util/dnsconfig: add new package to parse macOS DNS configuration
Signed-off-by: Andrew Dunham <andrew@du.nham.ca> Change-Id: I271b401af1b5e626b3afe291c6f7f15b319c601dandrew/util-dnsconfig
Andrew Dunham
parent
e1530cdfcc
commit
9058121b0c
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@ -0,0 +1,217 @@
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package dnsconfig
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/*
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#cgo LDFLAGS: -ldl
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#include <dlfcn.h>
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#include <stdlib.h>
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void* call_pointer(void* addr) {
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void* (*fn)(void) = addr;
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return fn();
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}
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void call_arg(void* addr, void* arg) {
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void (*fn)(void*) = addr;
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fn(arg);
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}
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*/
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import "C"
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import (
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"encoding/binary"
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"errors"
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"fmt"
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"net/netip"
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"sync"
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"syscall"
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"unsafe"
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)
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var (
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fptrOnce sync.Once
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dnsConfigurationCopyPtr unsafe.Pointer
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dnsConfigurationFreePtr unsafe.Pointer
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)
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func initPointers() {
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fptrOnce.Do(func() {
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sym := C.CString("dns_configuration_copy")
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defer C.free(unsafe.Pointer(sym))
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dnsConfigurationCopyPtr = C.dlsym(C.RTLD_DEFAULT, sym)
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sym = C.CString("dns_configuration_free")
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defer C.free(unsafe.Pointer(sym))
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dnsConfigurationFreePtr = C.dlsym(C.RTLD_DEFAULT, sym)
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})
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}
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var errSymbolNotFound = errors.New("symbol not found")
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func dnsConfigurationCopy() (*dnsConfig, error) {
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initPointers()
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if dnsConfigurationCopyPtr == nil {
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return nil, errSymbolNotFound
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}
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// Call through cgo so that the Go runtime switches to a C stack.
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ptr := C.call_pointer(dnsConfigurationCopyPtr)
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return (*dnsConfig)(ptr), nil
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}
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func dnsConfigurationFree(p *dnsConfig) error {
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initPointers()
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if dnsConfigurationFreePtr == nil {
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return errSymbolNotFound
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}
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// Call through cgo so that the Go runtime switches to a C stack.
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C.call_arg(dnsConfigurationFreePtr, unsafe.Pointer(p))
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return nil
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}
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// DNSConfig contains DNS configuration information as returned by macOS. It is
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// the Go version of the private dns_config_t type.
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type DNSConfig struct {
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Resolvers []*DNSResolver
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ScopedResolvers []*DNSResolver
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Generation uint64
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ServiceSpecificResolvers []*DNSResolver
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Version uint32
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}
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// DNSResolver contains DNS resolver-specific information as returned by macOS.
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// It is the Go version of the private dns_resolver_t type.
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type DNSResolver struct {
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Domain string
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Nameservers []netip.AddrPort
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Port uint16
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Search []string
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Options string
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Timeout uint32
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SearchOrder uint32
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IfIndex uint32
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Flags uint32
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ReachFlags uint32
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ServiceIdentifier uint32
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CID string
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IfName string
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// TODO: SortAddr []any?
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}
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// Get returns this system's DNS configuration, or an error.
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func Get() (*DNSConfig, error) {
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config, err := dnsConfigurationCopy()
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if err != nil {
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return nil, err
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}
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defer dnsConfigurationFree(config)
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// Verify that the version is what we expect. On newer versions of
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// macOS, we could check this and only load fields that are present,
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// instead of failing outright.
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version := binary.LittleEndian.Uint32(config.data[44 : 44+4])
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if version != 20170629 {
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return nil, fmt.Errorf("version mismatch: %d != 20170629", version)
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}
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ret := &DNSConfig{
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Generation: binary.LittleEndian.Uint64(config.data[24 : 24+8]),
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Version: version,
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}
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// Populate resolvers
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for _, resolver := range getResolvers(config.data[:], 0, 4) {
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ret.Resolvers = append(ret.Resolvers, parseResolver(resolver))
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}
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for _, resolver := range getResolvers(config.data[:], 12, 16) {
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ret.ScopedResolvers = append(ret.ScopedResolvers, parseResolver(resolver))
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}
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for _, resolver := range getResolvers(config.data[:], 32, 36) {
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ret.ServiceSpecificResolvers = append(ret.ServiceSpecificResolvers, parseResolver(resolver))
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}
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return ret, nil
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}
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func getResolvers(data []byte, numOff, arrOff int) []*dnsResolver {
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n := int(binary.LittleEndian.Uint32(data[numOff : numOff+4]))
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arr := unsafe.Pointer(uintptr(binary.LittleEndian.Uint64(data[arrOff : arrOff+8])))
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return unsafe.Slice((**dnsResolver)(arr), n)
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}
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func parseResolver(r *dnsResolver) *DNSResolver {
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ret := &DNSResolver{
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Domain: r.readCharPtr(0),
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Port: binary.LittleEndian.Uint16(r.data[20 : 20+2]),
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Options: r.readCharPtr(48),
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Timeout: r.readUint32(56),
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SearchOrder: r.readUint32(60),
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IfIndex: r.readUint32(64),
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Flags: r.readUint32(68),
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ReachFlags: r.readUint32(72),
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ServiceIdentifier: r.readUint32(76),
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CID: r.readCharPtr(80),
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IfName: r.readCharPtr(88),
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}
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// The actual nameservers for this DNS entry.
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nNameservers := int(binary.LittleEndian.Uint32(r.data[8 : 8+4]))
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arr := unsafe.Pointer(uintptr(binary.LittleEndian.Uint64(r.data[12 : 12+8])))
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for _, sockaddr := range unsafe.Slice((**syscall.RawSockaddr)(arr), nNameservers) {
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switch sockaddr.Family {
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case syscall.AF_INET:
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sa := (*syscall.RawSockaddrInet4)(unsafe.Pointer(sockaddr))
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ret.Nameservers = append(ret.Nameservers, netip.AddrPortFrom(
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netip.AddrFrom4(sa.Addr),
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sa.Port,
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))
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case syscall.AF_INET6:
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sa := (*syscall.RawSockaddrInet6)(unsafe.Pointer(sockaddr))
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ret.Nameservers = append(ret.Nameservers, netip.AddrPortFrom(
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netip.AddrFrom16(sa.Addr),
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sa.Port,
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))
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default:
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// Skip unknown address families
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// TODO: log?
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}
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}
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// Search domains
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nSearch := int(binary.LittleEndian.Uint32(r.data[24 : 24+4]))
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arr = unsafe.Pointer(uintptr(binary.LittleEndian.Uint64(r.data[28 : 28+8])))
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for _, ss := range unsafe.Slice((**C.char)(arr), nSearch) {
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ret.Search = append(ret.Search, C.GoString(ss))
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}
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return ret
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}
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// dnsConfig is the type returned from the dns_configuration_copy function. The
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// C header sets #pragma pack(4), which isn't easily represented in Go; we
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// instead use binary.Read to get fields from this structure.
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type dnsConfig struct {
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data [48]byte
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}
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// dnsResolver is the dns_resolver_t type; as above, since we can't represent
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// it in Go, we read fields from the structure manually.
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type dnsResolver struct {
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data [96]byte
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}
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func (d *dnsResolver) readCharPtr(off int) string {
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ptr := unsafe.Pointer(uintptr(binary.LittleEndian.Uint64(d.data[off : off+8])))
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return C.GoString((*C.char)(ptr))
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}
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func (d *dnsResolver) readInt32(off int) int32 {
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return int32(binary.LittleEndian.Uint32(d.data[off : off+4]))
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}
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func (d *dnsResolver) readUint32(off int) uint32 {
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return binary.LittleEndian.Uint32(d.data[off : off+4])
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}
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@ -0,0 +1,30 @@
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package dnsconfig
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import "testing"
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func TestGet(t *testing.T) {
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config, err := Get()
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if err != nil {
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t.Fatal(err)
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}
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if len(config.Resolvers) < 1 {
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t.Fatal("wanted at least one resolver")
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}
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// Sensibility check: do we have at least one nameserver?
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var nameservers int
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for _, resolver := range config.Resolvers {
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nameservers += len(resolver.Nameservers)
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}
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for _, resolver := range config.ScopedResolvers {
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nameservers += len(resolver.Nameservers)
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}
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for _, resolver := range config.ServiceSpecificResolvers {
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nameservers += len(resolver.Nameservers)
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}
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if nameservers == 0 {
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t.Fatal("wanted at least one nameserver, got 0")
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}
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}
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@ -0,0 +1,114 @@
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#include <sys/cdefs.h>
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#include <stddef.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <sys/types.h>
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#include <sys/socket.h>
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#include <netinet/in.h>
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/**************************************************
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* BEGIN TYPES FROM APPLE HEADER
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**************************************************/
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#define DNS_PTR(type, name) \
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union { \
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type name; \
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uint64_t _ ## name ## _p; \
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}
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#define DNS_VAR(type, name) \
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type name
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#pragma pack(4)
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typedef struct {
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struct in_addr address;
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struct in_addr mask;
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} dns_sortaddr_t;
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#pragma pack()
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#pragma pack(4)
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typedef struct {
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DNS_PTR(char *, domain); /* domain */
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DNS_VAR(int32_t, n_nameserver); /* # nameserver */
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DNS_PTR(struct sockaddr **, nameserver);
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DNS_VAR(uint16_t, port); /* port (in host byte order) */
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DNS_VAR(int32_t, n_search); /* # search */
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DNS_PTR(char **, search);
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DNS_VAR(int32_t, n_sortaddr); /* # sortaddr */
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DNS_PTR(dns_sortaddr_t **, sortaddr);
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DNS_PTR(char *, options); /* options */
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DNS_VAR(uint32_t, timeout); /* timeout */
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DNS_VAR(uint32_t, search_order); /* search_order */
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DNS_VAR(uint32_t, if_index);
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DNS_VAR(uint32_t, flags);
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DNS_VAR(uint32_t, reach_flags); /* SCNetworkReachabilityFlags */
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DNS_VAR(uint32_t, service_identifier);
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DNS_PTR(char *, cid); /* configuration identifer */
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DNS_PTR(char *, if_name); /* if_index interface name */
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} dns_resolver_t;
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#pragma pack()
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#pragma pack(4)
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typedef struct {
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DNS_VAR(int32_t, n_resolver); /* resolver configurations */
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DNS_PTR(dns_resolver_t **, resolver);
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DNS_VAR(int32_t, n_scoped_resolver); /* "scoped" resolver configurations */
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DNS_PTR(dns_resolver_t **, scoped_resolver);
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DNS_VAR(uint64_t, generation);
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DNS_VAR(int32_t, n_service_specific_resolver);
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DNS_PTR(dns_resolver_t **, service_specific_resolver);
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DNS_VAR(uint32_t, version);
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} dns_config_t;
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#pragma pack()
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/**************************************************
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* END TYPES FROM APPLE HEADER
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**************************************************/
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#define field_info(type, field) \
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printf("%-15s\t%-30s\toffset=%lu\tsizeof=%lu\n", \
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#type, \
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#field, \
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offsetof(type, field) , \
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sizeof ((type *)0)->field \
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)
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int main(void) {
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printf("sizeof(dns_config_t)=%lu\n", sizeof(dns_config_t));
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field_info(dns_config_t, n_resolver);
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field_info(dns_config_t, resolver);
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field_info(dns_config_t, n_scoped_resolver);
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field_info(dns_config_t, scoped_resolver);
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field_info(dns_config_t, generation);
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field_info(dns_config_t, n_service_specific_resolver);
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field_info(dns_config_t, service_specific_resolver);
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field_info(dns_config_t, version);
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printf("\n");
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printf("sizeof(dns_resolver_t)=%lu\n", sizeof(dns_resolver_t));
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field_info(dns_resolver_t, domain);
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field_info(dns_resolver_t, n_nameserver);
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field_info(dns_resolver_t, nameserver);
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field_info(dns_resolver_t, port);
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field_info(dns_resolver_t, n_search);
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field_info(dns_resolver_t, search);
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field_info(dns_resolver_t, n_sortaddr);
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field_info(dns_resolver_t, sortaddr);
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field_info(dns_resolver_t, options);
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field_info(dns_resolver_t, timeout);
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field_info(dns_resolver_t, search_order);
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field_info(dns_resolver_t, if_index);
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field_info(dns_resolver_t, flags);
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field_info(dns_resolver_t, reach_flags);
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field_info(dns_resolver_t, service_identifier);
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field_info(dns_resolver_t, cid);
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field_info(dns_resolver_t, if_name);
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printf("\n");
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field_info(struct sockaddr, sa_len);
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field_info(struct sockaddr, sa_family);
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field_info(struct sockaddr, sa_data);
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}
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