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Forum Discussion

dragonflymr's avatar
dragonflymr
Icon for Cirrostratus rankCirrostratus
Dec 07, 2015

Parsing IP to extract subnet

Hi,   I wonder if there is more elegant and fastest way to extract specific part of IP - like get only part of IP up to second dot, third dot.   I know that I can use scan to separate IP in fou...
devops
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StephanManthey's avatar
StephanManthey
Icon for Nacreous rankNacreous
Dec 08, 2015

Hi Piotr,

I hope you are doing well.

I am using 
getfield
to access the IP address digits i.e. as follows: 
when CLIENT_ACCEPTED {     
    snat 10.10.10.[expr ( [getfield [IP::client_addr] "." 4] % 32 ) + 1]
     snat 10.10.[getfield [IP::client_addr] "." 3].[getfield [IP::client_addr] "." 4]
}

The second line would apply a SNAT address based on replacing the first three digits the original IP address by "10.10.10." and calculates the last digit by reading the last digit from the original client IP and applies a modulus of 32 + 1 (i.e. turns a 47 into 16 to vary the SNATs in a range of 10.10.10.1 to 10.10.10.32 and persist to this value as long as client IP doesnt change). (Make sure to use a virtual address space or have "real" SNATs configured to avoid ARP issues.) The third line (commented) is using the original third and fourth digit of the original client IP to define a new SNAT. (Same ARP issues to expect as described above.)

Thanks, Stephan