I'm might embarrass myself with this question, because of something really obvious, but I'm puzzled.

My home broadband firewall log says it has been blocking incoming TCP traffic with a source (!) port of 443. What in the world might that be? If somebody was attempting an HTTPS connection, that would be destination port 443, not source port, right?

Here's the entry from the log file:

[UFW BLOCK] IN=enp3s0 OUT= MAC=xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx SRC=xx.xx.xx.x DST=xx.xx.xx.xx LEN=89 TOS=0x00 PREC=0x20 TTL=59 ID=58112 DF PROTO=TCP SPT=443 DPT=56419 WINDOW=7776 RES=0x00 ACK PSH URGP=0

What might this be trying to do?

  • 1
    Anyone can use any source port of their choice. The first 0-1024 reserved port are reserved, but nothing preventing them from making conscious decision to set the source port as 443.
    – Darius
    Feb 3 '14 at 22:20
  • Why would anybody do this though? Feb 3 '14 at 22:21
  • Isn't Port 443 the HTTPS port?
    – dotVezz
    Feb 3 '14 at 22:22
  • @dotVezz indeed, which is why it is so surprising that this port is the source (rather than the destination) port. Feb 4 '14 at 0:12
  • 1
    I'm seeing this a lot coming from Google IP addresses.
    – Michael
    May 27 '16 at 20:14

There is nothing restricting which source port you use when you open a TCP connection (it may require root/superuser/admin privileges to use a source port < 1024).

In any case most likely if you are seeing traffic that is originating externally with a source port of 443 what you are seeing is an attack (probably a bot running a script) that is hoping you will have a bug in your firewall configuration (firewall rules) and will let traffic in that originates from port 443 because you almost certainly do allow outbound traffic destined to port 443.


If it is a SYN packet with source port 443 then this is more than likely nefarious. Your packet is an ACK PSH, so it will be a connection you have dropped at your end but the web server you were connected to has not received a FIN or RST packet and keeps sending you data.


You should write more information If you want to know about it. That connection could have been any program. If you wanna know what TCP connections (on port 443) have your computer You can execute the netstat program and check the IP and the program.

# netstat -ntp | grep :443

Also, You can use a sniffer to capture the traffic and see the origin of this connection filtering by port 443 and IP. Additionally, You can do a whois with the IP to see where you are sending.

$ whois <ip>

If you want to see the TCP ports that Your PC has in listening mode execute:

# netstat -lnt

and If You want to see what program is listening use the argument -p

  • I understand what my computer is doing, that wasn't the question I'm afraid. Feb 4 '14 at 0:13

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