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#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);
}
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