dnsdist: How to send certain packets into the specified pool via "LuaFFIAction"

[jellybean13]

Hello,

I'm using "dnsdist 2.1.0-rc1 (Lua 5.1.4 [LuaJIT 2.1.1703358377])". I'd like to route requests to certain pool based on the value of a custom tag "tag_example", and all values of this tag are identical to the pool name (e.g. The tag value "dest_a" --> The pool name "dest_a"). I can simply achieve this using the following Lua function invoked by "LuaAction":

function lua_route_tag_to_pool(dq)
    return DNSAction.Pool, dq:getTag("tag_example")
end

However, if I switch "LuaAction" to "LuaFFIAction", the situation will change:

• I can't directly set the pool for "dq".
• Using the following Lua function will lead to a "No downstream server available" issue, regardless of whether a pool named "dest_a" exists.

function lua_route_tag_to_pool(dq)
    return DNSAction.Pool, "dest_a"
end

Here is an example of the relevant error logs:

msg="No downstream server available" subsystem="query-processing" level="0" ts="2026-06-30T16:05:06+0800" destination.address="192.168.0.1:53" dns.question.class="1" dns.question.flags="1" dns.question.id="0" dns.question.name="bing.com" dns.question.type="1" dnsdist.action="Dropped" proto="DoUDP" source.address="100.112.0.1:53878"

Maybe implementing a function like this could solve this problem:

void dnsdist_ffi_dnsquestion_set_pool(dnsdist_ffi_dnsquestion_t* dq, const char* poolName) __attribute__((visibility("default")));

Users can use this function like that:

local ffi = require("ffi")

function lua_route_tag_to_pool(dq)
    dnsdist_ffi_dnsquestion_set_pool(ffi.C.dnsdist_ffi_dnsquestion_get_tag(dq, "tag_example"))
    return DNSAction.Pool
end

[rgacogne]

You cannot use the DNSAction.Pool, "dest_a" syntax from a Lua FFI function, you need to use dnsdist_ffi_dnsquestion_set_result to set the second parameter and then return DNSAction.pool. I just added regression tests for this syntax in #17654 if you want an example.