devutils/ATFQ/book ATFQ git

Part II · Your system

19. Network

hub

Your network interfaces, their addresses, where traffic goes, and how names become addresses.

On your machine

alice@laptopexample
$ atfq network
  docker0   down · virtual · MAC 02:42:5c:11:aa:01
            172.17.0.1/16
  enp0s31f6 down · MAC 54:e1:ad:12:34:56
  lo        unknown · loopback
            127.0.0.1/8, ::1/128
  wlp0s20f3 up · wireless · MAC 8c:c6:81:2a:10:7f
            192.168.1.23/24, fe80::8ec6:81ff:fe2a:107f/64
  Route     default via 192.168.1.1 on wlp0s20f3

An example: Alice's laptop, one of the test machines. In your terminal, this block shows your own machine, read live.

Commands

ip -br addr
each interface and its addresses, one line each
ip route
where packets go, default gateway included
ping -c 3 example.com
is a host reachable, and how fast
ss -tulpn
ports open for connections 19.1
resolvectl status
DNS servers in use, per interface 19.2
nmcli device
NetworkManager's view, Wi-Fi included
curl -I https://example.com
test an HTTPS connection, headers only

Paths

/etc/hosts
names resolved without DNS
/etc/resolv.conf
DNS servers the system resolver asks
/sys/class/net/
one folder per interface: state, MAC, speed

Cards

  1. 19.1Who listens on which portA program that accepts connections listens on a port: 22 for SSH, 443 for HTTPS.
  2. 19.2DNS resolutionPrograms ask the system resolver to turn a name into an address. It tries the sources on the hosts: line of /etc/nsswitch.conf, in order.
  3. 19.3Interfaces and routesAn interface is a network connection point: enp3s0 (cable), wlp2s0 (Wi-Fi), lo (the machine itself).
  4. 19.4nftables vs iptables vs ufwThe kernel does the filtering (netfilter). iptables, nftables, ufw and firewalld are ways to write its rules.

Also about this

  1. 6.1Network layers: OSI and TCP/IPA network is built in layers, each using the one below: link (Ethernet, Wi-Fi), network (IP), transport (TCP, UDP), application (HTTP, DNS, SSH).
  2. 6.2What happens when you open a websiteDNS: the name becomes an IP address (19.2).
  3. 6.3IP addresses, subnets and NATAn IPv4 address is 32 bits, written 192.168.1.23. /24 means the first 24 bits name the network and the rest the machine.
  4. 6.4TCP, UDP and connectionsTCP gives a reliable, ordered stream: it numbers the bytes, waits for acknowledgements and sends again what was lost.
  5. 6.5Ethernet, MAC addresses and ARPOn a local network, frames travel from MAC address to MAC address: 48-bit hardware addresses such as f4:92:bf:0e:21:7a.