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 process associated at the pid.

Is UDP the connection is stateles, so, when i call netastat/lsof I can see it only if the UDP socket is opened and it's sending traffic.

I have tried with lsof -i UDP and with nestat -anpue, but I can't find wich process is doing that request because i need to call lsof/netstat exactly when the udp traffic is sended, if i call lsof/netstat before/after the udp datagram is sended is impossible to view the opened UDP socket.

Call netstat/lsof exactly when 3/4 udp packet is sended is IMPOSSIBLE.

How I can identify the infamous process? I have already inspected the traffic to try to identify the sended PID from the content of the packet, but is not possible to identify it from the contect of the traffic.

Anyone can help me ?

I'm root on this machine FEDORA 12 Linux noise.company.lan #1 SMP Wed Jul 7 04:49:59 UTC 2010 x86_64 x86_64 x86_64 GNU/Linux

7 Answers 7


Linux auditing can help. It will at least locate users and processes making datagram network connections. UDP packets are datagrams.

First, install the auditd framework on your platform and ensure that auditctl -l returns something, even if it says that no rules are defined.

Then, add a rule to watch the system call socket() and tag it for easy finding later (-k). I need to assume that you are on a 64-bit architecture, but you can substitute b32 in place of the b64 if you aren't.

auditctl -a exit,always -F arch=b64 -F a0=2 -F a1\&=2 -S socket -k SOCKET

You have to pick through man pages and header files to build this, but what it captures is essentially this system call: socket(PF_INET, SOCK_DGRAM|X, Y), where the third parameter is unspecified but frequently zero. PF_INET is 2 and SOCK_DGRAM is 2. TCP connections would use SOCK_STREAM which would set a1=1. (SOCK_DGRAM in the second parameter may be ORed with SOCK_NONBLOCK or SOCK_CLOEXEC, hence the &= comparison.) The -k SOCKET is our keyword we want to use when searching audit trails later. It can be anything, but I like to keep it simple.

Let a few moments go by and review the audit trails. Optionally, you could force a couple of packets by pinging a host out on the net, which will cause a DNS lookup to occur, which uses UDP, which should trip our audit alert.

ausearch -i -ts today -k SOCKET

And output similar to the section below will appear. I'm abbreviating it to highlight the important parts

type=SYSCALL ... arch=x86_64 syscall=socket success=yes exit=1 a0=2 a1=2 ... pid=14510 ... auid=zlagtime uid=zlagtime ... euid=zlagtime ... comm=ping exe=/usr/bin/ping key=SOCKET

In the above output, we can see that the ping command caused the socket to be opened. I could then run strace -p 14510 on the process, if it was still running. The ppid (parent process ID) is also listed in case it is a script that spawns the problem child a lot.

Now, if you have a lot of UDP traffic, this isn't going to be good enough and you'll have to resort to OProfile or SystemTap, both of which are currently beyond my expertise.

This should help narrow things down in the general case.

When you are done, remove the audit rule by using the same line you used to create it, only substitute -a with -d.

auditctl -d exit,always -F arch=b64 -F a0=2 -F a1\&=2 -S socket -k SOCKET
  • i will try it, but i think is the right answer.
    – boos
    Oct 21, 2010 at 8:57
  • This isn't picking up traffic being dropped by iptables, at least for me.
    – 2rs2ts
    Jul 13, 2016 at 20:56
  • 1
    +1 for the simplicity compared to the systemtap method (which is more analytical, but needs kernel devel packages)
    – basos
    May 5, 2017 at 14:17

You can use netstat, but you need the right flags, and it only works if the process that is sending the data is still alive. It won't find the traces of something that came briefly to life, sent UDP traffic, then went away. It also requires local root privileges. That said:

Here's me starting an ncat on my local host, sending UDP traffic to port 2345 on a (non-existent) machine

[madhatta@risby]$ ncat -u 2345 < /dev/urandom

Here's some tcpdump output proving that the traffic is going:

[root@risby ~]# tcpdump -n -n port 2345
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
12:41:32.391750 IP > UDP, length 8192
12:41:32.399723 IP > UDP, length 8192
12:41:32.401817 IP > UDP, length 8192
12:41:32.407051 IP > UDP, length 8192
12:41:32.413492 IP > UDP, length 8192
12:41:32.417417 IP > UDP, length 8192

Here's the useful bit, using netstat with the -a flag (to see port details) and the -p flag to see process ID details. It's the -p flag that requires root privileges:

[root@risby ~]# netstat -apn|grep -w 2345
udp        0      0            ESTABLISHED 9152/ncat     

As you can see, pid 9152 is fingered as having a connection open to port 2345 on the specified remote host. Netstat helpfully also runs that through ps and tells me the process name is ncat.

Hopefully that's of some use.

  • really nice done! :thumbup:
    – ThorstenS
    Oct 20, 2010 at 12:30
  • 5
    There's a catch though. If the problem is caused by a shell script spawning a subprocess that does the DNS lookup and that process quickly exits, then the source port (57550 above) will change all the time. In this case, the technique will not work and you'll have to take more drastic measures. Additionally, your netstat should have done a grep -w 57550 because multiple processes could be doing DNS lookups to the same server. Your method would not distinguish them. Oct 20, 2010 at 20:58
  • 1
    I agree with both of your objections, zerolagtime (but thanks for your kind words anyway, ThorstenS!).
    – MadHatter
    Oct 20, 2010 at 21:05

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, and Now, my network admin has mild OCD and he wanted to clean all the unnecessary traffic since we have our intern DNS caches. He wanted to disable outgoing port 53 for everyone except those cache servers.

So, after failing with auditctl, I dig into systemtap. I come up with the following script:

# cat >> udp_detect_domain.stp <<EOF
probe udp.sendmsg {
  if ( dport == 53 && daddr == "" ) {
    printf ("PID %5d (%s) sent UDP to %15s 53\n", pid(), execname(), daddr)

Then simply run:

stap -v udp_detect_domain.stp

This is the output that I got:

PID  3501 (python) sent UDP to 53
PID  3501 (python) sent UDP to 53
PID  3506 (python) sent UDP to 53

That's it! After changing resolv.conf those PIDs didn't pick up the changes.

Hope this helps :)


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) # or parametrize
      printf("%s[%d] %s:%d\n", execname(), pid(), daddr, dport)

I would use a net-sniffer like tcpdump or wireshark to view the DNS requests. The contents of the query can give an idea of what program is issuing them.

  • no information in the traffic sniffed i just already view.
    – boos
    Oct 20, 2010 at 10:57
  • No info? Empty packets? What I meant was, if something is attempting to resolve updates.java.sun.com or rss.cnn.com you might usefully infer something from it. Oct 20, 2010 at 11:54
  • dns query searching for the internal proxy. right now i found wich process is, but the question stil alive for a general problem solving technics
    – boos
    Oct 20, 2010 at 12:23
  • lsof -i | awk '/UDP/'
    – c4f4t0r
    Feb 13, 2014 at 18:07

Be aware that when using autitctl, nscd for example uses a slightly different parameter in the socket system call, when doing a DNS query:


So to make sure you catch those queries in addition to the ones that were mentioned above, you can add an additional filter, with the same name if you want:

auditctl -a exit,always -F arch=b64 -F a0=2  -F a1=2050 -S socket -k SOCKET

Here 2050 is a bitwise OR of SOCK_DGRAM (2) and SOCK_NONBLOCK (2048).

Then the search will find both those filters with the same key, SOCKET:

ausearch -i -ts today -k SOCKET

The hex values for the socket constants I found here: https://golang.org/pkg/syscall/#pkg-constants

As I have no reputation points to comment I added this.


I tried the Auditd framework, and initially it didn't look very promising, as all I thought it was telling me was that process sent a UDP packet.

Then tried SystemTap, and initially it worked, but then it started reporting the destination port and address as and 0 on some packets, but not others. Others have had that problem too. Rebooting and clearing caches didn't help, gave up.

Then looked into Dtrace, and quickly transitioned to Perf (thanks, Brendan Gregg!) However, couldn't figure out how to do the same task.

Then found another Auditd solution and apparently Auditd does actually record the destination ports and addresses, but embedded in hex:

$ sudo bash
# date +%s; \
  auditctl -a exit,always -F arch=b64  -S connect -k sendmsg; \
  nslookup serverfault.com > /dev/null; \
  auditctl -d exit,always -F arch=b64  -S connect -k sendmsg;\
  date +%s

# grep 1664829661 /var/log/audit/audit.log | grep -C1 -m1 08080808

type=SYSCALL msg=audit(1664829661.285:24390): arch=c000003e syscall=46 success=yes exit=33 a0=14 a1=7f0c3b3f59b0 a2=0 a3=7f0c41415170 items=0 ppid=1523 pid=10483 auid=1003 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="isc-worker0000" exe="/usr/bin/nslookup" key="sendmsg"
type=SOCKADDR msg=audit(1664829661.285:24390): saddr=02000035080808080000000000000000
type=PROCTITLE msg=audit(1664829661.285:24390): proctitle=6E736C6F6F6B7570007365727665726661756C742E636F6D00382E382E382E38

The saddr=02000035080808080000000000000000 breaks down into:

  • 0200 -- PF_INET
  • 0035 -- port 53 (DNS)
  • 08080808 -- ip address

The proctitle decodes as:

# echo 6E736C6F6F6B7570007365727665726661756C742E636F6D00382E382E382E38 | xxd -p -r | xargs -0

nslookup serverfault.com

Also confirmed it with arbitrary destination ip

type=SOCKADDR msg=audit(1664830858.835:24624): saddr=02000035010203040000000000000000
* 0200 -- PF_INET
* 0035 -- port 53 (DNS)
* 01020304 -- ip address

So... go have fun tracking down processes that send UDP packets! :-)

EDIT: Apparently ausearch is a darn useful tool; it converts the arguments to something approximately human-readable:

# ausearch -i -ts today -k sendmsg | more
type=PROCTITLE msg=audit(2022-10-03 13:58:51.203:24596) : proctitle=nslookup serverfault.com 
type=SOCKADDR msg=audit(2022-10-03 13:58:51.203:24596) : saddr={ fam=inet laddr= lport=53 } 
type=SYSCALL msg=audit(2022-10-03 13:58:51.203:24596) : arch=x86_64 syscall=sendmsg success=yes exit=33 a0=0x15 a1=0x7f42723ba2e0 a2=0x0 a3=0x7f42723ba520 items=0 ppid=1523 pid=15000 auid=randomuser uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=pts0 ses=2 comm=isc-worker0000 exe=/usr/bin/nslookup key=sendmsg 

EDIT2: Here's live printing of the requests, filtered to just destination port 53 (IDK why destination addr+port is laddr and lport, it just is):

# tail -f /var/log/audit/audit.log       \
  | ausearch -i -k sendmsg               \
  | grep -B1 --line-buffered 'lport=53 ' \
  | sed 's/ : / :\n   /'
type=PROCTITLE msg=audit(02/14/2023 16:20:05.022:135695) :
   proctitle=dig +short www.google.com @ 
type=SOCKADDR msg=audit(02/14/2023 16:20:05.022:135695) :
   saddr={ fam=inet laddr= lport=53 } 

EDIT3: And here's the TCP version, since DNS can do UDP or TCP:

# auditctl -a exit,always -F arch=b64 -S connect -k MYCONNECT
# tail -f /var/log/audit/audit.log \
  | ausearch -i -k MYCONNECT \
  | grep -B1 --line-buffered 'lport=53 ' \
  | sed 's/ : / :\n   /'

type=PROCTITLE msg=audit(02/14/2023 16:32:18.168:208235) :
   proctitle=/usr/bin/python3 /home/user/run_test.py 
type=SOCKADDR msg=audit(02/14/2023 16:32:18.168:208235) :
   saddr={ fam=inet laddr= lport=53 } 

Making it ignore is left as an exercise for the reader. :-)

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