quartz/content/notes/11-DHCP.md

title	tags
11-DHCP	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