TCP IP

TCP/IP: The foundations of the modern internet

sudo apt update && sudo apt install tcpdump -y

View TCP/IP traffic on an interface:

sudo tcpdump -i eth0 tcp

This shows TCP packets with source and destination addresses, ports, and flags. For IP traffic without a TCP filter:

sudo tcpdump -i eth0 ip

7. Use `tc` for traffic control based on TCP/IP principles

The `tc` (traffic control) tool in Linux works based on TCP/IP principles. The `tc ct` action communicates with the netfilter conntrack module to track connections, essential for stateful firewalling and NAT. View the current qdiscs:

tc qdisc show

Add a netem qdisc to simulate delay:

sudo tc qdisc add dev eth0 root netem delay 100ms

Remove the qdisc again:

sudo tc qdisc del dev eth0 root

8. Check checksums with `tc-csum`

`tc-csum` recalculates checksums after modifying packet data, needed to keep TCP/IP packets correct. Add a csum action to a filter:

sudo tc filter add dev eth0 parent 1: protocol ip prio 1 u32 match ip dst 192.168.1.1/32 action csum update

This recalculates the checksum for packets to 192.168.1.1. Check the filter:

tc filter show dev eth0

Verification

Check whether TCP/IP works correctly on your system:

ip addr show

Shows the IP addresses on your interfaces. Check connectivity:

ping -c 4 8.8.8.8

Shows ICMP echoes, which confirms that IP routing works. For TCP connectivity:

ss -tuln

Shows TCP and UDP listeners on your system. If these commands produce output, TCP/IP is running correctly.

Common problems

Problem: `tcpdump` shows no packets Cause: you have not selected the correct interface or lack root privileges. Solution: use `sudo` and check interfaces with `ip link show`. Select the correct interface, for example `tcpdump -i wlan0 tcp` for wifi.

Problem: `tc qdisc add` returns "RTNETLINK answers: Operation not permitted" Cause: you do not have root privileges or the interface does not support the qdisc. Solution: use `sudo` and check that the interface is up with `ip link set eth0 up`.

Problem: `tc filter add` fails with a "match" error Cause: the filter syntax is incorrect. Solution: check the IP addresses and use the correct protocol. Consult the manual with `man tc-u32` for the full syntax.

The protocol suite continues to evolve through new RFCs and better implementations, but the core — the combination of TCP and IP — has remained unchanged since the migration on ARPANET in 1983.

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