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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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 ...
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5answers
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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 ...
17
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2answers
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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 ...
14
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4answers
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Does HTTPS use TCP or UDP?

Does HTTPS use TCP or UDP?
14
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5answers
17k 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 ...
11
votes
3answers
16k views

how i can identify which process is making 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 ...
11
votes
2answers
9k 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 ...
11
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3answers
8k 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 ...
10
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4answers
959 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 ...
9
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5answers
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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 ...
7
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3answers
1k views

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 ...
7
votes
4answers
13k views

How can I test Multicast UDP connectivity between two servers?

I have a two servers on the same subnet. I have an application installed which uses Multicast UDP to propagate events between the servers and keep them in sync. This does not seem to be happening, ...
7
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2answers
12k views

listen to UDP data on local port with netcat

netcat -ul -p2115 fails with a usage statement. What am I doing wrong? Thanks!
7
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1answer
2k views

iperf max udp multicast performance peaking at 10Mbit/s?

I'm trying to test UDP multicast throughput via iperf but it seems like it's not sending more than 10Mbit/s from my dev machine: C:\> iperf -c 224.0.166.111 -u -T 1 -t 100 -i 1 -b 1000000000 ...
6
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3answers
2k 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 ...
6
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4answers
14k views

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 ...
5
votes
2answers
7k 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 ...
5
votes
1answer
262 views

Hardware/topology recommendations for low latency UDP network

I'm setting up a LAN comprised of 60 end nodes that are receiving a high volume of UPD datagrams from one master server. The server sends 60 unique datagrams every 100ms, each datagram is 64 bytes. ...
5
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1answer
1k 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 ...
5
votes
1answer
5k views

OpenVPN failing on self-signed certificate over udp, works over tcp

I have the following server.conf: # OpenVPN 2.x config proto tcp port 1194 dev tun-vpn dev-type tun server 10.8.0.0 255.255.0.0 push "route 172.16.0.0 255.255.0.0" push "dhcp-option DOMAIN ...
4
votes
2answers
6k views

Should I use TCP or UDP to run a web server

I have just installed Apache web server on my computer. I have managed to use it locally (I can open index.php from my computer using my web browser). But I would like to make my web site available ...
4
votes
2answers
979 views

Is OpenVPN UDP vulnerable to heartbleed?

Is OpenVPN UDP vulnerable to heartbleed? I need to decide if I'm going to rebuild some servers, but they are very carefully firewalled; 1194/TCP is one of the firewalled ports (yay whitelist!). ...
4
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4answers
3k 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 ...
4
votes
1answer
633 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 ...
4
votes
1answer
429 views

possible UDP attack on BIND?

hello everyone, i was surprised last month when my EC2 instance (ubuntu precise server), that is supposed to be under the free tier still, accumulated lots of traffic... today, while checking my ...
4
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1answer
142 views

Script to find the address udp packets are being sent to?

Here is the scenario. I have an IP address 1.2.3.4 port 2000 sending udp packets to one unknown IP. I would like to find the unknown IP so I can block it with iptables. Is there a way to do it with ...
4
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1answer
161 views

How does an Access Point handle broadcast packets from the wired side?

I have an Cisco AP 1242AG autonomous AP connected to L2 switches. Let's say I have 100 wired devices each sending 2 broadcast UDP packets per second. Obviously these packets will reach the Access ...
4
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2answers
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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 ...
4
votes
2answers
769 views

What is the equivalent of Cisco's 'ip virtual-reassembly' for a Juniper ISG 2000

I tested my dns servers with the oarc test and my size limit is at least 1403 bytes. I performed the same test before my Juniper ISG 2000 and the result is 2047 bytes. According to the chapter IP ...
3
votes
2answers
8k 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 ...
3
votes
1answer
200 views

Difference between /dev/udp and netcat

I have a syslog server listening on localhost:514 as UDP and would like to write messages to it on that port. (Using Ubuntu 14.04) If I run either of these commands this from bash it prints the date ...
3
votes
2answers
168 views

Finding the executable sending packets to a specific IP Address?

We host Game Servers off of Windows Server 2008 machines, and we just received a report that one of our customers is using their server to do some type of UDP attack. The person being attacked ...
3
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1answer
807 views

How to add a broadcast address to loopback with ifconfig on a OS/X?

I am trying to use ifconfig to turn on broadcast on my loopback interface. It currently reads: lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 As you can see, no broadcast address! ...
3
votes
4answers
2k 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 ...
3
votes
1answer
3k 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 ...
3
votes
1answer
984 views

tcpdump udp data capture latency

I am using tcpdump to capture UDP packets and analyze network latency between a UDP broadcaster and my server. To calculate latency I compare the source host timestamps reported in UDP application ...
3
votes
2answers
197 views

Port Knocking and TCP/IP within the OSI model

I have read that Port Knocking can be subject to false negative (type II error) due to the optimization techniques of TCP/IP. A correct port knock given by the client may be read as incorrect by the ...
3
votes
2answers
487 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 ...
3
votes
1answer
2k 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 ...
3
votes
1answer
598 views

Does Windows XP automatically reassemble UDP fragments?

I've got a Windows application that receives and processes XML messages transmitted via UDP. The application collects the data using Windows "raw" sockets, so the entire layer 3 packet is visible. ...
3
votes
2answers
115 views

NetBIOS over TCP/IP with no UDP

Is there a way to run NetBIOS entirely on TCP? I have an environment where UDP is not allowed. I need a way for the machines to talk to each other by name. I thought there might be a way to say that ...
3
votes
3answers
2k views

udp through nat

I've two private networks (each of them behind a typical dsl router). The routers are connected to the WWW. The extern interface of each router have one dynamic IP address. I want to stream data via ...
3
votes
2answers
984 views

Many clients on a wireless AP for UDP broadcast packets

I asked this question on StackOverflow and was directed over here, so I'd appreciate any advice. I'm deploying a smartphone application as part of a live music performance that depends on receiving ...
3
votes
2answers
145 views

Service to temporarily queue UDP packets

I have a service running on a linux box that receives a fairly constant stream of packets via UDP. The occasional loss or corruption of packets associated with using UDP is fine. However, the service ...
3
votes
1answer
431 views

one particular laptop does not receive UDP broadcast messages

I am trying to debug a problem with a particular laptop which is running Ubuntu. We have a service which uses broadcast UDP messages, and this laptop is not receiving them on its wireless interface, ...
3
votes
1answer
783 views

Reverse proxy for mixed tcp, udp, and http services

I'm building an architecture that will be hosting various services which individually maybe listening on a variable number of ports that could be mixed protocols. I want to map these services to ...
3
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0answers
241 views

UDP Overflow / UDP Drops on Standby Postgres Service

I am stumped trying to prevent an overflowing UDP buffer on a standby Postgres service. Any help would be most appreciated. Essentially a UDP buffer associated with the pg_standby process on my ...
3
votes
1answer
358 views

UDP Packet Size.. max, ping failing to 3G.. Recalcitrant software vendor

We have a network of 330 clients (Service Vans) connected via 3G to our central office. Currently these clients are connected via a small ruggedized 3G router that establishes the 3G connection and ...
2
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2answers
2k 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 ...
2
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3answers
922 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?