Where Are Mac Addresses Stored For Future Reference?

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The class thread represents a single thread of execution.Threads allow multiple functions to execute concurrently. Threads begin execution immediately upon construction of the associated thread object (pending any OS scheduling delays), starting at the top-level function provided as a constructor argument. MAC addresses are primarily assigned by device manufacturers, and are therefore often referred to as the burned-in address, or as an Ethernet hardware address, hardware address, or physical address. Each address can be stored in hardware, such as the card's read-only memory, or by a firmware mechanism. 'Where are MAC addresses stored for future reference? - MAC cache - Ethernet cache - ARP cache - NIC '- ARP cache. The physical machine address is also known as a Media Access Control or MAC address. The job of the ARP is essentially to translate 32-bit addresses to 48-bit addresses and vice-versa. This is necessary because in IP Version 4 (IPv4), the most common level of Internet Protocol ( IP ) in use today, an IP address is 32-bits long, but MAC. Where are MAC addresses stored for future reference? Attacker substituted invalid MAC address for network gateway-no users can access ext.

Where Are Mac Addresses Stored For Future Reference?Are

The socket buffer, or 'SKB', is the most fundamental datastructure in the Linux networking code. Every packet sentor received is handled using this data structure.

The most fundamental parts of the SKB structure are as follows:

The first two members implement list handling. Packets canexist on several kinds of lists and queues. For example, aTCP socket send queue. The third member says which list thepacket is on. Learn more about SKB list handlinghere.

This is where we record the socket assosciated with this SKB. When apacket is sent or received for a socket, the memory assosciated withthe packet must be charged to the socket for proper memory accounting.Read more about socket packet buffer memory accountinghere.

Here we record the timestamp for the packet, either when it arrivedor when it was sent. Calculating this is somewhat expensive, so thisvalue is only recorded if necessary. When something happens thatrequires that we start recording timestamps, net_enable_timestamp()is called. If that need goes away, net_disable_timestamp() is called.

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Timestamps are mostly used to packet sniffers. But they are also usedto implement certain socket options, and also some netfilter modulesmake use of this value as well.

These three members help keep track of the devices assosciatedwith a packet. The reason we have three different device pointersis that the main 'skb->dev' member can change as we encapsulateand decapsulate via a virtual device.

So if we are receiving a packet from a device which is part of abonding device instance, initially 'skb->dev' will be set to pointthe real underlying bonding slave. When the packet enters thenetworking (via 'netif_receive_skb()') we save 'skb->dev' awayin 'skb->real_dev' and update 'skb->dev' to point to the bondingdevice.

Likewise, the physical device receiving a packet always records itselfin 'skb->input_dev'. In this way, no matter how many layers ofvirtual devices end up being decapsulated, 'skb->input_dev' canalways be used to find the top-level device that actually receivedthis packet from the network.

Here we store the location of the various protocollayer headers as we build outgoing packets, and parseincoming ones. For example, 'skb->mac.raw' is set by'eth_type_trans()', when an eternet packet is received.Later, we can use this to find the location of the MACheader.

These members are potentially redundant, and could be removed.Read a discussion about that here.


This member is the generic route for the packet. It tells us howto get the packet to it's destination. Note that routes are usedfor both input and output. DST entries are about as complex asSKBs are, and thus probably deserve their own tutorial.

Here we store the security path traversed by the packet, if any.For example, on input IPSEC records each transformation whichhas been applied to the packet by a decapsulator. The recordsare an array of 'struct sec_decap_state' which each record thesecurity assosciation that matched and got applied. Later, whenwe are trying to validate the security policy against a packet, wemake sure that the transformations applied match the ones allowed bythe policy.

This is the SKB control block. It is an opaque storage area usableby protocols, and even some drivers, to store private per-packetinformation. TCP uses this, for example, to store sequence numbersand retransmission state for the frame.

The three length members are pretty straight-forward. The totalnumber of bytes in the packet is 'len'. SKBs are composed of alinear data buffer, and optionally a set of 1 or more page buffers.If there are page buffers, the total number of bytes in the pagebuffer area is 'data_len'. Therefore the number of bytes in thelinear buffer is 'skb->len - skb->data_len'. There is a shorthandfunction for this in 'skb_headlen()'.

The 'mac_len' holds the length of the MAC header. Normally, thisisn't really necessary to maintain, except to implement IPSECdecapsulation of IP tunnels properly. This field is initialized onceinside of 'netif_receive_skb()' to the formula'skb->nh.raw - skb->mac.raw'.

Since we only use this for one purpose, with some clever ideas wemay be able to eliminate this member in the future. For example,perhaps we can store the value in the 'struct sec_path'.

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Finally, 'csum' holds the checksum of the packet. When buildingsend packets, we copy the data in from userspace and calculate the16-bit two's complement sum in parallel for performance. This sumis accumulated in 'skb->csum'. This helps us compute the finalchecksum stored in the protocol packet header checksum field.This field can end up being ignored if, for example, the devicewill checksum the packet for us.

On input, the 'csum' field can be used to store a checksum calculatedby the device. If the device indicates 'CHECKSUM_HW' in the SKB'ip_summed' field, this means that 'csum' is the two's complementchecksum of the entire packet data area starting at 'skb->data'.This is generic enough such that both IPV4 and IPV6 checksumoffloading can be supported.

The 'local_df' field is used by the IPV4 protocol, and when setallows us to locally fragment frames which have already beenfragmented. This situation can arise, for example, with IPSEC.

In order to make quick references to SKB data, Linux has the conceptof SKB clones. When a clone of an SKB is made, all of the'struct sk_buff' structure members of the clone are private to theclone. The data, however, is shared between the primary SKB and it'sclone. When an SKB is cloned, the 'cloned' field will be set in boththe primary and clone SKB. Otherwise is will be zero.

Where Are Mac Addresses Stored For Future Reference?

The 'nohdr' field is used in the support of TCP Segmentation Offload('TSO' for short). Most devices supporting this feature need to makesome minor modifications to the TCP and IP headers of an outgoingpacket to get it in the right form for the hardware to process. Wedo not want these modifications to be seen by packet sniffers and thelike. So we use this 'nohdr' field and a special bit in the data areareference count to keep track of whether the device needs to replacethe data area before making the packet header modifications.

The type of the packet (basically, who is it for), is stored in the'pkt_type' field. It takes on one of the 'PACKET_*' values definedin the 'linux/if_packet.h' header file. For example, when an incomingethernet frame is to a destination MAC address matching the MACaddress of the ethernet device it arrived on, this field will be setto 'PACKET_HOST'. When a broadcast frame is received, it will be setto 'PACKET_BROADCAST'. And likewise when a multicast packet isreceived it will be set to 'PACKET_MULTICAST'.

Where Are Mac Addresses Stored For Future Reference?

The 'ip_summed' field describes what kind of checksumming assistencethe card has provided for a receive packet. It takes on one of threevalues: 'CHECKSUM_NONE' if the card provided no checksum assistence,'CHECKSUM_HW' if the two's complement checksum over the entire packethas been provides in 'skb->csum', and 'CHECKSUM_UNNECESSARY' if itis not necessary to verify the checksum of this packet. The latterusually occurs when the packet is received over the loopback device.'CHECKSUM_UNNECESSARY' can also be used when the device only providesa 'checksum OK' indication for receive packet checksum offload.

The 'priority' field is used in the implement of QoS. The packet'svalue of this field can be determined by, for example, the TOS fieldsetting in the IPV4 header. Then, the packet scheduler and classifierlayer can key off of this SKB priority value to schedule or classifythe packet, as configured by the administrator.

Where Are Mac Addresses Stored For Future References

The 'protocol' field is initialized by routines such as'eth_type_trans()'. It takes on one of the 'ETH_P_*' valuesdefined in the 'linux/if_ether.h' header file. Even non-ethernetdevices use these ethernet protocol type values to indicate whatprotocol should receive the packet. As long as we always have someethernet protocol value for each and every protocol, this shouldnot be a problem.

The 'security' field was meant to be used in the implementationof IP Security, but that never materialized. It can probablybe safely removed. Since the next field is a pointer, and thusneeds to be aligned properly, eliminating the 'security' field wouldunfortunately not buy us any space savings.

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The SKB 'destructor' and 'truesize' fields are used for socket bufferaccounting. See the SKB socket accountingpage for details.

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We reference count SKB objects using the 'users' field. Extrareferences can be obtained by invoking 'skb_get()'. An implicitsingle reference is present in the SKB (that is, 'users' has avalue of '1') when it is first allocated. References are droppedby invoking 'kfree_skb()'.

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These four pointers provide the core management of the linearpacket data area of an SKB. SKB data area handling is involvedenough to deserve it's very own tutorial. Check it outhere.