what does this mean "80/tcp filtered http"

Question

Our linux server(ubuntu) stop listing on port 80. this is what i got :

PORT     STATE    SERVICE
22/tcp   open     ssh
80/tcp   filtered http

what does this "80/tcp filtered http" means?
What are the reasons for a port become open to filtered?
How could we open port 80 again?

we run a node.js web applicatoin on this server. we've added port 80 on iptables but no help.

iptables -A INPUT -p tcp --dport 80 -j ACCEPT

Solution

For what I can read from the NMAP manual there are 6 states (at least for NMAP):

Open State

An application is actively accepting TCP connections, UDP datagrams or SCTP associations on this port. Finding these is often the primary goal of port scanning. Security-minded people know that each open port is an avenue for attack. Attackers and pen-testers want to exploit the open ports, while administrators try to close or protect them with firewalls without thwarting legitimate users. Open ports are also interesting for non-security scans because they show services available for use on the network.

Closed State

A closed port is accessible (it receives and responds to Nmap probe packets), but there is no application listening on it. They can be helpful in showing that a host is up on an IP address (host discovery, or ping scanning), and as part of OS detection. Because closed ports are reachable, it may be worth scanning later in case some open up. Administrators may want to consider blocking such ports with a firewall. Then they would appear in the filtered state, discussed next.

Filtered State

Nmap cannot determine whether the port is open because packet filtering prevents its probes from reaching the port. The filtering could be from a dedicated firewall device, router rules, or host-based firewall software. These ports frustrate attackers because they provide so little information. Sometimes they respond with ICMP error messages such as type 3 code 13 (destination unreachable: communication administratively prohibited), but filters that simply drop probes without responding are far more common. This forces Nmap to retry several times just in case the probe was dropped due to network congestion rather than filtering. This slows down the scan dramatically.

Unfiltered State

The unfiltered state means that a port is accessible, but Nmap is unable to determine whether it is open or closed. Only the ACK scan, which is used to map firewall rulesets, classifies ports into this state. Scanning unfiltered ports with other scan types such as Window scan, SYN scan, or FIN scan, may help resolve whether the port is open.

Open & Filtered State

Nmap places ports in this state when it is unable to determine whether a port is open or filtered. This occurs for scan types in which open ports give no response. The lack of response could also mean that a packet filter dropped the probe or any response it elicited. So Nmap does not know for sure whether the port is open or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans classify ports this way.

Closed & Filtered State

This state is used when Nmap is unable to determine whether a port is closed or filtered. It is only used for the IP ID idle scan.

So in your case, the filtered state might be because some packet filtering software might be blocking/preventing the detection. This could be a firewall, router, ip rules, etc.. You could start disabling each one, one at a time to quickly see which one is the one causing the problem.