Top 10 Essential Linux Commands
This article provides 10 commands that are vital for troubleshooting network issues on Linux systems with real-world examples.
CORE
·
Join the DZone community and get the full member experience.
Join For FreeAs a Linux administrator or even if you are a newbie who just started using Linux, having a good understanding of useful commands in troubleshooting network issues is paramount.
We'll explore the top 10 essential Linux commands for diagnosing and resolving common network problems. Each command will be accompanied by real-world examples to illustrate its usage and effectiveness.
1. ping
Example: ping google.com
test@ubuntu-server ~ % ping google.com -c 5
PING google.com (142.250.189.206): 56 data bytes
64 bytes from 142.250.189.206: icmp_seq=0 ttl=58 time=14.610 ms
64 bytes from 142.250.189.206: icmp_seq=1 ttl=58 time=18.005 ms
64 bytes from 142.250.189.206: icmp_seq=2 ttl=58 time=19.402 ms
64 bytes from 142.250.189.206: icmp_seq=3 ttl=58 time=22.450 ms
64 bytes from 142.250.189.206: icmp_seq=4 ttl=58 time=15.870 ms
--- google.com ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 14.610/18.067/22.450/2.749 ms
test@ubuntu-server ~ %Explanation
ping uses ICMP protocol, where ICMP stands for internet control message protocol and ICMP is a network layer protocol used by network devices to communicate. ping helps in testing the reachability of the host and it will also help in finding the latency between the source and destination.
2. traceroute
Example: traceroute google.com
test@ubuntu-server ~ % traceroute google.com
traceroute to google.com (142.250.189.238), 64 hops max, 52 byte packets
1 10.0.0.1 (10.0.0.1) 6.482 ms 3.309 ms 3.685 ms
2 96.120.90.197 (96.120.90.197) 13.094 ms 10.617 ms 11.351 ms
3 po-301-1221-rur01.fremont.ca.sfba.comcast.net (68.86.248.153) 12.627 ms 11.240 ms 12.020 ms
4 ae-236-rar01.santaclara.ca.sfba.comcast.net (162.151.87.245) 18.902 ms 44.432 ms 18.269 ms
5 be-299-ar01.santaclara.ca.sfba.comcast.net (68.86.143.93) 14.826 ms 13.161 ms 12.814 ms
6 69.241.75.42 (69.241.75.42) 12.236 ms 12.302 ms
69.241.75.46 (69.241.75.46) 15.215 ms
7 * * *
8 142.251.65.166 (142.251.65.166) 21.878 ms 14.087 ms
209.85.243.112 (209.85.243.112) 14.252 ms
9 nuq04s39-in-f14.1e100.net (142.250.189.238) 13.666 ms
192.178.87.152 (192.178.87.152) 12.657 ms 13.170 ms
test@ubuntu-server ~ %
Explanation
Traceroute shows the route packets take to reach a destination host. It displays the IP addresses of routers along the path and calculates the round-trip time (RTT) for each hop. Traceroute helps identify network congestion or routing issues.
3. netstat
Example: netstat -tulpn
test@ubuntu-server ~ % netstat -tuln
Active LOCAL (UNIX) domain sockets
Address Type Recv-Q Send-Q Inode Conn Refs Nextref Addr
aaf06ba76e4d0469 stream 0 0 0 aaf06ba76e4d03a1 0 0 /var/run/mDNSResponder
aaf06ba76e4d03a1 stream 0 0 0 aaf06ba76e4d0469 0 0
aaf06ba76e4cd4c1 stream 0 0 0 aaf06ba76e4ccdb9 0 0 /var/run/mDNSResponder
aaf06ba76e4cace9 stream 0 0 0 aaf06ba76e4c9e11 0 0 /var/run/mDNSResponder
aaf06ba76e4d0b71 stream 0 0 0 aaf06ba76e4d0aa9 0 0 /var/run/mDNSResponder
test@ubuntu-server ~ %
Explanation
Netstat displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. It's useful for troubleshooting network connectivity, identifying open ports, and monitoring network performance.
4. ifconfig/ip
Example: ifconfig or ifconfig <interface name>
test@ubuntu-server ~ % ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=6460<TSO4,TSO6,CHANNEL_IO,PARTIAL_CSUM,ZEROINVERT_CSUM>
ether 10:9f:41:ad:91:60
inet 10.0.0.24 netmask 0xffffff00 broadcast 10.0.0.255
inet6 fe80::870:c909:df17:7ed1%en0 prefixlen 64 secured scopeid 0xc
inet6 2601:641:300:e710:14ef:e605:4c8d:7e09 prefixlen 64 autoconf secured
inet6 2601:641:300:e710:d5ec:a0a0:cdbb:79a7 prefixlen 64 autoconf temporary
inet6 2601:641:300:e710::6cfc prefixlen 64 dynamic
nd6 options=201<PERFORMNUD,DAD>
media: autoselect
status: active
test@ubuntu-server ~ %
Explanation
ifconfig and ip commands are used to view and configure network parameters. They provide information about the IP address, subnet mask, MAC address, and network status of each interface.
5. tcpdump
Example:tcpdump -i en0 tcp port 80
test@ubuntu-server ~ % tcpdump -i en0 tcp port 80
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on en0, link-type EN10MB (Ethernet), snapshot length 524288 bytes
0 packets captured
55 packets received by filter
0 packets dropped by kernel
test@ubuntu-server ~ %
Explanation
Tcpdump is a packet analyzer that captures and displays network traffic in real-time. It's invaluable for troubleshooting network issues, analyzing packet contents, and identifying abnormal network behavior. Use tcpdump to inspect packets on specific interfaces or ports.
6. nslookup/dig
Example: nslookup google.com or dig
test@ubuntu-server ~ % nslookup google.com
Server: 2001:558:feed::1
Address: 2001:558:feed::1#53
Non-authoritative answer:
Name: google.com
Address: 172.217.12.110
test@ubuntu-server ~ %
test@ubuntu-server ~ % dig google.com
; <<>> DiG 9.10.6 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46600
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;google.com. IN A
;; ANSWER SECTION:
google.com. 164 IN A 142.250.189.206
;; Query time: 20 msec
;; SERVER: 2001:558:feed::1#53(2001:558:feed::1)
;; WHEN: Mon Apr 15 22:55:35 PDT 2024
;; MSG SIZE rcvd: 55
test@ubuntu-server ~ %
Explanation
nslookup and dig are DNS lookup tools used to query DNS servers for domain name resolution. They provide information about the IP address associated with a domain name and help diagnose DNS-related problems such as incorrect DNS configuration or server unavailability.
7. iptables/firewalld
Example: iptables -L or firewall-cmd --list-all
test@ubuntu-server ~# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy DROP)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
test@ubuntu-server ~#
Explanation
iptables and firewalld are firewall management tools used to configure packet filtering and network address translation (NAT) rules. They control incoming and outgoing traffic and protect the system from unauthorized access. Use them to diagnose firewall-related issues and ensure proper traffic flow.
8. ss
Example: ss -tulpn
test@ubuntu-server ~#
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
udp UNCONN 0 0 *:161 *:*
udp UNCONN 0 0 *:161 *:*
test@ubuntu-server ~#
Explanation
ss is a utility to investigate sockets. It displays information about TCP, UDP, and UNIX domain sockets, including listening and established connections, connection state, and process IDs. ss is useful for troubleshooting socket-related problems and monitoring network activity.
9. arp
Example: arp -a
test@ubuntu-server ~ % arp -a
? (10.0.0.1) at 80:da:c2:95:aa:f7 on en0 ifscope [ethernet]
? (10.0.0.57) at 1c:4d:66:bb:49:a on en0 ifscope [ethernet]
? (10.0.0.83) at 3a:4a:df:fe:66:58 on en0 ifscope [ethernet]
? (10.0.0.117) at 70:2a:d5:5a:cc:14 on en0 ifscope [ethernet]
? (10.0.0.127) at fe:e2:1c:4d:b3:f7 on en0 ifscope [ethernet]
? (10.0.0.132) at bc:d0:74:9a:51:85 on en0 ifscope [ethernet]
? (10.0.0.255) at ff:ff:ff:ff:ff:ff on en0 ifscope [ethernet]
mdns.mcast.net (224.0.0.251) at 1:0:5e:0:0:fb on en0 ifscope permanent [ethernet]
? (239.255.255.250) at 1:0:5e:7f:ff:fa on en0 ifscope permanent [ethernet]
test@ubuntu-server ~ %Explanation
arp (Address Resolution Protocol) displays and modifies the IP-to-MAC address translation tables used by the kernel. It resolves IP addresses to MAC addresses and vice versa. arp is helpful for troubleshooting issues related to network device discovery and address resolution.
10. mtr
Example: mtr
test.ubuntu.com (0.0.0.0) Tue Apr 16 14:46:40 2024
Keys: Help Display mode Restart statistics Order of fields quit Packets Ping
Host Loss% Snt Last Avg Best Wrst StDev
1. 10.0.0.10 0.0% 143 0.8 9.4 0.7 58.6 15.2
2. 10.0.2.10 0.0% 143 0.8 9.4 0.7 58.6 15.2
3. 192.168.0.233 0.0% 143 0.8 9.4 0.7 58.6 15.2
4. 142.251.225.178 0.0% 143 0.8 9.4 0.7 58.6 15.2
5. 142.251.225.177 0.0% 143 0.8 9.4 0.7 58.6 15.2
Explanation
mtr (My traceroute) combines the functionality of ping and traceroute into a single diagnostic tool. It continuously probes network paths between the host and a destination, displaying detailed statistics about packet loss, latency, and route changes. Mtr is ideal for diagnosing intermittent network problems and monitoring network performance over time.
Mastering these commands comes in handy for troubleshooting network issues on Linux hosts.
Opinions expressed by DZone contributors are their own.
Comments