#include "sock.h"
/* nslookup HOST - resolve a hostname to an IPv4 address via DNS over UDP.
   The kernel owns UDP; this program just builds the DNS query and parses the
   answer. Queries 1.1.1.1:53 (reachable through the bridge NAT). */

/* buf is first and large so 'name' lands past the arg string at 0xA000 */
static unsigned char buf[200];
static unsigned char name[64];
static unsigned char dnssrv[4];
static unsigned char ip[4];

/* build a DNS A-record query for 'host' into buf; return total length */
static unsigned char build_query(unsigned char *host) {
    unsigned char i = 0, p = 12, lblpos, len;
    buf[0] = 0x12; buf[1] = 0x34;          /* id */
    buf[2] = 0x01; buf[3] = 0x00;          /* flags: recursion desired */
    buf[4] = 0; buf[5] = 1;                /* qdcount = 1 */
    buf[6] = 0; buf[7] = 0; buf[8] = 0; buf[9] = 0; buf[10] = 0; buf[11] = 0;
    while (host[i]) {
        lblpos = p++; len = 0;
        while (host[i] && host[i] != '.') { buf[p++] = host[i]; i++; len++; }
        buf[lblpos] = len;
        if (host[i] == '.') i++;
    }
    buf[p++] = 0;                          /* root label */
    buf[p++] = 0; buf[p++] = 1;            /* QTYPE  = A  */
    buf[p++] = 0; buf[p++] = 1;            /* QCLASS = IN */
    return p;
}

/* advance past a DNS name at offset p (handles 0xC0 compression pointers) */
static unsigned int skip_name(unsigned int p) {
    for (;;) {
        if (buf[p] == 0) return p + 1;
        if ((buf[p] & 0xC0) == 0xC0) return p + 2;
        p += buf[p] + 1;
    }
}

/* find the first A record in the response; fill ip[]; return 1 on success */
static unsigned char parse_answer(void) {
    unsigned int p, a, an, type, rdlen;
    an = ((unsigned int)buf[6] << 8) | buf[7];   /* ancount */
    if (an == 0) return 0;
    p = skip_name(12) + 4;                        /* skip question (QNAME+QT+QC) */
    for (a = 0; a < an; a++) {
        p = skip_name(p);
        type = ((unsigned int)buf[p] << 8) | buf[p + 1];
        p += 8;                                   /* TYPE(2)+CLASS(2)+TTL(4) */
        rdlen = ((unsigned int)buf[p] << 8) | buf[p + 1];
        p += 2;
        if (type == 1 && rdlen == 4) {
            ip[0] = buf[p]; ip[1] = buf[p + 1]; ip[2] = buf[p + 2]; ip[3] = buf[p + 3];
            return 1;
        }
        p += rdlen;
    }
    return 0;
}

void main(void) {
    char *host;
    unsigned char s, i, qlen, rlen;
    host = getargs();
    if (!host || !*host) { puts("usage: nslookup HOST"); nl(); sexit(1); }
    for (i = 0; host[i] && i < 63; i++) name[i] = host[i];   /* arg -> safe buf */
    name[i] = 0;

    dnssrv[0] = 1; dnssrv[1] = 1; dnssrv[2] = 1; dnssrv[3] = 1;   /* 1.1.1.1 */
    s = net_socket(SOCK_UDP);
    if (s == 0xFF) { puts("nslookup: no socket"); nl(); sexit(1); }
    net_bind(s, 40000);
    net_connect(s, dnssrv, 53);

    qlen = build_query(name);
    net_send(s, buf, qlen);
    rlen = net_recv(s, buf, sizeof(buf));
    if (rlen == 0xFF) { puts("nslookup: timeout"); nl(); net_close(s); sexit(1); }

    if (parse_answer()) {
        puts((char *)name); puts(" -> ");
        for (i = 0; i < 4; i++) { putu(ip[i]); if (i < 3) putc('.'); }
        nl();
    } else {
        puts("nslookup: no A record"); nl();
    }
    net_close(s);
}