dnsresolv.inc - pub/scm/boot/syslinux/syslinux - Git at Google

 ; -*- fundamental -*-
 ; -----------------------------------------------------------------------
 ;
 ;   Copyright 2004-2008 H. Peter Anvin - All Rights Reserved
 ;
 ;   This program is free software; you can redistribute it and/or modify
 ;   it under the terms of the GNU General Public License as published by
 ;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
 ;   Bostom MA 02111-1307, USA; either version 2 of the License, or
 ;   (at your option) any later version; incorporated herein by reference.
 ;
 ; -----------------------------------------------------------------------

 ;
 ; dnsresolv.inc
 ;
 ; Very simple DNS resolver (assumes recursion-enabled DNS server;
 ; this should be the normal thing for client-serving DNS servers.)
 ;

 DNS_PORT	equ htons(53)		; Default DNS port
 DNS_MAX_PACKET	equ 512			; Defined by protocol
 ; TFTP uses the range 49152-57343
 DNS_LOCAL_PORT	equ htons(60053)	; All local DNS queries come from this port #
 DNS_MAX_SERVERS equ 4			; Max no of DNS servers

 		section .text

 ;
 ; Turn a string in DS:SI into a DNS "label set" in ES:DI.
 ; On return, DI points to the first byte after the label set,
 ; and SI to the terminating byte.
 ;
 ; On return, DX contains the number of dots encountered.
 ;
 dns_mangle:
 		push ax
 		push bx
 		xor dx,dx
 .isdot:
 		inc dx
 		xor al,al
 		mov bx,di
 		stosb
 .getbyte:
 		lodsb
 		and al,al
 		jz .endstring
 		cmp al,':'
 		jz .endstring
 		cmp al,'.'
 		je .isdot
 		inc byte [es:bx]
 		stosb
 		jmp .getbyte
 .endstring:
 		dec si
 		dec dx			; We always counted one high
 		cmp byte [es:bx],0
 		jz .done
 		xor al,al
 		stosb
 .done:
 		pop bx
 		pop ax
 		ret

 ;
 ; Compare two sets of DNS labels, in DS:SI and ES:DI; the one in SI
 ; is allowed pointers relative to a packet in DNSRecvBuf.
 ;
 ; Assumes DS == ES.  ZF = 1 if same; no registers changed.
 ; (Note: change reference to [di] to [es:di] to remove DS == ES assumption)
 ;
 dns_compare:
 		pusha
 %if 0

 .label:
 		lodsb
 		cmp al,0C0h
 		jb .noptr
 		and al,03Fh			; Get pointer value
 		mov ah,al			; ... in network byte order!
 		lodsb
 		mov si,DNSRecvBuf
 		add si,ax
 		jmp .label
 .noptr:
 		cmp al,[di]
 		jne .done			; Mismatch
 		inc di
 		movzx cx,al			; End label?
 		and cx,cx			; ZF = 1 if match
 		jz .done

 		; We have a string of bytes that need to match now
 		repe cmpsb
 		je .label

 .done:
 %else
 		xor ax,ax
 %endif
 		popa
 		ret

 ;
 ; Skip past a DNS label set in DS:SI.
 ;
 dns_skiplabel:
 		push ax
 		xor ax,ax			; AH == 0
 .loop:
 		lodsb
 		cmp al,0C0h			; Pointer?
 		jae .ptr
 		and al,al
 		jz .done
 		add si,ax
 		jmp .loop
 .ptr:
 		inc si				; Pointer is two bytes
 .done:
 		pop ax
 		ret

 		; DNS header format
 		struc dnshdr
 .id:		resw 1
 .flags:		resw 1
 .qdcount:	resw 1
 .ancount:	resw 1
 .nscount:	resw 1
 .arcount:	resw 1
 		endstruc

 		; DNS query
 		struc dnsquery
 .qtype:		resw 1
 .qclass:	resw 1
 		endstruc

 		; DNS RR
 		struc dnsrr
 .type:		resw 1
 .class:		resw 1
 .ttl:		resd 1
 .rdlength:	resw 1
 .rdata:		equ $
 		endstruc

 		section .bss2
 		alignb 2
 DNSSendBuf	resb DNS_MAX_PACKET
 DNSRecvBuf	resb DNS_MAX_PACKET
 LocalDomain	resb 256		; Max possible length
 DNSServers	resd DNS_MAX_SERVERS

 		section .data
 pxe_udp_write_pkt_dns:
 .status:	dw 0			; Status
 .sip:		dd 0			; Server IP
 .gip:		dd 0			; Gateway IP
 .lport:		dw DNS_LOCAL_PORT	; Local port
 .rport:		dw DNS_PORT		; Remote port
 .buffersize:	dw 0			; Size of packet
 .buffer:	dw DNSSendBuf, 0	; off, seg of buffer

 pxe_udp_read_pkt_dns:
 .status:	dw 0			; Status
 .sip:		dd 0			; Source IP
 .dip:		dd 0			; Destination (our) IP
 .rport:		dw DNS_PORT		; Remote port
 .lport:		dw DNS_LOCAL_PORT	; Local port
 .buffersize:	dw DNS_MAX_PACKET	; Max packet size
 .buffer:	dw DNSRecvBuf, 0	; off, seg of buffer

 LastDNSServer	dw DNSServers

 ; Actual resolver function
 ; Points to a null-terminated or :-terminated string in DS:SI
 ; and returns the name in EAX if it exists and can be found.
 ; If EAX = 0 on exit, the lookup failed.
 ;
 ; No segment assumptions permitted.
 ;
 		section .text
 dns_resolv:
 		push ds
 		push es
 		push di
 		push cx
 		push dx

 		push cs
 		pop es			; ES <- CS

 		; First, build query packet
 		mov di,DNSSendBuf+dnshdr.flags
 		inc word [es:di-2]	; New query ID
 		mov ax,htons(0100h)	; Recursion requested
 		stosw
 		mov ax,htons(1)		; One question
 		stosw
 		xor ax,ax		; No answers, NS or ARs
 		stosw
 		stosw
 		stosw

 		call dns_mangle		; Convert name to DNS labels

 		push cs			; DS <- CS
 		pop ds

 		push si			; Save pointer to after DNS string

 		; Initialize...
 		mov eax,[MyIP]
 		mov [pxe_udp_read_pkt_dns.dip],eax

 		and dx,dx
 		jnz .fqdn		; If we have dots, assume it's FQDN
 		dec di			; Remove final null
 		mov si,LocalDomain
 		call strcpy		; Uncompressed DNS label set so it ends in null
 .fqdn:

 		mov ax,htons(1)
 		stosw			; QTYPE  = 1 = A
 		stosw			; QCLASS = 1 = IN

 		sub di,DNSSendBuf
 		mov [pxe_udp_write_pkt_dns.buffersize],di

 		; Now, send it to the nameserver(s)
 		; Outer loop: exponential backoff
 		; Inner loop: scan the various DNS servers

 		mov dx,PKT_TIMEOUT
 		mov cx,PKT_RETRY
 .backoff:
 		mov si,DNSServers
 .servers:
 		cmp si,[LastDNSServer]
 		jb .moreservers

 .nomoreservers:
 		add dx,dx			; Exponential backoff
 		loop .backoff

 		xor eax,eax			; Nothing...
 .done:
 		pop si
 		pop dx
 		pop cx
 		pop di
 		pop es
 		pop ds
 		ret

 .moreservers:
 		lodsd				; EAX <- next server
 		push si
 		push cx
 		push dx

 		mov word [pxe_udp_write_pkt_dns.status],0

 		mov [pxe_udp_write_pkt_dns.sip],eax
 		mov [pxe_udp_read_pkt_dns.sip],eax
 		xor eax,[MyIP]
 		and eax,[Netmask]
 		jz .nogw
 		mov eax,[Gateway]
 .nogw:
 		mov [pxe_udp_write_pkt_dns.gip],eax

 		mov di,pxe_udp_write_pkt_dns
 		mov bx,PXENV_UDP_WRITE
 		call pxenv
 		jc .timeout				; Treat failed transmit as timeout
 		cmp word [pxe_udp_write_pkt_dns.status],0
 		jne .timeout

 		mov cx,[BIOS_timer]
 .waitrecv:
 		mov ax,[BIOS_timer]
 		sub ax,cx
 		cmp ax,dx
 		jae .timeout

 		mov word [pxe_udp_read_pkt_dns.status],0
 		mov word [pxe_udp_read_pkt_dns.buffersize],DNS_MAX_PACKET
 		mov di,pxe_udp_read_pkt_dns
 		mov bx,PXENV_UDP_READ
 		call pxenv
 		and ax,ax
 		jnz .waitrecv
 		cmp [pxe_udp_read_pkt_dns.status],ax
 		jnz .waitrecv

 		; Got a packet, deal with it...
 		mov si,DNSRecvBuf
 		lodsw
 		cmp ax,[DNSSendBuf]		; ID
 		jne .waitrecv			; Not ours

 		lodsw				; flags
 		xor al,80h			; Query#/Answer bit
 		test ax,htons(0F80Fh)
 		jnz .badness

 		lodsw
 		xchg ah,al			; ntohs
 		mov cx,ax			; Questions echoed
 		lodsw
 		xchg ah,al			; ntohs
 		push ax				; Replies
 		lodsw				; NS records
 		lodsw				; Authority records

 		jcxz .qskipped
 .skipq:
 		call dns_skiplabel		; Skip name
 		add si,4			; Skip question trailer
 		loop .skipq

 .qskipped:
 		pop cx				; Number of replies
 		jcxz .badness

 .parseanswer:
 		mov di,DNSSendBuf+dnshdr_size
 		call dns_compare
 		pushf
 		call dns_skiplabel
 		mov ax,[si+8]			; RDLENGTH
 		xchg ah,al			; ntohs
 		popf
 		jnz .notsame
 		cmp dword [si],htons(1)*0x10001	; TYPE = A, CLASS = IN?
 		jne .notsame
 		cmp ax,4			; RDLENGTH = 4?
 		jne .notsame
 		;
 		; We hit paydirt here...
 		;
 		mov eax,[si+10]
 .gotresult:
 		add sp,6			; Drop timeout information
 		jmp .done

 .notsame:
 		add si,10
 		add si,ax
 		loop .parseanswer

 .badness:
 		; We got back no data from this server.  Unfortunately, for a recursive,
 		; non-authoritative query there is no such thing as an NXDOMAIN reply,
 		; which technically means we can't draw any conclusions.  However,
 		; in practice that means the domain doesn't exist.  If this turns out
 		; to be a problem, we may want to add code to go through all the servers
 		; before giving up.

 		; If the DNS server wasn't capable of recursion, and isn't capable
 		; of giving us an authoritative reply (i.e. neither AA or RA set),
 		; then at least try a different setver...
 		test word [DNSRecvBuf+dnshdr.flags],htons(0480h)
 		jz .timeout

 		xor eax,eax
 		jmp .gotresult

 .timeout:
 		pop dx
 		pop cx
 		pop si
 		jmp .servers
	; -- fundamental --
	; -----------------------------------------------------------------------
	;
	; Copyright 2004-2008 H. Peter Anvin - All Rights Reserved
	;
	; This program is free software; you can redistribute it and/or modify
	; it under the terms of the GNU General Public License as published by
	; the Free Software Foundation, Inc., 53 Temple Place Ste 330,
	; Bostom MA 02111-1307, USA; either version 2 of the License, or
	; (at your option) any later version; incorporated herein by reference.
	;
	; -----------------------------------------------------------------------

	;
	; dnsresolv.inc
	;
	; Very simple DNS resolver (assumes recursion-enabled DNS server;
	; this should be the normal thing for client-serving DNS servers.)
	;

	DNS_PORT equ htons(53) ; Default DNS port
	DNS_MAX_PACKET equ 512 ; Defined by protocol
	; TFTP uses the range 49152-57343
	DNS_LOCAL_PORT equ htons(60053) ; All local DNS queries come from this port #
	DNS_MAX_SERVERS equ 4 ; Max no of DNS servers

	section .text

	;
	; Turn a string in DS:SI into a DNS "label set" in ES:DI.
	; On return, DI points to the first byte after the label set,
	; and SI to the terminating byte.
	;
	; On return, DX contains the number of dots encountered.
	;
	dns_mangle:
	push ax
	push bx
	xor dx,dx
	.isdot:
	inc dx
	xor al,al
	mov bx,di
	stosb
	.getbyte:
	lodsb
	and al,al
	jz .endstring
	cmp al,':'
	jz .endstring
	cmp al,'.'
	je .isdot
	inc byte [es:bx]
	stosb
	jmp .getbyte
	.endstring:
	dec si
	dec dx ; We always counted one high
	cmp byte [es:bx],0
	jz .done
	xor al,al
	stosb
	.done:
	pop bx
	pop ax
	ret

	;
	; Compare two sets of DNS labels, in DS:SI and ES:DI; the one in SI
	; is allowed pointers relative to a packet in DNSRecvBuf.
	;
	; Assumes DS == ES. ZF = 1 if same; no registers changed.
	; (Note: change reference to [di] to [es:di] to remove DS == ES assumption)
	;
	dns_compare:
	pusha
	%if 0

	.label:
	lodsb
	cmp al,0C0h
	jb .noptr
	and al,03Fh ; Get pointer value
	mov ah,al ; ... in network byte order!
	lodsb
	mov si,DNSRecvBuf
	add si,ax
	jmp .label
	.noptr:
	cmp al,[di]
	jne .done ; Mismatch
	inc di
	movzx cx,al ; End label?
	and cx,cx ; ZF = 1 if match
	jz .done

	; We have a string of bytes that need to match now
	repe cmpsb
	je .label

	.done:
	%else
	xor ax,ax
	%endif
	popa
	ret

	;
	; Skip past a DNS label set in DS:SI.
	;
	dns_skiplabel:
	push ax
	xor ax,ax ; AH == 0
	.loop:
	lodsb
	cmp al,0C0h ; Pointer?
	jae .ptr
	and al,al
	jz .done
	add si,ax
	jmp .loop
	.ptr:
	inc si ; Pointer is two bytes
	.done:
	pop ax
	ret

	; DNS header format
	struc dnshdr
	.id: resw 1
	.flags: resw 1
	.qdcount: resw 1
	.ancount: resw 1
	.nscount: resw 1
	.arcount: resw 1
	endstruc

	; DNS query
	struc dnsquery
	.qtype: resw 1
	.qclass: resw 1
	endstruc

	; DNS RR
	struc dnsrr
	.type: resw 1
	.class: resw 1
	.ttl: resd 1
	.rdlength: resw 1
	.rdata: equ $
	endstruc

	section .bss2
	alignb 2
	DNSSendBuf resb DNS_MAX_PACKET
	DNSRecvBuf resb DNS_MAX_PACKET
	LocalDomain resb 256 ; Max possible length
	DNSServers resd DNS_MAX_SERVERS

	section .data
	pxe_udp_write_pkt_dns:
	.status: dw 0 ; Status
	.sip: dd 0 ; Server IP
	.gip: dd 0 ; Gateway IP
	.lport: dw DNS_LOCAL_PORT ; Local port
	.rport: dw DNS_PORT ; Remote port
	.buffersize: dw 0 ; Size of packet
	.buffer: dw DNSSendBuf, 0 ; off, seg of buffer

	pxe_udp_read_pkt_dns:
	.status: dw 0 ; Status
	.sip: dd 0 ; Source IP
	.dip: dd 0 ; Destination (our) IP
	.rport: dw DNS_PORT ; Remote port
	.lport: dw DNS_LOCAL_PORT ; Local port
	.buffersize: dw DNS_MAX_PACKET ; Max packet size
	.buffer: dw DNSRecvBuf, 0 ; off, seg of buffer

	LastDNSServer dw DNSServers

	; Actual resolver function
	; Points to a null-terminated or :-terminated string in DS:SI
	; and returns the name in EAX if it exists and can be found.
	; If EAX = 0 on exit, the lookup failed.
	;
	; No segment assumptions permitted.
	;
	section .text
	dns_resolv:
	push ds
	push es
	push di
	push cx
	push dx

	push cs
	pop es ; ES <- CS

	; First, build query packet
	mov di,DNSSendBuf+dnshdr.flags
	inc word [es:di-2] ; New query ID
	mov ax,htons(0100h) ; Recursion requested
	stosw
	mov ax,htons(1) ; One question
	stosw
	xor ax,ax ; No answers, NS or ARs
	stosw
	stosw
	stosw

	call dns_mangle ; Convert name to DNS labels

	push cs ; DS <- CS
	pop ds

	push si ; Save pointer to after DNS string

	; Initialize...
	mov eax,[MyIP]
	mov [pxe_udp_read_pkt_dns.dip],eax

	and dx,dx
	jnz .fqdn ; If we have dots, assume it's FQDN
	dec di ; Remove final null
	mov si,LocalDomain
	call strcpy ; Uncompressed DNS label set so it ends in null
	.fqdn:

	mov ax,htons(1)
	stosw ; QTYPE = 1 = A
	stosw ; QCLASS = 1 = IN

	sub di,DNSSendBuf
	mov [pxe_udp_write_pkt_dns.buffersize],di

	; Now, send it to the nameserver(s)
	; Outer loop: exponential backoff
	; Inner loop: scan the various DNS servers

	mov dx,PKT_TIMEOUT
	mov cx,PKT_RETRY
	.backoff:
	mov si,DNSServers
	.servers:
	cmp si,[LastDNSServer]
	jb .moreservers

	.nomoreservers:
	add dx,dx ; Exponential backoff
	loop .backoff

	xor eax,eax ; Nothing...
	.done:
	pop si
	pop dx
	pop cx
	pop di
	pop es
	pop ds
	ret

	.moreservers:
	lodsd ; EAX <- next server
	push si
	push cx
	push dx

	mov word [pxe_udp_write_pkt_dns.status],0

	mov [pxe_udp_write_pkt_dns.sip],eax
	mov [pxe_udp_read_pkt_dns.sip],eax
	xor eax,[MyIP]
	and eax,[Netmask]
	jz .nogw
	mov eax,[Gateway]
	.nogw:
	mov [pxe_udp_write_pkt_dns.gip],eax

	mov di,pxe_udp_write_pkt_dns
	mov bx,PXENV_UDP_WRITE
	call pxenv
	jc .timeout ; Treat failed transmit as timeout
	cmp word [pxe_udp_write_pkt_dns.status],0
	jne .timeout

	mov cx,[BIOS_timer]
	.waitrecv:
	mov ax,[BIOS_timer]
	sub ax,cx
	cmp ax,dx
	jae .timeout

	mov word [pxe_udp_read_pkt_dns.status],0
	mov word [pxe_udp_read_pkt_dns.buffersize],DNS_MAX_PACKET
	mov di,pxe_udp_read_pkt_dns
	mov bx,PXENV_UDP_READ
	call pxenv
	and ax,ax
	jnz .waitrecv
	cmp [pxe_udp_read_pkt_dns.status],ax
	jnz .waitrecv

	; Got a packet, deal with it...
	mov si,DNSRecvBuf
	lodsw
	cmp ax,[DNSSendBuf] ; ID
	jne .waitrecv ; Not ours

	lodsw ; flags
	xor al,80h ; Query#/Answer bit
	test ax,htons(0F80Fh)
	jnz .badness

	lodsw
	xchg ah,al ; ntohs
	mov cx,ax ; Questions echoed
	lodsw
	xchg ah,al ; ntohs
	push ax ; Replies
	lodsw ; NS records
	lodsw ; Authority records

	jcxz .qskipped
	.skipq:
	call dns_skiplabel ; Skip name
	add si,4 ; Skip question trailer
	loop .skipq

	.qskipped:
	pop cx ; Number of replies
	jcxz .badness

	.parseanswer:
	mov di,DNSSendBuf+dnshdr_size
	call dns_compare
	pushf
	call dns_skiplabel
	mov ax,[si+8] ; RDLENGTH
	xchg ah,al ; ntohs
	popf
	jnz .notsame
	cmp dword [si],htons(1)*0x10001 ; TYPE = A, CLASS = IN?
	jne .notsame
	cmp ax,4 ; RDLENGTH = 4?
	jne .notsame
	;
	; We hit paydirt here...
	;
	mov eax,[si+10]
	.gotresult:
	add sp,6 ; Drop timeout information
	jmp .done

	.notsame:
	add si,10
	add si,ax
	loop .parseanswer

	.badness:
	; We got back no data from this server. Unfortunately, for a recursive,
	; non-authoritative query there is no such thing as an NXDOMAIN reply,
	; which technically means we can't draw any conclusions. However,
	; in practice that means the domain doesn't exist. If this turns out
	; to be a problem, we may want to add code to go through all the servers
	; before giving up.

	; If the DNS server wasn't capable of recursion, and isn't capable
	; of giving us an authoritative reply (i.e. neither AA or RA set),
	; then at least try a different setver...
	test word [DNSRecvBuf+dnshdr.flags],htons(0480h)
	jz .timeout

	xor eax,eax
	jmp .gotresult

	.timeout:
	pop dx
	pop cx
	pop si
	jmp .servers