Updating arp table

Many network devices send a gratuitous ARP packet when switching an IP address from one device to another.

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– For example, dst_confirm() calls neigh_confirm(). – dst_confirm() is called from tcp_ack() (net/ipv4/tcp_input.c) – and by udp_sendmsg() (net/ipv4/udp.c) and more.

– struct neigh_table ● struct neigh_table represents a neighboring table – (/include/net/neighbour.h) – The arp table (arp_tbl) is a neigh_table.

(by calling neigh_lookup()) – If we find an entry, we update the arp table by neigh_update().

● If there is no entry and there is NO support for unsolicited ARP we don't create an entry in the arp table.

● We can see the current neighbour states: ● Example : ● 192.168.0.254 dev eth0 lladdr :f1:a REACHABLE 192.168.0.152 dev eth0 lladdr :cc:bb:aa STALE 192.168.0.121 dev eth0 lladdr :1b:1c:14 PERMANENT 192.168.0.54 dev eth0 lladdr aa:ab:ac:ad:ae:af STALE ● == htonl(0x7f000000)) in linux/in.h ● If it is an ARP request (ARPOP_REQUEST) we call ip_route_input(). ● In case it is for us, (RTN_LOCAL) we send and ARP reply.

– arp_send(ARPOP_REPLY, ETH_P_ARP,sip,dev,tip,sha ,dev dev_addr,sha); – We also update our arp table with the sender entry (ip/mac). ● In case we receive an ARP reply – (ARPOP_REPLY) – We perform a lookup in the arp table.This is not the destination or source address on Layer 2 (Ethernet), but the ARP protocol field.In this scenario, you must send the ARP REPLY packet instead of ARP REQUEST.Typically, the switch does not wait for a MAC address expiration when a MAC move operation occurs.As soon as the switch detects the MAC address on the new interface, it immediately updates the table.– A multicast is sent in IPv6 (and not a broadcast).● If there is no answer in time to this arp request, then we will end up with sending back an ICMP error (Destination Host Unreachable).To resolve this issue, run the following command from the Net Scaler CLI on both Net Scaler appliances:set network L2param –garp Reply enabled Note: You must run this command on both the primary and secondary appliance in an high availability setup.Refer to Citrix e Docs - L2Param Command for more information.However, sometimes silent devices, such as SYSLOG servers or SNMP Trap receivers that receive UDP traffic but do not return acknowledgement (ACK ) messages to the traffic source, do not send gratuitous ARP packets when a device moves.If such a move occurs when the system administrator is not available to explicitly clear the affected interfaces by issuing the The ARP entry and the MAC address entry for the moved device expire within the times specified by the aging timer values.

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