why doesn't broadcast ping work?

Question

I have Freebsd as a router.
Local network is on interface sk0: inet 10.254.239.1 netmask 0xffffff00 broadcast 10.254.239.255
In my local network I have a computer(windows 7) that gets it's ip(10.254.239.2) from DHCP server on Freebsd.

• Ping to exact address 10.254.239.2 works fine.

When I try to ping bradcast 10.254.239.255 from Freebsd itself nothing happens:

--- 10.254.239.255 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss

• #arp -a says:
  ...
  ? (10.254.239.255) at (incomplete) on sk0 expired [ethernet]
  ...

• Firewall allows all on this interface

Where to look? What to do to make broadcast working?

Solution 1

If you want to perform host discovery, don't use broadcast pings, just simply ping each possible permutation for a given subnet. It sounds heavy, but it actually takes seconds (brackets are important):

$ time ( s=192.168.0 ; for i in $(seq 1 254) ; do ( ping -n -c 1 -w 1 $s.$i 1>/dev/null 2>&1 && printf "%-16s %s\n" $s.$i responded ) & done ; wait ; echo )
192.168.0.5      responded
192.168.0.11     responded
192.168.0.2      responded
192.168.0.254    responded
192.168.0.4      responded

real    0m1.317s
user    0m0.004s
sys 0m0.084s

Solution 2

Most operating systems simply ignore broadcast ICMP pings by default, for security reasons. You don't "make it work", and it has no effect on other kinds of broadcasts.

---

On FreeBSD, the net.inet.icmp.bmcastecho sysctl toggles this feature.

Solution 3

I know this question is very old, but it's one of the top result when googling 'ubuntu broadcast ping'.

I would like to add that you can send broadcast ping requests (ICMP echo-requests) very easily from a Linux host, with the option -b. Microsoft, for some reason, doesn't allow this.

Here's a broadcast ping sent from an Ubuntu host:

ping -b 192.168.1.255
WARNING: pinging broadcast address
PING 192.168.1.255 (192.168.1.255) 56(84) bytes of data.
64 bytes from 192.168.1.247: icmp_seq=1 ttl=64 time=0.031 ms
64 bytes from 192.168.1.254: icmp_seq=1 ttl=64 time=0.960 ms (DUP!)
64 bytes from 192.168.1.202: icmp_seq=1 ttl=64 time=92.3 ms (DUP!)

Note the -b option to allow broadcast. As you can see, several hosts on my network have replied, including a Cisco switch, my own Ubuntu host, and an Apple laptop.

And yes, as others have said before, broadcast pings are terrible for scanning a network. Some hosts simply ignore echo-requests. There are much better tools for this purpose (E.g. nmap).

Comments

[user714403]

While it does offer an alternative way to ping the network, it doesn't answer the questions posed as to "why doesn't broadcast ping work?"

[Craig]

True, but call it pragmatic intuition. I suspected there was an underlying reason as to why this person was trying to broadcast ping the network. I don't see any harm in going beyond the answer to provide a possible solution. ;-)

[Andre Miller]

And on Linux you can enable broadcast pings with: sysctl net.ipv4.icmp_echo_ignore_broadcasts=0

[Andrew Marshall]

Or, e.g. nmap -sn 192.168.1.0/24 (replace with appropriate subnet).