From eb92fb32b746f2104b0f370b5b295bb8dd4bd5e5 Mon Sep 17 00:00:00 2001

From: Simon Kelley <simon@thekelleys.org.uk>

Date: Tue, 7 Mar 2023 22:07:46 +0000

Subject: [PATCH] Set the default maximum DNS UDP packet size to 1232.

http://www.dnsflagday.net/2020/ refers.

Thanks to Xiang Li for the prompt.

---

 CHANGELOG     | 9 ++++++++-

 man/dnsmasq.8 | 3 ++-

 src/config.h  | 2 +-

 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/man/dnsmasq.8 b/man/dnsmasq.8

index 41e2e04..5acb935 100644

--- a/man/dnsmasq.8

+++ b/man/dnsmasq.8

@@ -183,7 +183,8 @@ to zero completely disables DNS function, leaving only DHCP and/or TFTP.

 .TP

 .B \-P, --edns-packet-max=<size>

 Specify the largest EDNS.0 UDP packet which is supported by the DNS

-forwarder. Defaults to 4096, which is the RFC5625-recommended size.

+forwarder. Defaults to 1232, which is the recommended size following the

+DNS flag day in 2020. Only increase if you know what you are doing.

 .TP

 .B \-Q, --query-port=<query_port>

 Send outbound DNS queries from, and listen for their replies on, the

diff --git a/src/config.h b/src/config.h

index 1e7b30f..37b374e 100644

--- a/src/config.h

+++ b/src/config.h

@@ -19,7 +19,7 @@

 #define CHILD_LIFETIME 150 /* secs 'till terminated (RFC1035 suggests > 120s) */

 #define TCP_MAX_QUERIES 100 /* Maximum number of queries per incoming TCP connection */

 #define TCP_BACKLOG 32  /* kernel backlog limit for TCP connections */

-#define EDNS_PKTSZ 4096 /* default max EDNS.0 UDP packet from RFC5625 */

+#define EDNS_PKTSZ 1232 /* default max EDNS.0 UDP packet from from  /dnsflagday.net/2020 */

 #define SAFE_PKTSZ 1232 /* "go anywhere" UDP packet size, see https://dnsflagday.net/2020/ */

 #define KEYBLOCK_LEN 40 /* choose to minimise fragmentation when storing DNSSEC keys */

 #define DNSSEC_WORK 50 /* Max number of queries to validate one question */

-- 

2.33.7