Local IP Services Topic Notes

DNS

DNS Overview

• Domain Name System
• Method of resolving host-names to IP addresses
• IOS supports the following DNS features:
  • DNS Authoritative Server
  • DNS Client
  • DNS Caching
  • DNS Spoofing
• Traffic sent over port UDP 53
• Some traffic replied sent over port TCP 53

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DNS Server

• Responds to DNS queries
• Hosts DNS mappings locally
  • (config)#ip host <name> <IP address>
• Can forward on requests
  • Command:
    • (config)#ip name-server <dns IP>
• Enabled globally
  •  Command:
    • (config)#ip dns server

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DNS Client

• Enabled by default
• Sets DNS queries to broadcast if no DNS server configured
• To configure a DNS server
  • Command:
    • (config)#ip name-server <dns IP>
• To disable DNS client capability
  • (config)#no ip domain-lookup

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DNS Proxy

• Can proxy request from a client that local DNS server isn't able to answer
• Can cache request locally
• Command:
  • (config)#ip name-server <dns IP>
• Can also proxy other protocols
  • Command:
    • (config)#ip forward-protocol <tcp | udp> <protocol>

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DHCP

DHCP Overview

• Dynamic Host Configuration Protocol
• Method of dynamically assigning IP addresses to hosts
• IOS Supports the following DHCP functions:
  • DHCP Server
    • Assigns IP addresses
    • Keeps database of address bindings
  • DHCP Client
    • Requests address allocation for interface from a DHCP server
  • DHCP Relay
    • Relays DHCP requests from clients to DHCP servers
  • DHCP Proxy
    • Requests IP addresses from servers on a hosts behalf
    • PPPoE address requests
    • IPIC request into DHCP
• Traffic sent to the DHCP Server is sent over port UDP 67
• Traffic sent from the DHCP Server to the client is sent over port UDP 68

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DORA Process

• 1. Discover
  • Broadcast from client to server
  • Locates a DHCP server
• 2. Offer
  • Unicast from server to client
  • Offers configuration parameters such as
    • IP address
    • Default Gateway
    • Domain Name
    • Lease time
    • Other DHCP options
• 3. Request
  • Broadcast from client to server
  • Sends a formal request for configuration parameters offered by DHCP server
• 4. Acknowledge
  • Unicast from server to client
  • Confirms information has been allocated to the client

• If the client receives multiple offers from different DHCP servers, it will accept the first offer it receives
• If configuration parameters missing from DHCP Offer message
  • Client will send a DHCP DECLINE broadcast message to server
  • Server will reply with a DHCP NAK broadcast message confirming configuration hasn't been allocated to client

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DHCP Server

•  Configure DHCP server using pools
  • Each pool is selected based on the following:
    • DHCP Client ID (Windows clients only)
    • Hardware address (MAC) is missing Client ID
    • Relaying gateway IP address
    • Interface receiving reqest
• Each pool has a separate configuration parameters such as subnet address
• Commands:
  • (config)#ip dhcp pool <dhcp-pool-name>
  • (dhcp-config)#network <subnet> <mask>
  • (dhcp-config)#default-router <gateway IP>
  • (dhcp-config)#dns-server <dns server IP>
  • (dhcp-config)#domain-name <domain-name>
  • (dhcp-config)#lease <days> <hours> <minutes>
• Need to exclude the local gateway address from the pool of addresses
  • Command:
    • (config)#ip dhcp-excluded-address <start address> <end address>

• Can assign individual addresses through DHCP based on Client ID
  • Client Command:
    • (config-if)#ip address dhcp client-id <interface>
  • Server Commands:
    • (config)#ip dhcp pool <pool-name>
    • (dhcp-config)#client-identifier <hardware address>
    • (dhcp-config)#host <host address> <CIDR>

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DHCP Client

•  Used to request an address for an interface from a DHCP server
• Can also set the Client ID manually in the request
  • Used by the server to determine different hosts
• Command:
  • (config-if)#ip address dhcp [client-id <client-id>]

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DHCP Relay

•  DHCP broadcasts can be converted to unicasts and relayed across an IP network
• Uses DHCP Helper function of IOS
• Configured on client incoming broadcast interface
• When broadcast is relayed
  • Relaying IOS device adds the "giaddr" field or gateway interface address
• Command:
  • (config-if)#ip helper-address <DHCP server IP>

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DHCP Troubleshooting Commands

• show ip dhcp bindings - Shows DHCP bindings database on DHCP server

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Network Time Protocol

Simple Network Management Protocol

SNMP v1

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SNMP v2c

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SNMP v3

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SYSLOG

Router IP Traffic Export

Embedded Event Manager

Remote Monitoring

Web Cache Communication Protocol (WCCP)

Trivial File Transfer Protocol

Secure Copy Protocol

HTTP and HTTP Access

Telnet and SSH Access