[UFW BLOCK] in dmesg

MilkTea · 06-10-2021, 07:36 PM

I have a rather strange problem that I couldn't figure out. On my server, I only enabled port 22 for SSH and port 6000 for an application. The default policies were deny incoming, allow outgoing, and deny routed. I noticed a bunch of [UFW BLOCK] from some external peers and the STP is 6000.

I couldn't figure out why ufw blocked those traffic, I changed the default for incoming to allow. Well, it didn't help. I then changed the default for routed to also allow. So now all incoming, outgoing, and routed are allowed. Still, I am seeing [UFW BLOCK] growing whenever I check dmesg.

I thought by allowing all is essentially not blocking anything. Why does ufw still block the incoming traffic?

ondoho · 06-11-2021, 01:44 AM

Quote:

Originally Posted by MilkTea

I have a rather strange problem that I couldn't figure out. On my server, I only enabled port 22 for SSH and port 6000 for an application. The default policies were deny incoming, allow outgoing, and deny routed. I noticed a bunch of [UFW BLOCK] from some external peers and the STP is 6000.

I couldn't figure out why ufw blocked those traffic, I changed the default for incoming to allow. Well, it didn't help. I then changed the default for routed to also allow. So now all incoming, outgoing, and routed are allowed. Still, I am seeing [UFW BLOCK] growing whenever I check dmesg.

I thought by allowing all is essentially not blocking anything. Why does ufw still block the incoming traffic?

Did you 'ufw reload' after making those changes?

Show us the (relevant part of the) logs, show us ufw configuration, e.g. 'ufw status verbose'.
Use code tags for code & command output (see my signature).

MilkTea · 06-11-2021, 08:02 AM

Yes, I did reload the rules. I, at one point, even restarted ufw service.
Here is a portion of dmesg -T

Code:

[Fri Jun 11 08:55:44 2021] [UFW BLOCK] IN=enp3s0 OUT= MAC=3c:7c:3f:21:e5:f0:00:f4:59:68:50:54:08:00 SRC=77.xx.yy.62 DST=192.168.2.2 LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=0 DF PROTO=TCP SPT=6000 DPT=48942 WINDOW=0 RES=0x00 RST URGP=0
[Fri Jun 11 08:55:44 2021] [UFW BLOCK] IN=enp3s0 OUT= MAC=3c:7c:3f:21:e5:f0:00:f4:59:68:50:54:08:00 SRC=77.xx.yy.62 DST=192.168.2.2 LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=0 DF PROTO=TCP SPT=6000 DPT=48942 WINDOW=0 RES=0x00 RST URGP=0
[Fri Jun 11 08:55:44 2021] [UFW BLOCK] IN=enp3s0 OUT= MAC=3c:7c:3f:21:e5:f0:00:f4:59:68:50:54:08:00 SRC=77.xx.yy.62 DST=192.168.2.2 LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=0 DF PROTO=TCP SPT=6000 DPT=48942 WINDOW=0 RES=0x00 RST URGP=0
[Fri Jun 11 08:55:44 2021] [UFW BLOCK] IN=enp3s0 OUT= MAC=3c:7c:3f:21:e5:f0:00:f4:59:68:50:54:08:00 SRC=77.xx.yy.62 DST=192.168.2.2 LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=0 DF PROTO=TCP SPT=6000 DPT=48942 WINDOW=0 RES=0x00 RST URGP=0

From ufw status verbose

Code:

Status: active
Logging: on (low)
Default: allow (incoming), allow (outgoing), allow (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
443/tcp                    ALLOW IN    Anywhere
6000/tcp                  ALLOW IN    Anywhere
22/tcp (OpenSSH)           ALLOW IN    Anywhere
443                        ALLOW IN    Anywhere
443/tcp (v6)               ALLOW IN    Anywhere (v6)
6000/tcp (v6)             ALLOW IN    Anywhere (v6)
22/tcp (OpenSSH (v6))      ALLOW IN    Anywhere (v6)
443 (v6)                   ALLOW IN    Anywhere (v6)

ondoho · 06-12-2021, 03:28 AM

Hmmm. My Default reads "deny (incoming), allow (outgoing), disabled (routed)" - yours reads like no firewall at all, tbh.
But I'm no expert.
I also get confused by SPT= and DPT= ...
Anyhow, it looks like it should be working (comparing to my own ufw setup where I succesfully punched some holes).

Only thing more stricter is that you specified the tcp protocol, so check out if your application uses only tcp.

Could it be that it's not UFW or your system's firewall at all, but your router instead?
I always have to open a port both on the server's firewall and the router.

MilkTea · 06-12-2021, 09:11 AM

As mentioned, it was "deny (incoming), allow (outgoing), disabled (routed)". I was trying to test why ufw blocked the incoming traffic so I changed it to "allow". The strange things is, like you pointed out, this is essentially no firewall. However, ufw still blocks some incoming traffic.

I believe SPT stands for "source port" and DPT stands for "destination port". So basically the incoming packets were from source:6000. From the message, I also think that opening only tcp should be sufficient. The block packages were using tcp (PROTO=TCP).

I have a physical firewall. However, since the dmesg says "[UFW BLOCK]", I think it is from ufw.

At this point, I have turned off ufw on my server because I have a physical firewall and this server is inside a private network. I don't think it is a huge concern from being attached from outside. The only ports that are opened, as you can see, are 443, 6000, and 22.

I simply want to learn and understand why ufw is blocking those packages.

ondoho · 06-13-2021, 02:50 AM

Maybe start with a fresh ufw profile.
Should default to deny incoming.
Then read the docs, and try punching a hole again.
Keep in mind that ufw has different commands to add opening rules: "allow,insert,prepend" and obviously they do different things.