Questions tagged [udp]

UDP stands for User Datagram Protocol. With UDP applications can send messages (datagrams) to other hosts on a network without requiring prior communications to set up special transmission channels or data paths.

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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 ...
Lock's user avatar
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76 votes
3 answers
255k views

Listen to UDP data on local port with netcat

netcat -ul -p2115 fails with a usage statement. What am I doing wrong?
Tyler's user avatar
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49 votes
7 answers
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How can I identify which processes are generating UDP traffic on Linux?

My machine is continously making udp dns traffic request. what i need to know is the PID of the process generating this traffic. The normal way in TCP connection is to use netstat/lsof and get the ...
boos's user avatar
  • 651
48 votes
11 answers
69k views

What is the difference between UDP and TCP?

My router has two protocols (and a "both" option) that I can select when setting up port forwarding: UDP and TCP. What is the difference between these two protocols and when would you select one over ...
Guy's user avatar
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48 votes
3 answers
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telnet counterpart for UDP

Is there anything that enables a "telnet-like" functionality for UDP? I know the difference between TCP and UDP, and why telnet itself won't work - but I'm wondering if there is something similar to ...
Dexter's user avatar
  • 617
47 votes
6 answers
130k views

Does HTTPS use TCP or UDP?

Does HTTPS use TCP or UDP?
Steven's user avatar
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44 votes
5 answers
40k views

Can TCP and UDP packets be split into pieces?

Can TCP packets arrive to receiver by pieces? For example, if I send 20 bytes using TCP protocol, can I be 100% sure that I will receive exactly 20 bytes at once, not 10 bytes then another 10 bytes ...
iamnp's user avatar
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40 votes
6 answers
110k views

Linux Kernel not passing through multicast UDP packets

Recently I've set up a new Ubuntu Server 10.04 and noticed my UDP server is no longer able to see any multicast data sent to the interface, even after joining the multicast group. I've got the exact ...
buecking's user avatar
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36 votes
4 answers
20k views

Do DNS queries always travel over UDP?

I've spent a bit of time researching this topic and can't seem to find an exact answer, so I'm fairly confident it's not a duplicate, and while my question is based on a security need, I think it's ...
Caderade's user avatar
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36 votes
2 answers
86k views

nginx: [emerg] "stream" directive is not allowed here

I have the latest NGINX from ppa installed on Ubuntu 16.04. nginx version: nginx/1.12.1 From my understanding, it should support stream and UDP load balancing. But I get this error message: ...
Houman's user avatar
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31 votes
2 answers
111k views

Does traceroute use UDP or ICMP or both?

When I do a trace on the www.google.com domain from my laptop, am I using icmp or udp ? I thought it was icmp type 11 but while searching for something else I came across rules where icmp type 30 was ...
Kris's user avatar
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27 votes
7 answers
132k views

How can I test Multicast UDP connectivity between two servers?

I have two servers on the same subnet. I have an application installed which uses multicast UDP to propagate events between the two servers and keep them in sync. This does not seem to be happening, ...
Greg B's user avatar
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24 votes
4 answers
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What is the "in-the-wire" size of a ethernet frame? 1518 or 1542?

According to the table here, it says that MTU = 1500 bytes and that the payload part is 1500 - 42 bytes or 1458 bytes (<- this is actually wrong!). Now on top of that you have to add IPv4 and UDP ...
chrisapotek's user avatar
23 votes
4 answers
43k views

Iptables: "-p udp --state ESTABLISHED"

let's look at these two iptables rules which are often used to allow outgoing DNS: iptables -A OUTPUT -p udp --sport 1024:65535 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A ...
Chris Lercher's user avatar
22 votes
3 answers
30k views

Sockets found by lsof but not by netstat

I have an application which is running out of file descriptors, apparently by opening sockets, but I can't find out exactly what these sockets do. These appear in the lsof output as java 9689 ...
Robert Munteanu's user avatar
21 votes
6 answers
72k views

How is the MTU is 65535 in UDP but ethernet does not allow frame size more than 1500 bytes

I am using a fast ethernet of 100 Mbps, whose frame size is less than 1500 bytes (1472 bytes for payload as per my textbook). In that, I was able to send and receive a UDP packet of message size 65507 ...
user avatar
20 votes
6 answers
13k views

Why would a university block incoming UDP traffic with destination port 53?

From my understanding DNS uses UDP and port 53. What undesirable things could happen if incoming UDP packets to port number 53 weren't blocked? UPDATE: Packets originate or are destined to the ...
Daniel Kobe's user avatar
19 votes
2 answers
57k views

Reduce firewall rules by half - one iptables rule for tcp and udp

I have a number of iptables rules on my firewall that look like this: iptables -A zone_lan_forward -p tcp -d 1.2.3.0/24 -j ACCEPT iptables -A zone_lan_forward -p udp -d 1.2.3.0/24 -j ACCEPT Is there ...
Big McLargeHuge's user avatar
18 votes
1 answer
590 views

Vendor whitepaper says: 5Mpps no prob. I'm already hitting a wall at 120kpps. Where's the bottleneck?

HP's whitepaper on their QLogic (fka Broadcom) NetXtreme II adapters, which includes the specific NIC that I'm testing, states (page 7) that their small packet performance for packets up to 256 bytes/...
Evgeniy Berezovsky's user avatar
17 votes
6 answers
13k views

Solution to route/proxy SNMP Traps (or Netflow, generic UDP, etc) for network monitoring?

I'm implementing a network monitoring solution for a very large network (approximately 5000 network devices). We'd like to have all devices on our network send SNMP traps to a single box (technically ...
Christopher Cashell's user avatar
16 votes
2 answers
115k views

Verify connectivity to a server on a UDP port

I want to know if a HP-UX machine can communicate to a remote machine (which I don't have control over) on a particular port using UDP. I tried telnet, but it doesn't look like it supports UDP. I ...
skinp's user avatar
  • 779
14 votes
3 answers
177k views

How to connect to a udp port command line?

This is what I tried,but seems not working : [root@ ~]# netstat -a|grep 48772 udp 0 0 *:48772 *:* [root@ ~]# telnet localhost ...
yum's user avatar
  • 589
14 votes
4 answers
2k views

tcpdump increases udp performance

I'm running a set of load tests to determine the performance of the following setup: Node.js test suite (client) --> StatsD (server) --> Graphite (server) In short, the node.js test suite ...
Ruben Homs's user avatar
12 votes
3 answers
8k views

Where is the documentation that states TCP and UDP source port should be over 1024 and random?

I am having a hard time to find where it is documented that the source port should be random and in the range 1024-65535. In which RFC is this documented? Edit: First reference for privileged ports ...
Mircea Vutcovici's user avatar
12 votes
2 answers
39k views

Extreme UDP packet loss at 300Mbit (14%), but TCP > 800Mbit w/o retransmits

I have a linux box I use as the iperf3 client, testing 2 identically equipped Windows 2012 R2 server boxes with Broadcom BCM5721, 1Gb adapters (2 ports, but only 1 used for the test). All machines are ...
Evgeniy Berezovsky's user avatar
11 votes
5 answers
45k views

How to list all requests to udp sockets?

I'm operating a couple of server daemons that use udp to communicate with large number of clients. How do I find and list out all the active udp "connections" that are talking to the servers in order ...
Flint's user avatar
  • 641
11 votes
2 answers
4k views

mosh-like port forwarding

This is on linux, connecting to linux servers: I love mosh, but it doesn't support port forwarding, and likely won't for a while since it's been almost a year now and it hasn't happened yet. port ...
Marc Merlin's user avatar
10 votes
3 answers
17k views

Load Balancing DNS Servers: UDP / TCP

I have been asked to rebuild our load balancing infrastructure in the data center. The original request was to load balance FTP servers. I tried doing that using the current load balancer (Piranha / ...
moshpitcodes's user avatar
10 votes
4 answers
2k views

How common are dropped packets in communications within a data center?

Let's say I have 2 machines in the same data center but not necessarily in the same rack. How common would dropped packets be when sent using UDP between these two machines? I'm asking under the ...
z8000's user avatar
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10 votes
3 answers
5k views

Windows Server 2012 - RDP over UDP not working

I have a Windows Server 2012 (not R2) machine, hosted within Hyper-V virtualization with RD Session Host & RD Gateway installed. It is used to run a desktop GIS application. The performance over ...
Mirko's user avatar
  • 101
10 votes
1 answer
50k views

OpenVPN Error : TLS Error: local/remote TLS keys are out of sync: [AF_INET]

Fist off thanks for reading this, I appreciate any and all suggestions. I am having some serious problems reconnecting to my OpenVPN client using Riseup.net's VPN. I have spent a few days banging my ...
Lucidity's user avatar
  • 103
10 votes
2 answers
12k views

Need a live UDP echo Server IP and port

I require to test my UDP program using echo server. I have tested this application in LAN but now I require it to test it in Internet. Is there and globally recognize freely available well known ...
Amit's user avatar
  • 221
9 votes
2 answers
9k views

Symmetric NAT and UDP Hole Punching

I've read this question, but the explanation of Symmetric NAT wasn't detailed enough. Please could someone help me to understand the following paragraphs? I read this about Symmetric NAT: Each ...
john's user avatar
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9 votes
2 answers
33k views

Higher rmem_max value leading to more packet loss

The rmem_max Linux setting defines the size of the buffer that receives UDP packets. When traffic becomes too busy, packet loss starts occurring. I made a graph showing how packet loss increases ...
Nicolas Raoul's user avatar
9 votes
1 answer
24k views

UDP packets seen on interface level but not delivered to application on RedHat

I have a problem with receiving UDP traffic on a RHEL6 server. When capturing the traffic with tcpdump, the packets are visible (checksums are correct, the destination IP and MAC match the machine's ...
wojciech_k's user avatar
8 votes
1 answer
6k views

HTTP, TCP, UDP and connectionless

I am a bit confused with HTTP lately. Some facts are that TCP can operate connection orientated or connectionless, this I understand. TCP though is connection-oriented while UDP is connectionless ...
What'sTheStoryWishBone's user avatar
8 votes
2 answers
60k views

How to start iperf3 server in UDP mode?

I would like to test UDP connectivity with iperf3 but do not know how to start it in UDP server mode. iperf3 -s only opens a TCP socket: root@srv ~# lsof -i -P | grep iperf3 iperf3 21030 ...
WoJ's user avatar
  • 3,657
8 votes
2 answers
16k views

How are UDP source ports selected?

I just did a packet capture from my machine and filtered out all the UDP connections. I saw certain connections using what appears to be a randomly generated UDP Source port, and certain connection ...
Eddie's user avatar
  • 103
8 votes
2 answers
12k views

UDP traversal through router automatically without router port forwarding?

Everyone knows the router is working like a firewall. Without port forwarding, a connection (no matter TCP or UDP) cannot be initiated from outside of the router. However, if a user initiate a TCP ...
user avatar
8 votes
2 answers
7k views

Is it possible to duplicate a UDP unicast stream with iptables?

I have a proprietary system which transmit a udp video stream from unit #1 (192.168.1.1) to unit #2 (.1.2). I can't make changes to this system, and I'm trying to clone this udp stream so I can access ...
Håkon K. Olafsen's user avatar
7 votes
2 answers
4k views

Are DNS queries encrypted?

I wonder if DNS queries are encrypted. If so, can you briefly explain the process as I suppose for an encryption to take place, there should at least be a minimum common ground between two parties (...
Ninja Bug's user avatar
  • 173
7 votes
3 answers
23k views

Netcat UDP File Transfer?

Is there any way to send a file (picture or video) using Netcat and UDP. It defaults as TCP, but I would like to send using UDP. I tried simply adding -u to the nc command, but that didn't work. Here ...
Vinnie's user avatar
  • 71
7 votes
1 answer
24k views

Tool for forwarding UDP packets Windows

Windows Server 2008 I use portproxy interface to forward traffic , but I looked into the msdn page and I noticed it doesn't support udp protocol . protocol Specifies the protocol to use. ...
bit8bug's user avatar
  • 95
7 votes
1 answer
17k views

Block all incoming DNS requests EXCEPT from IPs x,y,

I want to block all incoming requests on my two DNS servers APART FROM certain IP addresses e.g. IP of 1.2.3.4 will be allowed to make requests but NOBODY else will. How do you do this with iptables? ...
ale's user avatar
  • 933
7 votes
1 answer
17k views

How to create UDP proxy using iptables

I have a server running UDP server on port 50000. This server receives updates from clients, and pings back the clients every 5 seconds to the socket they were connected from. Now I want to redirect ...
arikfr's user avatar
  • 359
7 votes
1 answer
8k views

rsyslog udp forwarding truncates at 2048 characters

Rsyslog on a RHEL 6 server is receiving messages locally on port 514 UDP. These messages are sometimes much larger than normal syslog message sizes. I am seeing rsyslog handle all of the messages ...
nictrix's user avatar
  • 173
7 votes
2 answers
2k views

Why multicast UDP can be slower than unicast UDP?

I have two machines (with Centos 7.3 on both) connected via 1G switch. When I measure UDP thoughput between macines with iperf it gives about 850 Mbit/s. However when I measure multicast UDP thoughput ...
Alexei Osipov's user avatar
6 votes
3 answers
3k views

Why is it 3-way-handshake and not more?

TCP uses the 3-way-handshake mechanism. I understand how it works pretty well but I wonder why it's not 4, or even 5-way? Is it because that's too much overhead and is not needed?
IcySnow's user avatar
  • 203
6 votes
2 answers
8k views

How to determine which process is sending UDP packets once per hour?

I was doing a packet capture as part of a development project and saw some odd traffic coming from my machine in the capture file. About every 3600 seconds, a NAT-PMP request is being sent to the IP "...
fdmillion's user avatar
  • 419
6 votes
1 answer
4k views

UDP hole punching still required in IPv6, even without NAT?

Background (skip ahead for question) IPv4 needed NAT for address conservation. The firewalling properties of NAT were also beneficial for security. IPv4 NAT firewall rules are "block incoming ...
Peter Stock's user avatar

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