By the way, the necat command is trying to use port 53 TCP, not UDP, which may be the problem. – NerdOfLinux Aug 18 '17 at 2:31 True, but my dig command is also failing, which I suppose is more to the point.

Look man, you're talking a lot but the answer to the question remains that you only need port 53 open on a host that serves DNS to the network. Execute 'tcpdump -n -s 1500 -i eth0 udp port 53' to confirm that a client DNS request never uses port 53 on the localhost – venzen Feb 21 '13 at 6:26 Oct 21, 2019 · 53 – Domain Name System (DNS) 80 – Hypertext Transfer Protocol (HTTP) 110 – Post Office Protocol (POP3) 143 – Internet Message Access Protocol (IMAP) 443 – HTTP Secure (HTTPS) Since there are so many thousands of common port numbers, the easiest approach is to remember the ranges. Which will tell you if a given port is reserved or not. It is possible to bypass the rules of the remote firewall by sending UDP packets with a source port equal to 53. An attacker may use this flaw to inject UDP packets to the remote hosts, in spite of the presence of a firewall. Solution Either contact the vendor for an update or review the firewall rules settings. See Also What is really odd to me, however, is that I get the "UDP 53 Denied" message when I run the Basic Scan, then the Advanced Scan shows TCP 443 as the *only* port allowed and *all* other ports denied. But, maybe my Westell 6200 (AT&T) single-user modem/router combo and Linksys E2500 router have something worked out there. Remediating UDP Source Port Pass Firewall Vulnerability on ESXi servers ESXi uses a stateless firewall. Consequently, it has a rule to allow incoming DNS traffic (UDP) through source port 53. The easiest way to fix this vulnerability is to restrict the access on this port to the local DNS server IP addresses. Check UDP connection to any port of any IP or website from different places. For example you could check a DNS or game server for availability from many countries. Examples: domain://

Port 53 (UDP and TCP) Port 80 (TCP) Port 500 (UDP) Port 3544 (UDP) Port 4500 (UDP) Me too. 0 Kudos Report Inappropriate Content. Message 1 of 2 (1,648 Views)

If you are trying to see if connectivity works on DNS request (normally uses UDP/53), then the answer is no, telnet on port 53 will not work. If you are trying to test DNS resolution/request, then just use "nslookup" from command prompt, then specify the dns server: server , then test the resolution:

TCP port 21 — FTP (File Transfer Protocol) TCP port 22 — SSH (Secure Shell) TCP port 23 — Telnet. TCP port 25 — SMTP (Simple Mail Transfer Protocol) TCP and UDP port 53 — DNS (Domain Name System)

Port 53 should now be free on your Ubuntu system, and you shouldn't be getting errors like "listen tcp bind: address already in use" anymore. You can check to see if port 53 is in use or not by running sudo lsof -i :53 - if port 53 is not in use, this command shouldn't show any output. As we witnessed with the dhcpserver port on Felix, certain open UDP services can hide even from Nmap version detection. He has also only scanned the default ports so far, there are 64529 others that could possibly be open. For the record, 53 is the only open UDP port on Scanme. 2. Port 53: Port 53 is used by DNS. Let’s see one DNS packet capture. Here is trying to send DNS query. So destination port should be port 53. Now we put “udp.port == 53” as Wireshark filter and see only packets where port is 53. 3. Port 443: Port 443 is used by HTTPS. Let’s see one HTTPS packet capture. DNS primarily uses the User Datagram Protocol (UDP) on port number 53 to serve requests. DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. When the length of the answer exceeds 512 bytes and both client and server support EDNS, larger UDP packets are used.