Once every 5 minutes or so a process on my machine, with my main user id, tries to open a connection to an unknown IP address (no rDNS) at an unusual TCP port (>1k). I can see this in the computer's IP firewall log because the connection is rejected every time:

[243678.820911] Firewall: *TCP_OUT Blocked* IN= OUT=eth1 SRC= DST= LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=31984 DF PROTO=TCP SPT=31339 DPT=1234 WINDOW=64240 RES=0x00 SYN URGP=0 UID=1234 GID=1234

Now, I want to find out which process it is (a few hundred are running) in order to stop it from trying, and to see whether this is something I should be concerned about.

How can I wait for, and detect, up to the process name, which process opens a connection to at TCP port 3456 as user id 1234?

  • 1
    If the issue is regular ("every 5 minutes") then it may be a cron job, so looking at cron entries may be beneficial. Otherwise you could look at auditd and create an auditctl entry to log TCP session activity; eg auditctl -a exit,always -F arch=b64 -S connect -k FLAG will flag 64bit programs that call the connect(2) system call. You'll need a similar line for 32bit programs if you have both on your system – Stephen Harris Apr 7 at 19:53
  • Just to clarify: The firewall message is the PC's or your network's router? ... The IP address you listed in the question seems to belong to Samsung... So, if it's the router's firewall, I would say this is probably a Smart TV or phone or something trying to auto-update... you could use watch -n15 lsof -a -i4@ -u 1234 ... which will check every 15 seconds. – RubberStamp Apr 7 at 19:53
  • @RubberStamp Yes, they are on my machine, not the router. So, some Linux software component or web page in any of the 5 open browsers. – Ned64 Apr 9 at 21:09
  • @StephenHarris Thanks, I will look into the audit option, this may be just what I need. – Ned64 Apr 9 at 21:10

You could construct a tight loop with netstat like this:

while :; do netstat -np | awk '$5 ~ ":3456"  {print}'; done

It's not very efficient, but it should be able to capture the pid and name of your calling process.

  • Thanks, this might work, albeit at the expense of 100% CPU because of the busy waiting... still the netstat could miss the command - will try it out, though! – Ned64 Apr 9 at 21:11

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