[CentOS] testing a udp socket with netcat

Giles Coochey giles at coochey.net
Wed Mar 14 10:11:00 EDT 2012


On 14/03/2012 13:59, Arif Hossain wrote:
> On Wed, 2012-03-14 at 08:37 -0500, Les Mikesell wrote:
>> On Wed, Mar 14, 2012 at 8:25 AM, Arif Hossain<freefall1986 at gmail.com>  wrote:
>>> i've this udp daemon which is waiting for an incoming udp datagram. now
>>> i want test this daemon for random garbage to test how it behaves. My
>>> udp daemon is running because its shows on netstat. problem is if i
>>> issue following command for putting udp datagram :
>>> $nc -uvvz<host>  <port>
>>>
>>> it does not output any thing. i have straced the udp daemon. which does
>>> not returns from recv() call.
>>>
>>> my goal is to ppience regarding netcat so i'm asking if anyone can help me
>>> out with this.
>> Do you have iptables running?   The default config would probably
>> block your udp traffic.
>>
> here is my iptable-rules
>
> # Generated by iptables-save v1.4.7 on Wed Mar 14 19:58:13 2012
> *mangle
> :PREROUTING ACCEPT [329554:95268521]
> :INPUT ACCEPT [88918:46924677]
> :FORWARD ACCEPT [0:0]
> :OUTPUT ACCEPT [7296:1924138]
> :POSTROUTING ACCEPT [7296:1924138]
> COMMIT
> # Completed on Wed Mar 14 19:58:13 2012
> # Generated by iptables-save v1.4.7 on Wed Mar 14 19:58:13 2012
> *filter
> :INPUT ACCEPT [0:0]
> :FORWARD ACCEPT [0:0]
> :OUTPUT ACCEPT [206514:24743648]
> -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
> -A INPUT -p icmp -j ACCEPT
> -A INPUT -i lo -j ACCEPT
> -A INPUT -p tcp -m state --state NEW -m tcp --dport<port>  -j ACCEPT
> -A INPUT -j REJECT --reject-with icmp-host-prohibited
> -A FORWARD -j REJECT --reject-with icmp-host-prohibited
> COMMIT
> # Completed on Wed Mar 14 19:58:13 2012
>
And your INPUT chain rejects everything that is not matched by those 
lines above it.

You'll need a:

-A INPUT -p udp --dport <port> -j ACCEPT in there before you go to REJECT.

-- 
Best Regards,

Giles Coochey
NetSecSpec Ltd
UK Mobile: +44 7983 877 438
Business Email: giles.coochey at netsecspec.co.uk
Email/MSN/Live Messenger: giles at coochey.net
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