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I try to ping scan my local network using nmap, but it doesn't seem to find machines that are for sure alive. It responses to ping but on to ping scan.

Results for ping:

$ ping
PING ( 56 data bytes 64 bytes from icmp_seq=0 ttl=64 time=1.585 ms

And output of tcpdump:

$ tcpdump -i fxp0 -n host
12:43:07.624911 IP > ICMP echo request, id 61023, seq 0, length 64
12:43:07.625029 IP > ICMP echo reply, id 61023, seq 0, length 64

But when I try to use nmap it doesn't see it in spite of receiving response.

$ nmap -sn

Starting Nmap 5.61TEST5 ( ) at 2012-05-13 12:44 CEST
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in 3.02 seconds

Tcpdump output:

$ tcpdump -i fxp0 -n host
12:45:30.742933 IP > ICMP echo request, id 2715, seq 0, length 8
12:45:30.742974 IP > S 3607279099:3607279099(0) win 1024 <mss 1460>
12:45:30.743001 IP > . ack 3607279099 win 1024
12:45:30.743025 IP > ICMP time stamp query id 45449 seq 0, length 20
12:45:30.743061 IP > ICMP echo reply, id 2715, seq 0, length 8
12:45:30.743097 IP > S 2602942480:2602942480(0) ack 3607279100 win 5840 <mss 1460>
12:45:30.743130 IP > R 3607279100:3607279100(0) win 0
12:45:30.743134 IP > R 3607279099:3607279099(0) win 0
12:45:30.743148 IP > ICMP time stamp reply id 45449 seq 0: org 00:00:00.000, recv 11:04:04.243, xmit 11:04:04.243, length 20
12:45:32.745245 IP > ICMP time stamp query id 48722 seq 0, length 20
12:45:32.745281 IP > . ack 3607344634 win 1024
12:45:32.745307 IP > S 3607344634:3607344634(0) win 1024 <mss 1460>
12:45:32.745340 IP > ICMP time stamp reply id 48722 seq 0: org 00:00:00.000, recv 11:04:06.245, xmit 11:04:06.245, length 20
12:45:32.745380 IP > R 3607344634:3607344634(0) win 0
12:45:32.745401 IP > S 2629695507:2629695507(0) ack 3607344635 win 5840 <mss 1460>
12:45:32.745421 IP > R 3607344635:3607344635(0) win 0
12:45:32.745431 IP > ICMP echo request, id 38289, seq 0, length 8
12:45:32.745517 IP > ICMP echo reply, id 38289, seq 0, length 8

Why nmap says host seems down when it responds to requests?


Nmap version:

$ nmap --version

Nmap version 5.61TEST5 ( )
Platform: i386-portbld-freebsd7.4
Compiled with: liblua-5.1.5 openssl-0.9.8q libpcre-8.30 libpcap-0.9.8 nmap-libdnet-1.12 ipv6
Compiled without:

To be sure, first filter rule in firewall is:

pass quick on fxp0
share|improve this question
This is not normal behavior. If you check everything Tom H mentions in his answer below and still have the problem, send the output of nmap --version, nmap --iflist, and what you posted here to, the Nmap developers' mailing list. – bonsaiviking May 13 '12 at 13:04

2 Answers 2

Just tried this with my OpenBSD 5.1. Yes, you have FreeBSD 7.4, but we are talking quite a similar beast.

As a regular user, nmap -sn returns Host seems down for me, too, even though my test machine surely responds to ping as a regular user.

As root, however, nmap -sn returns Host is up as it should.

Looking at your prompt, you are likely also trying to nmap as a non-root user. Try if nmap works as root.

EDIT: Just fired up my FreeBSD 9.0, there nmap ping scan works fine as non-root.

share|improve this answer

My first observation would be that tcpdump -i fxp0 is dumping the packets that arrive on the fxp0, however nmap is running as a userspace process, hence a number of things could block nmap from seeing the response.

  1. such as an netfilter rule (user iptables -t filter -n -L)
  2. selinux (not seen this particular thing, but selinux is always a good bet when stupid sh*t happens
  3. also check the iptables nat tables to be sure iptables -t nat -n -L
share|improve this answer
umm. is fxp0 bsd? no idea then... – Tom H May 13 '12 at 11:45
if you are on linux try is under strace and see if it gives you more of a clue... strace -t -e trace=network nmap -sn – Tom H May 13 '12 at 11:50
OS is FreeBSD. I have updated original question with nmap version and info about firewall rule. – aadam May 15 '12 at 9:36

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