Networking / Stage 12 of 14

Diagnosing between two machines

Not running Starting usually ~20 s

Step 6 of 7 · lab · 30 min

Connect two VMs over the network

The site works on node2, but a request from node1 waits and gives up. Find on which machine and at which stage it gets lost, and restore access.

Situation

The lab has two machines: node1, your terminal, and node2, a server with a site on port 80. The node2 administrator writes: "The site opens for me." Yet from node1 curl http://node2/ waits and gives up with no answer.

Run commands on node2 from node1 over SSH: ssh node2 'command'. At every login ssh prints Warning: Permanently added 'node2'… — that is how the practice pair is set up, and it does not affect the commands.

Task

From node1, ping node2 gets a reply.
From node1, curl http://node2/ gets code 200, and the site on node2 keeps working.

There is no internet in the lab — only the network between node1 and node2. Everything you need is installed on both machines.

Lab · on your own

No commands are given in this lab: type them in the terminal yourself. Goals are checked on the machine after every command.

Goals

0 of 2

node2 replies to ping from node1 — not done yet
http://node2/ returns HTTP 200 to a request from node1 — not done yet

Ctrl+Enter

Starts the machine node1 · Ubuntu 24.04

The machine is starting

Hints

0 of 3

Report a problem

Lab goals

0 of 2

node2 replies to ping from node1 — not done yet
http://node2/ returns HTTP 200 to a request from node1 — not done yet

node1 Ubuntu 24.04

Lab goals

0 of 2

node2 replies to ping from node1 — not done yet
http://node2/ returns HTTP 200 to a request from node1 — not done yet

Network · picked for this lab

ip — addresses, interfaces and routes

Cheat sheet

$_ — insert into the terminal

ip -br a addresses, brief
ip -br l interfaces, brief
ip r routing table
ip route get 8.8.8.8 path a specific packet would take
ip -s link show eth0 counters and errors
ip neigh ARP
sudo ip a add 192.168.1.10/24 dev eth0 add an address temporarily
permanent: /etc/netplan/*.yaml + sudo netplan apply

Output modifiers (-br, -s, -c, -d, -h) go right after ip and only affect how the result is printed, not which object is selected: ip -br a and ip a -br are the same. -4/-6 narrow the output to one protocol version — useful when an interface has both IPv4 and IPv6 and the extra lines get in the way.

ip objects

Object	About	Common
`ip address` (`a`)	IP addresses on interfaces	`ip -br a`
`ip link` (`l`)	the interfaces themselves (up/down, MTU, MAC)	`ip -br l`
`ip route` (`r`)	the routing table	`ip r`
`ip neigh` (`n`)	ARP/neighbors (IP↔MAC)	`ip n`
`ip -s link`	packet/error counters	per interface
`ip route get`	which route will be chosen	`ip route get IP`

Commands

bash

ip -br a                          # addresses, brief (name · state · address)
ip -c a                           # with color
ip a show eth0                    # a single interface
ip -br l                          # interface list and link state
ip -s link show eth0              # RX/TX counters, errors, drops
ip r                              # routing table
ip route get 8.8.8.8              # which route/interface a packet takes
ip neigh                          # the ARP table (neighbors)

# changes (TEMPORARY, until reboot):
sudo ip link set eth0 up          # bring the interface up
sudo ip link set eth0 mtu 1400    # change MTU
sudo ip a add 192.168.1.10/24 dev eth0    # add an address
sudo ip a del 192.168.1.10/24 dev eth0    # remove an address
sudo ip r add default via 192.168.1.1     # default gateway
sudo ip r add 10.10.0.0/16 via 192.168.1.254   # static route
sudo ip neigh flush all           # clear the ARP cache

Changes made with ip ... are temporary (until reboot). For permanent configuration on Ubuntu Server, use netplan (/etc/netplan/*.yaml, sudo netplan apply).

Address, prefix and subnet

192.0.2.10/24 is an address and a prefix: the first 24 bits (192.0.2) are the network number, the rest is the host number. Addresses with the same network number form the subnet 192.0.2.0/24: from 192.0.2.0 to 192.0.2.255, hosts get .1–.254. Neighbours in a subnet exchange packets directly; another subnet needs a route — without one ping prints Network is unreachable at once (see ping).

bash

ip -4 -br a                       # addresses with prefixes
ip r                              # subnets you have a path to
ip route get 192.0.2.1            # which way a packet will go

Network diagnostics

bash

ip -br a                          # is there an address at all
ip -br l                          # is the link up? (NO-CARRIER = cable/port)
ip route get 8.8.8.8              # is there a route out, and through what
ping -c3 192.168.1.1              # is the gateway reachable
getent hosts example.com          # does DNS work (see dns-resolv)
ss -tlnp                          # what ports listen locally

The natural ladder: address → link → route/gateway → external IP → DNS → port (see network-diagnostics).

No address (`ip -br a` empty/down)

The interface is down (sudo ip link set <if> up), or DHCP/netplan didn't run (sudo netplan apply, journalctl -u systemd-networkd).

`NO-CARRIER` in `ip -br l`

No physical link: cable/port/switch. Check the hardware with ethtool.

There's an address, but "Network is unreachable"

No default route: ip r has no default line → add a gateway (in netplan).

Two interfaces, traffic goes "the wrong way"

ip route get <IP> shows the actual choice. It's governed by route metrics and order; fix it in netplan (metric:) or with policy routing.

IP pings, names don't

That's DNS, not ip: resolvectl status, getent hosts name (see dns-resolv).

Diagnosing between two machines

Situation

Task

Signing in is needed to start the machine

ip objects

Commands

Address, prefix and subnet

Network diagnostics

No address (ip -br a empty/down)

NO-CARRIER in ip -br l

There's an address, but "Network is unreachable"

Two interfaces, traffic goes "the wrong way"

IP pings, names don't

No address (`ip -br a` empty/down)

`NO-CARRIER` in `ip -br l`