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I am trying to do a routing rule to a bunch a subdomains matched by regular expression.

iptables -t nat -A PREROUTING -m string --string "/must-[a-z0-9-\.]+gonow[a-z\.]+/g" --algo regex -p tcp -m tcp --dport 12 -j DNAT --to-destination 1.1.1.1:12

getting error - iptables: No chain/target/match by that name.

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The problem is that you are attempting to use a relatively slow pattern matching algorithm in an application that MUST use a fast pattern matching algorithm. Also know that the entire use of the "string" filter can be compute intensive, and for that reason just a few years ago that module was not available by default. You will have to learn either the Boyer-Moore (bm) or Knuth-Pratt-Morris (kmp) fast string matching algorithms and use one of those.

See also the manual pages for iptables-extensions. Excerpt:

   --algo {bm|kmp}
          Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
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Some thoughts if not a solution:

  • Available parameters for the --algo option are bm or kmp. "regex" is not included, as noted.

  • I'm not certain either way whether an iptables command can incorporate regex. Perhaps not, by design, or perhaps so, because iptables presumably has access to the full range of shell/Bash syntax.

  • If regex is to work, one must attend closely to quoting. Regex often is hard/single quoted, for example.

  • I expect the issue depends on the distinction between an iptables command and an iptables rule. The command itself is not a rule but instead directs iptables to render a rule, which then resides in memory. The command may accept regex (e.g., in specifying a range, causing iptables to render a discrete rule for each member or increment of the range) simply because it is executed in a shell that permits it. It's another question, however, whether iptables permits passing regex to a rule, or evaluation of regex in a rule. @Doug's response suggests not, or at least that better alternatives exist. I expect at the least that getting regex to work in a rule will require some fancy and attentive quoting.

  • If you're simply looking to automate rule generation, perhaps consider a for-loop and incorporate your regex there or in a variable definition, e.g.,

dnsclients(){
local CLIENT
for CLIENT in $LAN; do
        iptables -A dns-clients -s $CLIENT -j dns-ports
done
}
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