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

[!INFO] RARP given mac address it finds the ip address. the opposite of ARP

[!INFO] ARP we need arp because we need the data link layer protocol. so we need the destination mac address. this allows us to send ethernet frame to a MAC address

[!INFO] BOOTP

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

[!INFO] DHCP more widely used. more compatible and generalised gives not only address. also tell you what is DNS server and gateway. ubiquitious radvd and slacc similar to DHCPv6 send broadcast message to network. the server takes these broadcast messages and returns and IP address, and the lease time of the address. broadcaster sends another messages to accept/reject the offer. this sequence of messages has a session ID

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

[!INFO]

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

S.D. with DNS-SD

• Service (SRV) records specify service type, transport protocol, and the domain.
• _smtp._tcp.domain returns Priority, Weight, Port and Address of mail servers, for browsing.
  • DNS Service Discovery (DNS-SD) allows for browsing service instances.
  • Most commonly used with mDNS.
• Not suitable for very dynamic data in traditional DNS. Why?

DNS-SD Example

• Examples taken from draft DNS-SD standard.
• What services are available on dns-sd.org? (provides a discovery starting point) dig +short -t any _services._dns-sd._udp.dnssd.org _ftp._tcp.dns-sd.org. _ssh._tcp.dns-sd.org. (and others...)
• What FTP services are available on dnssd.org? dig +short -t any _ftp._tcp.dnssd.org Apple\032QuickTime\032Files._ftp._tcp.d ns-sd.org. (and others...)