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Disable the Path MTU Discover on UDP Sockets

Browse Source 
Instead of disabling the fragmentation on the UDP sockets, we now
disable the Path MTU Discovery by setting IP(V6)_MTU_DISCOVER socket
option to IP_PMTUDISC_OMIT on Linux and disabling IP(V6)_DONTFRAG socket
option on FreeBSD.  This option sets DF=0 in the IP header and also
ignores the Path MTU Discovery.

As additional mitigation on Linux, we recommend setting
net.ipv4.ip_no_pmtu_disc to Mode 3:

    Mode 3 is a hardend pmtu discover mode. The kernel will only accept
    fragmentation-needed errors if the underlying protocol can verify
    them besides a plain socket lookup. Current protocols for which pmtu
    events will be honored are TCP, SCTP and DCCP as they verify
    e.g. the sequence number or the association. This mode should not be
    enabled globally but is only intended to secure e.g. name servers in
    namespaces where TCP path mtu must still work but path MTU
    information of other protocols should be discarded. If enabled
    globally this mode could break other protocols.
This commit is contained in:
Ondřej Surý
2021-07-28 15:55:46 +02:00
committed by Michał Kępień
parent 69243a0f56
commit 87d5c8ab7c
4 changed files with 22 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
lib/isc/netmgr/netmgr-int.h
View File

@@ -1779,9 +1779,10 @@ isc__nm_socket_incoming_cpu(uv_os_sock_t fd);
*/
isc_result_t
isc__nm_socket_dontfrag(uv_os_sock_t fd, sa_family_t sa_family);
isc__nm_socket_disable_pmtud(uv_os_sock_t fd, sa_family_t sa_family);
/*%<
* Set the SO_IP_DONTFRAG (or equivalent) socket option of the fd if available
* Disable the Path MTU Discovery, either by disabling IP(V6)_DONTFRAG socket
* option, or setting the IP(V6)_MTU_DISCOVER socket option to IP_PMTUDISC_OMIT
*/
isc_result_t

16
lib/isc/netmgr/netmgr.c
View File

@@ -3044,20 +3044,20 @@ isc__nm_socket_incoming_cpu(uv_os_sock_t fd) {
}
isc_result_t
isc__nm_socket_dontfrag(uv_os_sock_t fd, sa_family_t sa_family) {
isc__nm_socket_disable_pmtud(uv_os_sock_t fd, sa_family_t sa_family) {
/*
* Set the Don't Fragment flag on IP packets
* Disable the Path MTU Discovery on IP packets
*/
if (sa_family == AF_INET6) {
#if defined(IPV6_DONTFRAG)
if (setsockopt_on(fd, IPPROTO_IPV6, IPV6_DONTFRAG) == -1) {
if (setsockopt_off(fd, IPPROTO_IPV6, IPV6_DONTFRAG) == -1) {
return (ISC_R_FAILURE);
} else {
return (ISC_R_SUCCESS);
}
#elif defined(IPV6_MTU_DISCOVER)
#elif defined(IPV6_MTU_DISCOVER) && defined(IP_PMTUDISC_OMIT)
if (setsockopt(fd, IPPROTO_IPV6, IPV6_MTU_DISCOVER,
&(int){ IP_PMTUDISC_DO }, sizeof(int)) == -1)
&(int){ IP_PMTUDISC_OMIT }, sizeof(int)) == -1)
{
return (ISC_R_FAILURE);
} else {
@@ -3068,14 +3068,14 @@ isc__nm_socket_dontfrag(uv_os_sock_t fd, sa_family_t sa_family) {
#endif
} else if (sa_family == AF_INET) {
#if defined(IP_DONTFRAG)
if (setsockopt_on(fd, IPPROTO_IP, IP_DONTFRAG) == -1) {
if (setsockopt_off(fd, IPPROTO_IP, IP_DONTFRAG) == -1) {
return (ISC_R_FAILURE);
} else {
return (ISC_R_SUCCESS);
}
#elif defined(IP_MTU_DISCOVER)
#elif defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_OMIT)
if (setsockopt(fd, IPPROTO_IP, IP_MTU_DISCOVER,
&(int){ IP_PMTUDISC_DO }, sizeof(int)) == -1)
&(int){ IP_PMTUDISC_OMIT }, sizeof(int)) == -1)
{
return (ISC_R_FAILURE);
} else {

4
lib/isc/netmgr/udp.c
View File

@@ -65,7 +65,7 @@ isc__nm_udp_lb_socket(sa_family_t sa_family) {
RUNTIME_CHECK(result == ISC_R_SUCCESS);
(void)isc__nm_socket_incoming_cpu(sock);
(void)isc__nm_socket_dontfrag(sock, sa_family);
(void)isc__nm_socket_disable_pmtud(sock, sa_family);
result = isc__nm_socket_reuse(sock);
RUNTIME_CHECK(result == ISC_R_SUCCESS);
@@ -760,7 +760,7 @@ isc_nm_udpconnect(isc_nm_t *mgr, isc_sockaddr_t *local, isc_sockaddr_t *peer,
(void)isc__nm_socket_incoming_cpu(sock->fd);
(void)isc__nm_socket_dontfrag(sock->fd, sa_family);
(void)isc__nm_socket_disable_pmtud(sock->fd, sa_family);
event = isc__nm_get_netievent_udpconnect(mgr, sock, req);

18
lib/isc/socket.c
View File

@@ -2098,27 +2098,27 @@ set_tcp_maxseg(isc_socket_t *sock, int size) {
}
static void
set_ip_dontfrag(isc_socket_t *sock) {
set_ip_disable_pmtud(isc_socket_t *sock) {
/*
* Set the Don't Fragment flag on IP packets
* Disable Path MTU Discover on IP packets
*/
if (sock->pf == AF_INET6) {
#if defined(IPV6_DONTFRAG)
(void)setsockopt(sock->fd, IPPROTO_IPV6, IPV6_DONTFRAG,
&(int){ 1 }, sizeof(int));
&(int){ 0 }, sizeof(int));
#endif
#if defined(IPV6_MTU_DISCOVER)
#if defined(IPV6_MTU_DISCOVER) && defined(IP_PMTUDISC_OMIT)
(void)setsockopt(sock->fd, IPPROTO_IPV6, IPV6_MTU_DISCOVER,
&(int){ IP_PMTUDISC_DO }, sizeof(int));
&(int){ IP_PMTUDISC_OMIT }, sizeof(int));
#endif
} else if (sock->pf == AF_INET) {
#if defined(IP_DONTFRAG)
(void)setsockopt(sock->fd, IPPROTO_IP, IP_DONTFRAG, &(int){ 1 },
(void)setsockopt(sock->fd, IPPROTO_IP, IP_DONTFRAG, &(int){ 0 },
sizeof(int));
#endif
#if defined(IP_MTU_DISCOVER)
#if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_OMIT)
(void)setsockopt(sock->fd, IPPROTO_IP, IP_MTU_DISCOVER,
&(int){ IP_PMTUDISC_DO }, sizeof(int));
&(int){ IP_PMTUDISC_OMIT }, sizeof(int));
#endif
}
}
@@ -2409,7 +2409,7 @@ again:
#endif /* ifdef IP_RECVTOS */
#endif /* defined(USE_CMSG) || defined(SET_RCVBUF) || defined(SET_SNDBUF) */
set_ip_dontfrag(sock);
set_ip_disable_pmtud(sock);
setup_done:
inc_stats(manager->stats, sock->statsindex[STATID_OPEN]);