CDPA 網管訓練 駭客任務 2 Ethernet Switching ARP, IP, LAN, Subnet IP Header, Routing ICMP

今日目標 複習 & 補充 Ethernet Switching Ethernet Switching ARP ARP了解 IP Header IP Header Gateway (Routing) Gateway (Routing) Subnet Subnet ICMP Echo ICMP Echo工具 Nemesis Nemesis Tcpdump Tcpdump

Ethernet switching Ethernet HUB

Ethernet Switching Switch 123 A B C12 3 A -> C

Ethernet Switching Switch 123 A B C1A2 3C C -> A

Ethernet Switching Switch 123 A B C1A2 3C A -> C

ARP 00:00:00:00:00:0100:00:00:00:00: I want to communicate with Who has tell (00:00:00:00:00:01)

ARP 00:00:00:00:00:0100:00:00:00:00: I (00:00:00:00:00:02) have to (00:00:00:00:00:01)

Let’s try~~ 2 people a group Try to use computer A ping computer B. Try to use computer A ping computer B. Use computer C and the other two to listen. Use computer C and the other two to listen. Tool: tcpdump sudo tcpdump –I eth0 icmp or arp sudo tcpdump –I eth0 icmp or arp

IP Header

Version IPv4 -> value 4 IPv4 -> value 4 Internet Header Length (IHL) Internet Header Length (IHL) 4-bit Internet Header Length (IHL) telling the number of 32-bit words in the header. 4-bit Internet Header Length (IHL) telling the number of 32-bit words in the header. Value = Header Length (byte) / 4 Value = Header Length (byte) / 4

IP Header Type of Service In RFC 791, the following 8 bits were allocated to a Type of Service (ToS) field: In RFC 791, the following 8 bits were allocated to a Type of Service (ToS) field:RFC 791RFC 791 bits 0-2: precedence bit 3: 0 = Normal Delay, 1 = Low Delay bit 4: 0 = Normal Throughput, 1 = High Throughput bit 5: 0 = Normal Reliability, 1 = High Reliability bits 6-7: Reserved for future use

IP Header Total Length This field defines the entire datagram size, including header and data, in bytes. The minimum-length datagram is 20 bytes (20 bytes header + 0 bytes data) and the maximum is 65,535 — the maximum value of a 16-bit word. This field defines the entire datagram size, including header and data, in bytes. The minimum-length datagram is 20 bytes (20 bytes header + 0 bytes data) and the maximum is 65,535 — the maximum value of a 16-bit word.Identification Fragments of original datagram. Fragments of original datagram.

IP Header Flags (3 bits) Reserved, must be zero Reserved, must be zero Don't Fragment (DF) Don't Fragment (DF) More Fragments (MF) More Fragments (MF) Fragment Offset The offset of data from original datagram. The offset of data from original datagram. Time To Live (TTL) Time To Live (TTL) The maxim hop (router) datagram travels. The maxim hop (router) datagram travels.

IP Header Protocol TCP TCP UDP UDP ICMP ICMP Header Checksum Source, Destination Address Options

Subnet, netmask, CIDR CIDR Classless Inter-Domain Routing Classless Inter-Domain RoutingNetmask The continues bit length from most significant bit to least significant bit. The continues bit length from most significant bit to least significant bit. Used to determine the network id (LAN size). Used to determine the network id (LAN size).

subnet example IP Mask IP: IP: MASK: MASK: Network ID: Network ID: Host ID: Host ID: Same Meaning: /24 Same Meaning: /24 Same network ID: LAN LAN No need router. No need router.

Router When do we need router? Destination IP is not in LAN. Destination IP is not in LAN. ((Local IP) & (Netmask)) != ((Dst IP) & (Netmask)) Why do we need router? Destination host is not in LAN, can’t access it by Layer 2(Datalink Layer, MAC Address). Destination host is not in LAN, can’t access it by Layer 2(Datalink Layer, MAC Address). How do we use router? Simply use router’s MAC Address as Frame’s Destination MAC Address. Simply use router’s MAC Address as Frame’s Destination MAC Address.

Router How does router work? Use Destination IP Address to decide where the packet should go. Use Destination IP Address to decide where the packet should go.

Let’s try it~~~ Routing table netstat –rn netstat –rn Become a router sudo sysctl net.ipv4.conf.all.forwarding=1 sudo sysctl net.ipv4.conf.all.forwarding=1 Add new IP to interface sudo ifconfig eth0 inet add xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx sudo ifconfig eth0 inet add xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx Modify the interface IP sudo ifconfig eth0 inet xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx sudo ifconfig eth0 inet xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx Add a default gateway sudo route add default gw xxx.xxx.xxx.xxx sudo route add default gw xxx.xxx.xxx.xxx

ICMP Echo {request, reply} Type and Code must be set to 0. The Identifier and Sequence Number can be used by the client to manage which Echo Requests are associated with the Echo Replies. The Data received by the Echo Request must be entirely included in the Echo Reply.

Final exercise Use nemesis to send ICMP Echo Request To LAN To LAN To WAN To WAN Must use following args: Must use following args: -d –H –M, –S –D, -i -c -s -e Use tcpdump to listen ICMP Echo Reply. sudo tcpdump –i eth0 icmp sudo tcpdump –i eth0 icmp

Thanks for listening Any Question??