    const std = @import("std.zig");
    const builtin = @import("builtin");
    const assert = std.debug.assert;
    const net = @This();
    const mem = std.mem;
    const os = std.os;
    const fs = std.fs;
    const io = std.io;
    const native_endian = builtin.target.cpu.arch.endian();

    // Windows 10 added support for unix sockets in build 17063, redstone 4 is the

    // first release to support them.

    pub const has_unix_sockets = @hasDecl(os.sockaddr, "un") and
        (builtin.target.os.tag != .windows or
        builtin.os.version_range.windows.isAtLeast(.win10_rs4) orelse false);

    pub const Address = extern union {
        any: os.sockaddr,
        in: Ip4Address,
        in6: Ip6Address,
        un: if (has_unix_sockets) os.sockaddr.un else void,

        /// Parse the given IP address string into an Address value.
        /// It is recommended to use `resolveIp` instead, to handle
        /// IPv6 link-local unix addresses.
        pub fn parseIp(name: []const u8, port: u16) !Address {
            if (parseIp4(name, port)) |ip4| return ip4 else |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                error.NonCanonical,
                => {},
            }

            if (parseIp6(name, port)) |ip6| return ip6 else |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                error.InvalidIpv4Mapping,
                => {},
            }

            return error.InvalidIPAddressFormat;
        }

        pub fn resolveIp(name: []const u8, port: u16) !Address {
            if (parseIp4(name, port)) |ip4| return ip4 else |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                error.NonCanonical,
                => {},
            }

            if (resolveIp6(name, port)) |ip6| return ip6 else |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                error.InvalidIpv4Mapping,
                => {},
                else => return err,
            }

            return error.InvalidIPAddressFormat;
        }

        pub fn parseExpectingFamily(name: []const u8, family: os.sa_family_t, port: u16) !Address {
            switch (family) {
                os.AF.INET => return parseIp4(name, port),
                os.AF.INET6 => return parseIp6(name, port),
                os.AF.UNSPEC => return parseIp(name, port),
                else => unreachable,
            }
        }

        pub fn parseIp6(buf: []const u8, port: u16) !Address {
            return Address{ .in6 = try Ip6Address.parse(buf, port) };
        }

        pub fn resolveIp6(buf: []const u8, port: u16) !Address {
            return Address{ .in6 = try Ip6Address.resolve(buf, port) };
        }

        pub fn parseIp4(buf: []const u8, port: u16) !Address {
            return Address{ .in = try Ip4Address.parse(buf, port) };
        }

        pub fn initIp4(addr: [4]u8, port: u16) Address {
            return Address{ .in = Ip4Address.init(addr, port) };
        }

        pub fn initIp6(addr: [16]u8, port: u16, flowinfo: u32, scope_id: u32) Address {
            return Address{ .in6 = Ip6Address.init(addr, port, flowinfo, scope_id) };
        }

        pub fn initUnix(path: []const u8) !Address {
            var sock_addr = os.sockaddr.un{
                .family = os.AF.UNIX,
                .path = undefined,
            };

            // this enables us to have the proper length of the socket in getOsSockLen

            mem.set(u8, &sock_addr.path, 0);

            if (path.len > sock_addr.path.len) return error.NameTooLong;
            mem.copy(u8, &sock_addr.path, path);

            return Address{ .un = sock_addr };
        }

        /// Returns the port in native endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn getPort(self: Address) u16 {
            return switch (self.any.family) {
                os.AF.INET => self.in.getPort(),
                os.AF.INET6 => self.in6.getPort(),
                else => unreachable,
            };
        }

        /// `port` is native-endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn setPort(self: *Address, port: u16) void {
            switch (self.any.family) {
                os.AF.INET => self.in.setPort(port),
                os.AF.INET6 => self.in6.setPort(port),
                else => unreachable,
            }
        }

        /// Asserts that `addr` is an IP address.
        /// This function will read past the end of the pointer, with a size depending
        /// on the address family.
        pub fn initPosix(addr: *align(4) const os.sockaddr) Address {
            switch (addr.family) {
                os.AF.INET => return Address{ .in = Ip4Address{ .sa = @ptrCast(*const os.sockaddr.in, addr).* } },
                os.AF.INET6 => return Address{ .in6 = Ip6Address{ .sa = @ptrCast(*const os.sockaddr.in6, addr).* } },
                else => unreachable,
            }
        }

        pub fn format(
            self: Address,
            comptime fmt: []const u8,
            options: std.fmt.FormatOptions,
            out_stream: anytype,
        ) !void {
            switch (self.any.family) {
                os.AF.INET => try self.in.format(fmt, options, out_stream),
                os.AF.INET6 => try self.in6.format(fmt, options, out_stream),
                os.AF.UNIX => {
                    if (!has_unix_sockets) {
                        unreachable;
                    }

                    try std.fmt.format(out_stream, "{s}", .{std.mem.sliceTo(&self.un.path, 0)});
                },
                else => unreachable,
            }
        }

        pub fn eql(a: Address, b: Address) bool {
            const a_bytes = @ptrCast([*]const u8, &a.any)[0..a.getOsSockLen()];
            const b_bytes = @ptrCast([*]const u8, &b.any)[0..b.getOsSockLen()];
            return mem.eql(u8, a_bytes, b_bytes);
        }

        pub fn getOsSockLen(self: Address) os.socklen_t {
            switch (self.any.family) {
                os.AF.INET => return self.in.getOsSockLen(),
                os.AF.INET6 => return self.in6.getOsSockLen(),
                os.AF.UNIX => {
                    if (!has_unix_sockets) {
                        unreachable;
                    }

                    const path_len = std.mem.len(std.meta.assumeSentinel(&self.un.path, 0));
                    return @intCast(os.socklen_t, @sizeOf(os.sockaddr.un) - self.un.path.len + path_len);
                },
                else => unreachable,
            }
        }
    };

    pub const Ip4Address = extern struct {
        sa: os.sockaddr.in,

        pub fn parse(buf: []const u8, port: u16) !Ip4Address {
            var result = Ip4Address{
                .sa = .{
                    .port = mem.nativeToBig(u16, port),
                    .addr = undefined,
                },
            };
            const out_ptr = mem.asBytes(&result.sa.addr);

            var x: u8 = 0;
            var index: u8 = 0;
            var saw_any_digits = false;
            var has_zero_prefix = false;
            for (buf) |c| {
                if (c == '.') {
                    if (!saw_any_digits) {
                        return error.InvalidCharacter;
                    }
                    if (index == 3) {
                        return error.InvalidEnd;
                    }
                    out_ptr[index] = x;
                    index += 1;
                    x = 0;
                    saw_any_digits = false;
                    has_zero_prefix = false;
                } else if (c >= '0' and c <= '9') {
                    if (c == '0' and !saw_any_digits) {
                        has_zero_prefix = true;
                    } else if (has_zero_prefix) {
                        return error.NonCanonical;
                    }
                    saw_any_digits = true;
                    x = try std.math.mul(u8, x, 10);
                    x = try std.math.add(u8, x, c - '0');
                } else {
                    return error.InvalidCharacter;
                }
            }
            if (index == 3 and saw_any_digits) {
                out_ptr[index] = x;
                return result;
            }

            return error.Incomplete;
        }

        pub fn resolveIp(name: []const u8, port: u16) !Ip4Address {
            if (parse(name, port)) |ip4| return ip4 else |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                => {},
            }
            return error.InvalidIPAddressFormat;
        }

        pub fn init(addr: [4]u8, port: u16) Ip4Address {
            return Ip4Address{
                .sa = os.sockaddr.in{
                    .port = mem.nativeToBig(u16, port),
                    .addr = @ptrCast(*align(1) const u32, &addr).*,
                },
            };
        }

        /// Returns the port in native endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn getPort(self: Ip4Address) u16 {
            return mem.bigToNative(u16, self.sa.port);
        }

        /// `port` is native-endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn setPort(self: *Ip4Address, port: u16) void {
            self.sa.port = mem.nativeToBig(u16, port);
        }

        pub fn format(
            self: Ip4Address,
            comptime fmt: []const u8,
            options: std.fmt.FormatOptions,
            out_stream: anytype,
        ) !void {
            _ = fmt;
            _ = options;
            const bytes = @ptrCast(*const [4]u8, &self.sa.addr);
            try std.fmt.format(out_stream, "{}.{}.{}.{}:{}", .{
                bytes[0],
                bytes[1],
                bytes[2],
                bytes[3],
                self.getPort(),
            });
        }

        pub fn getOsSockLen(self: Ip4Address) os.socklen_t {
            _ = self;
            return @sizeOf(os.sockaddr.in);
        }
    };

    pub const Ip6Address = extern struct {
        sa: os.sockaddr.in6,

        /// Parse a given IPv6 address string into an Address.
        /// Assumes the Scope ID of the address is fully numeric.
        /// For non-numeric addresses, see `resolveIp6`.
        pub fn parse(buf: []const u8, port: u16) !Ip6Address {
            var result = Ip6Address{
                .sa = os.sockaddr.in6{
                    .scope_id = 0,
                    .port = mem.nativeToBig(u16, port),
                    .flowinfo = 0,
                    .addr = undefined,
                },
            };
            var ip_slice = result.sa.addr[0..];

            var tail: [16]u8 = undefined;

            var x: u16 = 0;
            var saw_any_digits = false;
            var index: u8 = 0;
            var scope_id = false;
            var abbrv = false;
            for (buf) |c, i| {
                if (scope_id) {
                    if (c >= '0' and c <= '9') {
                        const digit = c - '0';
                        if (@mulWithOverflow(u32, result.sa.scope_id, 10, &result.sa.scope_id)) {
                            return error.Overflow;
                        }
                        if (@addWithOverflow(u32, result.sa.scope_id, digit, &result.sa.scope_id)) {
                            return error.Overflow;
                        }
                    } else {
                        return error.InvalidCharacter;
                    }
                } else if (c == ':') {
                    if (!saw_any_digits) {
                        if (abbrv) return error.InvalidCharacter; // ':::'

                        if (i != 0) abbrv = true;
                        mem.set(u8, ip_slice[index..], 0);
                        ip_slice = tail[0..];
                        index = 0;
                        continue;
                    }
                    if (index == 14) {
                        return error.InvalidEnd;
                    }
                    ip_slice[index] = @truncate(u8, x >> 8);
                    index += 1;
                    ip_slice[index] = @truncate(u8, x);
                    index += 1;

                    x = 0;
                    saw_any_digits = false;
                } else if (c == '%') {
                    if (!saw_any_digits) {
                        return error.InvalidCharacter;
                    }
                    scope_id = true;
                    saw_any_digits = false;
                } else if (c == '.') {
                    if (!abbrv or ip_slice[0] != 0xff or ip_slice[1] != 0xff) {
                        // must start with '::ffff:'

                        return error.InvalidIpv4Mapping;
                    }
                    const start_index = mem.lastIndexOfScalar(u8, buf[0..i], ':').? + 1;
                    const addr = (Ip4Address.parse(buf[start_index..], 0) catch {
                        return error.InvalidIpv4Mapping;
                    }).sa.addr;
                    ip_slice = result.sa.addr[0..];
                    ip_slice[10] = 0xff;
                    ip_slice[11] = 0xff;

                    const ptr = mem.sliceAsBytes(@as(*const [1]u32, &addr)[0..]);

                    ip_slice[12] = ptr[0];
                    ip_slice[13] = ptr[1];
                    ip_slice[14] = ptr[2];
                    ip_slice[15] = ptr[3];
                    return result;
                } else {
                    const digit = try std.fmt.charToDigit(c, 16);
                    if (@mulWithOverflow(u16, x, 16, &x)) {
                        return error.Overflow;
                    }
                    if (@addWithOverflow(u16, x, digit, &x)) {
                        return error.Overflow;
                    }
                    saw_any_digits = true;
                }
            }

            if (!saw_any_digits and !abbrv) {
                return error.Incomplete;
            }

            if (index == 14) {
                ip_slice[14] = @truncate(u8, x >> 8);
                ip_slice[15] = @truncate(u8, x);
                return result;
            } else {
                ip_slice[index] = @truncate(u8, x >> 8);
                index += 1;
                ip_slice[index] = @truncate(u8, x);
                index += 1;
                mem.copy(u8, result.sa.addr[16 - index ..], ip_slice[0..index]);
                return result;
            }
        }

        pub fn resolve(buf: []const u8, port: u16) !Ip6Address {
            // TODO: Unify the implementations of resolveIp6 and parseIp6.

            var result = Ip6Address{
                .sa = os.sockaddr.in6{
                    .scope_id = 0,
                    .port = mem.nativeToBig(u16, port),
                    .flowinfo = 0,
                    .addr = undefined,
                },
            };
            var ip_slice = result.sa.addr[0..];

            var tail: [16]u8 = undefined;

            var x: u16 = 0;
            var saw_any_digits = false;
            var index: u8 = 0;
            var abbrv = false;

            var scope_id = false;
            var scope_id_value: [os.IFNAMESIZE - 1]u8 = undefined;
            var scope_id_index: usize = 0;

            for (buf) |c, i| {
                if (scope_id) {
                    // Handling of percent-encoding should be for an URI library.

                    if ((c >= '0' and c <= '9') or
                        (c >= 'A' and c <= 'Z') or
                        (c >= 'a' and c <= 'z') or
                        (c == '-') or (c == '.') or (c == '_') or (c == '~'))
                    {
                        if (scope_id_index >= scope_id_value.len) {
                            return error.Overflow;
                        }

                        scope_id_value[scope_id_index] = c;
                        scope_id_index += 1;
                    } else {
                        return error.InvalidCharacter;
                    }
                } else if (c == ':') {
                    if (!saw_any_digits) {
                        if (abbrv) return error.InvalidCharacter; // ':::'

                        if (i != 0) abbrv = true;
                        mem.set(u8, ip_slice[index..], 0);
                        ip_slice = tail[0..];
                        index = 0;
                        continue;
                    }
                    if (index == 14) {
                        return error.InvalidEnd;
                    }
                    ip_slice[index] = @truncate(u8, x >> 8);
                    index += 1;
                    ip_slice[index] = @truncate(u8, x);
                    index += 1;

                    x = 0;
                    saw_any_digits = false;
                } else if (c == '%') {
                    if (!saw_any_digits) {
                        return error.InvalidCharacter;
                    }
                    scope_id = true;
                    saw_any_digits = false;
                } else if (c == '.') {
                    if (!abbrv or ip_slice[0] != 0xff or ip_slice[1] != 0xff) {
                        // must start with '::ffff:'

                        return error.InvalidIpv4Mapping;
                    }
                    const start_index = mem.lastIndexOfScalar(u8, buf[0..i], ':').? + 1;
                    const addr = (Ip4Address.parse(buf[start_index..], 0) catch {
                        return error.InvalidIpv4Mapping;
                    }).sa.addr;
                    ip_slice = result.sa.addr[0..];
                    ip_slice[10] = 0xff;
                    ip_slice[11] = 0xff;

                    const ptr = mem.sliceAsBytes(@as(*const [1]u32, &addr)[0..]);

                    ip_slice[12] = ptr[0];
                    ip_slice[13] = ptr[1];
                    ip_slice[14] = ptr[2];
                    ip_slice[15] = ptr[3];
                    return result;
                } else {
                    const digit = try std.fmt.charToDigit(c, 16);
                    if (@mulWithOverflow(u16, x, 16, &x)) {
                        return error.Overflow;
                    }
                    if (@addWithOverflow(u16, x, digit, &x)) {
                        return error.Overflow;
                    }
                    saw_any_digits = true;
                }
            }

            if (!saw_any_digits and !abbrv) {
                return error.Incomplete;
            }

            if (scope_id and scope_id_index == 0) {
                return error.Incomplete;
            }

            var resolved_scope_id: u32 = 0;
            if (scope_id_index > 0) {
                const scope_id_str = scope_id_value[0..scope_id_index];
                resolved_scope_id = std.fmt.parseInt(u32, scope_id_str, 10) catch |err| blk: {
                    if (err != error.InvalidCharacter) return err;
                    break :blk try if_nametoindex(scope_id_str);
                };
            }

            result.sa.scope_id = resolved_scope_id;

            if (index == 14) {
                ip_slice[14] = @truncate(u8, x >> 8);
                ip_slice[15] = @truncate(u8, x);
                return result;
            } else {
                ip_slice[index] = @truncate(u8, x >> 8);
                index += 1;
                ip_slice[index] = @truncate(u8, x);
                index += 1;
                mem.copy(u8, result.sa.addr[16 - index ..], ip_slice[0..index]);
                return result;
            }
        }

        pub fn init(addr: [16]u8, port: u16, flowinfo: u32, scope_id: u32) Ip6Address {
            return Ip6Address{
                .sa = os.sockaddr.in6{
                    .addr = addr,
                    .port = mem.nativeToBig(u16, port),
                    .flowinfo = flowinfo,
                    .scope_id = scope_id,
                },
            };
        }

        /// Returns the port in native endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn getPort(self: Ip6Address) u16 {
            return mem.bigToNative(u16, self.sa.port);
        }

        /// `port` is native-endian.
        /// Asserts that the address is ip4 or ip6.
        pub fn setPort(self: *Ip6Address, port: u16) void {
            self.sa.port = mem.nativeToBig(u16, port);
        }

        pub fn format(
            self: Ip6Address,
            comptime fmt: []const u8,
            options: std.fmt.FormatOptions,
            out_stream: anytype,
        ) !void {
            _ = fmt;
            _ = options;
            const port = mem.bigToNative(u16, self.sa.port);
            if (mem.eql(u8, self.sa.addr[0..12], &[_]u8{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff })) {
                try std.fmt.format(out_stream, "[::ffff:{}.{}.{}.{}]:{}", .{
                    self.sa.addr[12],
                    self.sa.addr[13],
                    self.sa.addr[14],
                    self.sa.addr[15],
                    port,
                });
                return;
            }
            const big_endian_parts = @ptrCast(*align(1) const [8]u16, &self.sa.addr);
            const native_endian_parts = switch (native_endian) {
                .Big => big_endian_parts.*,
                .Little => blk: {
                    var buf: [8]u16 = undefined;
                    for (big_endian_parts) |part, i| {
                        buf[i] = mem.bigToNative(u16, part);
                    }
                    break :blk buf;
                },
            };
            try out_stream.writeAll("[");
            var i: usize = 0;
            var abbrv = false;
            while (i < native_endian_parts.len) : (i += 1) {
                if (native_endian_parts[i] == 0) {
                    if (!abbrv) {
                        try out_stream.writeAll(if (i == 0) "::" else ":");
                        abbrv = true;
                    }
                    continue;
                }
                try std.fmt.format(out_stream, "{x}", .{native_endian_parts[i]});
                if (i != native_endian_parts.len - 1) {
                    try out_stream.writeAll(":");
                }
            }
            try std.fmt.format(out_stream, "]:{}", .{port});
        }

        pub fn getOsSockLen(self: Ip6Address) os.socklen_t {
            _ = self;
            return @sizeOf(os.sockaddr.in6);
        }
    };

    pub fn connectUnixSocket(path: []const u8) !Stream {
        const opt_non_block = if (std.io.is_async) os.SOCK.NONBLOCK else 0;
        const sockfd = try os.socket(
            os.AF.UNIX,
            os.SOCK.STREAM | os.SOCK.CLOEXEC | opt_non_block,
            0,
        );
        errdefer os.closeSocket(sockfd);

        var addr = try std.net.Address.initUnix(path);

        if (std.io.is_async) {
            const loop = std.event.Loop.instance orelse return error.WouldBlock;
            try loop.connect(sockfd, &addr.any, addr.getOsSockLen());
        } else {
            try os.connect(sockfd, &addr.any, addr.getOsSockLen());
        }

        return Stream{
            .handle = sockfd,
        };
    }

    fn if_nametoindex(name: []const u8) !u32 {
        if (builtin.target.os.tag == .linux) {
            var ifr: os.ifreq = undefined;
            var sockfd = try os.socket(os.AF.UNIX, os.SOCK.DGRAM | os.SOCK.CLOEXEC, 0);
            defer os.closeSocket(sockfd);

            std.mem.copy(u8, &ifr.ifrn.name, name);
            ifr.ifrn.name[name.len] = 0;

            // TODO investigate if this needs to be integrated with evented I/O.

            try os.ioctl_SIOCGIFINDEX(sockfd, &ifr);

            return @bitCast(u32, ifr.ifru.ivalue);
        }

        if (comptime builtin.target.os.tag.isDarwin()) {
            if (name.len >= os.IFNAMESIZE)
                return error.NameTooLong;

            var if_name: [os.IFNAMESIZE:0]u8 = undefined;
            std.mem.copy(u8, &if_name, name);
            if_name[name.len] = 0;
            const if_slice = if_name[0..name.len :0];
            const index = os.system.if_nametoindex(if_slice);
            if (index == 0)
                return error.InterfaceNotFound;
            return @bitCast(u32, index);
        }

        @compileError("std.net.if_nametoindex unimplemented for this OS");
    }

    pub const AddressList = struct {
        arena: std.heap.ArenaAllocator,
        addrs: []Address,
        canon_name: ?[]u8,

        pub fn deinit(self: *AddressList) void {
            // Here we copy the arena allocator into stack memory, because

            // otherwise it would destroy itself while it was still working.

            var arena = self.arena;
            arena.deinit();
            // self is destroyed

        }
    };

    /// All memory allocated with `allocator` will be freed before this function returns.
    pub fn tcpConnectToHost(allocator: mem.Allocator, name: []const u8, port: u16) !Stream {
        const list = try getAddressList(allocator, name, port);
        defer list.deinit();

        if (list.addrs.len == 0) return error.UnknownHostName;

        for (list.addrs) |addr| {
            return tcpConnectToAddress(addr) catch |err| switch (err) {
                error.ConnectionRefused => {
                    continue;
                },
                else => return err,
            };
        }
        return std.os.ConnectError.ConnectionRefused;
    }

    pub fn tcpConnectToAddress(address: Address) !Stream {
        const nonblock = if (std.io.is_async) os.SOCK.NONBLOCK else 0;
        const sock_flags = os.SOCK.STREAM | nonblock |
            (if (builtin.target.os.tag == .windows) 0 else os.SOCK.CLOEXEC);
        const sockfd = try os.socket(address.any.family, sock_flags, os.IPPROTO.TCP);
        errdefer os.closeSocket(sockfd);

        if (std.io.is_async) {
            const loop = std.event.Loop.instance orelse return error.WouldBlock;
            try loop.connect(sockfd, &address.any, address.getOsSockLen());
        } else {
            try os.connect(sockfd, &address.any, address.getOsSockLen());
        }

        return Stream{ .handle = sockfd };
    }

    /// Call `AddressList.deinit` on the result.
    pub fn getAddressList(allocator: mem.Allocator, name: []const u8, port: u16) !*AddressList {
        const result = blk: {
            var arena = std.heap.ArenaAllocator.init(allocator);
            errdefer arena.deinit();

            const result = try arena.allocator().create(AddressList);
            result.* = AddressList{
                .arena = arena,
                .addrs = undefined,
                .canon_name = null,
            };
            break :blk result;
        };
        const arena = result.arena.allocator();
        errdefer result.deinit();

        if (builtin.target.os.tag == .windows or builtin.link_libc) {
            const name_c = try std.cstr.addNullByte(allocator, name);
            defer allocator.free(name_c);

            const port_c = try std.fmt.allocPrintZ(allocator, "{}", .{port});
            defer allocator.free(port_c);

            const sys = if (builtin.target.os.tag == .windows) os.windows.ws2_32 else os.system;
            const hints = os.addrinfo{
                .flags = sys.AI.NUMERICSERV,
                .family = os.AF.UNSPEC,
                .socktype = os.SOCK.STREAM,
                .protocol = os.IPPROTO.TCP,
                .canonname = null,
                .addr = null,
                .addrlen = 0,
                .next = null,
            };
            var res: *os.addrinfo = undefined;
            const rc = sys.getaddrinfo(name_c.ptr, port_c.ptr, &hints, &res);
            if (builtin.target.os.tag == .windows) switch (@intToEnum(os.windows.ws2_32.WinsockError, @intCast(u16, rc))) {
                @intToEnum(os.windows.ws2_32.WinsockError, 0) => {},
                .WSATRY_AGAIN => return error.TemporaryNameServerFailure,
                .WSANO_RECOVERY => return error.NameServerFailure,
                .WSAEAFNOSUPPORT => return error.AddressFamilyNotSupported,
                .WSA_NOT_ENOUGH_MEMORY => return error.OutOfMemory,
                .WSAHOST_NOT_FOUND => return error.UnknownHostName,
                .WSATYPE_NOT_FOUND => return error.ServiceUnavailable,
                .WSAEINVAL => unreachable,
                .WSAESOCKTNOSUPPORT => unreachable,
                else => |err| return os.windows.unexpectedWSAError(err),
            } else switch (rc) {
                @intToEnum(sys.EAI, 0) => {},
                .ADDRFAMILY => return error.HostLacksNetworkAddresses,
                .AGAIN => return error.TemporaryNameServerFailure,
                .BADFLAGS => unreachable, // Invalid hints

                .FAIL => return error.NameServerFailure,
                .FAMILY => return error.AddressFamilyNotSupported,
                .MEMORY => return error.OutOfMemory,
                .NODATA => return error.HostLacksNetworkAddresses,
                .NONAME => return error.UnknownHostName,
                .SERVICE => return error.ServiceUnavailable,
                .SOCKTYPE => unreachable, // Invalid socket type requested in hints

                .SYSTEM => switch (os.errno(-1)) {
                    else => |e| return os.unexpectedErrno(e),
                },
                else => unreachable,
            }
            defer sys.freeaddrinfo(res);

            const addr_count = blk: {
                var count: usize = 0;
                var it: ?*os.addrinfo = res;
                while (it) |info| : (it = info.next) {
                    if (info.addr != null) {
                        count += 1;
                    }
                }
                break :blk count;
            };
            result.addrs = try arena.alloc(Address, addr_count);

            var it: ?*os.addrinfo = res;
            var i: usize = 0;
            while (it) |info| : (it = info.next) {
                const addr = info.addr orelse continue;
                result.addrs[i] = Address.initPosix(@alignCast(4, addr));

                if (info.canonname) |n| {
                    if (result.canon_name == null) {
                        result.canon_name = try arena.dupe(u8, mem.sliceTo(n, 0));
                    }
                }
                i += 1;
            }

            return result;
        }
        if (builtin.target.os.tag == .linux) {
            const flags = std.c.AI.NUMERICSERV;
            const family = os.AF.UNSPEC;
            var lookup_addrs = std.ArrayList(LookupAddr).init(allocator);
            defer lookup_addrs.deinit();

            var canon = std.ArrayList(u8).init(arena);
            defer canon.deinit();

            try linuxLookupName(&lookup_addrs, &canon, name, family, flags, port);

            result.addrs = try arena.alloc(Address, lookup_addrs.items.len);
            if (canon.items.len != 0) {
                result.canon_name = canon.toOwnedSlice();
            }

            for (lookup_addrs.items) |lookup_addr, i| {
                result.addrs[i] = lookup_addr.addr;
                assert(result.addrs[i].getPort() == port);
            }

            return result;
        }
        @compileError("std.net.getAddressList unimplemented for this OS");
    }

    const LookupAddr = struct {
        addr: Address,
        sortkey: i32 = 0,
    };

    const DAS_USABLE = 0x40000000;
    const DAS_MATCHINGSCOPE = 0x20000000;
    const DAS_MATCHINGLABEL = 0x10000000;
    const DAS_PREC_SHIFT = 20;
    const DAS_SCOPE_SHIFT = 16;
    const DAS_PREFIX_SHIFT = 8;
    const DAS_ORDER_SHIFT = 0;

    fn linuxLookupName(
        addrs: *std.ArrayList(LookupAddr),
        canon: *std.ArrayList(u8),
        opt_name: ?[]const u8,
        family: os.sa_family_t,
        flags: u32,
        port: u16,
    ) !void {
        if (opt_name) |name| {
            // reject empty name and check len so it fits into temp bufs

            canon.items.len = 0;
            try canon.appendSlice(name);
            if (Address.parseExpectingFamily(name, family, port)) |addr| {
                try addrs.append(LookupAddr{ .addr = addr });
            } else |name_err| if ((flags & std.c.AI.NUMERICHOST) != 0) {
                return name_err;
            } else {
                try linuxLookupNameFromHosts(addrs, canon, name, family, port);
                if (addrs.items.len == 0) {
                    // RFC 6761 Section 6.3.3

                    // Name resolution APIs and libraries SHOULD recognize localhost

                    // names as special and SHOULD always return the IP loopback address

                    // for address queries and negative responses for all other query

                    // types.

                    // Check for equal to "localhost(.)" or ends in ".localhost(.)"

                    const localhost = if (name[name.len - 1] == '.') "localhost." else "localhost";
                    if (mem.endsWith(u8, name, localhost) and (name.len == localhost.len or name[name.len - localhost.len] == '.')) {
                        try addrs.append(LookupAddr{ .addr = .{ .in = Ip4Address.parse("127.0.0.1", port) catch unreachable } });
                        try addrs.append(LookupAddr{ .addr = .{ .in6 = Ip6Address.parse("::1", port) catch unreachable } });
                        return;
                    }

                    try linuxLookupNameFromDnsSearch(addrs, canon, name, family, port);
                }
            }
        } else {
            try canon.resize(0);
            try linuxLookupNameFromNull(addrs, family, flags, port);
        }
        if (addrs.items.len == 0) return error.UnknownHostName;

        // No further processing is needed if there are fewer than 2

        // results or if there are only IPv4 results.

        if (addrs.items.len == 1 or family == os.AF.INET) return;
        const all_ip4 = for (addrs.items) |addr| {
            if (addr.addr.any.family != os.AF.INET) break false;
        } else true;
        if (all_ip4) return;

        // The following implements a subset of RFC 3484/6724 destination

        // address selection by generating a single 31-bit sort key for

        // each address. Rules 3, 4, and 7 are omitted for having

        // excessive runtime and code size cost and dubious benefit.

        // So far the label/precedence table cannot be customized.

        // This implementation is ported from musl libc.

        // A more idiomatic "ziggy" implementation would be welcome.

        for (addrs.items) |*addr, i| {
            var key: i32 = 0;
            var sa6: os.sockaddr.in6 = undefined;
            @memset(@ptrCast([*]u8, &sa6), 0, @sizeOf(os.sockaddr.in6));
            var da6 = os.sockaddr.in6{
                .family = os.AF.INET6,
                .scope_id = addr.addr.in6.sa.scope_id,
                .port = 65535,
                .flowinfo = 0,
                .addr = [1]u8{0} ** 16,
            };
            var sa4: os.sockaddr.in = undefined;
            @memset(@ptrCast([*]u8, &sa4), 0, @sizeOf(os.sockaddr.in));
            var da4 = os.sockaddr.in{
                .family = os.AF.INET,
                .port = 65535,
                .addr = 0,
                .zero = [1]u8{0} ** 8,
            };
            var sa: *align(4) os.sockaddr = undefined;
            var da: *align(4) os.sockaddr = undefined;
            var salen: os.socklen_t = undefined;
            var dalen: os.socklen_t = undefined;
            if (addr.addr.any.family == os.AF.INET6) {
                mem.copy(u8, &da6.addr, &addr.addr.in6.sa.addr);
                da = @ptrCast(*os.sockaddr, &da6);
                dalen = @sizeOf(os.sockaddr.in6);
                sa = @ptrCast(*os.sockaddr, &sa6);
                salen = @sizeOf(os.sockaddr.in6);
            } else {
                mem.copy(u8, &sa6.addr, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff");
                mem.copy(u8, &da6.addr, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff");
                mem.writeIntNative(u32, da6.addr[12..], addr.addr.in.sa.addr);
                da4.addr = addr.addr.in.sa.addr;
                da = @ptrCast(*os.sockaddr, &da4);
                dalen = @sizeOf(os.sockaddr.in);
                sa = @ptrCast(*os.sockaddr, &sa4);
                salen = @sizeOf(os.sockaddr.in);
            }
            const dpolicy = policyOf(da6.addr);
            const dscope: i32 = scopeOf(da6.addr);
            const dlabel = dpolicy.label;
            const dprec: i32 = dpolicy.prec;
            const MAXADDRS = 3;
            var prefixlen: i32 = 0;
            const sock_flags = os.SOCK.DGRAM | os.SOCK.CLOEXEC;
            if (os.socket(addr.addr.any.family, sock_flags, os.IPPROTO.UDP)) |fd| syscalls: {
                defer os.closeSocket(fd);
                os.connect(fd, da, dalen) catch break :syscalls;
                key |= DAS_USABLE;
                os.getsockname(fd, sa, &salen) catch break :syscalls;
                if (addr.addr.any.family == os.AF.INET) {
                    // TODO sa6.addr[12..16] should return *[4]u8, making this cast unnecessary.

                    mem.writeIntNative(u32, @ptrCast(*[4]u8, &sa6.addr[12]), sa4.addr);
                }
                if (dscope == @as(i32, scopeOf(sa6.addr))) key |= DAS_MATCHINGSCOPE;
                if (dlabel == labelOf(sa6.addr)) key |= DAS_MATCHINGLABEL;
                prefixlen = prefixMatch(sa6.addr, da6.addr);
            } else |_| {}
            key |= dprec << DAS_PREC_SHIFT;
            key |= (15 - dscope) << DAS_SCOPE_SHIFT;
            key |= prefixlen << DAS_PREFIX_SHIFT;
            key |= (MAXADDRS - @intCast(i32, i)) << DAS_ORDER_SHIFT;
            addr.sortkey = key;
        }
        std.sort.sort(LookupAddr, addrs.items, {}, addrCmpLessThan);
    }

    const Policy = struct {
        addr: [16]u8,
        len: u8,
        mask: u8,
        prec: u8,
        label: u8,
    };

    const defined_policies = [_]Policy{
        Policy{
            .addr = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01".*,
            .len = 15,
            .mask = 0xff,
            .prec = 50,
            .label = 0,
        },
        Policy{
            .addr = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00".*,
            .len = 11,
            .mask = 0xff,
            .prec = 35,
            .label = 4,
        },
        Policy{
            .addr = "\x20\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00".*,
            .len = 1,
            .mask = 0xff,
            .prec = 30,
            .label = 2,
        },
        Policy{
            .addr = "\x20\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00".*,
            .len = 3,
            .mask = 0xff,
            .prec = 5,
            .label = 5,
        },
        Policy{
            .addr = "\xfc\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00".*,
            .len = 0,
            .mask = 0xfe,
            .prec = 3,
            .label = 13,
        },
        //  These are deprecated and/or returned to the address

        //  pool, so despite the RFC, treating them as special

        //  is probably wrong.

        // { "", 11, 0xff, 1, 3 },

        // { "\xfe\xc0", 1, 0xc0, 1, 11 },

        // { "\x3f\xfe", 1, 0xff, 1, 12 },

        // Last rule must match all addresses to stop loop.

        Policy{
            .addr = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00".*,
            .len = 0,
            .mask = 0,
            .prec = 40,
            .label = 1,
        },
    };

    fn policyOf(a: [16]u8) *const Policy {
        for (defined_policies) |*policy| {
            if (!mem.eql(u8, a[0..policy.len], policy.addr[0..policy.len])) continue;
            if ((a[policy.len] & policy.mask) != policy.addr[policy.len]) continue;
            return policy;
        }
        unreachable;
    }

    fn scopeOf(a: [16]u8) u8 {
        if (IN6_IS_ADDR_MULTICAST(a)) return a[1] & 15;
        if (IN6_IS_ADDR_LINKLOCAL(a)) return 2;
        if (IN6_IS_ADDR_LOOPBACK(a)) return 2;
        if (IN6_IS_ADDR_SITELOCAL(a)) return 5;
        return 14;
    }

    fn prefixMatch(s: [16]u8, d: [16]u8) u8 {
        // TODO: This FIXME inherited from porting from musl libc.

        // I don't want this to go into zig std lib 1.0.0.

        // FIXME: The common prefix length should be limited to no greater

        // than the nominal length of the prefix portion of the source

        // address. However the definition of the source prefix length is

        // not clear and thus this limiting is not yet implemented.

        var i: u8 = 0;
        while (i < 128 and ((s[i / 8] ^ d[i / 8]) & (@as(u8, 128) >> @intCast(u3, i % 8))) == 0) : (i += 1) {}
        return i;
    }

    fn labelOf(a: [16]u8) u8 {
        return policyOf(a).label;
    }

    fn IN6_IS_ADDR_MULTICAST(a: [16]u8) bool {
        return a[0] == 0xff;
    }

    fn IN6_IS_ADDR_LINKLOCAL(a: [16]u8) bool {
        return a[0] == 0xfe and (a[1] & 0xc0) == 0x80;
    }

    fn IN6_IS_ADDR_LOOPBACK(a: [16]u8) bool {
        return a[0] == 0 and a[1] == 0 and
            a[2] == 0 and
            a[12] == 0 and a[13] == 0 and
            a[14] == 0 and a[15] == 1;
    }

    fn IN6_IS_ADDR_SITELOCAL(a: [16]u8) bool {
        return a[0] == 0xfe and (a[1] & 0xc0) == 0xc0;
    }

    // Parameters `b` and `a` swapped to make this descending.

    fn addrCmpLessThan(context: void, b: LookupAddr, a: LookupAddr) bool {
        _ = context;
        return a.sortkey < b.sortkey;
    }

    fn linuxLookupNameFromNull(
        addrs: *std.ArrayList(LookupAddr),
        family: os.sa_family_t,
        flags: u32,
        port: u16,
    ) !void {
        if ((flags & std.c.AI.PASSIVE) != 0) {
            if (family != os.AF.INET6) {
                (try addrs.addOne()).* = LookupAddr{
                    .addr = Address.initIp4([1]u8{0} ** 4, port),
                };
            }
            if (family != os.AF.INET) {
                (try addrs.addOne()).* = LookupAddr{
                    .addr = Address.initIp6([1]u8{0} ** 16, port, 0, 0),
                };
            }
        } else {
            if (family != os.AF.INET6) {
                (try addrs.addOne()).* = LookupAddr{
                    .addr = Address.initIp4([4]u8{ 127, 0, 0, 1 }, port),
                };
            }
            if (family != os.AF.INET) {
                (try addrs.addOne()).* = LookupAddr{
                    .addr = Address.initIp6(([1]u8{0} ** 15) ++ [1]u8{1}, port, 0, 0),
                };
            }
        }
    }

    fn linuxLookupNameFromHosts(
        addrs: *std.ArrayList(LookupAddr),
        canon: *std.ArrayList(u8),
        name: []const u8,
        family: os.sa_family_t,
        port: u16,
    ) !void {
        const file = fs.openFileAbsoluteZ("/etc/hosts", .{}) catch |err| switch (err) {
            error.FileNotFound,
            error.NotDir,
            error.AccessDenied,
            => return,
            else => |e| return e,
        };
        defer file.close();

        var buffered_reader = std.io.bufferedReader(file.reader());
        const reader = buffered_reader.reader();
        var line_buf: [512]u8 = undefined;
        while (reader.readUntilDelimiterOrEof(&line_buf, '\n') catch |err| switch (err) {
            error.StreamTooLong => blk: {
                // Skip to the delimiter in the reader, to fix parsing

                try reader.skipUntilDelimiterOrEof('\n');
                // Use the truncated line. A truncated comment or hostname will be handled correctly.

                break :blk &line_buf;
            },
            else => |e| return e,
        }) |line| {
            var split_it = mem.split(u8, line, "#");
            const no_comment_line = split_it.first();

            var line_it = mem.tokenize(u8, no_comment_line, " \t");
            const ip_text = line_it.next() orelse continue;
            var first_name_text: ?[]const u8 = null;
            while (line_it.next()) |name_text| {
                if (first_name_text == null) first_name_text = name_text;
                if (mem.eql(u8, name_text, name)) {
                    break;
                }
            } else continue;

            const addr = Address.parseExpectingFamily(ip_text, family, port) catch |err| switch (err) {
                error.Overflow,
                error.InvalidEnd,
                error.InvalidCharacter,
                error.Incomplete,
                error.InvalidIPAddressFormat,
                error.InvalidIpv4Mapping,
                error.NonCanonical,
                => continue,
            };
            try addrs.append(LookupAddr{ .addr = addr });

            // first name is canonical name

            const name_text = first_name_text.?;
            if (isValidHostName(name_text)) {
                canon.items.len = 0;
                try canon.appendSlice(name_text);
            }
        }
    }

    pub fn isValidHostName(hostname: []const u8) bool {
        if (hostname.len >= 254) return false;
        if (!std.unicode.utf8ValidateSlice(hostname)) return false;
        for (hostname) |byte| {
            if (byte >= 0x80 or byte == '.' or byte == '-' or std.ascii.isAlNum(byte)) {
                continue;
            }
            return false;
        }
        return true;
    }

    fn linuxLookupNameFromDnsSearch(
        addrs: *std.ArrayList(LookupAddr),
        canon: *std.ArrayList(u8),
        name: []const u8,
        family: os.sa_family_t,
        port: u16,
    ) !void {
        var rc: ResolvConf = undefined;
        try getResolvConf(addrs.allocator, &rc);
        defer rc.deinit();

        // Count dots, suppress search when >=ndots or name ends in

        // a dot, which is an explicit request for global scope.

        var dots: usize = 0;
        for (name) |byte| {
            if (byte == '.') dots += 1;
        }

        const search = if (dots >= rc.ndots or mem.endsWith(u8, name, "."))
            ""
        else
            rc.search.items;

        var canon_name = name;

        // Strip final dot for canon, fail if multiple trailing dots.

        if (mem.endsWith(u8, canon_name, ".")) canon_name.len -= 1;
        if (mem.endsWith(u8, canon_name, ".")) return error.UnknownHostName;

        // Name with search domain appended is setup in canon[]. This both

        // provides the desired default canonical name (if the requested

        // name is not a CNAME record) and serves as a buffer for passing

        // the full requested name to name_from_dns.

        try canon.resize(canon_name.len);
        mem.copy(u8, canon.items, canon_name);
        try canon.append('.');

        var tok_it = mem.tokenize(u8, search, " \t");
        while (tok_it.next()) |tok| {
            canon.shrinkRetainingCapacity(canon_name.len + 1);
            try canon.appendSlice(tok);
            try linuxLookupNameFromDns(addrs, canon, canon.items, family, rc, port);
            if (addrs.items.len != 0) return;
        }

        canon.shrinkRetainingCapacity(canon_name.len);
        return linuxLookupNameFromDns(addrs, canon, name, family, rc, port);
    }

    const dpc_ctx = struct {
        addrs: *std.ArrayList(LookupAddr),
        canon: *std.ArrayList(u8),
        port: u16,
    };

    fn linuxLookupNameFromDns(
        addrs: *std.ArrayList(LookupAddr),
        canon: *std.ArrayList(u8),
        name: []const u8,
        family: os.sa_family_t,
        rc: ResolvConf,
        port: u16,
    ) !void {
        var ctx = dpc_ctx{
            .addrs = addrs,
            .canon = canon,
            .port = port,
        };
        const AfRr = struct {
            af: os.sa_family_t,
            rr: u8,
        };
        const afrrs = [_]AfRr{
            AfRr{ .af = os.AF.INET6, .rr = os.RR.A },
            AfRr{ .af = os.AF.INET, .rr = os.RR.AAAA },
        };
        var qbuf: [2][280]u8 = undefined;
        var abuf: [2][512]u8 = undefined;
        var qp: [2][]const u8 = undefined;
        const apbuf = [2][]u8{ &abuf[0], &abuf[1] };
        var nq: usize = 0;

        for (afrrs) |afrr| {
            if (family != afrr.af) {
                const len = os.res_mkquery(0, name, 1, afrr.rr, &[_]u8{}, null, &qbuf[nq]);
                qp[nq] = qbuf[nq][0..len];
                nq += 1;
            }
        }

        var ap = [2][]u8{ apbuf[0], apbuf[1] };
        ap[0].len = 0;
        ap[1].len = 0;

        try resMSendRc(qp[0..nq], ap[0..nq], apbuf[0..nq], rc);

        var i: usize = 0;
        while (i < nq) : (i += 1) {
            dnsParse(ap[i], ctx, dnsParseCallback) catch {};
        }

        if (addrs.items.len != 0) return;
        if (ap[0].len < 4 or (ap[0][3] & 15) == 2) return error.TemporaryNameServerFailure;
        if ((ap[0][3] & 15) == 0) return error.UnknownHostName;
        if ((ap[0][3] & 15) == 3) return;
        return error.NameServerFailure;
    }

    const ResolvConf = struct {
        attempts: u32,
        ndots: u32,
        timeout: u32,
        search: std.ArrayList(u8),
        ns: std.ArrayList(LookupAddr),

        fn deinit(rc: *ResolvConf) void {
            rc.ns.deinit();
            rc.search.deinit();
            rc.* = undefined;
        }
    };

    /// Ignores lines longer than 512 bytes.
    /// TODO: https://github.com/ziglang/zig/issues/2765 and https://github.com/ziglang/zig/issues/2761
    fn getResolvConf(allocator: mem.Allocator, rc: *ResolvConf) !void {
        rc.* = ResolvConf{
            .ns = std.ArrayList(LookupAddr).init(allocator),
            .search = std.ArrayList(u8).init(allocator),
            .ndots = 1,
            .timeout = 5,
            .attempts = 2,
        };
        errdefer rc.deinit();

        const file = fs.openFileAbsoluteZ("/etc/resolv.conf", .{}) catch |err| switch (err) {
            error.FileNotFound,
            error.NotDir,
            error.AccessDenied,
            => return linuxLookupNameFromNumericUnspec(&rc.ns, "127.0.0.1", 53),
            else => |e| return e,
        };
        defer file.close();

        var buf_reader = std.io.bufferedReader(file.reader());
        const stream = buf_reader.reader();
        var line_buf: [512]u8 = undefined;
        while (stream.readUntilDelimiterOrEof(&line_buf, '\n') catch |err| switch (err) {
            error.StreamTooLong => blk: {
                // Skip to the delimiter in the stream, to fix parsing

                try stream.skipUntilDelimiterOrEof('\n');
                // Give an empty line to the while loop, which will be skipped.

                break :blk line_buf[0..0];
            },
            else => |e| return e,
        }) |line| {
            const no_comment_line = no_comment_line: {
                var split = mem.split(u8, line, "#");
                break :no_comment_line split.first();
            };
            var line_it = mem.tokenize(u8, no_comment_line, " \t");

            const token = line_it.next() orelse continue;
            if (mem.eql(u8, token, "options")) {
                while (line_it.next()) |sub_tok| {
                    var colon_it = mem.split(u8, sub_tok, ":");
                    const name = colon_it.first();
                    const value_txt = colon_it.next() orelse continue;
                    const value = std.fmt.parseInt(u8, value_txt, 10) catch |err| switch (err) {
                        // TODO https://github.com/ziglang/zig/issues/11812

                        error.Overflow => @as(u8, 255),
                        error.InvalidCharacter => continue,
                    };
                    if (mem.eql(u8, name, "ndots")) {
                        rc.ndots = std.math.min(value, 15);
                    } else if (mem.eql(u8, name, "attempts")) {
                        rc.attempts = std.math.min(value, 10);
                    } else if (mem.eql(u8, name, "timeout")) {
                        rc.timeout = std.math.min(value, 60);
                    }
                }
            } else if (mem.eql(u8, token, "nameserver")) {
                const ip_txt = line_it.next() orelse continue;
                try linuxLookupNameFromNumericUnspec(&rc.ns, ip_txt, 53);
            } else if (mem.eql(u8, token, "domain") or mem.eql(u8, token, "search")) {
                rc.search.items.len = 0;
                try rc.search.appendSlice(line_it.rest());
            }
        }

        if (rc.ns.items.len == 0) {
            return linuxLookupNameFromNumericUnspec(&rc.ns, "127.0.0.1", 53);
        }
    }

    fn linuxLookupNameFromNumericUnspec(
        addrs: *std.ArrayList(LookupAddr),
        name: []const u8,
        port: u16,
    ) !void {
        const addr = try Address.resolveIp(name, port);
        (try addrs.addOne()).* = LookupAddr{ .addr = addr };
    }

    fn resMSendRc(
        queries: []const []const u8,
        answers: [][]u8,
        answer_bufs: []const []u8,
        rc: ResolvConf,
    ) !void {
        const timeout = 1000 * rc.timeout;
        const attempts = rc.attempts;

        var sl: os.socklen_t = @sizeOf(os.sockaddr.in);
        var family: os.sa_family_t = os.AF.INET;

        var ns_list = std.ArrayList(Address).init(rc.ns.allocator);
        defer ns_list.deinit();

        try ns_list.resize(rc.ns.items.len);
        const ns = ns_list.items;

        for (rc.ns.items) |iplit, i| {
            ns[i] = iplit.addr;
            assert(ns[i].getPort() == 53);
            if (iplit.addr.any.family != os.AF.INET) {
                sl = @sizeOf(os.sockaddr.in6);
                family = os.AF.INET6;
            }
        }

        // Get local address and open/bind a socket

        var sa: Address = undefined;
        @memset(@ptrCast([*]u8, &sa), 0, @sizeOf(Address));
        sa.any.family = family;
        const flags = os.SOCK.DGRAM | os.SOCK.CLOEXEC | os.SOCK.NONBLOCK;
        const fd = os.socket(family, flags, 0) catch |err| switch (err) {
            error.AddressFamilyNotSupported => blk: {
                // Handle case where system lacks IPv6 support

                if (family == os.AF.INET6) {
                    family = os.AF.INET;
                    break :blk try os.socket(os.AF.INET, flags, 0);
                }
                return err;
            },
            else => |e| return e,
        };
        defer os.closeSocket(fd);
        try os.bind(fd, &sa.any, sl);

        // Past this point, there are no errors. Each individual query will

        // yield either no reply (indicated by zero length) or an answer

        // packet which is up to the caller to interpret.

        // Convert any IPv4 addresses in a mixed environment to v4-mapped

        // TODO

        //if (family == AF.INET6) {

        //    setsockopt(fd, IPPROTO.IPV6, IPV6_V6ONLY, &(int){0}, sizeof 0);

        //    for (i=0; i<nns; i++) {

        //        if (ns[i].sin.sin_family != AF.INET) continue;

        //        memcpy(ns[i].sin6.sin6_addr.s6_addr+12,

        //            &ns[i].sin.sin_addr, 4);

        //        memcpy(ns[i].sin6.sin6_addr.s6_addr,

        //            "\0\0\0\0\0\0\0\0\0\0\xff\xff", 12);

        //        ns[i].sin6.sin6_family = AF.INET6;

        //        ns[i].sin6.sin6_flowinfo = 0;

        //        ns[i].sin6.sin6_scope_id = 0;

        //    }

        //}

        var pfd = [1]os.pollfd{os.pollfd{
            .fd = fd,
            .events = os.POLL.IN,
            .revents = undefined,
        }};
        const retry_interval = timeout / attempts;
        var next: u32 = 0;
        var t2: u64 = @bitCast(u64, std.time.milliTimestamp());
        var t0 = t2;
        var t1 = t2 - retry_interval;

        var servfail_retry: usize = undefined;

        outer: while (t2 - t0 < timeout) : (t2 = @bitCast(u64, std.time.milliTimestamp())) {
            if (t2 - t1 >= retry_interval) {
                // Query all configured nameservers in parallel

                var i: usize = 0;
                while (i < queries.len) : (i += 1) {
                    if (answers[i].len == 0) {
                        var j: usize = 0;
                        while (j < ns.len) : (j += 1) {
                            if (std.io.is_async) {
                                _ = std.event.Loop.instance.?.sendto(fd, queries[i], os.MSG.NOSIGNAL, &ns[j].any, sl) catch undefined;
                            } else {
                                _ = os.sendto(fd, queries[i], os.MSG.NOSIGNAL, &ns[j].any, sl) catch undefined;
                            }
                        }
                    }
                }
                t1 = t2;
                servfail_retry = 2 * queries.len;
            }

            // Wait for a response, or until time to retry

            const clamped_timeout = std.math.min(@as(u31, std.math.maxInt(u31)), t1 + retry_interval - t2);
            const nevents = os.poll(&pfd, clamped_timeout) catch 0;
            if (nevents == 0) continue;

            while (true) {
                var sl_copy = sl;
                const rlen = if (std.io.is_async)
                    std.event.Loop.instance.?.recvfrom(fd, answer_bufs[next], 0, &sa.any, &sl_copy) catch break
                else
                    os.recvfrom(fd, answer_bufs[next], 0, &sa.any, &sl_copy) catch break;

                // Ignore non-identifiable packets

                if (rlen < 4) continue;

                // Ignore replies from addresses we didn't send to

                var j: usize = 0;
                while (j < ns.len and !ns[j].eql(sa)) : (j += 1) {}
                if (j == ns.len) continue;

                // Find which query this answer goes with, if any

                var i: usize = next;
                while (i < queries.len and (answer_bufs[next][0] != queries[i][0] or
                    answer_bufs[next][1] != queries[i][1])) : (i += 1)
                {}

                if (i == queries.len) continue;
                if (answers[i].len != 0) continue;

                // Only accept positive or negative responses;

                // retry immediately on server failure, and ignore

                // all other codes such as refusal.

                switch (answer_bufs[next][3] & 15) {
                    0, 3 => {},
                    2 => if (servfail_retry != 0) {
                        servfail_retry -= 1;
                        if (std.io.is_async) {
                            _ = std.event.Loop.instance.?.sendto(fd, queries[i], os.MSG.NOSIGNAL, &ns[j].any, sl) catch undefined;
                        } else {
                            _ = os.sendto(fd, queries[i], os.MSG.NOSIGNAL, &ns[j].any, sl) catch undefined;
                        }
                    },
                    else => continue,
                }

                // Store answer in the right slot, or update next

                // available temp slot if it's already in place.

                answers[i].len = rlen;
                if (i == next) {
                    while (next < queries.len and answers[next].len != 0) : (next += 1) {}
                } else {
                    mem.copy(u8, answer_bufs[i], answer_bufs[next][0..rlen]);
                }

                if (next == queries.len) break :outer;
            }
        }
    }

    fn dnsParse(
        r: []const u8,
        ctx: anytype,
        comptime callback: anytype,
    ) !void {
        // This implementation is ported from musl libc.

        // A more idiomatic "ziggy" implementation would be welcome.

        if (r.len < 12) return error.InvalidDnsPacket;
        if ((r[3] & 15) != 0) return;
        var p = r.ptr + 12;
        var qdcount = r[4] * @as(usize, 256) + r[5];
        var ancount = r[6] * @as(usize, 256) + r[7];
        if (qdcount + ancount > 64) return error.InvalidDnsPacket;
        while (qdcount != 0) {
            qdcount -= 1;
            while (@ptrToInt(p) - @ptrToInt(r.ptr) < r.len and p[0] -% 1 < 127) p += 1;
            if (p[0] > 193 or (p[0] == 193 and p[1] > 254) or @ptrToInt(p) > @ptrToInt(r.ptr) + r.len - 6)
                return error.InvalidDnsPacket;
            p += @as(usize, 5) + @boolToInt(p[0] != 0);
        }
        while (ancount != 0) {
            ancount -= 1;
            while (@ptrToInt(p) - @ptrToInt(r.ptr) < r.len and p[0] -% 1 < 127) p += 1;
            if (p[0] > 193 or (p[0] == 193 and p[1] > 254) or @ptrToInt(p) > @ptrToInt(r.ptr) + r.len - 6)
                return error.InvalidDnsPacket;
            p += @as(usize, 1) + @boolToInt(p[0] != 0);
            const len = p[8] * @as(usize, 256) + p[9];
            if (@ptrToInt(p) + len > @ptrToInt(r.ptr) + r.len) return error.InvalidDnsPacket;
            try callback(ctx, p[1], p[10 .. 10 + len], r);
            p += 10 + len;
        }
    }

    fn dnsParseCallback(ctx: dpc_ctx, rr: u8, data: []const u8, packet: []const u8) !void {
        switch (rr) {
            os.RR.A => {
                if (data.len != 4) return error.InvalidDnsARecord;
                const new_addr = try ctx.addrs.addOne();
                new_addr.* = LookupAddr{
                    .addr = Address.initIp4(data[0..4].*, ctx.port),
                };
            },
            os.RR.AAAA => {
                if (data.len != 16) return error.InvalidDnsAAAARecord;
                const new_addr = try ctx.addrs.addOne();
                new_addr.* = LookupAddr{
                    .addr = Address.initIp6(data[0..16].*, ctx.port, 0, 0),
                };
            },
            os.RR.CNAME => {
                var tmp: [256]u8 = undefined;
                // Returns len of compressed name. strlen to get canon name.

                _ = try os.dn_expand(packet, data, &tmp);
                const canon_name = mem.sliceTo(std.meta.assumeSentinel(&tmp, 0), 0);
                if (isValidHostName(canon_name)) {
                    ctx.canon.items.len = 0;
                    try ctx.canon.appendSlice(canon_name);
                }
            },
            else => return,
        }
    }

    pub const Stream = struct {
        // Underlying socket descriptor.

        // Note that on some platforms this may not be interchangeable with a

        // regular files descriptor.

        handle: os.socket_t,

        pub fn close(self: Stream) void {
            os.closeSocket(self.handle);
        }

        pub const ReadError = os.ReadError;
        pub const WriteError = os.WriteError;

        pub const Reader = io.Reader(Stream, ReadError, read);
        pub const Writer = io.Writer(Stream, WriteError, write);

        pub fn reader(self: Stream) Reader {
            return .{ .context = self };
        }

        pub fn writer(self: Stream) Writer {
            return .{ .context = self };
        }

        pub fn read(self: Stream, buffer: []u8) ReadError!usize {
            if (builtin.os.tag == .windows) {
                return os.windows.ReadFile(self.handle, buffer, null, io.default_mode);
            }

            if (std.io.is_async) {
                return std.event.Loop.instance.?.read(self.handle, buffer, false);
            } else {
                return os.read(self.handle, buffer);
            }
        }

        /// TODO in evented I/O mode, this implementation incorrectly uses the event loop's
        /// file system thread instead of non-blocking. It needs to be reworked to properly
        /// use non-blocking I/O.
        pub fn write(self: Stream, buffer: []const u8) WriteError!usize {
            if (builtin.os.tag == .windows) {
                return os.windows.WriteFile(self.handle, buffer, null, io.default_mode);
            }

            if (std.io.is_async) {
                return std.event.Loop.instance.?.write(self.handle, buffer, false);
            } else {
                return os.write(self.handle, buffer);
            }
        }

        /// See https://github.com/ziglang/zig/issues/7699
        /// See equivalent function: `std.fs.File.writev`.
        pub fn writev(self: Stream, iovecs: []const os.iovec_const) WriteError!usize {
            if (std.io.is_async) {
                // TODO improve to actually take advantage of writev syscall, if available.

                if (iovecs.len == 0) return 0;
                const first_buffer = iovecs[0].iov_base[0..iovecs[0].iov_len];
                try self.write(first_buffer);
                return first_buffer.len;
            } else {
                return os.writev(self.handle, iovecs);
            }
        }

        /// The `iovecs` parameter is mutable because this function needs to mutate the fields in
        /// order to handle partial writes from the underlying OS layer.
        /// See https://github.com/ziglang/zig/issues/7699
        /// See equivalent function: `std.fs.File.writevAll`.
        pub fn writevAll(self: Stream, iovecs: []os.iovec_const) WriteError!void {
            if (iovecs.len == 0) return;

            var i: usize = 0;
            while (true) {
                var amt = try self.writev(iovecs[i..]);
                while (amt >= iovecs[i].iov_len) {
                    amt -= iovecs[i].iov_len;
                    i += 1;
                    if (i >= iovecs.len) return;
                }
                iovecs[i].iov_base += amt;
                iovecs[i].iov_len -= amt;
            }
        }
    };

    pub const StreamServer = struct {
        /// Copied from `Options` on `init`.
        kernel_backlog: u31,
        reuse_address: bool,

        /// `undefined` until `listen` returns successfully.
        listen_address: Address,

        sockfd: ?os.socket_t,

        pub const Options = struct {
            /// How many connections the kernel will accept on the application's behalf.
            /// If more than this many connections pool in the kernel, clients will start
            /// seeing "Connection refused".
            kernel_backlog: u31 = 128,

            /// Enable SO.REUSEADDR on the socket.
            reuse_address: bool = false,
        };

        /// After this call succeeds, resources have been acquired and must
        /// be released with `deinit`.
        pub fn init(options: Options) StreamServer {
            return StreamServer{
                .sockfd = null,
                .kernel_backlog = options.kernel_backlog,
                .reuse_address = options.reuse_address,
                .listen_address = undefined,
            };
        }

        /// Release all resources. The `StreamServer` memory becomes `undefined`.
        pub fn deinit(self: *StreamServer) void {
            self.close();
            self.* = undefined;
        }

        pub fn listen(self: *StreamServer, address: Address) !void {
            const nonblock = if (std.io.is_async) os.SOCK.NONBLOCK else 0;
            const sock_flags = os.SOCK.STREAM | os.SOCK.CLOEXEC | nonblock;
            const proto = if (address.any.family == os.AF.UNIX) @as(u32, 0) else os.IPPROTO.TCP;

            const sockfd = try os.socket(address.any.family, sock_flags, proto);
            self.sockfd = sockfd;
            errdefer {
                os.closeSocket(sockfd);
                self.sockfd = null;
            }

            if (self.reuse_address) {
                try os.setsockopt(
                    sockfd,
                    os.SOL.SOCKET,
                    os.SO.REUSEADDR,
                    &mem.toBytes(@as(c_int, 1)),
                );
            }

            var socklen = address.getOsSockLen();
            try os.bind(sockfd, &address.any, socklen);
            try os.listen(sockfd, self.kernel_backlog);
            try os.getsockname(sockfd, &self.listen_address.any, &socklen);
        }

        /// Stop listening. It is still necessary to call `deinit` after stopping listening.
        /// Calling `deinit` will automatically call `close`. It is safe to call `close` when
        /// not listening.
        pub fn close(self: *StreamServer) void {
            if (self.sockfd) |fd| {
                os.closeSocket(fd);
                self.sockfd = null;
                self.listen_address = undefined;
            }
        }

        pub const AcceptError = error{
            ConnectionAborted,

            /// The per-process limit on the number of open file descriptors has been reached.
            ProcessFdQuotaExceeded,

            /// The system-wide limit on the total number of open files has been reached.
            SystemFdQuotaExceeded,

            /// Not enough free memory.  This often means that the memory allocation  is  limited
            /// by the socket buffer limits, not by the system memory.
            SystemResources,

            /// Socket is not listening for new connections.
            SocketNotListening,

            ProtocolFailure,

            /// Firewall rules forbid connection.
            BlockedByFirewall,

            FileDescriptorNotASocket,

            ConnectionResetByPeer,

            NetworkSubsystemFailed,

            OperationNotSupported,
        } || os.UnexpectedError;

        pub const Connection = struct {
            stream: Stream,
            address: Address,
        };

        /// If this function succeeds, the returned `Connection` is a caller-managed resource.
        pub fn accept(self: *StreamServer) AcceptError!Connection {
            var accepted_addr: Address = undefined;
            var adr_len: os.socklen_t = @sizeOf(Address);
            const accept_result = blk: {
                if (std.io.is_async) {
                    const loop = std.event.Loop.instance orelse return error.UnexpectedError;
                    break :blk loop.accept(self.sockfd.?, &accepted_addr.any, &adr_len, os.SOCK.CLOEXEC);
                } else {
                    break :blk os.accept(self.sockfd.?, &accepted_addr.any, &adr_len, os.SOCK.CLOEXEC);
                }
            };

            if (accept_result) |fd| {
                return Connection{
                    .stream = Stream{ .handle = fd },
                    .address = accepted_addr,
                };
            } else |err| switch (err) {
                error.WouldBlock => unreachable,
                else => |e| return e,
            }
        }
    };

    test {
        _ = @import("net/test.zig");
    }