ADBulk/Services/DomainDiscovery.cs

using System.Net;
using ADBulkTool.Models;
using DnsClient;

namespace ADBulkTool.Services;

public static class DomainDiscovery
{
    public static async Task<DiscoveryResult> DiscoverAsync(TimeSpan timeout)
    {
        var steps = new List<string>();

        try
        {
            var domain = await GetSystemDomainAsync(timeout);
            if (string.IsNullOrWhiteSpace(domain))
            {
                return new DiscoveryResult(null, null, null, null, null,
                    "Could not detect a DNS domain from this machine. Check that you are on a domain-joined network and that /etc/resolv.conf has a search or domain entry.");
            }

            steps.Add($"Detected domain: {domain}");

            var searchBase = DomainToSearchBase(domain);
            steps.Add($"Derived search base: {searchBase}");

            var dnsServers = GetDnsServers();
            var dnsServer = dnsServers.FirstOrDefault();
            steps.Add(dnsServer is not null
                ? $"DNS server from /etc/resolv.conf: {dnsServer}"
                : "No DNS server found in /etc/resolv.conf, using system default.");

            var ldapHost = await FindDomainControllerViaSrvAsync(domain, dnsServer, timeout);

            if (ldapHost is not null)
            {
                steps.Add($"Found DC via SRV record: {ldapHost}");
                return new DiscoveryResult(domain, dnsServer, ldapHost, 636, searchBase,
                    string.Join(" | ", steps));
            }

            ldapHost = await FindDomainControllerViaLdapSrvAsync(domain, dnsServer, timeout);

            if (ldapHost is not null)
            {
                steps.Add($"Found DC via generic LDAP SRV: {ldapHost}");
                return new DiscoveryResult(domain, dnsServer, ldapHost, 636, searchBase,
                    string.Join(" | ", steps));
            }

            var guessedHost = $"dc01.{domain}";
            steps.Add($"SRV lookup failed. Guessed DC: {guessedHost}");
            return new DiscoveryResult(domain, dnsServer, guessedHost, 636, searchBase,
                string.Join(" | ", steps));
        }
        catch (Exception ex)
        {
            return new DiscoveryResult(null, null, null, null, null,
                $"Discovery failed: {ex.Message}");
        }
    }

    public static List<string> GetDnsServers()
    {
        var servers = new List<string>();

        try
        {
            var lines = File.ReadAllLines("/etc/resolv.conf");
            foreach (var line in lines)
            {
                var trimmed = line.Trim();
                if (trimmed.StartsWith("nameserver ", StringComparison.OrdinalIgnoreCase))
                {
                    var parts = trimmed.Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
                    if (parts.Length >= 2 && IPAddress.TryParse(parts[1], out _))
                    {
                        servers.Add(parts[1]);
                    }
                }
            }
        }
        catch
        {
        }

        return servers;
    }

    public static string? DomainToSearchBase(string? domain)
    {
        if (string.IsNullOrWhiteSpace(domain))
        {
            return null;
        }

        var parts = domain.Split('.', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
        if (parts.Length == 0)
        {
            return null;
        }

        return string.Join(",", parts.Select(p => $"DC={p}"));
    }

    private static async Task<string?> GetSystemDomainAsync(TimeSpan timeout)
    {
        try
        {
            using var cts = new CancellationTokenSource(timeout);

            var resolvDomain = await Task.Run(() => GetDomainFromResolvConf(), cts.Token);
            if (!string.IsNullOrWhiteSpace(resolvDomain))
            {
                return resolvDomain;
            }

            var hostDomain = await Task.Run(() => GetDomainFromHostname(), cts.Token);
            if (!string.IsNullOrWhiteSpace(hostDomain))
            {
                return hostDomain;
            }

            var ipDomain = await Task.Run(() =>
            {
                try { return System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().DomainName; }
                catch { return string.Empty; }
            }, cts.Token);
            if (!string.IsNullOrWhiteSpace(ipDomain) && !string.Equals(ipDomain, "localdomain", StringComparison.OrdinalIgnoreCase))
            {
                return ipDomain.TrimEnd('.');
            }

            return null;
        }
        catch
        {
            return null;
        }
    }

    private static string? GetDomainFromResolvConf()
    {
        try
        {
            var lines = File.ReadAllLines("/etc/resolv.conf");
            foreach (var line in lines)
            {
                var trimmed = line.Trim();

                if (trimmed.StartsWith("domain ", StringComparison.OrdinalIgnoreCase))
                {
                    var parts = trimmed.Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
                    if (parts.Length >= 2 && !string.IsNullOrWhiteSpace(parts[1]))
                    {
                        return parts[1].TrimEnd('.');
                    }
                }

                if (trimmed.StartsWith("search ", StringComparison.OrdinalIgnoreCase))
                {
                    var parts = trimmed.Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
                    if (parts.Length >= 2 && !string.IsNullOrWhiteSpace(parts[1]))
                    {
                        return parts[1].TrimEnd('.');
                    }
                }
            }
        }
        catch
        {
        }

        return null;
    }

    private static string? GetDomainFromHostname()
    {
        try
        {
            var hostname = Dns.GetHostEntry("localhost").HostName;
            var dotIndex = hostname.IndexOf('.');
            if (dotIndex > 0 && dotIndex < hostname.Length - 1)
            {
                var domain = hostname[(dotIndex + 1)..].TrimEnd('.');
                return string.IsNullOrWhiteSpace(domain) ? null : domain;
            }
        }
        catch
        {
        }

        try
        {
            var fullHostname = System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().HostName;
            var dotIndex = fullHostname.IndexOf('.');
            if (dotIndex > 0 && dotIndex < fullHostname.Length - 1)
            {
                var domain = fullHostname[(dotIndex + 1)..].TrimEnd('.');
                return string.IsNullOrWhiteSpace(domain) ? null : domain;
            }
        }
        catch
        {
        }

        return null;
    }

    private static async Task<string?> FindDomainControllerViaSrvAsync(string domain, string? dnsServer, TimeSpan timeout)
    {
        try
        {
            var srvQuery = $"_ldap._tcp.dc._msdcs.{domain}";
            var lookup = CreateLookup(dnsServer);
            using var cts = new CancellationTokenSource(timeout);
            var result = await lookup.QueryAsync(srvQuery, QueryType.SRV, QueryClass.IN, cts.Token);
            var record = result.Answers.SrvRecords().FirstOrDefault();
            return record?.DomainName.Value.TrimEnd('.');
        }
        catch
        {
            return null;
        }
    }

    private static async Task<string?> FindDomainControllerViaLdapSrvAsync(string domain, string? dnsServer, TimeSpan timeout)
    {
        try
        {
            var srvQuery = $"_ldap._tcp.{domain}";
            var lookup = CreateLookup(dnsServer);
            using var cts = new CancellationTokenSource(timeout);
            var result = await lookup.QueryAsync(srvQuery, QueryType.SRV, QueryClass.IN, cts.Token);
            var record = result.Answers.SrvRecords().FirstOrDefault();
            return record?.DomainName.Value.TrimEnd('.');
        }
        catch
        {
            return null;
        }
    }

    private static LookupClient CreateLookup(string? dnsServer)
    {
        if (dnsServer is not null && IPAddress.TryParse(dnsServer, out var ip))
        {
            return new LookupClient(new LookupClientOptions(ip) { UseCache = false, Timeout = TimeSpan.FromSeconds(5) });
        }

        return new LookupClient(new LookupClientOptions { UseCache = false, Timeout = TimeSpan.FromSeconds(5) });
    }
}