diff options
Diffstat (limited to 'c/nslookup.c')
| -rw-r--r-- | c/nslookup.c | 88 |
1 files changed, 88 insertions, 0 deletions
diff --git a/c/nslookup.c b/c/nslookup.c new file mode 100644 index 0000000..020a1c1 --- /dev/null +++ b/c/nslookup.c @@ -0,0 +1,88 @@ +#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); +} |