Differences for page hping getfield -- hping network security tool

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Differences for page hping getfield

Current version compared with version Wed Sep 29 06:06:26 GMT 2004

...
   hping3> hping getfield ip ttl "ip(saddr=1.2.3.4,daddr=5.6.7.8,ttl=64)"
   64
  
- If the layer does not exists, an empty string is returned.
+ If the layer does not exist, an empty string is returned.
  
  
  In complex packets the same layer can be present more times,
...

The following is the old page content

{hping getfield `layer` `field` `?skip?` `packet`}

[hping getfield] returns the value of the given `field` in the given
`layer`, for `packet`. For example:

hping3> hping getfield ip ttl "ip(saddr=1.2.3.4,daddr=5.6.7.8,ttl=64)"
 64

If the layer does not exists, an empty string is returned.

In complex packets the same layer can be present more times,
for example an [ICMP] error message can have its own [IP] header
and the [IP] header of the quoted packet. In order to specify
what is the right layer the `skip` argument of [hping getfield], [hping hasfield], [hping setfield]
is used. This argument specify how many layers of the specified
type to skip before to get the field value, so to specify a
`skip` value of 1 means to get the field from the second [IP] layer
and so on. Example:

hping3.0.0-alpha> set p [hping recv eth0]
 {ip(ihl=5,ver=4,tos=c0,totlen=58,id=62912,fragoff=0,mf=0,df=0,rf=0,ttl=64,proto=1,cksum=e500,saddr=192.168.1.7,daddr=192.168.1.6)+icmp(type=3,code=3,unused=0)+ip(ihl=5,ver=4,tos=00,totlen=30,id=60976,fragoff=0,mf=0,df=1,rf=0,ttl=64,proto=17,cksum=40c9,saddr=192.168.1.6,daddr=192.168.1.7)+udp(sport=33169,dport=10,len=10,cksum=94d6)+data(str=f\0a)}

Remember that [hping recv] returns a [Tcl] list, for default this list
is composed of only one element, but anyway we need to get extract
this element from the list:

hping3.0.0-alpha> set packet [lindex $p 0]
 ip(ihl=5,ver=4,tos=c0,totlen=58,id=62912,fragoff=0,mf=0,df=0,rf=0,ttl=64,proto=1,cksum=e500,saddr=192.168.1.7,daddr=192.168.1.6)+icmp(type=3,code=3,unused=0)+ip(ihl=5,ver=4,tos=00,totlen=30,id=60976,fragoff=0,mf=0,df=1,rf=0,ttl=64,proto=17,cksum=40c9,saddr=192.168.1.6,daddr=192.168.1.7)+udp(sport=33169,dport=10,len=10,cksum=94d6)+data(str=f\0a)

Now `packet` is set to an [ICMP] error obtained using netcat to send an [UDP] packet
to a non-bound port. We can ask for the checksum of the first ip layer:

hping3.0.0-alpha> hping getfield ip cksum $packet
 0xe500

If we want the second, we add a `skip` argument of 1:

hping3.0.0-alpha> hping getfield ip cksum 1 $packet
 0x40c9

A `skip` value of 0 is valid, and is equivalent to the
form without the skip argument.