Question

NMAP shows ports that I know nothing about in the "filtered" state

Asked by: psueoc

I noticed yesterday that when I run an NMAP against the external interface of my OpenBSD firewall it shows ports in the "filtered" state, that I know nothing about.  In the past it would only show the ports that I purposely opened in the PF.CONF file.  I don't know what changed, that's making me now see these "filtered" ports.  Below is output from NMAP.

Starting Nmap 4.10 ( http://www.insecure.org/nmap ) at 2008-02-28 08:18 Eastern Standard Time
Interesting ports on email.eoc.psu.edu (128.118.20.198):
Not shown: 1661 closed ports
PORT      STATE    SERVICE
21/tcp    open     ftp
22/tcp    open     ssh
25/tcp    open     smtp
80/tcp    open     http
554/tcp   filtered rtsp
1025/tcp  open     NFS-or-IIS
1720/tcp  filtered H.323/Q.931
2000/tcp  filtered callbook
3389/tcp  open     ms-term-serv
5060/tcp  filtered sip
49400/tcp filtered compaqdiag
50000/tcp filtered iiimsf
50002/tcp filtered iiimsf
54320/tcp filtered bo2k
61439/tcp filtered netprowler-manager
61440/tcp filtered netprowler-manager2
61441/tcp filtered netprowler-sensor
65301/tcp filtered pcanywhere

Nmap finished: 1 IP address (1 host up) scanned in 2.250 seconds

The ports in the OPEN state, are the ones that I opened on purpose.  The ones in the filtered state, I know nothing about.  Below is my pf.conf file

# VARIABLES
ext_if="xl1"
int_if="xl0"
tcp_services="{ 1025 }"
icmp_types="echoreq"
ssh="192.168.1.3"
web="192.168.1.3"
email="192.168.1.2"
ftp_server="192.168.1.3"

# OPTIONS
set block-policy return
set loginterface $ext_if
set skip on lo

# SCRUB
scrub in

# NAT/RDR
nat on $ext_if from !($ext_if) -> ($ext_if:0)

################# FTP ##################
nat-anchor "ftp-proxy/*"
rdr-anchor "ftp-proxy/*"
rdr on $ext_if proto tcp from any to any port 21 -> $ftp_server port 21
rdr on $ext_if proto tcp from any to any port 49152:65535 -> \
  $ftp_server port 49152:65535
################# FTP ##################

rdr on $ext_if proto tcp from any to any port 80 -> $web
rdr on $ext_if proto tcp from any to any port smtp -> $email
rdr on $ext_if proto tcp from any to any port 3389 -> $web
rdr on $ext_if proto tcp from any to any port 22 -> $ssh

# FILTER RULES
block in
pass out keep state
anchor "ftp-proxy/*"
antispoof quick for {lo $int_if }

pass in on $ext_if proto tcp from any to ($ext_if) \
  port $tcp_services flags S/SA keep state

pass in on $ext_if proto tcp from any to $web port 80 \
  flags S/SA synproxy state

pass in on $ext_if proto tcp from any to $web port 3389 \
  flags S/SA synproxy state

pass in on $ext_if proto tcp from any to $email port smtp \
  flags S/SA synproxy state

pass in on $ext_if proto tcp from any to $ssh port 22 \
  flags S/SA synproxy state

#################  FTP  #################
pass in quick on $ext_if proto tcp from any to $ftp_server \
  port 21 keep state
pass in quick on $ext_if proto tcp from any to $ftp_server \
  port > 49151 keep state
pass out quick on $int_if proto tcp from any to $ftp_server \
  port 21 keep state
pass out quick on $int_if proto tcp from any to $ftp_server \
  port > 49151 keep state
#################  FTP  #################

pass in inet proto icmp all icmp-type $icmp_types keep state
pass quick on $int_if



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Asked On
2008-02-28 at 05:30:27ID23200135
Tags

OpenBSD

,

Packet Filtering

,

3.9

Topics

OpenBSD

,

Unix Operating Systems

,

BSD Unix

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: ahoffmannPosted on 2008-03-01 at 12:43:03ID: 21023436

stolen from man nmap

       Filtered means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed

 

by: psueocPosted on 2008-03-03 at 05:18:30ID: 21031106

what would cause them to start showing up as filtered.  I didn't change anything.  Could my firewall be compromised?

I used to only see the OPEN ports.

 

by: ahoffmannPosted on 2008-03-04 at 13:20:50ID: 21045234

stolen from nmap's man page again:

       open|filtered
              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.

 

by: gheistPosted on 2008-03-11 at 23:58:56ID: 21103608

Your pf.conf does not return anything via "block in" rule, so nmap's "filtered" is expected.
No-one is compromised. Could it be you restarted system and firewall enabled finally :S

 

by: gheistPosted on 2008-06-06 at 05:44:59ID: 21728445

icmp rate limiting kicked in and you did not get response from some closed ports. why you are using nmap when you have system access including "netstat"

20120131-EE-VQP-002

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