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dhcpd: use unicast to reply messages
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,84 @@ | ||
| package dhcpd | ||
|
|
||
| import ( | ||
| "encoding/binary" | ||
| "fmt" | ||
| "net" | ||
| "syscall" | ||
|
|
||
| "github.com/insomniacslk/dhcp/dhcpv4" | ||
|
|
||
| "github.com/nadoo/glider/pkg/log" | ||
| ) | ||
|
|
||
| func reply(iface *net.Interface, resp *dhcpv4.DHCPv4) error { | ||
| p := [590]byte{12: 0x08, //ethernet layer: 14 bytes | ||
| 14: 0x45, 16: 0x02, 17: 0x40, 22: 0x40, 23: 0x11, //ip layer: 20 bytes | ||
| 35: 67, 37: 68, 38: 0x02, 39: 0x2c, //udp layer: 8 bytes | ||
| } | ||
|
|
||
| copy(p[0:], resp.ClientHWAddr[0:6]) | ||
| copy(p[6:], iface.HardwareAddr[0:6]) | ||
| copy(p[26:], resp.ServerIPAddr[0:4]) | ||
| copy(p[30:], resp.YourIPAddr[0:4]) | ||
|
|
||
| // ip layer checksum | ||
| checksum := checksum(p[14:34]) | ||
| binary.BigEndian.PutUint16(p[24:], checksum) | ||
|
|
||
| // dhcp payload | ||
| copy(p[42:], resp.ToBytes()) | ||
|
|
||
| // udp layer checksum, set to zero | ||
| // https://datatracker.ietf.org/doc/html/rfc768 | ||
| // An all zero transmitted checksum value means that the transmitter generated no | ||
| // checksum (for debugging or for higher level protocols that don't care). | ||
|
|
||
| fd, err := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_RAW, 0) | ||
| if err != nil { | ||
| return fmt.Errorf("cannot open socket: %v", err) | ||
| } | ||
| defer func() { | ||
| err = syscall.Close(fd) | ||
| if err != nil { | ||
| log.F("dhcpd: cannot close socket: %v", err) | ||
| } | ||
| }() | ||
|
|
||
| err = syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1) | ||
| if err != nil { | ||
| log.F("dhcpd: cannot set option for socket: %v", err) | ||
| } | ||
|
|
||
| var hwAddr [8]byte | ||
| copy(hwAddr[0:6], resp.ClientHWAddr[0:6]) | ||
| ethAddr := syscall.SockaddrLinklayer{ | ||
| Protocol: 0, | ||
| Ifindex: iface.Index, | ||
| Halen: 6, | ||
| Addr: hwAddr, | ||
| } | ||
| err = syscall.Sendto(fd, p[:], 0, ðAddr) | ||
| if err != nil { | ||
| return fmt.Errorf("cannot send frame via socket: %v", err) | ||
| } | ||
| return nil | ||
| } | ||
|
|
||
| func checksum(bytes []byte) uint16 { | ||
| var csum uint32 | ||
| for i := 0; i < len(bytes); i += 2 { | ||
| csum += uint32(bytes[i]) << 8 | ||
| csum += uint32(bytes[i+1]) | ||
| } | ||
| for { | ||
| // Break when sum is less or equals to 0xFFFF | ||
| if csum <= 65535 { | ||
| break | ||
| } | ||
| // Add carry to the sum | ||
| csum = (csum >> 16) + uint32(uint16(csum)) | ||
| } | ||
| // Flip all the bits | ||
| return ^uint16(csum) | ||
| } |