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JeremyCanfield
Asked: 2020-09-03 03:33:34 +0800 CST

firewalld causing nmap to return host seems down

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I've two machines, server1 and server2. On server2, I stop firewalld.

[root@server2 ~]# systemctl stop firewalld

From server1, nmap returns Host is up.

[root@server1 ~]$ nmap -sn server2

Starting Nmap 6.40 ( http://nmap.org ) at 2020-09-02 11:27 CDT
Nmap scan report for server2 (10.17.45.13)
Host is up (0.00045s latency).
Nmap done: 1 IP address (1 host up) scanned in 0.01 seconds

I enable firewalld on server2.

[root@server2 ~]# systemctl start firewalld

From server1, nmap returns Host seems down, thus something with firewalld on server2 is causing nmap to return Host seems down.

[root@server1 ~]$ nmap -sn server2

Starting Nmap 6.40 ( http://nmap.org ) at 2020-09-02 11:29 CDT
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in 0.01 seconds

From server1, ping/echo/ICMP works.

[root@server1 ~]$ ping -c4 server2
PING server2 (10.17.45.13) 56(84) bytes of data.
64 bytes from server2 (10.17.45.13): icmp_seq=1 ttl=64 time=0.436 ms
64 bytes from server2 (10.17.45.13): icmp_seq=2 ttl=64 time=0.388 ms
64 bytes from server2 (10.17.45.13): icmp_seq=3 ttl=64 time=0.338 ms
64 bytes from server2 (10.17.45.13): icmp_seq=4 ttl=64 time=0.390 ms

--- server2 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3000ms
rtt min/avg/max/mdev = 0.338/0.388/0.436/0.034 ms

Here are the firewalld settings for the public and drop zones on server2. icmp-block is empty meaning no ICMP types are being blocked, and icmp-block-inversion is set to no to allow all IMCP traffic.

[root@server2 ~]# firewall-cmd --zone=public --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eno16777984
  sources:
  services: ssh dhcpv6-client
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

[root@server2 ~]# firewall-cmd --zone=drop --list-all
drop
  target: DROP
  icmp-block-inversion: no
  interfaces:
  sources:
  services:
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

/var/log/firewalld is not logging anything for when nmap returns Host seems down.

ping icmp firewalld nmap

1 Answers

  1. Best Answer

    I don't know firewallcmd, but all the filtering in Linux is done by iptables. So, firewallcmd (like ufw and others) is a front-end for iptables.

    You can always check the active rules in the kernel by the command : iptables -L -nv, even you completely disable iptables service (as the firewall is managed by firewallcmd). If you don't disable the iptables service (or netfilter-persistent on Debian), you may have a collision between two managers.

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