Exercise: Map the network

You have just been handed servera and serverb. Before you change anything, find out how they are connected: their addresses, the network they share, the gateway, their neighbours, and what is listening.

Map the network

Read the address, prefix, gateway and DNS of a host.
Test reachability and trace a path.
Find the listening services of a host.

Addresses and interfaces

On servera, list all interfaces in brief form, with their MAC addresses. Which interface carries the address, and what is its prefix length?

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ ip -br addr
lo               UNKNOWN        127.0.0.1/8 ::1/128
enp5s0           UP             172.25.250.10/24 fe80::216:3eff:fe88:153c/64
[student@servera ~]$ ip -br link
lo               UNKNOWN        00:00:00:00:00:00 <LOOPBACK,UP,LOWER_UP>
enp5s0           UP             00:16:3e:88:15:3c <BROADCAST,MULTICAST,UP,LOWER_UP>

enp5s0 carries 172.25.250.10/24: prefix 24, which is the netmask 255.255.255.0. The interface name and MAC will differ on your lab.

Routes

Show the routing table. Which address is the default gateway, and which network is directly connected?

[student@servera ~]$ ip route
default via 172.25.250.254 dev enp5s0 proto dhcp src 172.25.250.10 metric 100
172.25.250.0/24 dev enp5s0 proto kernel scope link src 172.25.250.10 metric 100

The gateway is 172.25.250.254. The directly connected network is 172.25.250.0/24.

Reach the neighbours

Ping serverb and the gateway twice each. Then list the neighbours that servera now knows.

[student@servera ~]$ ping -c 2 serverb | tail -2
2 packets transmitted, 2 received, 0% packet loss, time 1006ms
rtt min/avg/max/mdev = 0.535/0.713/0.891/0.178 ms
[student@servera ~]$ ping -c 2 172.25.250.254 | tail -2
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 0.279/0.290/0.301/0.011 ms
[student@servera ~]$ ip neigh
172.25.250.254 dev enp5s0 lladdr 00:16:3e:42:55:32 REACHABLE
172.25.250.11 dev enp5s0 lladdr 00:16:3e:04:a5:a7 REACHABLE

Trace the path

Use tracepath -n to the gateway. How many hops are there?

[student@servera ~]$ tracepath -n -m 3 172.25.250.254
 1?: [LOCALHOST]                      pmtu 1500
 1:  172.25.250.254                                        0.301ms reached
     Resume: pmtu 1500 hops 1 back 1

One hop: the gateway is directly on servera's network.

What is listening?

Run sudo ss -tulpn. Which programs are listening, on which ports, and on which addresses? Which of them can be reached from another machine?

[student@servera ~]$ sudo ss -tulpn
Netid State  Recv-Q Send-Q Local Address:Port Peer Address:Port Process
udp   UNCONN 0      0          127.0.0.1:323       0.0.0.0:*   users:(("chronyd",pid=465,fd=5))
tcp   LISTEN 0      128          0.0.0.0:22        0.0.0.0:*   users:(("sshd",pid=641,fd=7))
tcp   LISTEN 0      128             [::]:22           [::]:*   users:(("sshd",pid=641,fd=8))

sshd listens on port 22 on all addresses, so it can be reached from other machines. chronyd's UDP port 323 is bound to 127.0.0.1 only: it is for local control, not for the network. You may also see other services, such as cockpit on 9090, depending on the image.

Compare with serverb

Run the same ip -br addr and ip route on serverb (from workstation: the servers have no ssh key for each other). What is the same, and what differs?

[student@servera ~]$ exit
[student@workstation ~]$ ssh student@serverb 'ip -br addr show enp5s0; ip route | head -1'
enp5s0           UP             172.25.250.11/24 fe80::216:3eff:fe04:a5a7/64
default via 172.25.250.254 dev enp5s0 proto dhcp src 172.25.250.11 metric 100

Same network and gateway; different host part of the address (.11) and a different MAC address.