Enable firewalling services with UFW on Ubuntu

By default the firewall is disabled on an Ubuntu server. We can verify this by looking at the iptables rules. In this example, the default policy is set to ACCEPT:

[email protected]:~$ sudo iptables --list -nv
Chain INPUT (policy ACCEPT 7523K packets, 10G bytes)
 pkts bytes target     prot opt in     out     source               destination
   69  2887 ACCEPT     udp  --  ens160 *       0.0.0.0/0            0.0.0.0/0            udp dpt:1194
    0     0 ACCEPT     all  --  tun0   *       0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  tun0   ens160  0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  ens160 tun0    0.0.0.0/0            0.0.0.0/0

To keep firewall management simple, ufw is used to manage the firewall. We can verify again, there is no firewall running by default:

[email protected]:~$ sudo ufw status
Status: inactive

Before changing the default policy to deny, make sure to allow remote management. On my system, SSH is allowed from the world. A more restrictive rule can be enabled if necessary:

[email protected]:~$ sudo ufw allow ssh
Rules updated
Rules updated (v6)

Change the default policy from allow to deny:

[email protected]:~$ sudo ufw default deny
Default incoming policy changed to 'deny'
(be sure to update your rules accordingly)

Enable the firewall:

[email protected]:~$ sudo ufw enable
Firewall is active and enabled on system startup

Validate the firewall is enabled:

[email protected]:~$ sudo ufw status verbose
Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), deny (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp                     ALLOW IN    Anywhere
60001:61000/udp            ALLOW IN    Anywhere
22/tcp (v6)                ALLOW IN    Anywhere (v6)
60001:61000/udp (v6)       ALLOW IN    Anywhere (v6)

ufw creates a lot of iptables rules:

[email protected]:~$ sudo iptables --list -nv
[sudo] password for jemurray:
Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
   69  2887 ACCEPT     udp  --  ens160 *       0.0.0.0/0            0.0.0.0/0            udp dpt:1194
    0     0 ACCEPT     all  --  tun0   *       0.0.0.0/0            0.0.0.0/0
 2568 1736K ufw-before-logging-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0
 2568 1736K ufw-before-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0
   25  6569 ufw-after-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    2   178 ufw-after-logging-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    2   178 ufw-reject-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    2   178 ufw-track-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  tun0   ens160  0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  ens160 tun0    0.0.0.0/0            0.0.0.0/0
    0     0 ufw-before-logging-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ufw-before-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ufw-after-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ufw-after-logging-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ufw-reject-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ufw-track-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain OUTPUT (policy ACCEPT 75 packets, 6300 bytes)
 pkts bytes target     prot opt in     out     source               destination
 2633 1773K ufw-before-logging-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0
 2633 1773K ufw-before-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0
  135 10596 ufw-after-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0
  135 10596 ufw-after-logging-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0
  135 10596 ufw-reject-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0
  135 10596 ufw-track-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-after-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-after-input (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:137
    1   229 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:138
    0     0 ufw-skip-to-policy-input  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:139
    0     0 ufw-skip-to-policy-input  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:445
   22  6162 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:67
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:68
    0     0 ufw-skip-to-policy-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type BROADCAST

Chain ufw-after-logging-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-after-logging-input (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-after-logging-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-after-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-before-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 3
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 11
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 12
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 8
    0     0 ufw-user-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-before-input (1 references)
 pkts bytes target     prot opt in     out     source               destination
 1323 1601K ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
 1211  128K ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ufw-logging-deny  all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 3
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 11
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 12
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 8
    2   656 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp spt:67 dpt:68
   30  6803 ufw-not-local  all  --  *      *       0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            224.0.0.251          udp dpt:5353
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            239.255.255.250      udp dpt:1900
   30  6803 ufw-user-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-before-logging-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-before-logging-input (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-before-logging-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-before-output (1 references)
 pkts bytes target     prot opt in     out     source               destination
 1323 1601K ACCEPT     all  --  *      lo      0.0.0.0/0            0.0.0.0/0
 1175  162K ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
  133 10418 ufw-user-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-logging-allow (0 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW ALLOW] "

Chain ufw-logging-deny (2 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID limit: avg 3/min burst 10
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-not-local (1 references)
 pkts bytes target     prot opt in     out     source               destination
    7   412 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type LOCAL
    0     0 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type MULTICAST
   23  6391 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type BROADCAST
    0     0 ufw-logging-deny  all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-reject-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-reject-input (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-reject-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-skip-to-policy-forward (0 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-skip-to-policy-input (7 references)
 pkts bytes target     prot opt in     out     source               destination
   23  6391 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-skip-to-policy-output (0 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-track-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-track-input (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-track-output (1 references)
 pkts bytes target     prot opt in     out     source               destination
   24  1440 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate NEW
   34  2678 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate NEW

Chain ufw-user-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-user-input (1 references)
 pkts bytes target     prot opt in     out     source               destination
    7   412 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:22

Chain ufw-user-limit (0 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 5 LOG flags 0 level 4 prefix "[UFW LIMIT BLOCK] "
    0     0 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            reject-with icmp-port-unreachable

Chain ufw-user-limit-accept (0 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0

Chain ufw-user-logging-forward (0 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-user-logging-input (0 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-user-logging-output (0 references)
 pkts bytes target     prot opt in     out     source               destination

Chain ufw-user-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

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