questions

Set overview

1. What does systemctl enable do?

Continue your preparation

2. What package manager does RHEL/CentOS use?

3. What is curl primarily used for?

4. What does netstat -tuln show?

5. What does the ping command do?

6. Explain firewall configuration in Linux using iptables and firewalld. How do you allow or block network traffic?

7. Explain DNS configuration in Linux including /etc/hosts, /etc/resolv.conf, and DNS troubleshooting commands.

8. Compare different methods for transferring files between Linux systems: scp, rsync, and sftp. When should you use each?

9. Describe a systematic approach to troubleshooting network connectivity issues. Include commands and diagnostic steps.

10. Which command updates package lists on Debian/Ubuntu systems?

11. Explain SSH key-based authentication and security best practices for SSH configuration. How do you set up and secure SSH access?

12. Explain advanced package management including repository configuration, package dependencies, and resolving package conflicts.

13. Explain systemd service management including creating custom service units, service dependencies, and troubleshooting service failures.

14. How do you ensure package security in Linux? Discuss package verification, security updates, and vulnerability scanning.

15. Which command displays network interface configuration in Linux?

16. What is SSH used for in Linux?

17. Explain advanced network diagnostic tools including traceroute, mtr, tcpdump, and netcat. Provide use cases for each.

More sets in Linux & Shell Scripting

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