Ciro Santilli @cirosantilli 35

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Linux networking HOWTO by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Computer network by scale by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Andrew Wiles by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Video 1.

Beauty Is Suffering

. Source.

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At sign by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Double quotes by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Local area network by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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iproute2 by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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tcpdump by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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To test it, let's get two computers on the same local area network, e.g. connected to Wi-Fi on the same home modem router.

On computer B:

find computer IP with the ip CLI tool. Suppose it is 192.168.1.102
then run Ciro's nc HTTP test server

On computer A, run on terminal 1:

sudo tcpdump ip src 192.168.1.102 or dst 192.168.1.102

Then on terminal 2 make a test request:

curl 192.168.1.102:8000

Output on terminal 1:

17:14:22.017001 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [S], seq 2563867413, win 64240, options [mss 1460,sackOK,TS val 303966323 ecr 0,nop,wscale 7], length 0
17:14:22.073957 IP 192.168.1.102.8000 > ciro-p14s.55798: Flags [S.], seq 1371418143, ack 2563867414, win 65160, options [mss 1460,sackOK,TS val 171832817 ecr 303966323,nop,wscale 7], length 0
17:14:22.074002 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [.], ack 1, win 502, options [nop,nop,TS val 303966380 ecr 171832817], length 0
17:14:22.074195 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [P.], seq 1:82, ack 1, win 502, options [nop,nop,TS val 303966380 ecr 171832817], length 81
17:14:22.076710 IP 192.168.1.102.8000 > ciro-p14s.55798: Flags [P.], seq 1:80, ack 1, win 510, options [nop,nop,TS val 171832821 ecr 303966380], length 79
17:14:22.076710 IP 192.168.1.102.8000 > ciro-p14s.55798: Flags [.], ack 82, win 510, options [nop,nop,TS val 171832821 ecr 303966380], length 0
17:14:22.076727 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [.], ack 80, win 502, options [nop,nop,TS val 303966383 ecr 171832821], length 0
17:14:22.077006 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [F.], seq 82, ack 80, win 502, options [nop,nop,TS val 303966383 ecr 171832821], length 0
17:14:22.077564 IP 192.168.1.102.8000 > ciro-p14s.55798: Flags [F.], seq 80, ack 82, win 510, options [nop,nop,TS val 171832821 ecr 303966380], length 0
17:14:22.077578 IP ciro-p14s.55798 > 192.168.1.102.8000: Flags [.], ack 81, win 502, options [nop,nop,TS val 303966384 ecr 171832821], length 0
17:14:22.079429 IP 192.168.1.102.8000 > ciro-p14s.55798: Flags [.], ack 83, win 510, options [nop,nop,TS val 171832824 ecr 303966383], length 0

TODO understand them all! Possibly correlate with Wireshark, or use -A option to dump content.

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Wireshark by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Amazing tool that captures packets and disassembles them. Allows you to click an interactive tree that represents Ethernet, TCP/IP and application layer like HTTP.

Start capture immediately from CLI, capture packets to/from 192.168.1.102:

sudo wireshark -f 'host 192.168.1.102' -k

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Connect to other computer in LAN by hostname by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Connect to other computer in LAN by hostname with DHCP by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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unix.stackexchange.com/questions/16890/how-to-make-a-machine-accessible-from-the-lan-using-its-hostname

TODO didn't manage to get it working with TP Link ARCHER VR2800 even though it shows DHCP as enabled and it also shows MAC addresses and corresponding hostnames in the router management interface.

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Connect two computers directly without a router with an Ethernet cable by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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For IP-level communication, askubuntu.com/questions/22835/how-to-network-two-ubuntu-computers-using-ethernet-without-a-router/116680#116680 just worked between P51 and P14s both on Ubuntu 23.10 connected with a regular Cat 5e cable.

On both machines, first we found the Ethernet cable interface name with the ip CLI tool:

ip a

which outputs on the P41s:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp1s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether fc:5c:ee:24:fb:b4 brd ff:ff:ff:ff:ff:ff
3: wlp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 04:7b:cb:cc:1b:10 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.123/24 brd 192.168.1.255 scope global dynamic noprefixroute wlp2s0
       valid_lft 61284sec preferred_lft 61284sec
    inet6 fe80::3597:15d8:74ff:e112/64 scope link noprefixroute
       valid_lft forever preferred_lft forever

so the interface was enp1s0f0, because wlp is wireless and lo is localhost.

So on the P14s we assign an IP of 10.0.0.10 to the P51:

sudo ip address add 10.0.0.10/24 dev enp1s0f0

Then on the P51 analogously, giving IP of 10.0.0.20 to the P14s:

sudo ip address add 10.0.0.20/24 dev enp0s31f6

And after that, P14s can:

ping 10.0.0.10

and P51 can:

ping 10.0.0.20

TODO after a few seconds, the settings appear to be forgotten, and ping stops working unless you do sudo ip address add on the local machine again. This seems to happen after a popup appears saying "Activation of network connection failed" as it fails to obtain Internet from the cable.

TODO list and delete such manual assignments we've made.

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Data link layer by