From db63b4b63158d95399fa9109b9802b195df515f8 Mon Sep 17 00:00:00 2001
From: Otto <otto.moerbeek@open-xchange.com>
Date: Tue, 6 Apr 2021 10:41:39 +0200
Subject: [PATCH] Also disable PMTU for v6

---
 pdns/dnsdist.cc       |  4 ++--
 pdns/iputils.cc       | 41 ++++++++++++++++++++++++++++++++---------
 pdns/iputils.hh       |  2 +-
 pdns/nameserver.cc    | 12 +++++-------
 pdns/pdns_recursor.cc | 12 +++++-------
 5 files changed, 45 insertions(+), 26 deletions(-)

diff --git a/pdns/dnsdist.cc b/pdns/dnsdist.cc
index 5eee451c37..f474c79c91 100644
--- a/pdns/dnsdist.cc
+++ b/pdns/dnsdist.cc
@@ -1903,9 +1903,9 @@ static void setUpLocalBind(std::unique_ptr<ClientState>& cs)
   /* Only set this on IPv4 UDP sockets.
      Don't set it for DNSCrypt binds. DNSCrypt pads queries for privacy
      purposes, so we do receive large, sometimes fragmented datagrams. */
-  if (!cs->tcp && cs->local.isIPv4() && !cs->dnscryptCtx) {
+  if (!cs->tcp && !cs->dnscryptCtx) {
     try {
-      setSocketIgnorePMTU(cs->udpFD);
+      setSocketIgnorePMTU(cs->udpFD, cs->local.sin4.sin_family);
     }
     catch(const std::exception& e) {
       warnlog("Failed to set IP_MTU_DISCOVER on UDP server socket for local address '%s': %s", cs->local.toStringWithPort(), e.what());
diff --git a/pdns/iputils.cc b/pdns/iputils.cc
index 019eff2dd2..216945b7cc 100644
--- a/pdns/iputils.cc
+++ b/pdns/iputils.cc
@@ -136,28 +136,51 @@ int SSetsockopt(int sockfd, int level, int opname, int value)
   return ret;
 }
 
-void setSocketIgnorePMTU(int sockfd)
+void setSocketIgnorePMTU(int sockfd, int family)
 {
+  if (family == AF_INET) {
 #if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT)
 #ifdef IP_PMTUDISC_OMIT
   /* Linux 3.15+ has IP_PMTUDISC_OMIT, which discards PMTU information to prevent
      poisoning, but still allows fragmentation if the packet size exceeds the
      outgoing interface MTU, which is good.
   */
-  try {
-    SSetsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_OMIT);
-    return;
-  }
-  catch(const std::exception& e) {
-    /* failed, let's try IP_PMTUDISC_DONT instead */
-  }
+    try {
+      SSetsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_OMIT);
+      return;
+    }
+    catch(const std::exception& e) {
+      /* failed, let's try IP_PMTUDISC_DONT instead */
+    }
 #endif /* IP_PMTUDISC_OMIT */
 
   /* IP_PMTUDISC_DONT disables Path MTU discovery */
-  SSetsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DONT);
+    SSetsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DONT);
 #endif /* defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT) */
+  }
+  else {
+    #if defined(IPV6_MTU_DISCOVER) && defined(IPV6_PMTUDISC_DONT)
+#ifdef IPV6_PMTUDISC_OMIT
+  /* Linux 3.15+ has IPV6_PMTUDISC_OMIT, which discards PMTU information to prevent
+     poisoning, but still allows fragmentation if the packet size exceeds the
+     outgoing interface MTU, which is good.
+  */
+    try {
+      SSetsockopt(sockfd, IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_OMIT);
+      return;
+    }
+    catch(const std::exception& e) {
+      /* failed, let's try IP_PMTUDISC_DONT instead */
+    }
+#endif /* IPV6_PMTUDISC_OMIT */
+
+  /* IPV6_PMTUDISC_DONT disables Path MTU discovery */
+    SSetsockopt(sockfd, IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_DONT);
+#endif /* defined(IPV6_MTU_DISCOVER) && defined(IPV6_PMTUDISC_DONT) */
+  }
 }
 
+
 bool setReusePort(int sockfd)
 {
 #if defined(SO_REUSEPORT_LB)
diff --git a/pdns/iputils.hh b/pdns/iputils.hh
index b0999f01ff..92055f8c2c 100644
--- a/pdns/iputils.hh
+++ b/pdns/iputils.hh
@@ -1433,7 +1433,7 @@ int SBind(int sockfd, const ComboAddress& local);
 int SAccept(int sockfd, ComboAddress& remote);
 int SListen(int sockfd, int limit);
 int SSetsockopt(int sockfd, int level, int opname, int value);
-void setSocketIgnorePMTU(int sockfd);
+void setSocketIgnorePMTU(int sockfd, int family);
 bool setReusePort(int sockfd);
 
 #if defined(IP_PKTINFO)
diff --git a/pdns/nameserver.cc b/pdns/nameserver.cc
index a09953e635..3f03eb3ff5 100644
--- a/pdns/nameserver.cc
+++ b/pdns/nameserver.cc
@@ -128,13 +128,11 @@ void UDPNameserver::bindAddresses()
     if (!setSocketTimestamps(s))
       g_log<<Logger::Warning<<"Unable to enable timestamp reporting for socket "<<locala.toStringWithPort()<<endl;
 
-    if (locala.isIPv4()) {
-      try {
-        setSocketIgnorePMTU(s);
-      }
-      catch(const std::exception& e) {
-        g_log<<Logger::Warning<<"Failed to set IP_MTU_DISCOVER on UDP server socket: "<<e.what()<<endl;
-      }
+    try {
+      setSocketIgnorePMTU(s, locala.sin4.sin_family);
+    }
+    catch(const std::exception& e) {
+      g_log<<Logger::Warning<<"Failed to set IP_MTU_DISCOVER on UDP server socket: "<<e.what()<<endl;
     }
 
     if (d_can_reuseport) {
diff --git a/pdns/pdns_recursor.cc b/pdns/pdns_recursor.cc
index 5d8b3c959f..76330cba71 100644
--- a/pdns/pdns_recursor.cc
+++ b/pdns/pdns_recursor.cc
@@ -3427,13 +3427,11 @@ static void makeUDPServerSockets(deferredAdd_t& deferredAdds)
 #endif
     }
 
-    if (sin.isIPv4()) {
-      try {
-        setSocketIgnorePMTU(fd);
-      }
-      catch(const std::exception& e) {
-        g_log<<Logger::Warning<<"Failed to set IP_MTU_DISCOVER on UDP server socket: "<<e.what()<<endl;
-      }
+    try {
+      setSocketIgnorePMTU(fd, sin.sin4.sin_family);
+    }
+    catch(const std::exception& e) {
+      g_log<<Logger::Warning<<"Failed to set IP_MTU_DISCOVER on UDP server socket: "<<e.what()<<endl;
     }
 
     socklen_t socklen=sin.getSocklen();
-- 
2.47.2