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firewall - Why open 123 udp in both directions? - Super User (Linux PCs being able to use NTP, but Windows PCs mysteriously not receiving any replies, and it always turned out to be a port-123 firewall rule ) Dynamically assigned ports are usually chosen from the "ephemeral" range, so they generally don't collide with any service worth blocking (as service ports are typically chosen from a lower range)
Why does NTP require bi-directional firewall access to UDP port 123? When operating in symmetric modes (1 and 2), this field must contain the NTP port number PORT (123) assigned by the IANA Since the client's source port is 123, when the server sends the response back it'll send it to port 123 Naturally, in order to be able to receive that response the client must allow incoming responses on port 123
NTP: Common issues and troubleshooting - Cisco Community Confirm UDP port 123 is open throughout the network for NTP packets Alternate design option If you have a high end router, use it as a "NTP master" and have it poll to public time servers, and everyone else poll this high end router
Solved: Open UDP Ports 123 161 500 - Cisco Community Solved: Hallo, I have a few C1100 routers that seem to have some UDP ports open Unless I disable the protocols e g NTP, the port are as below after an nmap scan: 123 udp open ntp 161 udp open snmp 500 udp open isakmp Our SP says that there is
Change source port of NTP requests on router using iptables iptables -t nat -I POSTROUTING -p udp -m udp --sport 123 -j MASQUERADE --to-ports 60000-61000 AT T blocks outbound UDP traffic with source port 123, to mitigate common NTP reflection attacks from compromised machines on customer networks Destination port 123 is allowed, but most NTP clients use 123 for the source port as well
NTP Rule is bidirectional - Cisco Community Hi, I confuse how to work NTP traffic My network device need NTP from window server the firewall is between my network device and Window server I enable NTP server service on window So i would like to know If i open NTP port 123 on firewall for the traffic from my network device to window server
ubuntu - What are the iptables rules to permit ntp? - Super User in any case, NTP is UDP port 123, so, assuming you are a CLIENT and want to access NTP servers you'd do: iptables -A OUTPUT -p udp --dport 123 -j ACCEPT iptables -A INPUT -p udp --sport 123 -j ACCEPT these will append the rules to the end of your OUTPUT and INPUT chains Assuming you want to be a server, you'd do
Can I setup my OpenVPN home server on port 123 udp? You can set up OpenVPN on any UDP port number you like, it doesn't care; but it might break Synology's own NTP access, depending on which NTP software it uses, because an often-used NTP "symmetric" mode actually uses ports '123→123' instead of the typical 'random→123' (Windows NTP client is one example but I've seen this on Linux as well)
Opening UDP port 123 - Cisco Community Good morning, I've been having problems with external NTP time servers not responding, causing time sync problems in our network Can someone tell me the best way to handle allowing traffic on UDP port 123?
Linux: change default port for NTPD via iptables - Super User OP specifically mentioned the src_port, and DNAT does the opposite Yes, it is actually not uncommon for NTP packets to have both sport=123 and dport=123, usually when the NTP client is working in "peer" mode of some kind, and yes, I have also seen this conflict with ISP-level blocking (when the intent is to prevent customers from accidentally running open NTP servers)