net: UDP sockets in the kernel + a DNS resolver (real hostname lookups)

Adds UDP to the kernel socket layer on top of the ICMP core:
- net_sum() (raw folded sum) split out of net_cksum() so a UDP pseudo-header
  (src/dst IP + proto + length) can seed the segment checksum.
- udp_send: builds IP+UDP with the pseudo-header checksum; NET_BIND sets the
  local/source port; net_connect sets the peer.
- udp_in: demuxes inbound UDP by destination port to the bound socket
  (net_find_udp), delivers the payload + source addr.

Also fixes a real recv bug: net_pump clobbers BC/DE/HL, so the old recv timeout
counted in registers and was effectively random. recv now counts in WRAM.

New `nslookup HOST` (PROG_NSLOOKUP=28, bank 30): builds a DNS A query and parses
the answer (with 0xC0 name-compression) entirely in userland over a UDP socket -
the kernel never sees DNS, just UDP. Verified through the bridge NAT:
    /# nslookup example.com   ->  example.com -> 172.66.147.243

This gives us name resolution for the TCP/HTTP demo next.

2 files changed, 328 insertions, 21 deletions

diff --git a/src/programs.asm b/src/programs.asm
index 854b2d2..9ae7f58 100644
--- a/src/programs.asm
+++ b/src/programs.asm
@@ -124,6 +124,9 @@ ProgNetd:
 SECTION "prog_ping",   ROMX[$4000], BANK[29]
 ProgPing:
     INCBIN "c/ping.bin"
+SECTION "prog_nslookup", ROMX[$4000], BANK[30]
+ProgNslookup:
+    INCBIN "c/nslookup.bin"
 
 ; -----------------------------------------------------------------------------
 ; PROG_SH (bank 3) - the shell, now written in C (c/sh.c): parses >/</| and
@@ -222,6 +225,8 @@ ProgramTable::
     dw ProgNetd
     db LOW(BANK(ProgPing)), HIGH(BANK(ProgPing))
     dw ProgPing
+    db LOW(BANK(ProgNslookup)), HIGH(BANK(ProgNslookup))
+    dw ProgNslookup
 
 ; -----------------------------------------------------------------------------
 ; NameTable (ROM0) - command name -> program id, searched by sys_lookup.
@@ -284,4 +289,6 @@ NameTable::
     db PROG_NETD
     db "ping", 0
     db PROG_PING
+    db "nslookup", 0
+    db PROG_NSLOOKUP
     db 0
diff --git a/src/socket.asm b/src/socket.asm
index f0d8444..ef9d4b9 100644
--- a/src/socket.asm
+++ b/src/socket.asm
@@ -35,6 +35,8 @@ wNetTx::       DS 256      ; tx build buffer
 wNetRxBuf::    DS 256      ; rx frame assembly / parse
 wNetRxLen::    DS 1
 wNetRxEsc::    DS 1
+wNetUdpPort::  DS 2      ; scratch: UDP dst port being demuxed
+wNetTO::       DS 3      ; recv timeout counter (net_pump clobbers registers)
 
 SECTION "socket", ROM0
 
@@ -123,9 +125,10 @@ net_slip_send:
     ret
 
 ; -----------------------------------------------------------------------------
-; net_cksum(HL = buf, B = len, DE = seed) -> HL = inverted 16-bit checksum.
-; One's-complement sum (RFC 1071), carry-folded each add. len <= 255.
-net_cksum:
+; net_sum(HL = buf, B = len, DE = seed) -> DE = folded 16-bit one's-complement
+; sum (RFC 1071), carry-folded each add. len <= 255. Used to chain a pseudo-
+; header into a segment checksum.
+net_sum:
 .loop
     ld a, b
     cp 2
@@ -147,15 +150,19 @@ net_cksum:
 .tail
     ld a, b
     or a
-    jr z, .fold
+    ret z
     ld a, [hl]                  ; last odd byte -> high half
     add d
     ld d, a
-    jr nc, .fold
+    ret nc
     inc e
-    jr nz, .fold
+    ret nz
     inc d
-.fold
+    ret
+
+; net_cksum(HL, B, DE=seed) -> HL = inverted 16-bit checksum.
+net_cksum:
+    call net_sum
     ld a, d
     cpl
     ld h, a
@@ -201,11 +208,30 @@ sys_net::
     jp z, net_op_recv
     cp NET_CLOSE
     jp z, net_op_close
+    cp NET_BIND
+    jp z, net_op_bind
     cp NET_POLL
     jp z, net_op_poll
     ld a, $FF
     ret
 
+; ---- bind(sock, port) - set local port -----------------------------------
+net_op_bind:
+    ld a, [wNetReq + NR_SOCK]
+    call net_sock_ptr
+    ld a, l
+    add SK_LPORT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [wNetReq + NR_PORT]
+    ld [hl+], a
+    ld a, [wNetReq + NR_PORT + 1]
+    ld [hl], a
+    xor a
+    ret
+
 ; ---- socket(type) -> A = sockid or $FF ---------------------------------------
 net_op_socket:
     ld c, 0
@@ -281,6 +307,8 @@ net_op_send:
     ld a, [hl]                  ; SK_TYPE
     cp SOCK_ICMP
     jp z, icmp_send
+    cp SOCK_UDP
+    jp z, udp_send
     ld a, $FF
     ret
 
@@ -292,21 +320,27 @@ net_op_recv:
     ld [wNetSockPtr], a
     ld a, h
     ld [wNetSockPtr+1], a
-    ld b, 200                   ; outer timeout
-.outer
-    ld de, 0
-.inner
+    xor a                       ; 24-bit timeout counter in WRAM (net_pump
+    ld [wNetTO], a              ; clobbers BC/DE/HL, so we can't count in regs)
+    ld [wNetTO+1], a
+    ld [wNetTO+2], a
+.wait
     call net_pump
-    call sock_has_rx            ; Z clear (nz) if HASRX set
+    call sock_has_rx            ; nz if a datagram is buffered
     jr nz, .got
-    dec de
-    ld a, d
-    or e
-    jr nz, .inner
-    call SchedYield
-    dec b
-    jr nz, .outer
-    ld a, $FF                   ; timeout
+    ld hl, wNetTO
+    inc [hl]
+    jr nz, .wait                ; low byte didn't wrap -> spin (no yield)
+    call SchedYield             ; every 256 pumps, yield to the scheduler
+    inc hl
+    inc [hl]
+    jr nz, .wait
+    inc hl
+    inc [hl]
+    ld a, [hl]
+    cp 250                      ; timeout after ~250*65536 pumps
+    jr c, .wait
+    ld a, $FF
     ret
 .got
     ; copy sock.RXBUF -> process NR_BUF, len = sock.RXLEN, NR_IP = sock.SRCIP
@@ -493,6 +527,153 @@ icmp_send:
     ret
 
 ; =============================================================================
+; udp_send - build + transmit a UDP datagram from the current socket.
+; =============================================================================
+udp_send:
+    ld a, $45
+    ld [wNetTx+0], a
+    xor a
+    ld [wNetTx+1], a
+    ld [wNetTx+2], a
+    ld a, [wNetReq+NR_LEN]
+    add 28                      ; 20 IP + 8 UDP + payload
+    ld [wNetTx+3], a
+    xor a
+    ld [wNetTx+4], a
+    ld [wNetTx+5], a
+    ld [wNetTx+6], a
+    ld [wNetTx+7], a
+    ld a, 64
+    ld [wNetTx+8], a
+    ld a, 17
+    ld [wNetTx+9], a           ; proto UDP
+    xor a
+    ld [wNetTx+10], a
+    ld [wNetTx+11], a
+    ld a, [wNetOurIP+0]
+    ld [wNetTx+12], a
+    ld a, [wNetOurIP+1]
+    ld [wNetTx+13], a
+    ld a, [wNetOurIP+2]
+    ld [wNetTx+14], a
+    ld a, [wNetOurIP+3]
+    ld [wNetTx+15], a
+    ; dst IP = sock.RIP
+    call sock_ptr_hl
+    ld a, l
+    add SK_RIP
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld de, wNetTx+16
+    ld b, 4
+.dip
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec b
+    jr nz, .dip
+    ; UDP header at wNetTx+20: src port = SK_LPORT
+    call sock_ptr_hl
+    ld a, l
+    add SK_LPORT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [hl+]
+    ld [wNetTx+20], a
+    ld a, [hl]
+    ld [wNetTx+21], a
+    ; dst port = SK_RPORT
+    call sock_ptr_hl
+    ld a, l
+    add SK_RPORT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [hl+]
+    ld [wNetTx+22], a
+    ld a, [hl]
+    ld [wNetTx+23], a
+    ; length = 8 + payload
+    xor a
+    ld [wNetTx+24], a
+    ld a, [wNetReq+NR_LEN]
+    add 8
+    ld [wNetTx+25], a
+    xor a
+    ld [wNetTx+26], a          ; cksum = 0
+    ld [wNetTx+27], a
+    ; payload from process buf
+    ld a, [wNetReq+NR_BUF]
+    ld l, a
+    ld a, [wNetReq+NR_BUF+1]
+    ld h, a
+    ld de, wNetTx+28
+    ld a, [wNetReq+NR_LEN]
+    ld b, a
+    or a
+    jr z, .nopl
+.cppl
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec b
+    jr nz, .cppl
+.nopl
+    ; UDP checksum with pseudo-header: sum(src+dst IP) + proto + udplen, then seg
+    ld hl, wNetTx+12
+    ld b, 8
+    ld de, 0
+    call net_sum               ; DE = IP-address sum
+    ld a, e
+    add 17                     ; + protocol
+    ld e, a
+    ld a, d
+    adc 0
+    ld d, a
+    ld a, [wNetReq+NR_LEN]
+    add 8                      ; + udp length
+    add e
+    ld e, a
+    ld a, d
+    adc 0
+    ld d, a                    ; DE = pseudo-header seed
+    ld a, [wNetReq+NR_LEN]
+    add 8
+    ld b, a
+    ld hl, wNetTx+20
+    call net_cksum             ; HL = inverted checksum
+    ld a, h
+    or l
+    jr nz, .cknz
+    ld hl, $FFFF               ; 0 means "no checksum" -> send all-ones
+.cknz
+    ld a, h
+    ld [wNetTx+26], a
+    ld a, l
+    ld [wNetTx+27], a
+    ; IP checksum
+    ld hl, wNetTx
+    ld b, 20
+    ld de, 0
+    call net_cksum
+    ld a, h
+    ld [wNetTx+10], a
+    ld a, l
+    ld [wNetTx+11], a
+    ld a, [wNetReq+NR_LEN]
+    add 28
+    ld b, a
+    ld hl, wNetTx
+    call net_slip_send
+    xor a
+    ret
+
+; =============================================================================
 ; net_pump - read available serial bytes, assemble SLIP frames, process each.
 ; Non-blocking: returns when the link has no more bytes ready.
 ; =============================================================================
@@ -576,7 +757,9 @@ process_frame:
     ld [wNetIhl], a             ; ihl in bytes
     ld a, [wNetRxBuf+9]         ; protocol
     cp 1
-    jr z, icmp_in
+    jp z, icmp_in
+    cp 17
+    jp z, udp_in
     ret
 
 ; -----------------------------------------------------------------------------
@@ -736,6 +919,123 @@ icmp_reply:
     jr nz, .cp
     ret
 
+; -----------------------------------------------------------------------------
+; udp_in - deliver an inbound UDP datagram to the socket bound to its dst port.
+udp_in:
+    ld a, [wNetIhl]
+    add 2
+    ld e, a
+    ld d, 0
+    ld hl, wNetRxBuf
+    add hl, de                  ; HL = &udp.dstport
+    ld a, [hl+]
+    ld [wNetUdpPort], a
+    ld a, [hl]
+    ld [wNetUdpPort+1], a
+    call net_find_udp           ; HL = &sock, CF if none
+    ret c
+    ; deliver: HASRX=1, SRCIP, RXLEN + RXBUF (payload after ihl+8)
+    push hl
+    ld a, l
+    add SK_HASRX
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, 1
+    ld [hl], a
+    pop hl
+    push hl
+    ld a, l
+    add SK_SRCIP
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld de, wNetRxBuf+12
+    ld b, 4
+.sip
+    ld a, [de]
+    ld [hl+], a
+    inc de
+    dec b
+    jr nz, .sip
+    pop hl
+    ; payload offset = ihl + 8, length = rxlen - ihl - 8
+    ld a, [wNetIhl]
+    add 8
+    ld e, a
+    ld d, 0
+    ld a, [wNetRxLen]
+    sub e
+    ld c, a                     ; C = payload length
+    push hl
+    ld a, l
+    add SK_RXLEN
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, c
+    ld [hl], a
+    pop hl
+    ld a, l
+    add SK_RXBUF
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a                     ; HL = &sock.RXBUF (dest)
+    push hl
+    ld hl, wNetRxBuf
+    add hl, de                  ; HL = &payload (src)
+    pop de                      ; DE = dest
+    ld a, c
+    or a
+    ret z
+.cp
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec c
+    jr nz, .cp
+    ret
+
+; net_find_udp -> HL = &UDP socket bound to wNetUdpPort, CF set if none
+net_find_udp:
+    ld c, 0
+.l
+    ld a, c
+    call net_sock_ptr           ; HL = &sock (clobbers B, DE)
+    ld a, [hl]
+    cp SOCK_UDP
+    jr nz, .nx
+    push hl
+    ld a, l
+    add SK_LPORT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [wNetUdpPort]
+    cp [hl]
+    jr nz, .nomatch
+    inc hl
+    ld a, [wNetUdpPort+1]
+    cp [hl]
+    jr nz, .nomatch
+    pop hl                      ; match
+    or a                        ; CF = 0
+    ret
+.nomatch
+    pop hl
+.nx
+    inc c
+    ld a, c
+    cp MAX_SOCKS
+    jr c, .l
+    scf
+    ret
+
 ; net_swap_ip - swap the src/dst IP fields of the frame in wNetRxBuf
 net_swap_ip:
     ld hl, wNetRxBuf+12