Networking commands

In this chapter, we will learn about a few basic networking commands, which will help us in our daily Linux usage.

Finding the IP address

The ip command can be used to find the IP address of the system.

$ 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: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1454 qdisc fq_codel state UP group default qlen 1000
    link/ether fa:16:3e:3c:ed:27 brd ff:ff:ff:ff:ff:ff
    inet 172.10.95.253/20 brd 172.10.111.255 scope global dynamic eth0
    valid_lft 57021sec preferred_lft 57021sec
    inet6 fe80::f816:3eff:fe3c:ed27/64 scope link
    valid_lft forever preferred_lft forever

Here lo is a special device which points to the same system (also known as localhost). The IP 127.0.0.1 always points to the the localhost. eth0 is our ethernet device which connects to the network.

Finding ARP table

ARP stands for Address Resolution Protocol, it is used to find the hardware addresses (MAC address) of the computers in the local network. Every operating system keeps a local cache (as a table) of this information. You can see the details using the following command.

$ ip neighbour

ping command

ping is simple way to find if you are connected to Internet or not. We can also ping any particular computer to find if the computer is connected to the network or not. Press Ctrl+c to stop the loop.

$ ping google.com
PING google.com (216.58.201.142) 56(84) bytes of data.
64 bytes from mad06s25-in-f142.1e100.net (216.58.201.142): icmp_seq=1 ttl=44 time=157 ms
64 bytes from mad06s25-in-f142.1e100.net (216.58.201.142): icmp_seq=2 ttl=44 time=156 ms
64 bytes from mad06s25-in-f142.1e100.net (216.58.201.142): icmp_seq=3 ttl=44 time=156 ms
^C
--- google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2000ms
rtt min/avg/max/mdev = 156.373/156.811/157.566/0.704 ms

Short note about DNS

DNS or Domain Name System is a decentralized naming system for systems which are connected to Internet (can be for private networks too). This is the way a computer knows, which other computer to connect to, when we type google.com in our browser, or in the ping command. There are servers known as dns servers, and for every domain name it needs to find, the client system generally connects to these dns servers, and finds out the IP address of the computer at that domain name.

/etc/hosts

The system looks at this file first for any name resolution. If it can not find the DNS entry, then the system looks at the /etc/resolv.conf, and connects to the DNS server.

You can update /etc/hosts file to add a domain to any particular IP address. Say, you want to be able to reach a server at IP address x.x.x.x with the name datastore, so you add an entry like the following to the file.

x.x.x.x    datastore

/etc/resolv.conf

/etc/resolv.conf is the configuration file which contains the DNS server address to use for DNS queries.

$ cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 1.1.1.1
nameserver 8.8.8.8

Here you can see that 1.1.1.1 & 8.8.8.8 are two DNS servers are being in this machine. The 1.1.1.1 is the DNS server from Cloudflare, and 8.8.8.8 is the DNS server hosted by Google.

systemd-resolved controlled name resolution

In most of the modern systems you will find the /etc/resolv.conf looks a bit different and actually a symbolic link. The example below is from Ubuntu 20.04.

$ ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Jul 31  2020 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf

$ cat /etc/resolv.conf
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "resolvectl status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 127.0.0.53
options edns0 trust-ad
search localdomain

That strange IP address, 127.0.0.53 is a special one managed by systemd-resolved service, where it listens for DNS queries. By default it picks up the DNS server addresses provided by the DHCP service, in case you want to manually set that up, you can configure them at the /etc/systemd/resolved.conf file. Here we are setting 1.1.1.1 as the primary DNS server, 8.8.8.8 as the fallback server, and also enabling DNS over TLS.

#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
#
# Entries in this file show the compile time defaults.
# You can change settings by editing this file.
# Defaults can be restored by simply deleting this file.
#
# See resolved.conf(5) for details

[Resolve]
DNS= 1.1.1.1
FallbackDNS= 8.8.8.8
#Domains=
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
DNSOverTLS=yes
#Cache=no-negative
#DNSStubListener=yes
#ReadEtcHosts=yes

You can learn about all the settings from the man page, man resolved.conf.

If want to go back to the simple /etc/resolv.conf file, you can remove the symlink and put in a new file at /etc/resolv.conf.

resolvectl command

The resolvectl command helps us to query via the systemd-resolved service. To check the current settings, use the status flag.

$ resolvectl status
Global
       LLMNR setting: no
MulticastDNS setting: no
  DNSOverTLS setting: yes
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 1.1.1.1
Fallback DNS Servers: 8.8.8.8
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 2 (ens33)
      Current Scopes: DNS
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: yes
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 192.168.195.1
          DNS Domain: ~.
                      localdomain

To query the IP address of a domain:

resolvectl query fedoraproject.org
fedoraproject.org: 140.211.169.196             -- link: ens33
                   140.211.169.206             -- link: ens33
                   152.19.134.198              -- link: ens33
                   38.145.60.21                -- link: ens33
                   8.43.85.67                  -- link: ens33
                   152.19.134.142              -- link: ens33
                   209.132.190.2               -- link: ens33
                   38.145.60.20                -- link: ens33
                   67.219.144.68               -- link: ens33
                   8.43.85.73                  -- link: ens33

-- Information acquired via protocol DNS in 1.4ms.
-- Data is authenticated: no

To view the TXT record:

$ resolvectl query -t TXT fedoraproject.org
fedoraproject.org IN TXT "v=spf1 a a:mailers.fedoraproject.org ip4:38.145.60.11 ip4:38.145.60.12 ?all" -- link: ens33

-- Information acquired via protocol DNS in 289.7ms.
-- Data is authenticated: no

resolvectl command can do many more things. Please have a look at the man page for more examples.

host command

The host command is a simple DNS lookup utility. It will show you the IP address of any given hostname.

$ host www.example.com
www.example.com has address 93.184.216.34
www.example.com has IPv6 address 2606:2800:220:1:248:1893:25c8:1946

dig command

dig command can tell us DNS records, MX details (used to send emails) and other information for a given domain name. It is one of the primary command people use to query DNS records.

$ dig kushaldas.in

; <<>> DiG 9.10.4-P8-RedHat-9.10.4-5.P8.fc25 <<>> kushaldas.in
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50750
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;kushaldas.in.                      IN      A

;; ANSWER SECTION:
kushaldas.in.               5528    IN      A       208.113.152.208

;; Query time: 66 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sun Jun 25 11:37:00 IST 2017
;; MSG SIZE  rcvd: 57

If you want to print only the IP address in the output, you can use +short as argument.

$ dig +short kushaldas.in
208.113.152.208

You can also specify any particular type of record from the DNS server. For example, if I want to get the TXT records, I can do the following command.

$ dig +short kushaldas.in TXT
"google-site-verification=DPpUk-OfBLT-5PkbSR9VM2Uht3eXaksthROvS-L9iKY"
"kolab-verify=35f0040cd1ebb20fb7f0b3fade0e1c8e"

You can use any of the following options instead of TXT: A|MX|TXT|CNAME|NS.

If you want to specify a DNS server to use, you can do that with the address specified at the end of the command along with a @ sign.

$ dig rtnpro.com @208.67.222.222

; <<>> DiG 9.10.4-P8-RedHat-9.10.4-5.P8.fc25 <<>> rtnpro.com @208.67.222.222
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27312
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;rtnpro.com.                        IN      A

;; AUTHORITY SECTION:
rtnpro.com.         3600    IN      SOA     dns1.bigrock.in. rtnpro.gmail.com. 2017021401 7200 7200 172800 38400

;; Query time: 899 msec
;; SERVER: 208.67.222.222#53(208.67.222.222)
;; WHEN: Sun Jun 25 11:40:01 IST 2017
;; MSG SIZE  rcvd: 106

If you want learn about the full path of the DNS query, you can find that out by +trace flag. For the following example where we are trying to find the IP address for anweshadas.in, dig first connects to the root name servers to find the correct DNS resolvers for the .in servers, then they tell dig to go those servers (in this case Dreamhost name servers) which contain the exact IP address for the domain.

$ dig +trace anweshadas.in @8.8.8.8

; <<>> DiG 9.11.5-P4-5.1+deb10u5-Debian <<>> +trace anweshadas.in @8.8.8.8
;; global options: +cmd
.                   47041   IN      NS      m.root-servers.net.
.                   47041   IN      NS      b.root-servers.net.
.                   47041   IN      NS      c.root-servers.net.
.                   47041   IN      NS      d.root-servers.net.
.                   47041   IN      NS      e.root-servers.net.
.                   47041   IN      NS      f.root-servers.net.
.                   47041   IN      NS      g.root-servers.net.
.                   47041   IN      NS      h.root-servers.net.
.                   47041   IN      NS      a.root-servers.net.
.                   47041   IN      NS      i.root-servers.net.
.                   47041   IN      NS      j.root-servers.net.
.                   47041   IN      NS      k.root-servers.net.
.                   47041   IN      NS      l.root-servers.net.
.                   47041   IN      RRSIG   NS 8 0 518400 20210730050000 20210717040000 26838 . MFT2Q71k1LZVfXyH2qKWLoS7a7j5aSVdlp4SrIptZXP0ydjav7y5sLv/ Yz76Ki+3PU0G3SagwbC61bdi6sNV5DiBpxIzny8Mavx23P6XKsbetFr1 RgkwlzyGJmd0kLA4ydgjrzRh2hhvQkBDWtzBpVLUo7tDmwodE/zi/RUA CMofG9YIkgxSX0/5qUUKXhijHocYXQU++x7RbFqTxJBEW8Fn6GDTtg1Z pTT0UYpmMX5NHiRlneYb6ChHGQLfbQ1kBblxuQlsPb46dJBKaXT3wr3/ SXUXQCZ+ADCsolK+LhGeQtByqBEXryjuT/U2WK8mqcTAs/d1bToRwrH5 nxizXg==
;; Received 525 bytes from 8.8.8.8#53(8.8.8.8) in 1 ms

in.                 172800  IN      NS      ns1.registry.in.
in.                 172800  IN      NS      ns2.registry.in.
in.                 172800  IN      NS      ns3.registry.in.
in.                 172800  IN      NS      ns4.registry.in.
in.                 172800  IN      NS      ns5.registry.in.
in.                 172800  IN      NS      ns6.registry.in.
in.                 86400   IN      DS      54739 8 1 2B5CA455A0E65769FF9DF9E75EC40EE1EC1CDCA9
in.                 86400   IN      DS      54739 8 2 9F122CFD6604AE6DEDA0FE09F27BE340A318F06AFAC11714A73409D4 3136472C
in.                 86400   IN      RRSIG   DS 8 1 86400 20210730170000 20210717160000 26838 . i6toEqveLqwB/W4Z/77bfGyFyYJRepGi8uYoQ0jEZM1I95qxsqeMCtdV cr3foafFJKaCkvH2eAfIUrHH8GMn/t9lVDrHwikLisoaPaSahgoQAOPm ClR/VDcAxkVwE+07Ir6ROt+qXn5jse4gnB+nezI4Q+rakearp8D9AaxJ ubWnAMfHOqKBLDMGNrm6/XRk6HA43nrMIUKNCFbhpKo5gkvy+S768uQu ySRdLTUxN0ELO9Qv7fBqQxamRyZ1N5LKTpjkKNKYwnihOVIWvktqt4p7 xoJL56z0XE9HhhI807GOBcpLBeaRKZXOA8GKU77pm9lDLHSuG4epF3zD X9Vayw==
;; Received 794 bytes from 193.0.14.129#53(k.root-servers.net) in 1 ms

anweshadas.in.              86400   IN      NS      ns3.dreamhost.com.
anweshadas.in.              86400   IN      NS      ns2.dreamhost.com.
anweshadas.in.              86400   IN      NS      ns1.dreamhost.com.
bo801o0uciino3vfr38lrljcrv2ucohi.in. 1800 IN NSEC3 1 1 1 00763C64 BO9UQ54VB22M3J37NR3N6GRC6J4RVUTV NS SOA RRSIG DNSKEY NSEC3PARAM
bo801o0uciino3vfr38lrljcrv2ucohi.in. 1800 IN RRSIG NSEC3 8 2 1800 20210813034112 20210714030559 65169 in. gZ3NODrbaP6/GV1McvgHTD4wn9w2w5CCqjoI+JyjRpNVweGuDex5A/ls OznLptg/nmmJlx3835suy9I79h0tOjDjWNXxLQ9scKCtYZJSFqIdnRS9 QP5egjVJnZ3zOLN0lO//hQa/gIhKCSqYpLCWLS1RoFn3B5uvF96VopKU YYfjXFbqYCjyx4T8oZi72xFUChr/yi/dVkHbM0OvwLCJRg==
8679tah9aq7s760bquasj6clf332vb3e.in. 1800 IN NSEC3 1 1 1 00763C64 869E0HEFFMPE89PM1VHLGQHH72K7IGRM NS DS RRSIG
8679tah9aq7s760bquasj6clf332vb3e.in. 1800 IN RRSIG NSEC3 8 2 1800 20210815211611 20210716202452 65169 in. GahrJsYIVpR5+eaykB/DuaIPSUeR+rX3DxR7yR3mMN/7pVSekbZ/Fw4I q5NZLKhBp2WKq0aiwxYX4+VfKjdLyQLKeGoXYeFpwps6KiCf8gLjDJwO Hx1PCgg5CnUEYw+iXd3GGx46ZlyHHbuSNa4YbVBEHevcmo/3oc3ubiMf VKTcuR+upzBQmLzNId6LB9qZBpFfe6GPCy/sMMaCKB0OwQ==
;; Received 664 bytes from 37.209.196.12#53(ns3.registry.in) in 2 ms

anweshadas.in.              14400   IN      A       159.89.209.77
;; Received 58 bytes from 162.159.26.14#53(ns1.dreamhost.com) in 160 ms

You can even print the response in YAML format.

$ dig +yaml kushaldas.in
-
type: MESSAGE
message:
type: RECURSIVE_RESPONSE
query_time: !!timestamp 2022-04-23T18:52:23.844Z
response_time: !!timestamp 2022-04-23T18:52:24.046Z
message_size: 124b
socket_family: INET
socket_protocol: UDP
response_address: 127.0.0.53
response_port: 53
query_address: 0.0.0.0
query_port: 56777
response_message_data:
  opcode: QUERY
  status: NOERROR
  id: 12591
  flags: qr rd ra
  QUESTION: 1
  ANSWER: 1
  AUTHORITY: 3
  ADDITIONAL: 1
  OPT_PSEUDOSECTION:
    EDNS:
      version: 0
      flags:
      udp: 65494
  QUESTION_SECTION:
    - kushaldas.in. IN A
  ANSWER_SECTION:
    - kushaldas.in. 300 IN A 51.159.23.159
  AUTHORITY_SECTION:
    - kushaldas.in. 2876 IN NS ns3.dreamhost.com.
    - kushaldas.in. 2876 IN NS ns2.dreamhost.com.
    - kushaldas.in. 2876 IN NS ns1.dreamhost.com.

Hint

Please remember that dig returns 0 as exit code even if it can not find any information for the domain (NXDOMAIN). This will save you some time for your scripts.`1` is for usage error, 9 is used for server error & 10 as an internal error for itself.

ss command

ss command shows us socket statistics from the system. This command replaces the older netstat command. Read the man page of the command to know more about the different arguments we can pass at the command line.

$ ss -putn
Netid State      Recv-Q Send-Q                           Local Address:Port                                          Peer Address:Port
tcp   ESTAB      0      0                                 192.168.1.101:51496                                       162.125.34.129:443                 users:(("dropbox",pid=28797,fd=80))
tcp   ESTAB      0      0                                 192.168.1.101:47864                                       74.125.200.189:443                 users:(("chrome",pid=22112,fd=385))
tcp   ESTAB      0      0                                 192.168.1.101:59524                                      209.12.123.55:22                  users:(("ssh",pid=26621,fd=3))
... long output

You can also learn various other statistics about sockets, for example, if you want to know all the sockets from your computer connected to any machine at port 443, you can use the following command.

$ ss -pt dst :443

traceroute command

The traceroute command is used to show the full route of a network packet from the system to any given host.

$ traceroute www.rtnpro.com
traceroute to www.rtnpro.com (146.185.181.157), 30 hops max, 60 byte packets
1  gateway (192.168.1.1)  1.434 ms  1.920 ms  1.891 ms
2  45.113.248.3 (45.113.248.3)  7.478 ms  10.335 ms  10.343 ms
3  45.113.248.1 (45.113.248.1)  10.319 ms  10.293 ms  10.274 ms
4  121.244.26.1.static-pune.vsnl.net.in (121.244.26.1)  26.938 ms  26.608 ms  27.165 ms
5  172.31.183.162 (172.31.183.162)  9.883 ms  10.133 ms  10.122 ms
6  172.31.19.201 (172.31.19.201)  10.591 ms 172.29.250.33 (172.29.250.33)  6.894 ms 172.31.19.201 (172.31.19.201)  8.203 ms
7  ix-ae-0-4.tcore1.MLV-Mumbai.as6453.net (180.87.38.5)  9.378 ms  8.886 ms  9.240 ms
8  if-ae-9-5.tcore1.WYN-Marseille.as6453.net (80.231.217.77)  159.550 ms if-ae-5-2.tcore1.WYN-Marseille.as6453.net (180.87.38.126)  159.614 ms if-ae-9-5.tcore1.WYN-Marseille.as6453.net (80.231.217.77)  159.506 ms
9  if-ae-8-1600.tcore1.PYE-Paris.as6453.net (80.231.217.6)  159.392 ms  159.474 ms  159.405 ms
10  if-ae-15-2.tcore1.AV2-Amsterdam.as6453.net (195.219.194.145)  159.327 ms  158.355 ms  122.520 ms
11  195.219.194.26 (195.219.194.26)  133.216 ms  134.168 ms  134.683 ms
12  138.197.250.29 (138.197.250.29)  192.236 ms  192.125 ms 138.197.250.23 (138.197.250.23)  192.083 ms
13  * 146.185.181.157 (146.185.181.157)  191.831 ms  191.861 ms

tracepath command

The tracepath command traces a path to a network host discovering MTU along the path. This is a modern replacement of the traceroute command, and also does not need superuser privileges to execute.

$ tracepath www.rtnpro.com
1?: [LOCALHOST]                      pmtu 1500
1:  gateway                                               0.950ms
1:  gateway                                               0.715ms
2:  gateway                                               0.689ms pmtu 1492
2:  45.113.248.3                                          3.564ms
3:  45.113.248.1                                          4.639ms
4:  121.244.26.1.static-pune.vsnl.net.in                  4.132ms
5:  172.31.183.162                                        4.733ms asymm  7
6:  172.29.250.33                                        12.524ms asymm  7
7:  ix-ae-0-4.tcore1.MLV-Mumbai.as6453.net                7.208ms asymm  8
8:  if-ae-5-2.tcore1.WYN-Marseille.as6453.net           125.727ms asymm 12
9:  if-ae-8-1600.tcore1.PYE-Paris.as6453.net            128.893ms asymm 11
10:  if-ae-15-2.tcore1.AV2-Amsterdam.as6453.net          126.019ms asymm  9
11:  195.219.194.26                                      136.373ms asymm 10
12:  138.197.250.27                                      130.198ms
13:  146.185.181.157                                     131.040ms reached
    Resume: pmtu 1492 hops 13 back 13

Remote login to a computer using ssh tool

We use the ssh command to login to remote computers. The remote computer must have the sshd service running, and should also allow clients to connect to this service. Let’s try to connect to localhost itself. Remember to start the sshd service before this step.

$ ssh kdas@localhost
kdas@localhost's password:
Last login: Wed Jun 21 08:44:40 2017 from 192.168.1.101
$

As you can see, the command syntax is ssh followed by user@hostname. If your remote system’s user name is same as your current one, then you can omit the username and just use the hostname (IP address or domain name).

$ ssh localhost
kdas@localhost's password:
$

ssh key generation

ssh keys are used in the daily life of a Linux user or developer. In simple terms, it helps us to securely login to other computers. In the following example, we will create a new key for our user.

$ ssh-keygen -t rsa -b 4096 -C "kushaldas@gmail.com"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/fedora/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/fedora/.ssh/id_rsa.
Your public key has been saved in /home/fedora/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:O6Rxir7lpFBQsBnvs+NJRU8Ih01ffVBvLTE8s5TpxLQ kushaldas@gmail.com
The key's randomart image is:
+---[RSA 4096]----+
|  o.o+o   ...*=o |
|   *.o.o .  . @=.|
|  + . o o    =E++|
|   o . o      oo |
|    + o S        |
|   . = * .       |
|  . = = o        |
|   = B   .       |
|    *..          |
+----[SHA256]-----+

As you can see in the output, the key has been saved in the ~/.ssh directory. You can also find out that these files are only readable by the owner.

$ ls -l .ssh
total 12
-rw-------. 1 fedora fedora 3326 Jun 25 06:25 id_rsa
-rw-r--r--. 1 fedora fedora  745 Jun 25 06:25 id_rsa.pub

Each key has two parts. The id_rsa.pub is the public key and id_rsa is the private part of the key. One can safely upload or use the public key anywhere. But the private key, should be kept in a safe manner, because if people get access to your private key, they can also access all of your information from any system using that key.

In other words, do not give the private key to anyone, or do not randomly copy the .ssh directory to a USB drive and then forget about it.

ssh-copy-id

ssh-copy-id command copies the keys to a given remote system. After this step we can use the ssh key to login to the box directly, instead of the usual username / password method.

$ ssh-copy-id fedora@209.12.123.55
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 2 key(s) remain to be installed -- if you are prompted now it is to install the new keys

fedora@209.12.123.55's password:

Number of key(s) added: 2

Now try logging into the machine, with:   "ssh 'fedora@209.12.123.55'"
and check to make sure that only the key(s) you wanted were added.

Stop and disable the sshd service

If you don’t need ssh access to your computer (say, your laptop), you should always stop and disable the sshd service in the computer.

Disable password based login for ssh

Remember, this step can be dangerous. Unless you’re really, really sure that you can access a computer by either logging in physically or using your ssh key (and you have a backup of the key somewhere), you should not do this step.

By disabling password based login in the sshd service, you make sure that only people with the right private key can login to the computer. This helps greatly when people try to break into the system by guessing the password. This is also really helpful in case your computer is connected to some network, and you still need to access it over ssh.

We will use vim to open the /etc/ssh/sshd_config file, which is the configuration file for sshd service.

$ sudo vim /etc/ssh/sshd_config

Search for the term PasswordAuthentication, and change the value to no. Below I have added a new line to do the same. You can also understand, that the lines starting with # are comments in this configuration file. This configuration will disable password based authentication for the sshd service. You should remember to restart the sshd service after this step for the change to take place.

_images/passwordauthno.png

How to find active (open) network connections from your computer?

$ sudo lsof -i -n -P

The lsof command shows open files, using -i we are asking to list of all Internet and x.25 (HP-UX) network files. To know more, read the man page of the lsof command.

To know about ports

If you want to know more about popular ports used by the applications/protocols, you should look into /etc/services file on your computer.