I am working to a CentOS 6 server with nonstandard iptables system without rule for ACCEPT ESTABLISHED connections. All tables and chains empty (flush by legacy custom script) so only filter/INPUT chain has rules (also fail2ban chain):
Chain INPUT (policy ACCEPT) target prot opt source destination f2b-postfix tcp -- 0.0.0.0/0 0.0.0.0/0 ACCEPT all -- 192.168.0.0/16 0.0.0.0/0 ACCEPT all -- 127.0.0.0/8 0.0.0.0/0 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:25 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:443 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:587 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:993 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:995 DROP tcp -- 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02
Chain f2b-postfix (1 references) target prot opt source destination REJECT all -- 200.23.235.30 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- 177.11.167.57 0.0.0.0/0 reject-with icmp-port-unreachable RETURN all -- 0.0.0.0/0 0.0.0.0/0
When fail2ban block a IP address, established connections are allowed to continue, but with no rule to accept established connections how is that possible? Why doesn't f2b first rule block established connections?
On 2019-06-26 02:41, MRob wrote:
I am working to a CentOS 6 server with nonstandard iptables system without rule for ACCEPT ESTABLISHED connections. All tables and chains empty (flush by legacy custom script) so only filter/INPUT chain has rules (also fail2ban chain):
Chain INPUT (policy ACCEPT) target prot opt source destination f2b-postfix tcp -- 0.0.0.0/0 0.0.0.0/0 ACCEPT all -- 192.168.0.0/16 0.0.0.0/0 ACCEPT all -- 127.0.0.0/8 0.0.0.0/0 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:25 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:443 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:587 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:993 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:995 DROP tcp -- 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02
Chain f2b-postfix (1 references) target prot opt source destination REJECT all -- 200.23.235.30 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- 177.11.167.57 0.0.0.0/0 reject-with icmp-port-unreachable RETURN all -- 0.0.0.0/0 0.0.0.0/0
When fail2ban block a IP address, established connections are allowed to continue, but with no rule to accept established connections how is that possible? Why doesn't f2b first rule block established connections?
The short answer is that the firewall rules REJECT...Fail2Ban only tells the firewall what to reject, at the point of entry.
Think of it this way:
Fail2Ban is the manager of a popular dance club. He determines the list of who may or may not be admitted to the club.
The firewall is the guy at the door of a popular club. He's doing his job, checking IDs, checking against the list of allowed or rejected guests and acting accordingly.
If the manager updates the list, it's not the door guy's job to go back through the club to find anyone who may have been admitted prior to the list having been updated. That's the job of a bouncer.
If you want the door guy to also be a bouncer, you'll need to configure your Fail2Ban actions to add iptables rules which invoke DROP instead of REJECT.
On 6/26/19 8:41 AM, MRob wrote:
I am working to a CentOS 6 server with nonstandard iptables system without rule for ACCEPT ESTABLISHED connections. All tables and chains empty (flush by legacy custom script) so only filter/INPUT chain has rules (also fail2ban chain):
Chain INPUT (policy ACCEPT) target prot opt source destination f2b-postfix tcp -- 0.0.0.0/0 0.0.0.0/0
[...]
Chain f2b-postfix (1 references) target prot opt source destination REJECT all -- 200.23.235.30 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- 177.11.167.57 0.0.0.0/0 reject-with icmp-port-unreachable RETURN all -- 0.0.0.0/0 0.0.0.0/0
[...]
When fail2ban block a IP address, established connections are allowed to continue, but with no rule to accept established connections how is that possible? Why doesn't f2b first rule block established connections?
The first rule should indeed block traffic for the established connections too. It matches tcp only, are your connections really on tcp (and ipv4)?
Try adding the "-v" option to iptables so you can see the packet counters. Do some experiments and check if the counters grow or not when you expect packets to be dropped.
Regards.
On 6/25/19 11:41 PM, MRob wrote:
When fail2ban block a IP address, established connections are allowed to continue, but with no rule to accept established connections how is that possible?
It doesn't look like it would be.
1: Open a connection that will demonstrate the problem later. 2: Trigger a block from an address that you control. 3: Check the output of "iptables -L -v" to demonstrate that the address is blocked. 4: Use "tcpdump -nn -i any host <address>" to watch traffic from that address. 5: Send a command over the connection from step 1. tcpdump should show packets in both directions, and your session should be usable, according to the problem you described. 6: Check the output of "iptables -L -v" again and look at the counters on each rule to see which rule is being matched.
On 6/26/2019 2:41 AM, MRob wrote:
I am working to a CentOS 6 server with nonstandard iptables system without rule for ACCEPT ESTABLISHED connections. All tables and chains empty (flush by legacy custom script) so only filter/INPUT chain has rules (also fail2ban chain):
Chain INPUT (policy ACCEPT) target prot opt source destination f2b-postfix tcp -- 0.0.0.0/0 0.0.0.0/0 ACCEPT all -- 192.168.0.0/16 0.0.0.0/0 ACCEPT all -- 127.0.0.0/8 0.0.0.0/0 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:25 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:443 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:587 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:993 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:995 DROP tcp -- 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02
Chain f2b-postfix (1 references) target prot opt source destination REJECT all -- 200.23.235.30 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- 177.11.167.57 0.0.0.0/0 reject-with icmp-port-unreachable RETURN all -- 0.0.0.0/0 0.0.0.0/0
When fail2ban block a IP address, established connections are allowed to continue, but with no rule to accept established connections how is that possible? Why doesn't f2b first rule block established connections?
The way I solved this problem was using conntrack. I added entries to the fail2ban action to delete connections where the source or destination matched the IP I was trying to block. This results in all communications from that IP being dropped immediately. I used a .local file to redefine the actionban. It looks like this:
$ cat firewallcmd-allports.local [Definition] actionban = firewall-cmd --direct --add-rule ipv4 filter f2b-<name> 0 -s <ip> -j <blocktype> (conntrack -D -s <ip>; exit 0) (conntrack -D -d <ip>; exit 0)
You have to install the conntrack-tools package to use the conntrack command, but I don't remember having to do anything else to make it work.
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