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intermittent failures and queries sent over TCP

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bind 9.11.5.P4 on Debian 10

Greetings. I recently had to migrate a nameserver from FreeBSD to
Debian. It works fine most of the time but I've noticed a few
intermittent resolution failures.

After "gmail.com" failed to resolve I took a packet capture using
tcpdump to listen to the result of the command "dig -t mx gmail.com" and
here's what I found:

1. That query over UDP, with responses over UDP pointing to Google's
nameservers

2. Nearly 200 attempts to reach root servers over TCP, followed
immediately by RST messages from the root servers.

Some time later, gmail.com started resolving succesfully again, clearing
up the issue for now.

AFAIK there's nothing in the BIND configs that would force the use of
TCP queries. I checked the docs for various TCP options and didn't see
any applied here. I don't know if the TCP queries are related to the
gmail.com resolution failure but I suspect they are (and in any event
inability to reach root servers is a problem).

This server is authoritative for several domains. It gets its zones from
a hidden primary. The system's firewall permits inbound TCP and UDP
traffic on port 53 and AFAIK does not block outbound UDP (the firewall
is nftables, which is new to me, but since I see UDP queries in the
packet capture I think it works).

What would cause the server to send queries over TCP?

Thanks in advance for troubleshooting clues.


dn



CONFIG FILES

(named.conf is just pointers to .local and .options and .default-zones)

// named.conf.local

acl "xfer" {
// redacted -- a list of IPv4 and IPv6 addresses I trust
};

controls {
        inet 127.0.0.1 port 953 allow { 127.0.0.1; };
};

logging {
	channel simple_log {
		file "/var/log/named/named.log" versions 30 size 1m;
		severity info;
		print-time yes;
		print-severity yes;
		print-category yes;
	};
	category default { simple_log; };
	category update { simple_log; };
	category update-security { simple_log; };
	category security { simple_log; };
	category queries { simple_log; };
	category lame-servers { null; };
};

zone  "example1.org" in {
        type slave;
        file "example1.org.bak";
        masters { 198.18.0.53; }; // not the real address
        allow-query { any; };
        allow-transfer { xfer; };
};

zone  "example2.org" in {
        type slave;
        file "example2.org.bak";
        masters { 198.18.0.53; }; // not the real address
        allow-query { any; };
        allow-transfer { xfer; };
};

// etc.


// named.conf.options

acl "trusted" {

        // redacted -- a list of IPv4 and IPv6 addresses I trust
};

options {
	directory "/var/cache/bind";
	pid-file        "/var/run/named/named.pid";
        statistics-file "/var/run/named/named.stats";
        transfer-format many-answers;
        masterfile-format text;
        max-transfer-time-in 60;
        allow-query { any; };
        allow-recursion { trusted; };
        allow-query-cache { trusted; };
        allow-transfer { xfer; };
        version none;

		disable-empty-zone "255.255.255.255.IN-ADDR.ARPA";
		disable-empty-zone
"0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA";
		disable-empty-zone
"1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA";


	querylog yes;


};

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intermittent failures and queries sent over TCP David Newman <dnewman@networktest.com> - 2020-08-18 17:34 -0700

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