Linux check udp ports

Testing UDP port connectivity

I am trying to test whether I can get to a particular port on a remote server (both of which I have access to) through UDP. Both servers are internet facing. I am using netcat to have a certain port listening. I then use nmap to check for that port to see if it is open, but it doesn’t appear to be. Iptables is turned off. Any suggestions why this could be? I am eventually going to setup a VPN tunnel, but because I’m very new to tunnels, I want to make sure I have connectivity on port UDP 1194 before advancing.

I’ve respondend to the «Testing UDP port connectivity» question. But I suggest to concentrate on the more specific «make sure OpenVPN receives my UDP packets» part — that could be easily achieved by looking at OpenVPN logs.

10 Answers 10

There is no such thing as an «open» UDP port, at least not in the sense most people are used to think (which is answering something like «OK, I’ve accepted your connection»). UDP is session-less, so «a port» (read: the UDP protocol in the operating system IP stack) will never respond «success» on its own.

UDP ports only have two states: listening or not. That usually translates to «having a socket open on it by a process» or «not having any socket open». The latter case should be easy to detect since the system should respond with an ICMP Destination Unreachable packet with code=3 (Port unreachable). Unfortunately many firewalls could drop those packets so if you don’t get anything back you don’t know for sure if the port is in this state or not. And let’s not forget that ICMP is session-less too and doesn’t do retransmissions: the Port Unreachable packet could very well be lost somewhere on the net.

A UDP port in the «listening» state may not respond at all (the process listening on it just receives the packet and doesn’t transmit anything) or it could send something back (if the process does act upon reception and if it acts by responding via UDP to the original sender IP:port). So again, you never know for sure what’s the state if you don’t get anything back.

You say you can have control of the receiving host: that makes you able to construct your own protocol to check UDP port reachability: just put a process on the receiving host that’ll listen on the given UDP port and respond back (or send you an email, or just freak out and unlink() everything on the host file system. anything that’ll trigger your attention will do).

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How to check if TCP / UDP port is open on Linux & Windows Cloud Servers

Verifying which all ports (TCP/UDP) are open and listening on the network interfaces of a server is very important when it comes to server security as well as troubleshooting service-related issues. Vulnerable open ports can be the cause of severe security breaches in a server. It is a must that such ports are found out and closed/disabled.

In case of service-related issues, checking which all ports are in use can be used as a troubleshooting mechanism to find if multiple services are listening on the same port.

The following are the common port numbers:

• Ports 0 to 1023 are Well-Known Ports
• Ports 1024 to 49151 are Registered Ports (*Often registered by a software developer to designate a particular port for their application)
• Ports 49152 to 65535 are Public Ports

Port	Name of the Service	Transport protocol
20,21	FTP	TCP
22	SSH	TCP and UDP
23	Telnet	TCP
25	SMTP	TCP
50,51	IPSec	/
53	DNS	TCP and UDP
67,68	DHCP	UDP
69	TFTP	UDP
80	HTTP	TCP
110	POP3	TCP
119	NNTP	TCP
123	NTP	TCP
135-139	NetBIOS	TCP and UDP
143	IMAP	TCP and UDP
161,162	SNMP	TCP and UDP
389	Lightweight Directory Access	TCP and UDP
443	HTTPS	TCP and UDP
465	SMTP over SSL	TCP
989	FTP Protocol (data) over TLS/SSL	TCP and UDP
990	FTP Protocol (data) over TLS/SSL	TCP and UDP
993	IMAP over SSL	TCP
995	POP3 over SSL	TCP
3389	Remote Desktop	TCP and UDP

This guide outlines the basic steps to determine which all ports are open in a service using commands such as lsof , netstat and nmap in Linux server and netstat on Windows server.

Linux

An example of this is when both Apache and Nginx services run on the same server.

Method 1 — Using lsof command

lsof (list open files) is a command that is used to display the list of all open files in a server and the services that have opened them.

The general syntax of the lsof command is as below:

Using pipe and grep commands, the result of the above command can be filtered to show the result of files that are listening on different ports in the server.

# sudo lsof -i -P -n | grep LISTEN # doas lsof -i -P -n | grep LISTEN (for OpenBSD systems) 

Taking the last line from sample output, the result can be explained as below:

named 812 named 23u IPv4 16018 0t0 TCP 123.123.123.12:53 (LISTEN) 

• named : name of the service.
• 123.123.123.12 : IP on which the named service is bound to.
• 53 : TCP port of the service that is being used.
• 812 : Process ID of the service.

Method 2 — Using netstat command

netstat (network statistics) is a command-line tool that can be used to monitor both incoming and outgoing network connections in a server.

The netstat command along with the grep command to check the listening services can be used in the below syntax

# netstat -tulpn | grep LISTEN # netstat -nat | grep LISTEN (for OpenBSD systems) 

netstat command has been deprecated in the latest versions of Linux distribution. The ss command has taken its place.
The syntax for using the ss command is as provided below:

The switches for the ss command mean as follows:

• t: Show only TCP sockets.
• u: Show only UDP sockets.
• l: Show listening sockets.
• p: Show the name of the process that opened the socket.
• n: Do not try to resolve service names.

Method 3 — Using nmap command

nmap (network mapper) is a network scanner command tool that can be used to find out hosts and services on a server. This is done by sending packets to the server and analyzing the results further.

The general syntax of the nmap command that can be executed from within the server is as follows:

# sudo nmap -sT -O localhost # sudo nmap -sTU -O 123.123.123.12 (scan both TCP and UDP for server) 

Windows

In Windows servers, the netstat command can be used to check the ports that are currently in use in the server.

The syntax for the netstat command to be used in Windows servers is as below:

> netstat -bano | more > netstat -bano | findstr /R /C:"[LISTENING]" 

Each field in the above result mean as below:

• Proto : The protocol being used for the socket (TCP/UDP).
• Local address : Source IP address and port to which the service is listening to.
• State : The current state of the service.
• PID : Process ID of the service, followed by the service name.

Related Tutorials

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How does netcat know if a UDP port is open?

Unlike TCP, UDP is connectionless (fire and forget). So at a high level does anyone know how netcat knows the UDP port is open? Does it ask for a reply or something like that?

4 Answers 4

In fact, it doesn’t. You can check by doing:

$ nc -vz -u 8.8.8.8 53 Connection to 8.8.8.8 53 port [udp/domain] succeeded! $ nc -vz -u 8.8.8.8 54 Connection to 8.8.8.8 54 port [udp/*] succeeded! $ nc -vz -u 8.8.8.8 59 Connection to 8.8.8.8 59 port [udp/*] succeeded! $ 

So with UDP, it’s not something you can really check unless it will give you information back.

Judging by the specific output Connection to Connection to 10.1.0.100 53 port [udp/domain] succeeded! you are using openbsd-netcat.

Looking at the code for that the test is to bind to the UDP socket, i.e. there is an open connection:

 if (vflag || zflag) < /* For UDP, make sure we are connected. */ if (uflag) < if (udptest(s) == -1) < ret = 1; continue; >> /* Don't look up port if -n. */ if (nflag) sv = NULL; else < sv = getservbyport( ntohs(atoi(portlist[i])), uflag ? "udp" : "tcp"); >fprintf(stderr, "Connection to %s %s port [%s/%s] " "succeeded!\n", host, portlist[i], uflag ? "udp" : "tcp", sv ? sv->s_name : "*"); 

udptest issues around 3 writes to the open socket. There is a note that this doesn’t work for IPv6 and fails after around 100 ports checked.

So while the other suggestion may be valid, I don’t think that’s happening in this particular case.

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How to Test Port [TCP/UDP] Connectivity from a Linux Server

Here is a short post to check port [TCP/UDP] connectivity from a Linux server. A TCP/IP network connection may be either blocked, dropped, open, or filtered. These actions are generally controlled by the IPtables firewall the system uses and is independent of any process or program that may be listening on a network port.

Telnet and nc are common tools used to test port connectivity from Linux server. Telnet can be used to test tcp port connections, where as nc can be used to test both tcp/udp ports connectivity. Make sure telnet and nc tools are installed on the Linux server you are trying to test connectivity.

# yum install nc # yum install telnet

Testing TCP port connectivity with telnet

Lets see how we can use telnet command to test the TCP port connectivity. The syntax to use the telnet command is as follows:

# telnet [hostname/IP address] [port number]

Example of successful connection:

# telnet 192.168.12.10 22 Trying 192.168.12.10. Connected to 192.168.12.10. Escape character is '^]'. SSH-2.0-OpenSSH_6.6.1 Protocol mismatch. Connection closed by foreign host.

Example of unsuccessful connection:

# telnet 192.168.12.10 22 Trying 192.168.12.10. telnet: connect to address 192.168.12.10: No route to host

Using nc command to test TCP port connectivity

The syntax to use nc command for testing TCP post connectivity is as follows:

# nc -z -v [hostname/IP address] [port number]

Example of successful connection:

# nc -z -v 192.168.10.12 22 Connection to 192.118.20.95 22 port [tcp/ssh] succeeded!

Example of unsuccessful connection:

# nc -z -v 192.168.10.12 22 nc: connect to 192.118.20.95 port 22 (tcp) failed: No route to host

Testing UDP port connectivity wit nc command

The syntax to test UDP port connectivity with nc command is as follows:

# nc -z -v -u [hostname/IP address] [port number]

Example of successful connection:

# nc -z -v -u 192.168.10.12 123 Connection to 192.118.20.95 123 port [udp/ntp] succeeded!

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