Can't Close Port 21

Question

I have this strange issue where I can't close port 21. I'm using Digital Ocean running Debian Stretch 9. I run nmap against my domain and it shows port 21 is open.

For reference the domain is: https://fadverts.co.uk

I have a firewall setup to only allow the following ports: 22, 80, 443. I have tried both iptables and ufw to no avail.

Here's the output from ufw status numbered:

[ 1] 22/tcp                     LIMIT IN    Anywhere
[ 2] 80/tcp                     ALLOW IN    Anywhere
[ 3] 443/tcp                    ALLOW IN    Anywhere
[ 4] 22/tcp (v6)                LIMIT IN    Anywhere (v6)
[ 5] 80/tcp (v6)                ALLOW IN    Anywhere (v6)
[ 6] 443/tcp (v6)               ALLOW IN    Anywhere (v6)

Here's the output from iptables:

Chain INPUT (policy DROP)
target     prot opt source               destination
ufw-before-logging-input  all  --  0.0.0.0/0            0.0.0.0/0
ufw-before-input  all  --  0.0.0.0/0            0.0.0.0/0
ufw-after-input  all  --  0.0.0.0/0            0.0.0.0/0
ufw-after-logging-input  all  --  0.0.0.0/0            0.0.0.0/0
ufw-reject-input  all  --  0.0.0.0/0            0.0.0.0/0
ufw-track-input  all  --  0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy DROP)
target     prot opt source               destination
ufw-before-logging-forward  all  --  0.0.0.0/0            0.0.0.0/0
ufw-before-forward  all  --  0.0.0.0/0            0.0.0.0/0
ufw-after-forward  all  --  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
ufw-reject-forward  all  --  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)
target     prot opt source               destination
ufw-before-logging-output  all  --  0.0.0.0/0            0.0.0.0/0
ufw-before-output  all  --  0.0.0.0/0            0.0.0.0/0
ufw-after-output  all  --  0.0.0.0/0            0.0.0.0/0
ufw-after-logging-output  all  --  0.0.0.0/0            0.0.0.0/0
ufw-reject-output  all  --  0.0.0.0/0            0.0.0.0/0
ufw-track-output  all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-after-forward (1 references)
target     prot opt source               destination

Chain ufw-after-input (1 references)
target     prot opt source               destination
ufw-skip-to-policy-input  udp  --  0.0.0.0/0            0.0.0.0/0            udp dpt:137
ufw-skip-to-policy-input  udp  --  0.0.0.0/0            0.0.0.0/0            udp dpt:138
ufw-skip-to-policy-input  tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:139
ufw-skip-to-policy-input  tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:445
ufw-skip-to-policy-input  udp  --  0.0.0.0/0            0.0.0.0/0            udp dpt:67
ufw-skip-to-policy-input  udp  --  0.0.0.0/0            0.0.0.0/0            udp dpt:68
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)
target     prot opt source               destination
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)
target     prot opt source               destination
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)
target     prot opt source               destination

Chain ufw-after-output (1 references)
target     prot opt source               destination

Chain ufw-before-forward (1 references)
target     prot opt source               destination
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 3
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 4
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 11
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 12
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 8
ufw-user-forward  all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-before-input (1 references)
target     prot opt source               destination
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
ufw-logging-deny  all  --  0.0.0.0/0            0.0.0.0/0            ctstate INVALID
DROP       all  --  0.0.0.0/0            0.0.0.0/0            ctstate INVALID
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 3
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 4
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 11
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 12
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 8
ACCEPT     udp  --  0.0.0.0/0            0.0.0.0/0            udp spt:67 dpt:68
ufw-not-local  all  --  0.0.0.0/0            0.0.0.0/0
ACCEPT     udp  --  0.0.0.0/0            224.0.0.251          udp dpt:5353
ACCEPT     udp  --  0.0.0.0/0            239.255.255.250      udp dpt:1900
ufw-user-input  all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-before-logging-forward (1 references)
target     prot opt source               destination

Chain ufw-before-logging-input (1 references)
target     prot opt source               destination

Chain ufw-before-logging-output (1 references)
target     prot opt source               destination

Chain ufw-before-output (1 references)
target     prot opt source               destination
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
ufw-user-output  all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-logging-allow (0 references)
target     prot opt source               destination
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)
target     prot opt source               destination
RETURN     all  --  0.0.0.0/0            0.0.0.0/0            ctstate INVALID limit: avg 3/min burst 10
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)
target     prot opt source               destination
RETURN     all  --  0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type LOCAL
RETURN     all  --  0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type MULTICAST
RETURN     all  --  0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type BROADCAST
ufw-logging-deny  all  --  0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10
DROP       all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-reject-forward (1 references)
target     prot opt source               destination

Chain ufw-reject-input (1 references)
target     prot opt source               destination

Chain ufw-reject-output (1 references)
target     prot opt source               destination

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

Chain ufw-skip-to-policy-input (7 references)
target     prot opt source               destination
DROP       all  --  0.0.0.0/0            0.0.0.0/0

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

Chain ufw-track-forward (1 references)
target     prot opt source               destination

Chain ufw-track-input (1 references)
target     prot opt source               destination

Chain ufw-track-output (1 references)
target     prot opt source               destination
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            ctstate NEW
ACCEPT     udp  --  0.0.0.0/0            0.0.0.0/0            ctstate NEW

Chain ufw-user-forward (1 references)
target     prot opt source               destination

Chain ufw-user-input (1 references)
target     prot opt source               destination
           tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:22 ctstate NEW recent: SET name: DEFAULT side: source mask: 255.255.255.255
ufw-user-limit  tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:22 ctstate NEW recent: UPDATE seconds: 30 hit_count: 6 name: DEFAULT side: source mask: 255.255.255.255
ufw-user-limit-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:80
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:443

Chain ufw-user-limit (1 references)
target     prot opt source               destination
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] "
REJECT     all  --  0.0.0.0/0            0.0.0.0/0            reject-with icmp-port-unreachable

Chain ufw-user-limit-accept (1 references)
target     prot opt source               destination
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0

Chain ufw-user-logging-forward (0 references)
target     prot opt source               destination

Chain ufw-user-logging-input (0 references)
target     prot opt source               destination

Chain ufw-user-logging-output (0 references)
target     prot opt source               destination

Chain ufw-user-output (1 references)
target     prot opt source               destination

Right now I'm using ufw but I have the same problem when using iptables. Here is a configuration I tested, but still port 21 is showing as open:

iptables -t filter -F
iptables -t filter -X
iptables -t filter -P FORWARD DROP
iptables -t filter -P INPUT DROP
iptables -t filter -P OUTPUT DROP
iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -t filter -A INPUT -i lo -j ACCEPT
iptables -t filter -A OUTPUT -o lo -j ACCEPT
iptables -t filter -A OUTPUT -p tcp --dport 53 -j ACCEPT
iptables -t filter -A OUTPUT -p udp --dport 53 -j ACCEPT
iptables -t filter -A INPUT -p tcp --dport 53 -j ACCEPT
iptables -t filter -A INPUT -p udp --dport 53 -j ACCEPT
iptables -t filter -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -t filter -A OUTPUT -p tcp --dport 22 -j ACCEPT
iptables -t filter -A OUTPUT -p tcp --dport 80 -j ACCEPT
iptables -t filter -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -t filter -A OUTPUT -p tcp --dport 443 -j ACCEPT
iptables -t filter -A INPUT -p tcp --dport 443 -j ACCEPT

The firewall works because if I create a server using nc -l -p 5001 and scan the port from the outside it's closed.

Also if I telnet to port 21 it connects:

telnet fadverts.co.uk 21
Trying 139.59.172.62...
Connected to fadverts.co.uk.
Escape character is '^]'.

Any ideas what's going on here?

My home machine. I'm scanning my domain name which points to a digital ocean server in London: `nmap mydomain.com -p 21` — BugHunterUK, Jun 28 '17 at 16:01
May be you have any `PREROUTING` entries in `*nat` section? They work before `*filter`. could you show output for `iptables-save`. — Alexander Tolkachev, Jun 28 '17 at 21:04
A couple of friends from different countries ran some nmap scans and said port 21 was closed and they couldn't connect via telnet. It's strange because any device in this house shows it's open. No idea why, but I think it's a false positive. I can reproduce it too, if I create a new digital ocean box the same will happen but to everyone else it's closed. — BugHunterUK, Jun 29 '17 at 11:42
@BugHunterUK try to scan any other site. May be it's your ISP do something with 21/tcp port and it's shown as opened. — Alexander Tolkachev, Jul 01 '17 at 08:26

Can't Close Port 21

0 Answers0