A Linux Sysadmin’s Guide to Network Management, Troubleshooting and Debugging

Network Configuration, Troubleshooting and Debugging Tools

1. ifconfig Command

ifconfig is a command line interface tool for network interface configuration and also used to initialize an interfaces at system boot time. Once a server is up and running, it can be used to assign an IP Address to an interface and enable or disable the interface on demand.

It is also used to view the status IP Address, Hardware / MAC address, as well as MTU (Maximum Transmission Unit) size of the currently active interfaces. ifconfig is thus useful for debugging or performing system tuning.

Here is an example to display status of all active network interfaces.

$ ifconfig

enp1s0 Link encap:Ethernet HWaddr 28:d2:44:eb:bd:98

inet addr:192.168.0.103 Bcast:192.168.0.255 Mask:255.255.255.0

inet6 addr: fe80::8f0c:7825:8057:5eec/64 Scope:Link

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:169854 errors:0 dropped:0 overruns:0 frame:0

TX packets:125995 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:1000

RX bytes:174146270 (174.1 MB) TX bytes:21062129 (21.0 MB)

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

inet6 addr: ::1/128 Scope:Host

UP LOOPBACK RUNNING MTU:65536 Metric:1

RX packets:15793 errors:0 dropped:0 overruns:0 frame:0

TX packets:15793 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:1

RX bytes:2898946 (2.8 MB) TX bytes:2898946 (2.8 MB)

To list all interfaces which are currently available, whether up or down, use the -a flag.

$ ifconfig -a

To assign an IP address to an interface, use the following command.

$ sudo ifconfig eth0 192.168.56.5 netmask 255.255.255.0

To activate an network interface, type.

$ sudo ifconfig up eth0

To deactivate or shut down an network interface, type.

$ sudo ifconfig down eth0

Note: Although ifconfig is a great tool, it is now obsolete (deprecated), its replacement is ip command which is explained below.

2. IP Command

ip command is another useful command line utility for displaying and manipulating routing, network devices, interfaces. It is a replacement for ifconfig and many other networking commands. The following command will show the IP address and other information about an network interface.

$ ip addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1

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: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000

link/ether 28:d2:44:eb:bd:98 brd ff:ff:ff:ff:ff:ff

inet 192.168.0.103/24 brd 192.168.0.255 scope global dynamic enp1s0

valid_lft 5772sec preferred_lft 5772sec

inet6 fe80::8f0c:7825:8057:5eec/64 scope link

valid_lft forever preferred_lft forever

3: wlp2s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000

link/ether 38:b1:db:7c:78:c7 brd ff:ff:ff:ff:ff:ff

...

To temporarily assign IP Address to a specific network interface (eth0), type.

$ sudo ip addr add 192.168.56.1 dev eth0

To remove an assigned IP address from an network interface (eth0), type.

$ sudo ip addr del 192.168.56.15/24 dev eth0

To show the current neighbour table in kernel, type.

$ ip neigh

192.168.0.1 dev enp1s0 lladdr 10:fe:ed:3d:f3:82 REACHABLE

3. ifup, ifdown, and ifquery command

ifup command actives a network interface, making it available to transfer and receive data.

$ sudo ifup eth0

ifdown command disables a network interface, keeping it in a state where it cannot transfer or receive data.

$ sudo ifdown eth0

ifquery command used to parse the network interface configuration, enabling you to receive answers to query about how it is currently configured.

$ sudo ifquery eth0

4. Ethtool Command

ethtool is a command line utility for querying and modifying network interface controller parameters and device drivers. The example below shows the usage of ethtool and a command to view the parameters for the network interface.

$ sudo ethtool enp0s3

Settings for enp0s3:

Supported ports: [ TP ]

Supported link modes: 10baseT/Half 10baseT/Full

100baseT/Half 100baseT/Full

1000baseT/Full

Supported pause frame use: No

Supports auto-negotiation: Yes

Advertised link modes: 10baseT/Half 10baseT/Full

100baseT/Half 100baseT/Full

1000baseT/Full

Advertised pause frame use: No

Advertised auto-negotiation: Yes

Speed: 1000Mb/s

Duplex: Full

Port: Twisted Pair

PHYAD: 0

Transceiver: internal

Auto-negotiation: on

MDI-X: off (auto)

Supports Wake-on: umbg

Wake-on: d

Current message level: 0x00000007 (7)

drv probe link

Link detected: yes

5. Ping Command

ping (Packet INternet Groper) is a utility normally used for testing connectivity between two systems on a network (Local Area Network (LAN) or Wide Area Network (WAN)). It use ICMP (Internet Control Message Protocol) to communicate to nodes on a network.

To test connectivity to another node, simply provide its IP or host name, for example.

$ ping 192.168.0.103

PING 192.168.0.103 (192.168.0.103) 56(84) bytes of data.

64 bytes from 192.168.0.103: icmp_seq=1 ttl=64 time=0.191 ms

64 bytes from 192.168.0.103: icmp_seq=2 ttl=64 time=0.156 ms

64 bytes from 192.168.0.103: icmp_seq=3 ttl=64 time=0.179 ms

64 bytes from 192.168.0.103: icmp_seq=4 ttl=64 time=0.182 ms

64 bytes from 192.168.0.103: icmp_seq=5 ttl=64 time=0.207 ms

64 bytes from 192.168.0.103: icmp_seq=6 ttl=64 time=0.157 ms

--- 192.168.0.103 ping statistics ---

6 packets transmitted, 6 received, 0% packet loss, time 5099ms

rtt min/avg/max/mdev = 0.156/0.178/0.207/0.023 ms

You can also tell ping to exit after a specified number of ECHO_REQUEST packets, using the -c flag as shown.

$ ping -c 4 192.168.0.103

PING 192.168.0.103 (192.168.0.103) 56(84) bytes of data.

64 bytes from 192.168.0.103: icmp_seq=1 ttl=64 time=1.09 ms

64 bytes from 192.168.0.103: icmp_seq=2 ttl=64 time=0.157 ms

64 bytes from 192.168.0.103: icmp_seq=3 ttl=64 time=0.163 ms

64 bytes from 192.168.0.103: icmp_seq=4 ttl=64 time=0.190 ms

--- 192.168.0.103 ping statistics ---

4 packets transmitted, 4 received, 0% packet loss, time 3029ms

rtt min/avg/max/mdev = 0.157/0.402/1.098/0.402 ms

6. Traceroute Command

Traceroute is a command line utility for tracing the full path from your local system to another network system. It prints number of hops (router IP’s) in that path you travel to reach the end server. It is an easy-to-use network troubleshooting utility after ping command.

In this example, we are tracing the route packets take from the local system to one of Google’s servers with IP address 216.58.204.46.

$ traceroute 216.58.204.46

traceroute to 216.58.204.46 (216.58.204.46), 30 hops max, 60 byte packets

1 gateway (192.168.0.1) 0.487 ms 0.277 ms 0.269 ms

2 5.5.5.215 (5.5.5.215) 1.846 ms 1.631 ms 1.553 ms

3 * * *

4 72.14.194.226 (72.14.194.226) 3.762 ms 3.683 ms 3.577 ms

5 108.170.248.179 (108.170.248.179) 4.666 ms 108.170.248.162 (108.170.248.162) 4.869 ms 108.170.248.194 (108.170.248.194) 4.245 ms

6 72.14.235.133 (72.14.235.133) 72.443 ms 209.85.241.175 (209.85.241.175) 62.738 ms 72.14.235.133 (72.14.235.133) 65.809 ms

7 66.249.94.140 (66.249.94.140) 128.726 ms 127.506 ms 209.85.248.5 (209.85.248.5) 127.330 ms

8 74.125.251.181 (74.125.251.181) 127.219 ms 108.170.236.124 (108.170.236.124) 212.544 ms 74.125.251.181 (74.125.251.181) 127.249 ms

9 216.239.49.134 (216.239.49.134) 236.906 ms 209.85.242.80 (209.85.242.80) 254.810 ms 254.735 ms

10 209.85.251.138 (209.85.251.138) 252.002 ms 216.239.43.227 (216.239.43.227) 251.975 ms 209.85.242.80 (209.85.242.80) 236.343 ms

11 216.239.43.227 (216.239.43.227) 251.452 ms 72.14.234.8 (72.14.234.8) 279.650 ms 277.492 ms

12 209.85.250.9 (209.85.250.9) 274.521 ms 274.450 ms 209.85.253.249 (209.85.253.249) 270.558 ms

13 209.85.250.9 (209.85.250.9) 269.147 ms 209.85.254.244 (209.85.254.244) 347.046 ms 209.85.250.9 (209.85.250.9) 285.265 ms

14 64.233.175.112 (64.233.175.112) 344.852 ms 216.239.57.236 (216.239.57.236) 343.786 ms 64.233.175.112 (64.233.175.112) 345.273 ms

15 108.170.246.129 (108.170.246.129) 345.054 ms 345.342 ms 64.233.175.112 (64.233.175.112) 343.706 ms

16 108.170.238.119 (108.170.238.119) 345.610 ms 108.170.246.161 (108.170.246.161) 344.726 ms 108.170.238.117 (108.170.238.117) 345.536 ms

17 lhr25s12-in-f46.1e100.net (216.58.204.46) 345.382 ms 345.031 ms 344.884 ms

7. MTR Network Diagnostic Tool

MTR is a modern command-line network diagnostic tool that combines the functionality of ping and traceroute into a single diagnostic tool. Its output is updated in real-time, by default until you exit the program by pressing q.

The easiest way of running mtr is to provide it a host name or IP address as an argument, as follows.

$ mtr google.com

$ mtr 216.58.223.78

Sample Output

tecmint.com (0.0.0.0) Thu Jul 12 08:58:27 2018

First TTL: 1

Host Loss% Snt Last Avg Best Wrst StDev

1. 192.168.0.1 0.0% 41 0.5 0.6 0.4 1.7 0.2

2. 5.5.5.215 0.0% 40 1.9 1.5 0.8 7.3 1.0

3. 209.snat-111-91-120.hns.net.in 23.1% 40 1.9 2.7 1.7 10.5 1.6

4. 72.14.194.226 0.0% 40 89.1 5.2 2.2 89.1 13.7

5. 108.170.248.193 0.0% 40 3.0 4.1 2.4 52.4 7.8

6. 108.170.237.43 0.0% 40 2.9 5.3 2.5 94.1 14.4

7. bom07s10-in-f174.1e100.net 0.0% 40 2.6 6.7 2.3 79.7 16.

You can limit the number of pings to a specific value and exit mtr after those pings, using the -c flag as shown.

$ mtr -c 4 google.com

8. Route Command

route is a command line utility for displaying or manipulating the IP routing table of a Linux system. It is mainly used to configure static routes to specific hosts or networks via an interface.

You can view Kernel IP routing table by typing.

$ route

Destination Gateway Genmask Flags Metric Ref Use Iface

default gateway 0.0.0.0 UG 100 0 0 enp0s3

192.168.0.0 0.0.0.0 255.255.255.0 U 100 0 0 enp0s3

192.168.122.0 0.0.0.0 255.255.255.0 U 0 0 0 virbr0

There are numerous commands you can use to configure routing. Here are some useful ones:

Add a default gateway to the routing table.

$ sudo route add default gw <gateway-ip>

Add a network route to the routing table.

$ sudo route add -net <network ip/cidr> gw <gateway ip> <interface>

Delete a specific route entry from the routing table.

$ sudo route del -net <network ip/cidr>

9. Nmcli Command

Nmcli is an easy-to-use, scriptable command-line tool to report network status, manage network connections, and control the NetworkManager.

To view all your network devices, type.

$ nmcli dev status

DEVICE TYPE STATE CONNECTION

virbr0 bridge connected virbr0

enp0s3 ethernet connected Wired connection 1

To check network connections on your system, type.

$ nmcli con show

Wired connection 1 bc3638ff-205a-3bbb-8845-5a4b0f7eef91 802-3-ethernet enp0s3

virbr0 00f5d53e-fd51-41d3-b069-bdfd2dde062b bridge virbr0

To see only the active connections, add the -a flag.

$ nmcli con show -a

Network Scanning and Performance Analysis Tools

10. Netstat Command

netstat is a command line tool that displays useful information such as network connections, routing tables, interface statistics, and much more, concerning the Linux networking subsystem. It is useful for network troubleshooting and performance analysis.

Additionally, it is also a fundamental network service debugging tool used to check which programs are listening on what ports. For instance, the following command will show all TCP ports in listening mode and what programs are listening on them.

$ sudo netstat -tnlp

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name

tcp 0 0 0.0.0.0:587 0.0.0.0:* LISTEN 1257/master

tcp 0 0 127.0.0.1:5003 0.0.0.0:* LISTEN 1/systemd

tcp 0 0 0.0.0.0:110 0.0.0.0:* LISTEN 1015/dovecot

tcp 0 0 0.0.0.0:143 0.0.0.0:* LISTEN 1015/dovecot

tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN 1/systemd

tcp 0 0 0.0.0.0:465 0.0.0.0:* LISTEN 1257/master

tcp 0 0 0.0.0.0:53 0.0.0.0:* LISTEN 1404/pdns_server

tcp 0 0 0.0.0.0:21 0.0.0.0:* LISTEN 1064/pure-ftpd (SER

tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 972/sshd

tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 975/cupsd

tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 1257/master

tcp 0 0 0.0.0.0:8090 0.0.0.0:* LISTEN 636/lscpd (lscpd -

tcp 0 0 0.0.0.0:993 0.0.0.0:* LISTEN 1015/dovecot

tcp 0 0 0.0.0.0:995 0.0.0.0:* LISTEN 1015/dovecot

tcp6 0 0 :::3306 :::* LISTEN 1053/mysqld

tcp6 0 0 :::3307 :::* LISTEN 1211/mysqld

tcp6 0 0 :::587 :::* LISTEN 1257/master

tcp6 0 0 :::110 :::* LISTEN 1015/dovecot

tcp6 0 0 :::143 :::* LISTEN 1015/dovecot

tcp6 0 0 :::111 :::* LISTEN 1/systemd

tcp6 0 0 :::80 :::* LISTEN 990/httpd

tcp6 0 0 :::465 :::* LISTEN 1257/master

tcp6 0 0 :::53 :::* LISTEN 1404/pdns_server

tcp6 0 0 :::21 :::* LISTEN 1064/pure-ftpd (SER

tcp6 0 0 :::22 :::* LISTEN 972/sshd

tcp6 0 0 ::1:631 :::* LISTEN 975/cupsd

tcp6 0 0 :::25 :::* LISTEN 1257/master

tcp6 0 0 :::993 :::* LISTEN 1015/dovecot

tcp6 0 0 :::995 :::* LISTEN 1015/dovecot

To view kernel routing table, use the -r flag (which is equivalent to running route command above).

$ netstat -r

Destination Gateway Genmask Flags MSS Window irtt Iface

default gateway 0.0.0.0 UG 0 0 0 enp0s3

192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 enp0s3

192.168.122.0 0.0.0.0 255.255.255.0 U 0 0 0 virbr0

Note: Although Netstat is a great tool, it is now obsolete (deprecated), its replacement is ss command which is explained below.

11. ss Command

ss (socket statistics) is a powerful command line utility to investigate sockets. It dumps socket statistics and displays information similar to netstat. In addition, it shows more TCP and state information compared to other similar utilities.

The following example show how to list all TCP ports (sockets) that are open on a server.

$ ss -ta

State Recv-Q Send-Q Local Address:Port Peer Address:Port

LISTEN 0 100 *:submission *:*

LISTEN 0 128 127.0.0.1:fmpro-internal *:*

LISTEN 0 100 *:pop3 *:*

LISTEN 0 100 *:imap *:*

LISTEN 0 128 *:sunrpc *:*

LISTEN 0 100 *:urd *:*

LISTEN 0 128 *:domain *:*

LISTEN 0 9 *:ftp *:*

LISTEN 0 128 *:ssh *:*

LISTEN 0 128 127.0.0.1:ipp *:*

LISTEN 0 100 *:smtp *:*

LISTEN 0 128 *:8090 *:*

LISTEN 0 100 *:imaps *:*

LISTEN 0 100 *:pop3s *:*

ESTAB 0 0 192.168.0.104:ssh 192.168.0.103:36398

ESTAB 0 0 127.0.0.1:34642 127.0.0.1:opsession-prxy

ESTAB 0 0 127.0.0.1:34638 127.0.0.1:opsession-prxy

ESTAB 0 0 127.0.0.1:34644 127.0.0.1:opsession-prxy

ESTAB 0 0 127.0.0.1:34640 127.0.0.1:opsession-prxy

LISTEN 0 80 :::mysql :::*

...

To display all active TCP connections together with their timers, run the following command.

$ ss -to

12. NC Command

NC (NetCat) also referred to as the “Network Swiss Army knife”, is a powerful utility used for almost any task related to TCP, UDP, or UNIX-domain sockets. It is used open TCP connections, listen on arbitrary TCP and UDP ports, perform port scanning plus more.

You can also use it as a simple TCP proxies, for network daemon testing, to check if remote ports are reachable and much more. Furthermore, you can employ nc together with pv command to transfer files between two computers.

The following example, will show how to scan a list of ports.

$ nc -zv server2.tecmint.lan 21 22 80 443 3000

You can also specify a range of ports as shown.

$ nc -zv server2.tecmint.lan 20-90

The following example shows how to use nc to open a TCP connection to port 5000 on server2.tecmint.lan, using port 3000 as the source port, with a timeout of 10 seconds.

$ nc -p 3000 -w 10 server2.tecmint.lan 5000

13. Nmap Command

Nmap (Network Mapper) is a powerful and extremely versatile tool for Linux system/network administrators. It is used gather information about a single host or explore networks an entire network. Nmap is also used to perform security scans, network audit and finding open ports on remote hosts and so much more.

You can scan a host using its host name or IP address, for instance.

$ nmap google.com

Starting Nmap 6.40 ( http://nmap.org ) at 2018-07-12 09:23 BST

Nmap scan report for google.com (172.217.166.78)

Host is up (0.0036s latency).

rDNS record for 172.217.166.78: bom05s15-in-f14.1e100.net

Not shown: 998 filtered ports

PORT STATE SERVICE

80/tcp open http

443/tcp open https

Nmap done: 1 IP address (1 host up) scanned in 4.92 seconds

Alternatively, use an IP address as shown.

$ nmap 192.168.0.103

Starting Nmap 6.40 ( http://nmap.org ) at 2018-07-12 09:24 BST

Nmap scan report for 192.168.0.103

Host is up (0.000051s latency).

Not shown: 994 closed ports

PORT STATE SERVICE

22/tcp open ssh

25/tcp open smtp

902/tcp open iss-realsecure

4242/tcp open vrml-multi-use

5900/tcp open vnc

8080/tcp open http-proxy

MAC Address: 28:D2:44:EB:BD:98 (Lcfc(hefei) Electronics Technology Co.)

Nmap done: 1 IP address (1 host up) scanned in 0.13 seconds

DNS Lookup Utilities

14. host Command

host command is a simple utility for carrying out DNS lookups, it translates host names to IP addresses and vice versa.

$ host google.com

google.com has address 172.217.166.78

google.com mail is handled by 20 alt1.aspmx.l.google.com.

google.com mail is handled by 30 alt2.aspmx.l.google.com.

google.com mail is handled by 40 alt3.aspmx.l.google.com.

google.com mail is handled by 50 alt4.aspmx.l.google.com.

google.com mail is handled by 10 aspmx.l.google.com.

15. dig Command

dig (domain information groper) is also another simple DNS lookup utility, that is used to query DNS related information such as A Record, CNAME, MX Record etc, for example:

$ dig google.com

; <<>> DiG 9.9.4-RedHat-9.9.4-51.el7 <<>> google.com

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23083

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 13, ADDITIONAL: 14

;; OPT PSEUDOSECTION:

; EDNS: version: 0, flags:; udp: 4096

;; QUESTION SECTION:

;google.com. IN A

;; ANSWER SECTION:

google.com. 72 IN A 172.217.166.78

;; AUTHORITY SECTION:

com. 13482 IN NS c.gtld-servers.net.

com. 13482 IN NS d.gtld-servers.net.

com. 13482 IN NS e.gtld-servers.net.

com. 13482 IN NS f.gtld-servers.net.

com. 13482 IN NS g.gtld-servers.net.

com. 13482 IN NS h.gtld-servers.net.

com. 13482 IN NS i.gtld-servers.net.

com. 13482 IN NS j.gtld-servers.net.

com. 13482 IN NS k.gtld-servers.net.

com. 13482 IN NS l.gtld-servers.net.

com. 13482 IN NS m.gtld-servers.net.

com. 13482 IN NS a.gtld-servers.net.

com. 13482 IN NS b.gtld-servers.net.

;; ADDITIONAL SECTION:

a.gtld-servers.net. 81883 IN A 192.5.6.30

b.gtld-servers.net. 3999 IN A 192.33.14.30

c.gtld-servers.net. 14876 IN A 192.26.92.30

d.gtld-servers.net. 85172 IN A 192.31.80.30

e.gtld-servers.net. 95861 IN A 192.12.94.30

f.gtld-servers.net. 78471 IN A 192.35.51.30

g.gtld-servers.net. 5217 IN A 192.42.93.30

h.gtld-servers.net. 111531 IN A 192.54.112.30

i.gtld-servers.net. 93017 IN A 192.43.172.30

j.gtld-servers.net. 93542 IN A 192.48.79.30

k.gtld-servers.net. 107218 IN A 192.52.178.30

l.gtld-servers.net. 6280 IN A 192.41.162.30

m.gtld-servers.net. 2689 IN A 192.55.83.30

;; Query time: 4 msec

;; SERVER: 192.168.0.1#53(192.168.0.1)

;; WHEN: Thu Jul 12 09:30:57 BST 2018

;; MSG SIZE rcvd: 487

16. NSLookup Command

Nslookup is also a popular command line utility to query DNS servers both interactively and non-interactively. It is used to query DNS resource records (RR). You can find out “A” record (IP address) of a domain as shown.

$ nslookup google.com

Server: 192.168.0.1

Address: 192.168.0.1#53

Non-authoritative answer:

Name: google.com

Address: 172.217.166.78

You can also perform a reverse domain lookup as shown.

$ nslookup 216.58.208.174

Server: 192.168.0.1

Address: 192.168.0.1#53

Non-authoritative answer:

174.208.58.216.in-addr.arpa name = lhr25s09-in-f14.1e100.net.

174.208.58.216.in-addr.arpa name = lhr25s09-in-f174.1e100.net.

Authoritative answers can be found from:

in-addr.arpa nameserver = e.in-addr-servers.arpa.

in-addr.arpa nameserver = f.in-addr-servers.arpa.

in-addr.arpa nameserver = a.in-addr-servers.arpa.

in-addr.arpa nameserver = b.in-addr-servers.arpa.

in-addr.arpa nameserver = c.in-addr-servers.arpa.

in-addr.arpa nameserver = d.in-addr-servers.arpa.

a.in-addr-servers.arpa internet address = 199.180.182.53

b.in-addr-servers.arpa internet address = 199.253.183.183

c.in-addr-servers.arpa internet address = 196.216.169.10

d.in-addr-servers.arpa internet address = 200.10.60.53

e.in-addr-servers.arpa internet address = 203.119.86.101

f.in-addr-servers.arpa internet address = 193.0.9.1

Linux Network Packet Analyzers

17. Tcpdump Command

Tcpdump is a very powerful and widely used command-line network sniffer. It is used to capture and analyze TCP/IP packets transmitted or received over a network on a specific interface.

To capture packets from a given interface, specify it using the -i option.

$ tcpdump -i eth1

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on enp0s3, link-type EN10MB (Ethernet), capture size 262144 bytes

09:35:40.287439 IP tecmint.com.ssh > 192.168.0.103.36398: Flags [P.], seq 4152360356:4152360552, ack 306922699, win 270, options [nop,nop,TS val 2211778668 ecr 2019055], length 196

09:35:40.287655 IP 192.168.0.103.36398 > tecmint.com.ssh: Flags [.], ack 196, win 5202, options [nop,nop,TS val 2019058 ecr 2211778668], length 0

09:35:40.288269 IP tecmint.com.54899 > gateway.domain: 43760+ PTR? 103.0.168.192.in-addr.arpa. (44)

09:35:40.333763 IP gateway.domain > tecmint.com.54899: 43760 NXDomain* 0/1/0 (94)

09:35:40.335311 IP tecmint.com.52036 > gateway.domain: 44289+ PTR? 1.0.168.192.in-addr.arpa. (42)

To capture a specific number of packets, use the -c option to enter the desired number.

$ tcpdump -c 5 -i eth1

You can also capture and save packets to a file for later analysis, use the -w flag to specify the output file.

$ tcpdump -w captured.pacs -i eth1

18. Wireshark Utility

Wireshark is a popular, powerful, versatile and easy to use tool for capturing and analyzing packets in a packet-switched network, in real-time.

You can also save data it has captured to a file for later inspection. It is used by system administrators and network engineers to monitor and inspect the packets for security and troubleshooting purposes.

19. Bmon Tool

bmon is a powerful, command line based network monitoring and debugging utility for Unix-like systems, it captures networking related statistics and prints them visually in a human friendly format. It is a reliable and effective real-time bandwidth monitor and rate estimator.

20. UFW (Uncomplicated Firewall)

UFW is a well known and default firewall configuration tool on Debian and Ubuntu Linux distributions. It is used top enable/disable system firewall, add/delete/modify/reset packet filtering rules and much more.

To check UFW firewall status, type.

$ sudo ufw status

If UFW firewall is not active, you can activate or enable it using the following command.

$ sudo ufw enable

To disable UFW firewall, use the following command.

$ sudo ufw disable

If you want to find more information about a particular program, you can consult its man pages as shown.

$ man programs_name

That’s all for now! In this comprehensive guide, we reviewed some of the most used command-line tools and utilities for network management in Linux, under different categories, for system administrators, and equally useful to full-time network administrators/engineers.

You can share your thoughts about this guide via the comment form below. If we have missed any frequently used and important Linux networking tools/utilities or any useful related information, also let us know.