Important Port numbers for the Linux system administrator

<div dir="ltr" style="text-align: left;" trbidi="on"> <h2 style="text-align: left;"> This is an important question mostly asked during Linux Interview.</h2> <h4 style="text-align: left;"> To understanding the ports, their details and numbers, I have collected some info.</h4> <h4 style="text-align: left;"> <span style="color: blue;">Linux Important Ports information </span></h4> <div> <h4 style="text-align: left;"> FAQ related to PORTS in Linux !!</h4> </div> <div style="text-align: left;"> <b>Q. What is a port?</b><br /> <br /> A port is piece of software which is used as docking point in your machine, where remote application can communicate. This is analogy to the physical ports for entering in to a country from different sea ports.<br /> <br /> <b>Q. What is hardware port?</b><br /> <br /> This is a physical peripheral connecting point to a machine from a physical device.<br /> <br /> <b>Q. What is a socket?</b><br /> <br /> Socket is combination of software Port and IP address.<br /> <br /> <b>Q. What is the range of ports or how many ports are there?</b><br /> <br /> Port numbers can vary from 0 to 65535, so total we can get 65536 ports<br /> <br /> <b>Q. Why port numbers are just 65536?</b><br /> <br /> This is because limitation in TCP/IP stack where the port number field is just 16bit size. So we get only 2^16(2 to the power of 16) ports which are equal to 65536 available ports<br /> <br /> <b>Q.What are the well-known ports or assigned ports or default ports?</b><br /> <br /> Well known ports are from 0 to 1023(total 2^10=1024 ports)<br /> <br /> <b>Q.What do you mean by default port?</b><br /> <br /> Default port is a designated port for particular well-known service such as web server, mail server, ftp server etc. By default FTP uses 21 port, DNS uses 53 and Apache uses 80 port.<br /> <b><br />Q.Can we change default port for a service(example Apache, squid)?</b><br /> <br /> Yes, we can change. In Apache and DNS we can change this using listen configuration entry in httpd.conf and named.conf. Squid have port entry in its squid.conf file to mention port number.<br /> <br /> <b>Q.What are the protocol numbers for TCP and UDP?</b><br /> <br /> Do not confuse this one with port numbers. TCP and UDP have their own numbers in TCP/IP stack.<br /> <br /> TCP protocol number:6<br /> <br /> UDP protocol number:17<br /> <br /> <b>Q. Is there any way I can see all the port information in Linux?</b><br /> <br /> Yes, you can get that from /etc/services files.<br /> <br /> <b>Q. How can I see open ports in Linux?</b><br /> <br /> Use nmap command.</div> <h4 style="text-align: left;"> Well known ports</h4> <div style="text-align: left;"> <br /> 20 – FTP Data (For transferring FTP data)<br /> <br /> 21 – FTP Control (For starting FTP connection)<br /> <br /> 22 – SSH(For secure remote administration which uses SSL to encrypt the transmission)<br /> <br /> 23 – Telnet (For insecure remote administration<br /> <br /> 25 – SMTP(Mail Transfer Agent for e-mail server such as SEND mail)<br /> <br /> 53 – DNS(Special service which uses both TCP and UDP)<br /> <br /> 67 – Bootp<br /> <br /> 68 – DHCP<br /> <br /> 69 – TFTP(Trivial file transfer protocol uses udp protocol for connection less transmission of data)<br /> <br /> 80 – HTTP/WWW(apache)<br /> <br /> 88 – Kerberos<br /> <br /> 110 – POP3(Mail delivery Agent)<br /> <br /> 123 – NTP(Network time protocol used for time syncing uses UDP protocol)<br /> <br /> 137 – NetBIOS(nmbd)<br /> <br /> 139 – SMB-Samba(smbd)<br /> <br /> 143 – IMAP<br /> <br /> 161 – SNMP(For network monitoring)<br /> <br /> 389 – LDAP(For centralized administration)<br /> <br /> 443 – HTTPS(HTTP+SSL for secure web access)<br /> <br /> 514 – Syslogd(udp port)<br /> <br /> 636 – ldaps(both tcp and udp)<br /> <br /> 873 – rsync<br /> <br /> 989 – FTPS-data<br /> <br /> 990 – FTPS<br /> <br /> 993 – IMAPS<br /> <br /> 1194 – openVPN<br /> <br /> 1812 – RADIUS<br /> <br /> 995 – POP3s<br /> <br /> 2049 – NFS(nfsd, rpc.nfsd, rpc, portmap)<br /> <br /> 2401 – CVS server<br /> <br /> 3306 – MySql<br /> <br /> 3690 – SVN<br /> <br /> 6000-6063-X11<br /> <br /> <b>Note1</b>:If protocol(TCP or UDP) is not mention then the above port are solely for TCP. Some service use UDP as mention in above list.<br /> <br /> <b>Note2</b>:X11 use 6000 to 6063.. ports for connecting X11 from remote server.</div> </div>

Important Port numbers for the Linux system administrator

This is an important question mostly asked during Linux Interview. To understanding the ports, their details and numbers, I have collect...

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