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109 lines
4.6 KiB
Markdown
109 lines
4.6 KiB
Markdown
---
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title: "11-DHCP"
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tags:
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- lecture
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- cosc301
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---
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Address Assignment
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- First there was Reverse ARP (RARP)
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- Only IP address, no options for DNS etc.
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- Then the Bootstrap Protocol (BOOTP)
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- Included concept of tagged options
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- Useful for software like X-Terminals,
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- Allow direct network booting
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- Like RARP, constrained to single subnet, but perform at UDP/IP, instead of MAC layer
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DHCP
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- Dynamic Host Configuration Protocol (DHCP)
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- Compatible with BOOTP (uses same ports)
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- Can efficiently use a shared pool of addresses
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- IP addresses can be tied to MAC address, or client IP name
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- Ubiquitous (at least in the IPv4 sense!)
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- Limited to a single subnet, but routers can incorporate relay agents
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- Successor: DHCPv6, less useful in IPv6
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Static vs. Dynamic
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- Static address is assigned manually by system admin in the DHCP configuration file using the client’s MAC address
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- Server machines’ addresses should be static
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- Dynamically allocated addresses have a lease time period before they are re-assigned
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- Dynamically allocated address can be assigned to the same client as it keeps a table of past IP addresses and their clients MAC addresses.
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- Layer 3 (IP) change breaks existing connections if IP addresses change in a long session such as downloaders, terminal sessions
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How DHCP Works
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- Client broadcasts UDP request to 255.255.255.255 port 67
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- Routers can relay using a relay agent
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- The first DHCP server may send an offering
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- Corollary: there should usually only be one DHCP server in a subnet (broadcast zone)
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- Offer can contain many types of options.
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- Netmask, default router, DNS server, lease time, etc.
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- https://tools.ietf.org/html/rfc2132
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DHCP client/server source code
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- https://github.com/samueldotj/dhcp-client
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- https://www.oryxembedded.com/doc/dhcp__server_8c_sourc e.html
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- https://github.com/isc-projects/dhcp
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Manual Assignment vs. DHCP
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- Of course, you could do it manually. Why?
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- PRO Protection against multiple/rogue DHCP servers!
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- CON Change management
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- PRO / CON: Better control, but bigger problems when mistakes are made, e.g., IP address clash
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- DHCP is still fairly reliable.
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- Short outages are generally unnoticeable
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Ad-hoc Assignment
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- ZeroConf (Zero Configuration Networking) Link Local Addresses
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- 169.254.0.0/16
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- Single subnet, no routing (no internet)
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- Get an address by selecting an address and testing for duplicates
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- Useful for ad-hoc networks, and unconfigured network devices
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- Similar to the principle of IPv6 link local addresses
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Service Offerings
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- Network configuration
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- Current offerings like DHCP are suitable
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- Service location and configuration
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- Current offerings/support not enough
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- E.g. name resolution service for ad-hoc network
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- How to find a web proxy or email gateway in an institute?
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- Should services should be resolved or searched based on Physical Location? Network Location? User? Class? Device?
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Ad-hoc Name Resolution
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- IP traditionally lacks this
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- Proprietary LAN-based protocols have supported it for years
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- AppleTalk, NetBIOS
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- Useful when infrastructure has no local knowledge (e.g. no DNS)
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- In IP world: Multicast DNS (mDNS)
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Multicast DNS
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- mDNS queries are the same as DNS, except queries are targeted at 224.0.0.251:5353
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- Queries are made under ‘.local’, so ‘.local’ should never be used for traditional DNS
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- <device_name>.local.
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- Client resolvers must recognise .local queries
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- Special treatment: not upstream DNS
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- Also Microsoft’s LLMNR (Link-Local Multicast Name Resolution)
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Service Discovery
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- Locate the services we need automatically, or by browsing, searching/filtering or provisioning
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- Indistinct services all behave the same way and can be assigned automatically
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- Distinct services provide different behavior and need to browsed, searched, or provisioned
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- Need to consider network context
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- Operates within a specified network scope
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- Many protocols were proposed for service discovery
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- DNS-SD in Zero Configuration Networking, DHCP options, SLP, Directory Services like LDAP
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Well known service names
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- http://wpad.domain/wpad.dat for Web proxy auto-configuration
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- Other common names include smtp (or mail), pop3, imap, ftp, www, ns1, ns2, time
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- These should be aliases, so they can be redirected to other machines easily
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- Most useful for human-based configuration
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S.D. with DHCP
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- DHCP has various, diverse options: Syslog, DNS, LPR, WINS, NTP, LDAP (!)
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- Think carefully about security
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- Requires client support, in DHCP client or application
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- You can provision based on the machine or subnet, or a single group
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