vault backup: 2023-04-05 10:52:35

content/notes/11-DHCP.md

@@ -5,6 +5,8 @@ tags:
 - cosc301
 ---
 
+> [!DANGER]  NEED TO STUDY
+
 Address Assignment 
 - First there was Reverse ARP (RARP) 
 	- Only IP address, no options for DNS etc. 

content/notes/11-DHCP.sync-conflict-20230404-164926-OX6NNIH.md

@@ -0,0 +1,142 @@
+---
+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...)