Supports custom DNS server addresses

[xieliuhao]

For example, when making a query, it supports specifying DNS server addresses such as 8.8.8.8 and 114.114.114.114. This is because, in some networks, the default DNS server may not be able to retrieve the records.

[alessionossa]

I think it is already possible to do this by initialising the resolver like the following:

var options = CAresDNSResolver.Options.default
options.servers = ["8.8.8.8", "114.114.114.114"]
let resolver = try AsyncDNSResolver(options: options)

[bci]

This works, with only ipv6 an issue.

My current application allows the user to select from a list of public DNS servers:

• 1.1.1.1
• 8.8.8.8
• 208.67.220.220

All work.

• 7.7.7.7 fails as expected. This is not a DNS server.

Error: <DnsQueries.swift> getMxWithDomainName(domainName:altDnsServer:) [#161]| The operation couldn’t be completed. (AsyncDNSResolver.AsyncDNSResolver.Error error 1.)

• [2620:119:35::35]
• 2620:119:35::35

IPv6 addresses for dns.opendns.com - I expected this to work as ipv6 is documented. I was unsure in brackets were required, so I tried both versions. Same error:

Error: <DnsQueries.swift> getMxWithDomainName(domainName:altDnsServer:) [#161]| The operation couldn’t be completed. (AsyncDNSResolver.AsyncDNSResolver.Error error 1.)

My function:

func getMxWithDomainName(domainName: String, altDnsServer: String = "") async throws -> [String] {
    var mxRecords: [MXRecord]
    do {
        var options = CAresDNSResolver.Options.default
        options.timeoutMillis = 3000
        if altDnsServer != "" {
            debugLog(object: "Setting options.servers to [\(altDnsServer)")
            options.servers = [altDnsServer]
        }
        let resolver = try AsyncDNSResolver(options: options)
        mxRecords = try await resolver.queryMX(name: domainName)
    } catch {
        debugLog(object: error.localizedDescription)
        throw error
    }
    let mxStrings = mxRecords.map { "\($0.host) (\($0.priority))" }
    return mxStrings
}

I just need to figure out how to get better error strings ;)