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I discovered a lot of unauthorized login attempts in /var/log/auth.log

...
Sep 26 22:15:34 hostname sshd[3072475]: Failed password for invalid user user from x.y.z.w port 51056 ssh2
Sep 26 22:15:39 hostname sshd[3072519]: Failed password for invalid user user from x.y.z.w  port 62354 ssh2
Sep 26 22:16:51 hostname sshd[3072643]: Failed password for invalid user user from x.y.z.w port 10596 ssh2
...

I am puzzled why this happens, since I have configured portforwarding on my internet router (zyxel VMG3925-B10B) so that e.g. port 54321 is mapped to port 22 on internal ip address of my ubuntu-box.

If I try to ssh-login from outside home - so using an outside IP to anything but port 54321 I will get rejected. So why is it possible for somebody to get though my router firewall to the internal ip of my ubuntu box.

I realize that this question potentially is more a question for the manufacturer of my router. However my it-security knowledge is somewhat limited and i would like to hear other peoples strategies

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    Back when I was in computer security, i ALWAYS did my nmap reconnaissance scans on ports 1-65535 (all of them). Simply using port 54321 hides nothing. There is a package called fail2ban that might help
    – waltinator
    Oct 1, 2021 at 22:31

1 Answer 1

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By poking around, the bad guys have found out that your external port 54321 is your ssh access port. The ports listed in your log are their source ports, not the destination ports. You should find that ssh login attempts on your port 54321 occur at a much much lower rate than if it were port 22.

You can mitigate the issue via iptables rules, or fail2ban (however it is spelled), or other. I use the recent module in itpables:

# Dynamic Badguy List. Detect and DROP Bad IPs that do password attacks on SSH.
# Once they are on the BADGUY list then DROP all packets from them.
#$IPTABLES -A INPUT -i $EXTIF -m recent --update --hitcount 3 --seconds 5400 --name BADGUY_SSH -j LOG --log-prefix "SSH BAD:" --log-level info
#$IPTABLES -A INPUT -i $EXTIF -m recent --update --hitcount 3 --seconds 5400 --name BADGUY_SSH -j DROP
# Sometimes make the lock time very long. Typically to try to get rid of coordinated attacks from China.
$IPTABLES -A INPUT -i $EXTIF -m recent --mask $BIT_MASK --update --hitcount 3 --seconds 90000 --name BADGUY_SSH -j LOG --log-prefix "SSH BAD:" --log-level info
$IPTABLES -A INPUT -i $EXTIF -m recent --mask $BIT_MASK --update --hitcount 3 --seconds 90000 --name BADGUY_SSH -j DROP
$IPTABLES -A INPUT -i $EXTIF -p tcp -m tcp --dport 22 -m recent --mask $BIT_MASK --set --name BADGUY_SSH -j ACCEPT

I now use a BIT_MASK (currently "255.255.252.0"), because attackers have become clever and often merely switch to another ip address on the same sub-net. $EXTIF is my WAN facing NIC.

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  • Hi @Doug Thanks for your answers. I started by installing fail2ban, which seems like a good start. So far I have relied on ufw with a few rules for opening some ports. I like the idea that you posted, where iptables are used with bitmask. I tried looking up how to set up iptables. It looked as if iptables rules are cleared on reboot unless you do something like ``` apt-get install iptables-persistent ``` but may i ask how you configure and preserve the iptables rules? I wouldn't be able to remember this. Where do you store files that contain e.g . $BIT_MASK and rules
    – jedo-tm
    Oct 1, 2021 at 23:34
  • I use a script to load my iptables rules after boot. It is called from a post-boot service. By using a script I can also deal with some directly related stuff that isn't actually iptables. I am not a fan of iptables-persistent. Oct 2, 2021 at 3:31
  • Thanks @dough :)
    – jedo-tm
    Oct 3, 2021 at 11:25

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