library(data.table)

## this basic aysnc lookup is a modified version of an idea described in
## http://rud.is/b/2013/08/12/reverse-ip-address-lookups-with-r-from-simple-to-bulkasynchronous/
ip.to.host <- function(ips) {
  ## store ip list in a temp file
  tf <- tempfile()
  cat(ips, sep='\n', file=tf)
  ## use the adns filter to resolve them asynchronously (see man page for timeouts and other options)
  host.names <- system(paste("adnsresfilter <", tf) ,intern=TRUE, ignore.stderr=TRUE)
  ## cleanup the temp file
  file.remove(tf)
  return(host.names)
}

## now extend the above to implement a  ip to name cache
ip.cached.lookup <- function(ips, reset.cache=FALSE) {
  cache.file <- "~/.ip.cache.rda"

  ## if the cache file exists: load it
  if (!reset.cache & !file.access(cache.file,4)){
      load(cache.file)
      message("ip cache entries loaded :", nrow(host))
  } else {
      ## create an empty table (with just locahost)
      host <- data.table(hip="127.0.0.1", hname="localhost")
  }

  ## prepare a table of query ip and name
  qh <- data.table(hip=as.character(ips),hname=NA)

  ## keep them sorted by ip to speedup data.table lookups
  setkey(host,hip)

  ## resolve all known host name from the cache
  qh$hname <- host[qh]$hname

  ## collect the list of unique ips which did not get resolved yet
  new.ips <- unique(qh[is.na(qh$hname)]$hip)

  ## if not empty, resolve the rest
  if (length(new.ips) > 0) {
    ## add the new ips to the cache table
    host <- rbind(host, list(hip=new.ips,hname=NA))

    ## find locations which need resolution (either new or expired)
    need.resolving <- is.na(host$hname)

    message("new ips to resolve: ", sum(need.resolving))
    ## and resolve them
    host$hname[need.resolving] <- ip.to.host(host[need.resolving]$hip)

    ## need to set key again after rbind above..
    setkey(host,hip)

    ## .. to do the remaining lookups
    qh$hname <- host[qh]$hname

    ## save the new cache status
    save(host, file = cache.file)
  }

  return(qh$hname)
}

## with this function you can easily add a host.name column to your
## weblog data.table from the previous posts to get started with
## the real log analysis

w$host.name <- ip.cached.lookup(w$host)