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content/notes/11-DHCP.md
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- cosc301 - cosc301
--- ---
> [!DANGER] NEED TO STUDY
Address Assignment Address Assignment
- First there was Reverse ARP (RARP) - First there was Reverse ARP (RARP)
- Only IP address, no options for DNS etc. - Only IP address, no options for DNS etc.

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---
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
> [!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] RFC
> docs of all protocols
> [!INFO]
> static more stable
> dynamic susceptible to man in the middle attacks, from the gateway.
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...)