I have a KVM host which bridges its eth0 as br0. I have one guest which uses the bridge adapter (rather than NAT) with its own public IP.

If I setup basic firewall rules on the host, it suddenly begins taking a long time to connect to ssh (or other services, such as http) the guest on its IP - whether locally (from the host) or from another host entirely. Long time I mean maybe 20 seconds instead of 1 second. Also its outbound connections take a long time to open as well (connecting from guest to another host).

I simply have all forwarding enabled for simplicity; without that statement no traffic is getting to or from the guest. If I flush all these rules, then connectivity returns to normal.

Questions: 1. What gives? 2. How to troubleshoot further - is there a way to log all rejections?

These are my rule statements:


#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -d -j REJECT

#  Accept all established inbound connections

#  Allow all outbound traffic - you can modify this to only allow certain traffic

# Forward all traffic through the bridge interface 

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
#allow check_nrpe & check_mk
-A INPUT -p tcp --dport 5666 -j ACCEPT
-A INPUT -p tcp --dport 6556 -j ACCEPT
-A INPUT -p tcp --dport 2220 -j ACCEPT

#  Allow SSH connections
#  The -dport number should be the same port number you set in sshd_config
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Reject all other inbound - default deny unless explicitly allowed policy


1 Answer 1


Did you setup the services to try to reverse DNS incoming connections (as is the default often with ssh), then proceed to block DNS queries via your firewall rules?

  • Of course I did! The kvm host runs dnsmasq and uses the local nameserver as the first in the list. I should have realized this but hadn't thought of the RDNS explanation. Thanks!
    – Jeremy
    May 6, 2012 at 21:36

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