Subject: [PATCH] nslookup: ensure unique transaction IDs for the DNS queries
From: Uwe Kleine-König <[email-masked]>
Date: Sat, 8 Oct 2022 19:22:52 +0200

The transaction IDs generated by res_mkquery() for both glibc and musl only
depends on the state of the monotonic clock.
For some machines (here: a TP-Link RE200 powered by a MediaTek MT7620A)
the monotonic clock has a coarse resolution (here: 20 µs) and it can happen
that the requests for A and AAAA share the same transaction ID.

In that case the mapping from received responses to the sent queries
doesn't work and name resolution fails as follows:

        # /bin/busybox nslookup heise.de
        Server:         127.0.0.1
        Address:        127.0.0.1:53

        Non-authoritative answer:
        Name:   heise.de
        Address: 193.99.144.80

        *** Can't find heise.de: No answer

because the AAAA reply is dropped as a duplicate reply to the A query.

To prevent this make sure the transaction IDs are unique.

Forwarded: http://lists.busybox.net/pipermail/busybox/2022-October/089911.html
---
--- a/networking/nslookup.c
+++ b/networking/nslookup.c
@@ -978,6 +978,10 @@ int nslookup_main(int argc UNUSED_PARAM,
 		}
 	}
 
+	/* Ensure the Transaction IDs are unique */
+	for (rc = 1; rc < G.query_count; rc++)
+		G.query[rc].query[1] = G.query[rc - 1].query[1] + 1;
+
 	for (rc = 0; rc < G.serv_count;) {
 		int c;