This change introduces a new sysctl parameter, arp_evict_nocarrier. When set (default) the ARP cache will be cleared on a NOCARRIER event. This new option has been defaulted to '1' which maintains existing behavior. Clearing the ARP cache on NOCARRIER is relatively new, introduced by: commit 859bd2ef1fc1110a8031b967ee656c53a6260a76 Author: David Ahern <dsahern@gmail.com> Date: Thu Oct 11 20:33:49 2018 -0700 net: Evict neighbor entries on carrier down The reason for this changes is to prevent the ARP cache from being cleared when a wireless device roams. Specifically for wireless roams the ARP cache should not be cleared because the underlying network has not changed. Clearing the ARP cache in this case can introduce significant delays sending out packets after a roam. A user reported such a situation here: https://lore.kernel.org/linux-wireless/CACsRnHWa47zpx3D1oDq9JYnZWniS8yBwW1h0WAVZ6vrbwL_S0w@mail.gmail.com/ After some investigation it was found that the kernel was holding onto packets until ARP finished which resulted in this 1 second delay. It was also found that the first ARP who-has was never responded to, which is actually what caues the delay. This change is more or less working around this behavior, but again, there is no reason to clear the cache on a roam anyways. As for the unanswered who-has, we know the packet made it OTA since it was seen while monitoring. Why it never received a response is unknown. In any case, since this is a problem on the AP side of things all that can be done is to work around it until it is solved. Some background on testing/reproducing the packet delay: Hardware: - 2 access points configured for Fast BSS Transition (Though I don't see why regular reassociation wouldn't have the same behavior) - Wireless station running IWD as supplicant - A device on network able to respond to pings (I used one of the APs) Procedure: - Connect to first AP - Ping once to establish an ARP entry - Start a tcpdump - Roam to second AP - Wait for operstate UP event, and note the timestamp - Start pinging Results: Below is the tcpdump after UP. It was recorded the interface went UP at 10:42:01.432875. 10:42:01.461871 ARP, Request who-has 192.168.254.1 tell 192.168.254.71, length 28 10:42:02.497976 ARP, Request who-has 192.168.254.1 tell 192.168.254.71, length 28 10:42:02.507162 ARP, Reply 192.168.254.1 is-at ac:86:74:55:b0:20, length 46 10:42:02.507185 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 1, length 64 10:42:02.507205 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 2, length 64 10:42:02.507212 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 3, length 64 10:42:02.507219 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 4, length 64 10:42:02.507225 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 5, length 64 10:42:02.507232 IP 192.168.254.71 > 192.168.254.1: ICMP echo request, id 52792, seq 6, length 64 10:42:02.515373 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 1, length 64 10:42:02.521399 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 2, length 64 10:42:02.521612 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 3, length 64 10:42:02.521941 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 4, length 64 10:42:02.522419 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 5, length 64 10:42:02.523085 IP 192.168.254.1 > 192.168.254.71: ICMP echo reply, id 52792, seq 6, length 64 You can see the first ARP who-has went out very quickly after UP, but was never responded to. Nearly a second later the kernel retries and gets a response. Only then do the ping packets go out. If an ARP entry is manually added prior to UP (after the cache is cleared) it is seen that the first ping is never responded to, so its not only an issue with ARP but with data packets in general. As mentioned prior, the wireless interface was also monitored to verify the ping/ARP packet made it OTA which was observed to be true.
Unified · Split
| @@ -1611,6 +1611,15 @@ arp_accept - BOOLEAN · 1610 unmodified lines | |||
| 1611 | gratuitous arp frame, the arp table will be updated regardless | 1611 | gratuitous arp frame, the arp table will be updated regardless |
| 1612 | if this setting is on or off. | 1612 | if this setting is on or off. |
| 1613 | 1613 | ||
| 1614 | arp_evict_nocarrier - BOOLEAN | ||
| 1615 | Clears the ARP cache on NOCARRIER events. This option is important for | ||
| 1616 | wireless devices where the ARP cache should not be cleared when roaming | ||
| 1617 | between access points on the same network. In most cases this should | ||
| 1618 | remain as the default (1). | ||
| 1619 | |||
| 1620 | - 1 - (default): Clear the ARP cache on NOCARRIER events | ||
| 1621 | - 0 - Do not clear ARP cache on NOCARRIER events | ||
| 1622 | |||
| 1614 | mcast_solicit - INTEGER | 1623 | mcast_solicit - INTEGER |
| 1615 | The maximum number of multicast probes in INCOMPLETE state, | 1624 | The maximum number of multicast probes in INCOMPLETE state, |
| 1616 | when the associated hardware address is unknown. Defaults | 1625 | when the associated hardware address is unknown. Defaults |
| @@ -133,6 +133,8 @@ static inline void ipv4_devconf_setall(struct in_device *in_dev) · 132 unmodified lines | |||
| 133 | #define IN_DEV_ARP_ANNOUNCE(in_dev) IN_DEV_MAXCONF((in_dev), ARP_ANNOUNCE) | 133 | #define IN_DEV_ARP_ANNOUNCE(in_dev) IN_DEV_MAXCONF((in_dev), ARP_ANNOUNCE) |
| 134 | #define IN_DEV_ARP_IGNORE(in_dev) IN_DEV_MAXCONF((in_dev), ARP_IGNORE) | 134 | #define IN_DEV_ARP_IGNORE(in_dev) IN_DEV_MAXCONF((in_dev), ARP_IGNORE) |
| 135 | #define IN_DEV_ARP_NOTIFY(in_dev) IN_DEV_MAXCONF((in_dev), ARP_NOTIFY) | 135 | #define IN_DEV_ARP_NOTIFY(in_dev) IN_DEV_MAXCONF((in_dev), ARP_NOTIFY) |
| 136 | #define IN_DEV_ARP_EVICT_NOCARRIER(in_dev) IN_DEV_ANDCONF((in_dev), \ | ||
| 137 | ARP_EVICT_NOCARRIER) | ||
| 136 | 138 | ||
| 137 | struct in_ifaddr { | 139 | struct in_ifaddr { |
| 138 | struct hlist_node hash; | 140 | struct hlist_node hash; |
| @@ -169,6 +169,7 @@ enum · 168 unmodified lines | |||
| 169 | IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST, | 169 | IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST, |
| 170 | IPV4_DEVCONF_DROP_GRATUITOUS_ARP, | 170 | IPV4_DEVCONF_DROP_GRATUITOUS_ARP, |
| 171 | IPV4_DEVCONF_BC_FORWARDING, | 171 | IPV4_DEVCONF_BC_FORWARDING, |
| 172 | IPV4_DEVCONF_ARP_EVICT_NOCARRIER, | ||
| 172 | __IPV4_DEVCONF_MAX | 173 | __IPV4_DEVCONF_MAX |
| 173 | }; | 174 | }; |
| 174 | 175 | ||
| @@ -482,6 +482,7 @@ enum · 481 unmodified lines | |||
| 482 | NET_IPV4_CONF_PROMOTE_SECONDARIES=20, | 482 | NET_IPV4_CONF_PROMOTE_SECONDARIES=20, |
| 483 | NET_IPV4_CONF_ARP_ACCEPT=21, | 483 | NET_IPV4_CONF_ARP_ACCEPT=21, |
| 484 | NET_IPV4_CONF_ARP_NOTIFY=22, | 484 | NET_IPV4_CONF_ARP_NOTIFY=22, |
| 485 | NET_IPV4_CONF_ARP_EVICT_NOCARRIER=23, | ||
| 485 | }; | 486 | }; |
| 486 | 487 | ||
| 487 | /* /proc/sys/net/ipv4/netfilter */ | 488 | /* /proc/sys/net/ipv4/netfilter */ |
| @@ -1247,6 +1247,8 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event, · 1246 unmodified lines | |||
| 1247 | { | 1247 | { |
| 1248 | struct net_device *dev = netdev_notifier_info_to_dev(ptr); | 1248 | struct net_device *dev = netdev_notifier_info_to_dev(ptr); |
| 1249 | struct netdev_notifier_change_info *change_info; | 1249 | struct netdev_notifier_change_info *change_info; |
| 1250 | struct in_device *in_dev; | ||
| 1251 | bool evict_nocarrier; | ||
| 1250 | 1252 | ||
| 1251 | switch (event) { | 1253 | switch (event) { |
| 1252 | case NETDEV_CHANGEADDR: | 1254 | case NETDEV_CHANGEADDR: |
| @@ -1257,7 +1259,14 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event, · 4 unmodified lines | |||
| 1257 | change_info = ptr; | 1259 | change_info = ptr; |
| 1258 | if (change_info->flags_changed & IFF_NOARP) | 1260 | if (change_info->flags_changed & IFF_NOARP) |
| 1259 | neigh_changeaddr(&arp_tbl, dev); | 1261 | neigh_changeaddr(&arp_tbl, dev); |
| 1260 | if (!netif_carrier_ok(dev)) | 1262 | |
| 1263 | in_dev = __in_dev_get_rtnl(dev); | ||
| 1264 | if (!in_dev) | ||
| 1265 | evict_nocarrier = true; | ||
| 1266 | else | ||
| 1267 | evict_nocarrier = IN_DEV_ARP_EVICT_NOCARRIER(in_dev); | ||
| 1268 | |||
| 1269 | if (evict_nocarrier && !netif_carrier_ok(dev)) | ||
| 1261 | neigh_carrier_down(&arp_tbl, dev); | 1270 | neigh_carrier_down(&arp_tbl, dev); |
| 1262 | break; | 1271 | break; |
| 1263 | default: | 1272 | default: |
| @@ -75,6 +75,7 @@ static struct ipv4_devconf ipv4_devconf = { · 74 unmodified lines | |||
| 75 | [IPV4_DEVCONF_SHARED_MEDIA - 1] = 1, | 75 | [IPV4_DEVCONF_SHARED_MEDIA - 1] = 1, |
| 76 | [IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/, | 76 | [IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/, |
| 77 | [IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] = 1000 /*ms*/, | 77 | [IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] = 1000 /*ms*/, |
| 78 | [IPV4_DEVCONF_ARP_EVICT_NOCARRIER - 1] = 1, | ||
| 78 | }, | 79 | }, |
| 79 | }; | 80 | }; |
| 80 | 81 | ||
| @@ -87,6 +88,7 @@ static struct ipv4_devconf ipv4_devconf_dflt = { · 6 unmodified lines | |||
| 87 | [IPV4_DEVCONF_ACCEPT_SOURCE_ROUTE - 1] = 1, | 88 | [IPV4_DEVCONF_ACCEPT_SOURCE_ROUTE - 1] = 1, |
| 88 | [IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/, | 89 | [IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/, |
| 89 | [IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] = 1000 /*ms*/, | 90 | [IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] = 1000 /*ms*/, |
| 91 | [IPV4_DEVCONF_ARP_EVICT_NOCARRIER - 1] = 1, | ||
| 90 | }, | 92 | }, |
| 91 | }; | 93 | }; |
| 92 | 94 | ||
| @@ -2532,6 +2534,8 @@ static struct devinet_sysctl_table { · 2439 unmodified lines | |||
| 2532 | DEVINET_SYSCTL_RW_ENTRY(ARP_IGNORE, "arp_ignore"), | 2534 | DEVINET_SYSCTL_RW_ENTRY(ARP_IGNORE, "arp_ignore"), |
| 2533 | DEVINET_SYSCTL_RW_ENTRY(ARP_ACCEPT, "arp_accept"), | 2535 | DEVINET_SYSCTL_RW_ENTRY(ARP_ACCEPT, "arp_accept"), |
| 2534 | DEVINET_SYSCTL_RW_ENTRY(ARP_NOTIFY, "arp_notify"), | 2536 | DEVINET_SYSCTL_RW_ENTRY(ARP_NOTIFY, "arp_notify"), |
| 2537 | DEVINET_SYSCTL_RW_ENTRY(ARP_EVICT_NOCARRIER, | ||
| 2538 | "arp_evict_nocarrier"), | ||
| 2535 | DEVINET_SYSCTL_RW_ENTRY(PROXY_ARP_PVLAN, "proxy_arp_pvlan"), | 2539 | DEVINET_SYSCTL_RW_ENTRY(PROXY_ARP_PVLAN, "proxy_arp_pvlan"), |
| 2536 | DEVINET_SYSCTL_RW_ENTRY(FORCE_IGMP_VERSION, | 2540 | DEVINET_SYSCTL_RW_ENTRY(FORCE_IGMP_VERSION, |
| 2537 | "force_igmp_version"), | 2541 | "force_igmp_version"), |