0 bytes pin using check_ping

[Andreas Ericsson]

Andy Shellam (Mailing Lists) wrote:
> Why do you need to make a "0 byte" ping?  (I'm not particularly up on 
> networking, but a 0 byte ping would in theory send nothing, right?)
> 

No, it would send the IP and ICMP headers, but with a 0-byte data part 
of the icmp echo request.

> Tried doing this on my Fedora system, and you still get a minimum of 8 
> bytes back:
> 
> /bin/ping -s 0 www.google.com
> PING www.l.google.com (216.239.59.99) 0(28) bytes of data.
> 8 bytes from 216.239.59.99: icmp_seq=1 ttl=243
> 

That is the struct timeval that ping includes after the ICMP header. 8 
bytes of data is the absolute minimum to send in an ICMP packet. It's 
normally defined as ICMP_MINLEN in /usr/include/netinet/ip_icmp.h.

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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