Cannot parse DNS IPv6 address

[MarsSec773]

Improve Docs

Continue with #3184
I got an error of "local DNS is empty" when I try to deploy a APISIX pod.
I specified DNS resolver as IPv6 address.

Please describe which part of docs should be improved or typo fixed

Source code with the error:
https://github.com/apache/apisix/blob/924a30db36268ee1d7fab255ddebfc23a7110314/apisix/cli/ops.lua

    local dns_resolver = sys_conf["dns_resolver"]
    if not dns_resolver or #dns_resolver == 0 then
        local dns_addrs, err = local_dns_resolver("/etc/resolv.conf")
        if not dns_addrs then
            util.die("failed to import local DNS: ", err, "\n")
        end

        if #dns_addrs == 0 then
            util.die("local DNS is empty\n")
        end

        sys_conf["dns_resolver"] = dns_addrs
    end

DNS address is not read because local_dns_resolver function doesn't parse IPv6 address.
We can see the regex.
https://github.com/apache/apisix/blob/924a30db36268ee1d7fab255ddebfc23a7110314/apisix/cli/ops.lua

local function local_dns_resolver(file_path)
    local file, err = io_open(file_path, "rb")
    if not file then
        return false, "failed to open file: " .. file_path .. ", error info:" .. err
    end

    local dns_addrs = {}
    for line in file:lines() do
        local addr, n = line:gsub("^nameserver%s+(%d+%.%d+%.%d+%.%d+)%s*$", "%1")
        if n == 1 then
            table_insert(dns_addrs, addr)
        end
    end

    file:close()
    return dns_addrs
end

I hope this improvement could be applied, thanks.

Describe the solution you'd like

Add a regex for IPv6 DNS resolver.

[spacewander]

@MarsSec773
Can you try #3187?

[MarsSec773]

@MarsSec773
Can you try #3187?

Thank you very much for your quick response.
Everything works great.

By adding [ ] to enclose the IPv6 address additionally, I got running state of apisix server.

NAME                             READY   STATUS    RESTARTS   AGE
apisix-server-65fd5bb697-d8xv6   1/1     Running   0          26s

I don't have experience of coding Lua. FYI, let me provide my final script.

local function local_dns_resolver(file_path)
    local file, err = io_open(file_path, "rb")
    if not file then
        return false, "failed to open file: " .. file_path .. ", error info:" .. err
    end

    local dns_addrs = {}
    for line in file:lines() do
        local addr, n = line:gsub("^nameserver%s+(%d+%.%d+%.%d+%.%d+)%s*$", "%1")
        local addr6, n6 = line:gsub("^nameserver%s+([^%s]+)%s*$", "%1")
        if n == 1 then
            table_insert(dns_addrs, addr)
        end
        if n6 == 1 then
            table_insert(dns_addrs, "[" .. addr6 .. "]")
        end
    end

    file:close()
    return dns_addrs
end

Thank you so much @spacewander .