Networking on tplat

Berkeley Packet Filters

Mon, 01 Jan 0001 00:00:00 +0000

Berkeley Packet Filters

BPF is a filtering syntax used to match traffic by host, network, port, protocol, and other packet attributes.

Primitives

Primitive filter	Description
`[src\|dst] host <host>`	Matches a host as the IP source, destination, or either.
`ether [src\|dst] host <ehost>`	Matches a host as the Ethernet source, destination, or either.
`[src\|dst] net <network>`	Matches packets to or from the source and destination network, or either.
`[src\|dst] net <network> mask <netmask>` or `[src\|dst] net <network>/<len>`	Matches packets with a specific netmask.
`[src\|dst] port <port>` or `[tcp\|udp] [src\|dst] port <port>`	Matches packets sent to or from a port.
`[src\|dst] portrange <p1>-<p2>` or `[tcp\|udp] [src\|dst] portrange <p1>-<p2>`	Matches packets to or from a port range.
`less <length>`	Matches packets less than or equal to the length.
`greater <length>`	Matches packets greater than or equal to the length.
`(ether\|ip\|ip6) proto <protocol>`	Matches an Ethernet, IPv4, or IPv6 protocol.
`(ip\|ip6) protochain <protocol>`	Matches IPv4 or IPv6 packets with a protocol header in the chain, for example `ip6 protochain 6`.
`(ether\|ip) broadcast`	Matches Ethernet or IPv4 broadcasts.
`(ether\|ip\|ip6) multicast`	Matches Ethernet, IPv4, or IPv6 multicasts.
`vlan [<vlan>]`	Matches 802.1Q frames with a VLAN ID.
`mpls [<label>]`	Matches MPLS packets with a label.

Protocols and Operators

Common protocols include arp, ether, fddi, icmp, ip, ip6, link, ppp, radio, rarp, slip, tcp, tr, udp, and wlan.

Protocol Data Unit

Mon, 01 Jan 0001 00:00:00 +0000

Protocol Data Unit

A protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network.

flowchart LR

subgraph Bit
    Physical
end

subgraph Frame
    Link
end

subgraph Packet
    Network
end

subgraph Segment[Segment / Datagram]
    Transport
end

subgraph Data
    Session
    Presentation
    Application
end

Bit --> Frame --> Packet --> Segment --> Data

Network Security

Mon, 01 Jan 0001 00:00:00 +0000

Network Security

Network security focuses on protecting availability, integrity, and confidentiality in connected systems. Common attacks target bandwidth, routing, name resolution, and device access, so defenses usually combine filtering, redundancy, and monitoring.

Firewalls

A firewall controls traffic between network segments by applying a rule set to each connection attempt. It can be implemented in software, hardware, or both, and it decides whether packets are allowed, dropped, or blocked. The goal is to prevent unauthorized or unwanted connections.