What is LDAP service in Linux? LDAP (Lightweight Directory Access Protocol) is a directory service used to store and manage centralized information, such as: * Users and passwords * Groups * Computers * Email addresses * Application authentication data On Linux, LDAP is commonly used for: * Centralized user login * Authentication and authorization * Integration with applications (GitLab, Jenkins, mail servers) * Active Directory integration Think of LDAP as a central phonebook for users and systems 📒 ========================== How LDAP works (simple flow) 1. Client asks LDAP server: “Is this user valid?” 2. LDAP server checks directory 3. Server responds with yes/no and user details Common LDAP service in Linux * OpenLDAP (slapd) → most widely used LDAP server on Linux Service name: slapd LDAP Port Numbers Protocol Port Description LDAP 389/TCP Plain LDAP LDAPS 636/TCP Secure LDAP (SSL/TLS) LDAP + STARTTLS 389/TCP Encrypted over standard LDAP 📌 389 = default
📌 636 = secure (legacy LDAPS) Check LDAP port in Linux ss -tulnp | grep ldap or netstat -tulnp | grep 389 LDAP Configuration files (OpenLDAP) * Main config:



/etc/ldap/slapd.d/ * 

 * Old-style config (deprecated):



/etc/ldap/slapd.conf * 

 Interview-ready answer LDAP is a directory service used to centrally manage users, groups, and authentication. In Linux, OpenLDAP provides LDAP services. The default LDAP port is 389, and the secure LDAPS port is 636. =================== DHCP Service : Alright 👍 let’s do this step-by-step, clean and exam/real-world friendly.
I’ll show DHCP server installation + configuration on Linux, covering Ubuntu/Debian and RHEL/CentOS. What is DHCP? DHCP (Dynamic Host Configuration Protocol) automatically provides: * IP address * Subnet mask * Gateway * DNS to client machines. DHCP Server Installation & Configuration 1. Install DHCP server Ubuntu / Debian sudo apt update sudo apt install isc-dhcp-server -y RHEL / CentOS / Rocky / Alma sudo dnf install dhcp-server -y 2. Select network interface Ubuntu / Debian Edit: sudo nano /etc/default/isc-dhcp-server Set your interface name: INTERFACESv4="eth0" (Check interface name using ip a) RHEL / CentOS Interface is defined directly in config (next step). 3. Configure DHCP server Edit DHCP config file: sudo nano /etc/dhcp/dhcpd.conf Basic DHCP configuration option domain-name "example.com"; option domain-name-servers 8.8.8.8, 8.8.4.4; default-lease-time 600; max-lease-time 7200; authoritative; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.100 192.168.1.200; option routers 192.168.1.1; option subnet-mask 255.255.255.0; option broadcast-address 192.168.1.255; } 📌 Adjust IP range and gateway based on your network. 4. Assign static IP (reservation) – optional host client1 { hardware ethernet 00:11:22:33:44:55; fixed-address 192.168.1.50; } 5. Start and enable DHCP service Ubuntu / Debian sudo systemctl restart isc-dhcp-server sudo systemctl enable isc-dhcp-server RHEL / CentOS sudo systemctl restart dhcpd sudo systemctl enable dhcpd 6. Check DHCP service status systemctl status isc-dhcp-server or systemctl status dhcpd 7. Allow DHCP in firewall Ubuntu (UFW) sudo ufw allow 67/udp sudo ufw reload RHEL / CentOS (firewalld) sudo firewall-cmd --add-service=dhcp --permanent sudo firewall-cmd --reload 8. Verify DHCP leases cat /var/lib/dhcp/dhcpd.leases 9. Test from client machine On a client: dhclient Check IP: ip a Common Problems & Fixes ❌ DHCP failed to start * Wrong interface name * Subnet mismatch ✔ Fix: ip a Ensure subnet matches interface network. ❌ Clients not getting IP * Firewall blocking port 67 * Wrong gateway IP Interview-friendly summary * DHCP package: isc-dhcp-server / dhcp-server * Config file: /etc/dhcp/dhcpd.conf * Port: 67/UDP * Lease file: /var/lib/dhcp/dhcpd.leases * Command to test: dhclient If you want next: * DHCP relay agent * DHCP failover * PXE boot using DHCP * Real-time troubleshooting scenarios Just tell me 😄 Get smarter responses, upload files and images, and more. Log in Sign up for free ============================== Linux Interview questions: =============================== On RHEL / CentOS / Rocky / AlmaLinux yum install bind bind-utils -y ✅ On Ubuntu / Debian apt update apt install bind9 bind9utils -y 🔹 Start and enable DNS service systemctl start named # RHEL/CentOS systemctl enable named systemctl start bind9 # Ubuntu systemctl enable bind9 🔹 Check DNS service status systemctl status named or systemctl status bind9 🔹 Main configuration files File Purpose /etc/named.conf Main DNS config (RHEL) /etc/bind/named.conf Main DNS config (Ubuntu) /var/named/ Zone files /etc/resolv.conf DNS resolver config 🔹 Open DNS port (important) DNS uses port 53 (TCP & UDP) firewall-cmd --add-service=dns --permanent firewall-cmd --reload 🔹 Test DNS nslookup google.com or dig google.com ⭐ One-line interview answer DNS service in Linux is installed using BIND, configured via named.conf, started with systemctl, and listens on port 53. ⭐ Very short interview version Install BIND, configure named.conf, start the named service, and open port 53. If you want next, I can explain: * DNS forward zone & reverse zone * Sample zone file * DNS troubleshooting interview questions * Difference between DNS client vs server Just tell me 👍 =================================================================

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