fields in sc_w3c.log c-dns

[chophouse]

What is the purpose of capturing the c-dns address when you already capture the c-ip ?

Also can you please explain the abbreviations (and units) of the log fields? For instance why are some fields pre-pended with 'c' and others with "cs"? What is "sc" and "x"?

c-ip
c-dns
cs-uri-stem
c-status
cs(User-Agent)
sc-bytes
x-duration
avgbandwidth

Thanks