---
title: "11-DHCP"
tags: 
- lecture
- cosc301
---

Address Assignment 
- First there was Reverse ARP (RARP) 
	- Only IP address, no options for DNS etc. 
- Then the Bootstrap Protocol (BOOTP) 
	- Included concept of tagged options 
	- Useful for software like X-Terminals, 
	- Allow direct network booting 
	- Like RARP, constrained to single subnet, but perform at UDP/IP, instead of MAC layer

DHCP 
- Dynamic Host Configuration Protocol (DHCP) 
- Compatible with BOOTP (uses same ports) 
- Can efficiently use a shared pool of addresses 
- IP addresses can be tied to MAC address, or client IP name 
- Ubiquitous (at least in the IPv4 sense!) 
- Limited to a single subnet, but routers can incorporate relay agents 
- Successor: DHCPv6, less useful in IPv6

Static vs. Dynamic 
- Static address is assigned manually by system admin in the DHCP configuration file using the client’s MAC address 
	- Server machines’ addresses should be static 
- Dynamically allocated addresses have a lease time period before they are re-assigned 
	- Dynamically allocated address can be assigned to the same client as it keeps a table of past IP addresses and their clients MAC addresses. 
	- Layer 3 (IP) change breaks existing connections if IP addresses change in a long session such as downloaders, terminal sessions

How DHCP Works 
- Client broadcasts UDP request to 255.255.255.255 port 67 
	- Routers can relay using a relay agent 
- The first DHCP server may send an offering 
	- Corollary: there should usually only be one DHCP server in a subnet (broadcast zone) 
- Offer can contain many types of options. 
	- Netmask, default router, DNS server, lease time, etc. 
	- https://tools.ietf.org/html/rfc2132

DHCP client/server source code 
- https://github.com/samueldotj/dhcp-client 
- https://www.oryxembedded.com/doc/dhcp__server_8c_sourc e.html 
- https://github.com/isc-projects/dhcp

Manual Assignment vs. DHCP 
- Of course, you could do it manually. Why? 
	- PRO Protection against multiple/rogue DHCP servers! 
	- CON Change management 
	- PRO / CON: Better control, but bigger problems when mistakes are made, e.g., IP address clash 
- DHCP is still fairly reliable. 
	- Short outages are generally unnoticeable

Ad-hoc Assignment 
- ZeroConf (Zero Configuration Networking) Link Local Addresses 
	- 169.254.0.0/16 
	- Single subnet, no routing (no internet) 
- Get an address by selecting an address and testing for duplicates 
- Useful for ad-hoc networks, and unconfigured network devices 
- Similar to the principle of IPv6 link local addresses

Service Offerings 
- Network configuration 
	- Current offerings like DHCP are suitable 
- Service location and configuration 
	- Current offerings/support not enough 
- E.g. name resolution service for ad-hoc network 
	- How to find a web proxy or email gateway in an institute? 
	- Should services should be resolved or searched based on Physical Location? Network Location? User? Class? Device?

Ad-hoc Name Resolution 
- IP traditionally lacks this 
	- Proprietary LAN-based protocols have supported it for years 
	- AppleTalk, NetBIOS 
- Useful when infrastructure has no local knowledge (e.g. no DNS) 
- In IP world: Multicast DNS (mDNS)

Multicast DNS 
- mDNS queries are the same as DNS, except queries are targeted at 224.0.0.251:5353 
	- Queries are made under ‘.local’, so ‘.local’ should never be used for traditional DNS 
		- <device_name>.local. 
	- Client resolvers must recognise .local queries 
		- Special treatment: not upstream DNS 
- Also Microsoft’s LLMNR (Link-Local Multicast Name Resolution)


Service Discovery 
- Locate the services we need automatically, or by browsing, searching/filtering or provisioning 
	- Indistinct services all behave the same way and can be assigned automatically 
	- Distinct services provide different behavior and need to browsed, searched, or provisioned 
- Need to consider network context 
	- Operates within a specified network scope 
- Many protocols were proposed for service discovery 
	- DNS-SD in Zero Configuration Networking, DHCP options, SLP, Directory Services like LDAP

Well known service names 
- http://wpad.domain/wpad.dat for Web proxy auto-configuration 
- Other common names include smtp (or mail), pop3, imap, ftp, www, ns1, ns2, time 
	- These should be aliases, so they can be redirected to other machines easily 
- Most useful for human-based configuration

S.D. with DHCP 
- DHCP has various, diverse options: Syslog, DNS, LPR, WINS, NTP, LDAP (!) 
	- Think carefully about security 
- Requires client support, in DHCP client or application 
- You can provision based on the machine or subnet, or a single group