101
votes
Testing UDP port connectivity
To test if udp port is responding, use netcat.
An example from the man page:
nc -v -u -z -w 3 example.host 20-30
Send UDP packets to ports 20-30 of example.host, and report which ones
did ...
- 1,691
76
votes
Testing UDP port connectivity
both on client ans server install nc:
yum install nc (for centos)
on server listen UDP port: nc -ul 6111 (add the -6 option if you're testing an ipv6 connection)
on client nc -u <server> 6111
...
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60
votes
Accepted
nginx: [emerg] "stream" directive is not allowed here
stream needs to be on the same level as http block so like
http { foo }
stream { bar }
My guess is your include for /etc/nginx/conf.d/*.conf is located in the http {} block and not outside of it. ...
- 22.2k
40
votes
Accepted
Why would a university block incoming UDP traffic with destination port 53?
The logic works like this:
Only authoritative DNS servers that provide records to the internet are required to be exposed.
Open recursive servers that are exposed to the internet will inevitably be ...
- 32.2k
40
votes
Accepted
Do DNS queries always travel over UDP?
Normal DNS queries use UDP port 53, but longer queries (> 512 octets) will receive a 'truncated' reply, that results in a TCP 53 conversation to facilitate sending/receiving the entire query. Also, ...
- 814
29
votes
Do DNS queries always travel over UDP?
This started as a comment to George's answer, but it got long. The larger picture is somewhat complicated, as it requires understanding some history.
RFC 1035 originally called for a limit of 512 ...
- 32.2k
24
votes
Why would a university block incoming UDP traffic with destination port 53?
For example, attackers could use university's DNS server as transit host for DNS Amplification DDoS Attack
- 1,642
23
votes
How can I identify which processes are generating UDP traffic on Linux?
I had exactly the same problem and unfortunately auditd didn't do much for me.
I had traffic from some of my servers going towards google DNS addresses, 8.8.8.8 and 8.8.4.4. Now, my network admin ...
- 5,257
15
votes
Accepted
Load Balancing DNS Servers: UDP / TCP
Don't load balance your DNS.
It's an incredibly light protocol - you'd need an enormous amount of traffic to need more than one box (in which case you'll just be bottlenecking on your load balancer ...
- 114k
14
votes
Accepted
Vendor whitepaper says: 5Mpps no prob. I'm already hitting a wall at 120kpps. Where's the bottleneck?
RSS too is enabled in the NIC settings, with 8 queues.
Which unfortunately did not mean that RSS was being employed, as
netsh int tcp show global
showed:
TCP Global Parameters
---------------------...
13
votes
How is the MTU is 65535 in UDP but ethernet does not allow frame size more than 1500 bytes
UDP doesn't know anything about MTU. UDP packets can have any size from 8 to 65535 bytes. The protocol layers below UDP either can send a packet of a specific size or will reject to send that packet ...
- 859
12
votes
Does HTTPS use TCP or UDP?
Nowadays HTTPS can run above either TCP or UDP.
The new "QUIC" protocol aims to replace multiple TCP connections with one multiplexed UDP connection, and hence can handle SSL and HTTPS:
HTTPS &...
- 515
12
votes
Listen to UDP data on local port with netcat
For some reason, Andrew's solution didn't work for me. With further research, I learned that we need to add -p flag when working locally. So the following command worked for me.
nc -ulp 2115
- 221
12
votes
tcpdump increases udp performance
When tcpdump is running, it will be fairly prompt at reading in the incoming frames. My hypothesis is that the NIC's packet ring buffer settings may be a bit on the small size; when tcpdump is running ...
- 3,959
12
votes
Accepted
Does UDP includes of source address?
Both UDP and TCP does not include source address
Your question is based on a misunderstanding. It's true that neither UDP or TCP headers include a source address, but that's because they don't have ...
- 32.2k
12
votes
How to determine which process is sending UDP packets once per hour?
You can easily accomplish this with SysInternals' Process Monitor. Run it as Administrator, then configure it as follows:
On the Filter menu click Filter...
In the first drop down box, select ...
- 3,111
11
votes
Load Balancing DNS Servers: UDP / TCP
I'm uncomfortable with this Q&A because it hasn't really been established what type of DNS server you're talking about. There are some significant misconceptions when it comes to the resiliency of ...
- 32.2k
11
votes
Why would a university block incoming UDP traffic with destination port 53?
Andrew B's answer is excellent. What he said.
To answer the question "What undesirable things could happen if incoming UDP packets to port number 53 weren't blocked?" more specifically, I googled "...
- 18.6k
11
votes
Are DNS queries encrypted?
DNS, by default and when created, does not offer neither authenticity (being sure you get a response from the true authoritative nameserver of the zone) nor confidentiality (making sure no one on the ...
- 9,721
10
votes
Verify connectivity to a server on a UDP port
You can use netcat. In OSX, this is my command:
$ nc -vnzu <IP> <PORT>
found 0 associations
found 1 connections:
1: flags=82<CONNECTED,PREFERRED>
outif (null)
src <SOURCE IP> ...
- 406
9
votes
Extreme UDP packet loss at 300Mbit (14%), but TCP > 800Mbit w/o retransmits
There is no problem. This is normal and expected behaviour.
The reason for the packet loss is that UDP doesn't have any congestion control. In tcp when congestion control algorithms kick in, it ...
- 14.4k
9
votes
Accepted
UDP hole punching still required in IPv6, even without NAT?
Before answering the questions, I'd like to address some of the assumptions in it.
The firewalling properties of NAT were also beneficial for security.
This is often-repeated, but simply not true; ...
- 79.2k
8
votes
Accepted
How to block 111 udp port via iptables?
Depending on how nice you want to be to the client, a possible solution can be:
iptables -I INPUT -p udp --dport 111 -j DROP
or
iptables -I INPUT -p udp --dport 111 -j REJECT
- 4,453
7
votes
Testing UDP port connectivity
You can scan udp ports by using following command
nmap -sU -v <hostname or ip>
- 171
7
votes
Troubleshooting UDP multicast on Windows
If you have a VirtualBox Host-Only Ethernet Adapter activated on your system that might be the problem, which it was for me, so try disabling it. See https://www.virtualbox.org/ticket/8698.
- 171
7
votes
How can I identify which processes are generating UDP traffic on Linux?
Here's a systemtap option, using the netfilter probes available in stap verson 1.8 and later. See also man probe::netfilter.ip.local_out.
# stap -e 'probe netfilter.ip.local_out {
if (dport == 53) ...
- 301
7
votes
Accepted
php-fpm php_network_getaddresses calls randomly start failing with bad udp cksum
You have a defective security fix installed -- this sounds like the issue from USN-3239-2.
A security update for GNU libc that addressed (among other things)...
an unbounded stack allocation in ...
- 22.4k
7
votes
Accepted
netdata ipv4 UDP errors
netdata uses statsd as the metrics collection system. This is a UDP based protocol which is incredibly fast and efficient, but at high rates can overflow the recv_buffer of the ingress node. The ...
- 1,693
7
votes
Accepted
Are DNS queries encrypted?
Standard DNS is not encrypted anywhere. DNSSEC has cryptographically signed (but still not encrypted) responses. There have been some non-standard ideas and implementations over the years, but nothing ...
- 413
7
votes
Accepted
How to start iperf3 server in UDP mode?
The syntax is a bit different for iperf3. Example 1Mbps udp test:
server side:
iperf3 --server
client side:
iperf3 --udp --client client.ip.address --bitrate 1M
What I find really interesting is the ...
- 86
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