Nftables rules for hardening Discourse installation

Installation
1

Hello all,

this is my first post here. My background: I have been administering Linux servers for over a decade. About a week ago I did a Discourse installation on a server (Debian Bullseye). I like it so far great!

Now I wanted to implement the usual hardening measures on the host system (eg. webserver). This includes nftables rulesets among other things. I generally use these:

#!/usr/sbin/nft -f

#################### 
# Purge/Flush      # 
#################### 
flush ruleset

#################### 
# Incoming Traffic # 
#################### 
table inet filter { 	    
   chain input { 		       
      type filter hook input priority 0; policy drop;
      
      # Allow loopback interface 		       
      iifname lo accept

      # Rate limit ICMPv4|ICMPv6 traffic       
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } limit rate over 5/second drop       
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } limit rate over 5/second drop

      # Allow packets to established/related connections        
      ct state established,related accept         
  
      # Drop invalid connections        
      ct state invalid drop
      
      # Allow ICMPv4: Ping requests | Error messages | Router selection messages       
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } accept              
   
      # Allow ICMPv6 traffic (https://tools.ietf.org/html/rfc4890#page-18)       
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } accept

      # Allow SSH access on port 2222 [rate limit]       
      tcp dport 2222 ct state new limit rate 3/minute accept

      # Allow HTTP / HTTPS traffic
      tcp dport { http, https } accept
      
      # Reject other packets       
      ip protocol tcp reject with tcp reset
      ip6 nexthdr tcp reject with tcp reset    
   }
#################### 
# Forward Traffic  # 
####################    
   chain forward { 		       
      type filter hook forward priority 0; policy drop; 	    
   } 
#################### 
# Outgoing Traffic # 
####################    
   chain output { 		       
      type filter hook output priority 0; policy accept;  	
      
      # Allow loopback interface 		       
      oifname lo accept	    
   } 
}

However, after activation Discourse stopped working. I suspect the packages are not being passed to the Docker installation of Discourse. This rule in particular is likely to be problematic:

#################### 
# Forward Traffic  # 
####################    
   chain forward { 		       
      type filter hook forward priority 0; policy drop; 	    
   }

But before I start tinkering now, I wanted to ask if anyone has already dealt with this issue and has working firewall rules for the base system. Do these rules even make sense for a Docker Discourd installation? So far I have had little to do with Docker.

Some more information. These firewall rules are automatically added by Docker (docker-ce):

table ip nat {
	chain DOCKER {
		iifname "docker0" counter packets 0 bytes 0 return
		iifname != "docker0" meta l4proto tcp tcp dport 443 counter packets 155 bytes 7070 dnat to 172.17.0.2:443
		iifname != "docker0" meta l4proto tcp tcp dport 80 counter packets 128 bytes 6263 dnat to 172.17.0.2:80
	}

	chain POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		oifname != "docker0" ip saddr 172.17.0.0/16 counter packets 34 bytes 2188 masquerade 
		meta l4proto tcp ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 443 counter packets 0 bytes 0 masquerade 
		meta l4proto tcp ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 80 counter packets 0 bytes 0 masquerade 
	}

	chain PREROUTING {
		type nat hook prerouting priority dstnat; policy accept;
		fib daddr type local counter packets 11439 bytes 550595 jump DOCKER
	}

	chain OUTPUT {
		type nat hook output priority -100; policy accept;
		ip daddr != 127.0.0.0/8 fib daddr type local counter packets 0 bytes 0 jump DOCKER
	}
}
table ip filter {
	chain DOCKER {
		iifname != "docker0" oifname "docker0" meta l4proto tcp ip daddr 172.17.0.2 tcp dport 443 counter packets 155 bytes 7070 accept
		iifname != "docker0" oifname "docker0" meta l4proto tcp ip daddr 172.17.0.2 tcp dport 80 counter packets 128 bytes 6263 accept
	}

	chain DOCKER-ISOLATION-STAGE-1 {
		iifname "docker0" oifname != "docker0" counter packets 588 bytes 44199 jump DOCKER-ISOLATION-STAGE-2
		counter packets 1187 bytes 428425 return
	}

	chain DOCKER-ISOLATION-STAGE-2 {
		oifname "docker0" counter packets 0 bytes 0 drop
		counter packets 588 bytes 44199 return
	}

	chain FORWARD {
		type filter hook forward priority filter; policy drop;
		counter packets 1187 bytes 428425 jump DOCKER-USER
		counter packets 1187 bytes 428425 jump DOCKER-ISOLATION-STAGE-1
		oifname "docker0" ct state related,established counter packets 316 bytes 370893 accept
		oifname "docker0" counter packets 283 bytes 13333 jump DOCKER
		iifname "docker0" oifname != "docker0" counter packets 588 bytes 44199 accept
		iifname "docker0" oifname "docker0" counter packets 0 bytes 0 accept
	}

	chain DOCKER-USER {
		counter packets 1187 bytes 428425 return
	}

	chain INPUT {
		type filter hook input priority filter; policy accept;		
	}
}

Cheers

2

Hey @OrkoGrayskull did you find

In your searches? There’s a little more backstory there on how docker interacts with firewalls.

2 Likes
3

I have now read the thread and tested something myself. Docker brings its own iptables rules and applies them after installation or when the docker service is (re)started/loaded:

service docker restart

Without these rulesets the installation of Discourse will not work. Means: when installing, rebuilding or restarting the container, these rules must be present. Otherwise the error message appears:

Error response from daemon: driver failed programming external connectivity on endpoint app (e4d4d3cc812a11862ee6aaa6ab453e61b95da1e6d90f9a76a71959148d228476):  (iptables failed: iptables --wait -t nat -A DOCKER -p tcp -d 0/0 --dport 443 -j DNAT --to-destination 172.17.0.2:443 ! -i docker0: iptables: No chain/target/match by that name.
 (exit status 1))

However, once the Discourse installation is complete or the container is restarted after an update, the Docker rules are apparently no longer needed. Then the following nftables rules can be loaded:

#!/usr/sbin/nft -f

#################### 
# Purge/Flush      # 
#################### 
flush ruleset

#################### 
# Incoming Traffic # 
#################### 
table inet filter { 	    
   chain input { 		       
      type filter hook input priority 0; policy drop;
      
      # Allow loopback interface 		       
      iifname lo accept

      # Rate limit ICMPv4|ICMPv6 traffic       
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } limit rate over 5/second drop       
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } limit rate over 5/second drop

      # Allow packets to established/related connections        
      ct state established,related accept         
  
      # Drop invalid connections        
      ct state invalid drop
      
      # Allow ICMPv4: Ping requests | Error messages | Router selection messages       
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } accept              
   
      # Allow ICMPv6 traffic (https://tools.ietf.org/html/rfc4890#page-18)       
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } accept

      # Allow SSH access on port 7777 [rate limit]       
      tcp dport 7777 ct state new limit rate 3/minute accept

      # Allow HTTP / HTTPS traffic
      tcp dport { http, https } accept
      
      # Reject other packets       
      ip protocol tcp reject with tcp reset
      ip6 nexthdr tcp reject with tcp reset    
   }
#################### 
# Forward Traffic  # 
####################    
   chain forward { 		       
      type filter hook forward priority 0; policy drop; 	    
   } 
#################### 
# Outgoing Traffic # 
####################    
   chain output { 		       
      type filter hook output priority 0; policy accept;  	
      
      # Allow loopback interface 		       
      oifname lo accept	    
   } 
}

This now brings me to another question: what to do?

4

Ok. I think I’ve got it now. Working ntftables ruleset with Discourse (Docker).

After initial Discourse installation do the following on Debian (Bullseye):

[1] Save Docker ruleset after installation:

nft flush ruleset
systemctl restart docker
iptables-save > iptables-docker.conf
iptables-restore-translate -f iptables-docker.conf > docker.nft

[2] Apply and show ruleset:

nft -f docker.nft
nft list ruleset

This is the exported ruleset after Discourse installation:

table ip filter {
	chain INPUT {
		type filter hook input priority filter; policy accept;
	}

	chain FORWARD {
		type filter hook forward priority filter; policy accept;
		counter jump DOCKER-USER
		counter jump DOCKER-ISOLATION-STAGE-1
		oifname "docker0" ct state established,related counter accept
		oifname "docker0" counter DOCKER
		iifname "docker0" oifname != "docker0" counter accept
		iifname "docker0" oifname "docker0" counter accept
	}

	chain OUTPUT {
		type filter hook output priority filter; policy accept;
	}

	chain DOCKER {
		iifname != "docker0" oifname "docker0" ip daddr 172.17.0.2 tcp dport 443 counter accept
		iifname != "docker0" oifname "docker0" ip daddr 172.17.0.2 tcp dport 80 counter accept
	}

	chain DOCKER-ISOLATION-STAGE-1 {
		iifname "docker0" oifname != "docker0" counter packets 0 bytes 0 jump DOCKER-ISOLATION-STAGE-2
		counter return
	}

	chain DOCKER-ISOLATION-STAGE-2 {
		oifname "docker0" drop
		counter return
	}

	chain DOCKER-USER {
		counter return
	}
}
table ip nat {
	chain PREROUTING {
		type nat hook prerouting priority dstnat; policy accept;
		fib daddr type local counter jump DOCKER
	}

	chain INPUT {
		type nat hook input priority 100; policy accept;
	}

	chain OUTPUT {
		type nat hook output priority -100; policy accept;
		ip daddr != 127.0.0.0/8 fib daddr type local counter jump DOCKER
	}

	chain POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		oifname != "docker0" ip saddr 172.17.0.0/16 counter masquerade
		ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 443 masquerade
		ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 80 masquerade
	}

	chain DOCKER {
		iifname "docker0" counter return
		iifname != "docker0" tcp dport 443 counter dnat to 172.17.0.2:443
		iifname != "docker0" tcp dport 80 counter dnat to 172.17.0.2:80
	}
}

[3] Integrate/Adapt ruleset:
nano /etc/nftables.conf

#!/usr/sbin/nft -f

####################
# Purge/Flush      #
####################
flush ruleset

####################
# IPv4/IPv6        #
####################
table inet filter {
####################
# Incoming Traffic #
####################	    
   chain input {       
      type filter hook input priority 0; policy drop;
      
      # Allow loopback interface	       
      iifname lo accept

      # Rate limit ICMPv4|ICMPv6 traffic 
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } limit rate over 5/second drop 
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } limit rate over 5/second drop

      # Allow packets to established/related connections    
      ct state established,related accept   
  
      # Drop invalid connections  
      ct state invalid drop
      
      # Allow ICMPv4: Ping requests | Error messages | Router selection messages       
      ip protocol icmp icmp type { echo-request, echo-reply, destination-unreachable, time-exceeded, parameter-problem, router-solicitation, router-advertisement } accept              
   
      # Allow ICMPv6 traffic (https://tools.ietf.org/html/rfc4890#page-18)       
      ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, echo-request, parameter-problem, echo-reply, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } accept

      # Allow SSH access on port 7777 [rate limit]       
      tcp dport 7777 ct state new limit rate 3/minute accept

      # Allow HTTP / HTTPS traffic [needed for IPv6 Discourse access]
      tcp dport { http, https } accept
      
      # Reject other packets       
      ip protocol tcp reject with tcp reset
      ip6 nexthdr tcp reject with tcp reset    
   }
#################### 
# Outgoing Traffic # 
####################    
   chain output { 		       
      type filter hook output priority 0; policy accept;  	
      
      # Allow loopback interface 		       
      oifname lo accept	    
   } 
}

#################### 
# IPv6             # 
#################### 
table ip6 filter { 
#################### 
# Forward Traffic  # 
####################   
   chain forward {       
      type filter hook forward priority 0; policy drop;    
   } 
} 

#################### 
# IPv4             # 
#################### 
table ip filter { 
#################### 
# Forward Traffic  # 
####################    
   chain forward {       
      type filter hook forward priority 0; policy drop;       
      jump DOCKER-USER       
      jump DOCKER-ISOLATION-STAGE-1       
      oifname "docker0" ct state established,related accept       
      oifname "docker0" jump DOCKER       
      iifname "docker0" oifname != "docker0" accept       
      iifname "docker0" oifname "docker0" accept    
   } 
#################### 
# Docker Traffic   # 
####################    
   chain DOCKER {       
      iifname != "docker0" oifname "docker0" ip daddr 172.17.0.2 tcp dport 443 accept       
      iifname != "docker0" oifname "docker0" ip daddr 172.17.0.2 tcp dport 80 accept    
   }    
   chain DOCKER-ISOLATION-STAGE-1 {       
      iifname "docker0" oifname != "docker0" jump DOCKER-ISOLATION-STAGE-2       
      return    
   }    
   chain DOCKER-ISOLATION-STAGE-2 {       
      oifname "docker0" drop       
      return    
   }    
   chain DOCKER-USER {       
      return    
   } 
} 

#################### 
# NAT IPv4         # 
#################### 
table ip nat { 
#################### 
# Docker NAT       # 
####################    
   chain PREROUTING {       
      type nat hook prerouting priority dstnat; policy accept;       
      fib daddr type local jump DOCKER    
   }
   chain INPUT {      
      type nat hook input priority 100; policy accept;    
   }
   chain OUTPUT {       
      type nat hook output priority -100; policy accept;       
      ip daddr != 127.0.0.0/8 fib daddr type local jump DOCKER    
   }
   chain POSTROUTING {       
      type nat hook postrouting priority srcnat; policy accept;       
      oifname != "docker0" ip saddr 172.17.0.0/16 masquerade       
      ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 443 masquerade       
      ip saddr 172.17.0.2 ip daddr 172.17.0.2 tcp dport 80 masquerade    
   }
   chain DOCKER {       
      iifname "docker0" return       
      iifname != "docker0" tcp dport 443 dnat to 172.17.0.2:443       
      iifname != "docker0" tcp dport 80 dnat to 172.17.0.2:80    
   } 
}

[4] Disable Docker iptables rules after restarting docker service:
nano /etc/docker/daemon.json

{
   "iptables": false
}

systemctl restart docker
nft list ruleset

[5] Apply your ruleset:

nft -f /etc/nftables.conf

That’s it. Checked. Works. Maybe this helps someone.