udnspkt: A module to create/parse DNS packets to resolve hostnames.

This module implements "Sans I/O" approach, where packets are created/
parsed using BytesIO objects, and all networking happens outside the
module.

The module implements enough functionality to resolve a domain name
into IP address, for both IPv4 and IPv6.

Other DNS functionality is outside the scope of this module, that's
why it's called *u*dnspkt.

The API is experimental and subject to change.

This module requires .readbin()/.writebin() methods on a stream.

udnspkt/udnspkt.py

@@ -0,0 +1,78 @@
+import uio
+
+
+def write_fqdn(buf, name):
+    parts = name.split(".")
+    for p in parts:
+        buf.writebin("B", len(p))
+        buf.write(p)
+    buf.writebin("B", 0)
+
+
+def skip_fqdn(buf):
+    while True:
+        sz = buf.readbin("B")
+        if not sz:
+            break
+        if sz >= 0xc0:
+            buf.readbin("B")
+            break
+        buf.read(sz)
+
+
+def make_req(buf, fqdn, is_ipv6):
+    typ = 1  # A
+    if is_ipv6:
+        typ = 28  # AAAA
+
+    buf.writebin(">H", 0)
+    buf.writebin(">H", 0x100)
+    # q count
+    buf.writebin(">H", 1)
+    buf.writebin(">H", 0)
+    # squashed together
+    buf.writebin(">I", 0)
+
+    write_fqdn(buf, fqdn)
+    buf.writebin(">H", typ)
+    buf.writebin(">H", 1)  # Class
+
+
+def parse_resp(buf, is_ipv6):
+    typ = 1  # A
+    if is_ipv6:
+        typ = 28  # AAAA
+
+    id = buf.readbin(">H")
+    flags = buf.readbin(">H")
+    assert flags & 0x8000
+    qcnt = buf.readbin(">H")
+    acnt = buf.readbin(">H")
+    nscnt = buf.readbin(">H")
+    addcnt = buf.readbin(">H")
+    #print(qcnt, acnt, nscnt, addcnt)
+
+    skip_fqdn(buf)
+    v = buf.readbin(">H")
+    #print(v)
+    v = buf.readbin(">H")
+    #print(v)
+
+    for i in range(acnt):
+        #print("Resp #%d" % i)
+        #v = read_fqdn(buf)
+        #print(v)
+        skip_fqdn(buf)
+        t = buf.readbin(">H")
+        #print("Type", t)
+        v = buf.readbin(">H")
+        #print("Class", v)
+        v = buf.readbin(">I")
+        #print("TTL", v)
+        rlen = buf.readbin(">H")
+        #print("rlen", rlen)
+        rval = buf.read(rlen)
+        #print(rval)
+
+        if t == typ:
+            return rval