Practice Exercise: Networking Configuration and Tools

Objectives

Explore networking configuration on a Linux system.
Learn how to use various networking tools to troubleshoot and manage network connections.

Scenario

As a Linux enthusiast, understanding networking configuration and tools is crucial for managing network connections and troubleshooting network issues. This exercise will help you practice key networking concepts and tools commonly used in Linux.

Tasks

Task 1: Display ARP Cache

Use the arp command to display the ARP cache table.

Identify the MAC addresses associated with IP addresses in the cache.

[intern@intern-a1t-inf-lnx1 ~]$ arp
Address                  HWtype  HWaddress           Flags Mask            Iface
gw-li2013.linode.com     ether   00:00:0c:9f:f0:08   C                     eth0

Task 2: Trace Route to a Host

Use the traceroute command to trace the route to a remote host, e.g., traceroute linux.org.

Examine the list of hops between your system and the remote host.

[intern@intern-a1t-inf-lnx1 ~]$ traceroute linux.org
traceroute to linux.org (172.67.73.26), 30 hops max, 60 byte packets
 1  10.209.3.88 (10.209.3.88)  0.256 ms  0.241 ms  0.238 ms
 2  10.209.35.54 (10.209.35.54)  0.232 ms  0.227 ms  0.220 ms
 3  10.209.32.2 (10.209.32.2)  0.207 ms 10.209.32.1 (10.209.32.1)  3.817 ms  3.813 ms
 4  lo0-0.gw2.sin1.sg.linode.com (139.162.0.102)  0.537 ms lo0-0.gw1.sin1.sg.linode.com (139.162.0.101)  0.328 ms lo0-0.gw2.sin1.sg.linode.com (139.162.0.102)  0.300 ms
 5  ae25.r02.sin01.ien.netarch.akamai.com (23.56.138.24)  0.473 ms  0.455 ms  0.456 ms
 6  * * *
 7  93.186.133.94 (93.186.133.94)  0.716 ms cloudflare-ic-350354.ip.twelve99-cust.net (62.115.181.151)  1.270 ms 93.186.133.94 (93.186.133.94)  0.687 ms
 8  93.186.133.17 (93.186.133.17)  1.139 ms 93.186.133.93 (93.186.133.93)  1.223 ms *
 9  172.67.73.26 (172.67.73.26)  1.254 ms *  0.946 ms

Task 3: Check Listening Ports

Use the netstat or ss command to display a list of listening ports on your system.
Identify which services are listening on specific ports.

Note any suspicious or unexpected ports.

[intern@intern-a1t-inf-lnx1 ~]$ netstat -ln
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:10248         0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp6       0      0 :::37579                :::*                    LISTEN
tcp6       0      0 :::10250                :::*                    LISTEN
tcp6       0      0 :::22                   :::*                    LISTEN
udp        0      0 127.0.0.53:53           0.0.0.0:*
udp        0      0 192.168.64.2:68         0.0.0.0:*
udp6       0      0 fe80::f0d0:95ff:fe0:546 :::*
raw6       0      0 :::58                   :::*                    7
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  2      [ ACC ]     STREAM     LISTENING     145542   /run/user/1000/systemd/private
unix  2      [ ACC ]     STREAM     LISTENING     145549   /run/user/1000/bus
[intern@intern-a1t-inf-lnx1 ~]$ ss | head
Netid State    Recv-Q Send-Q                   Local Address:Port      Peer Address:Port  Process
u_dgr ESTAB    0      0                  /run/systemd/notify 17942                * 0
u_dgr ESTAB    0      0         /run/systemd/journal/dev-log 17968                * 0
u_dgr ESTAB    0      0          /run/systemd/journal/socket 17970                * 0
u_str ESTAB    0      0                                    * 402629               * 402630
u_str ESTAB    0      0                                    * 140675               * 140676
u_dgr ESTAB    0      0                                    * 19657                * 19656
u_str ESTAB    0      0                                    * 18285                * 18424
u_str ESTAB    0      0          /run/dbus/system_bus_socket 20643                * 20320
u_str ESTAB    0      0          /run/systemd/journal/stdout 20389                * 20382

Task 4: Test DNS Resolution

Use the nslookup or dig command to test DNS resolution for a domain name, e.g., nslookup linuxfoundation.org.

Observe the DNS server's response.

[intern@intern-a1t-inf-lnx1 ~]$ nslookup linuxfoundation.org
Server:     127.0.0.53
Address:    127.0.0.53#53

Non-authoritative answer:
Name:   linuxfoundation.org
Address: 3.13.31.214

[intern@intern-a1t-inf-lnx1 ~]$ dig !$
dig linuxfoundation.org

; <<>> DiG 9.11.36-RedHat-9.11.36-9.el8 <<>> linuxfoundation.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44615
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;linuxfoundation.org.       IN  A

;; ANSWER SECTION:
linuxfoundation.org.    596 IN  A   3.13.31.214

;; Query time: 0 msec
;; SERVER: 139.162.10.8#53(139.162.10.8)
;; WHEN: Tue Oct 03 05:58:45 UTC 2023
;; MSG SIZE  rcvd: 64

Task 5: Configure a Static IP Address

If you have a spare virtual machine or test system, try configuring a static IP address for one of its network interfaces.
Use the ifconfig or ip command to set the IP address, subnet mask, and gateway.
Check for your network interface

In my case I'll be updating the network interface enp0s1

[intern@intern-a1t-inf-lnx1 ~]$ ip a
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: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether f2:d0:95:0d:20:04 brd ff:ff:ff:ff:ff:ff
    inet 192.168.64.2/24 metric 100 brd 192.168.64.255 scope global dynamic enp0s1
       valid_lft 43805sec preferred_lft 43805sec
    inet6 fd76:221f:c594:440:f0d0:95ff:fe0d:2004/64 scope global dynamic mngtmpaddr noprefixroute
       valid_lft 2591899sec preferred_lft 604699sec
    inet6 fe80::f0d0:95ff:fe0d:2004/64 scope link
       valid_lft forever preferred_lft forever
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
    link/ether 02:42:d8:29:4d:a9 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever
[intern@intern-a1t-inf-lnx1 ~]$ sudo ip addr add 192.168.64.3/24 dev enp0s1
[sudo] password for intern:
[intern@intern-a1t-inf-lnx1 ~]$ sudo ip link set enp0s1 up
[intern@intern-a1t-inf-lnx1 ~]$ ip a show enp0s1
2: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether f2:d0:95:0d:20:04 brd ff:ff:ff:ff:ff:ff
    inet 192.168.64.2/24 metric 100 brd 192.168.64.255 scope global dynamic enp0s1
       valid_lft 43601sec preferred_lft 43601sec
    inet 192.168.64.3/24 scope global secondary enp0s1
       valid_lft forever preferred_lft forever
    inet6 fd76:221f:c594:440:f0d0:95ff:fe0d:2004/64 scope global dynamic mngtmpaddr noprefixroute
       valid_lft 2591914sec preferred_lft 604714sec
    inet6 fe80::f0d0:95ff:fe0d:2004/64 scope link
       valid_lft forever preferred_lft forever

Note that this changes won't persist between reboots

Conclusion

By completing these exercises, you've gained practical experience in networking configuration and troubleshooting using various Linux networking tools. These skills are essential for managing and maintaining network connections on Linux systems.