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3.11 if_attach Function

The three interface initialization functions shown earlier each call if_attach to complete initialization of the interface's ifnet structure and to insert the structure on the list of previously configured interfaces. Also, in if_attach, the kernel initializes and assigns each interface a link-level address. Figure 3.32 illustrates the data structures constructed by if_attach.

03fig32.gifFigure 3.32. ifnet list.

In Figure 3.32, if_attach has been called three times: from leattach with an le_softc structure, from slattach with an sl_softc structure, and from loopattach with a generic ifnet structure. Each time it is called it adds another ifnet structure to the ifnet list, creates a link-level ifaddr structure for the interface (which contains two sockaddr_d1 structures, Figure 3.33), and initializes an entry in the ifnet_addrs array.

03fig33.gifFigure 3.33. sockaddr_dl structure.

The structures contained within le_softc[0] and sl_softc[0] are nested as shown in Figure 3.20.

After this initialization, the interfaces are configured only with link-level addresses. IP addresses, for example, are not configured until much later by the ifconfig program (Section 6.6).

The link-level address contains a logical address for the interface and a hardware address if supported by the network (e.g., a 48-bit Ethernet address for le0). The hardware address is used by ARP and the OSI protocols, while the logical address within a sockaddr_dl contains a name and numeric index for the interface within the kernel, which supports a table lookup for converting between an interface index and the associated ifaddr structure (ifa_ifwithnet, Figure 6.32).

The sockaddr_dl structure is shown in Figure 3.33.


Recall from Figure 3.18 that sdl_len specifies the length of the entire address and sdl_family specifies the address family, in this case AF_LINK.


sdl_index identifies the interface within the kernel. In Figure 3.32 the Ethernet interface would have an index of 1, the SLIP interface an index of 2, and the loopback interface an index of 3. The global integer if_index contains the last index assigned by the kernel.


sdl_type is initialized from the if_type member of the ifnet structure associated with this datalink address.


In addition to a numeric index, each interface has a text name formed from the if_name and if_unit members of the ifnet structure. For example, the first SLIP interface is called "sl0" and the second is called "sl1". The text name is stored at the front of the sdl_data array, and sdl_nlen is the length of this name in bytes (3 in our SLIP example).

The datalink address is also stored in the structure. The macro LLADDR converts a pointer to a sockaddr_dl structure into a pointer to the first byte beyond the text name. sdl_alen is the length of the hardware address. For an Ethernet device, the 48-bit hardware address appears in the sockaddr_dl structure beyond the text name. Figure 3.38 shows an initialized sockaddr_dl structure.

Net/3 does not use sdl_slen.

if_attach updates two global variables. The first, if_index, holds the index of the last interface in the system and the second, ifnet_addrs, points to an array of ifaddr pointers. Each entry in the array points to the link-level address of an interface. The array provides quick access to the link-level address for every interface in the system.

The if_attach function is long and consists of several tricky assignment statements. We describe it in four parts, starting with Figure 3.34.

03fig34.gifFigure 3.34. if_attach function: assign interface index.


if_attach has a single argument, ifp, a pointer to the ifnet structure that has been initialized by a network device driver. Net/3 keeps all the ifnet structures on a linked list headed by the global pointer ifnet. The while loop locates the end of the list and saves the address of the null pointer at the end of the list in p. After the loop, the new ifnet structure is attached to the end of the ifnet list, if_index is incremented, and the new index is assigned to ifp->if_index.

Resize ifnet_addrs array if necessary


The first time through if_attach, the ifnet_addrs array doesn't exist so space for 16 entries (16 = 8 << 1) is allocated. When the array becomes full, a new array of twice the size is allocated and the entries from the old array are copied to the new array.

if_indexlim is a static variable private to if_attach. if_indexlim is updated by the <<= operator.

The malloc and free functions in Figure 3.34 are not the standard C library functions of the same name. The second argument in the kernel versions specifies a type, which is used by optional diagnostic code in the kernel to detect programming errors. If the third argument to malloc is M_WAITOK, the function blocks the calling process if it needs to wait for free memory to become available. If the third argument is M_DONTWAIT, the function does not block and returns a null pointer when no memory is available.

The next section of if_attach, shown in Figure 3.35, prepares a text name for the interface and computes the size of the link-level address.

03fig35.gifFigure 3.35. if_attach function: compute size of link-level address.

Create link-level name and compute size of link-level address


if_attach constructs the name of the interface from if_unit and if_name. The function sprint_d converts the numeric value of if_unit to a string stored in workbuf. masklen is the number of bytes occupied by the information before sdl_data in the sockaddr_dl array plus the size of the text name for the interface (namelen + unitlen). The function rounds socksize, which is masklen plus the hardware address length (if_addrlen), up to the boundary of a long integer (ROUNDUP). If this is less than the size of a sockaddr_dl structure, the standard sockaddr_dl structure is used, ifasize is the size of an ifaddr structure plus two times socksize, so it can hold the sockaddr_dl structures.

In the next section, if_attach allocates and links the structures together, as shown in Figure 3.36.

03fig36.gifFigure 3.36. The link-level address and mask assigned during if_attach.

In Figure 3.36 there is a gap between the ifaddr structure and the two sockaddr_dl structures to illustrate that they are allocated in a contiguous area of memory but that they are not defined by a single C structure.

The organization shown in Figure 3.36 is repeated in the in_ifaddr structure; the pointers in the generic ifaddr portion of the structure point to specialized sockaddr structures allocated in the device-specific portion of the structure, in this case, sockaddr_dl structures. Figure 3.37 shows the initialization of these structures.

03fig37.gifFigure 3.37. if_attach function: allocate and initialize link-level address.

The address


If enough memory is available, bzero fills the new structure with 0s and sdl points to the first sockaddr_dl just after the ifaddr structure. If no memory is available, the code is skipped.

sdl_len is set to the length of the sockaddr_dl structure, and sdl_family is set to AF_LINK. A text name is constructed within sdl_data from if_name and unitname, and the length is saved in sdl_nlen. The interface's index is copied into sdl_index as well as the interface type into sdl_type. The allocated structure is inserted into the ifnet_addrs array and linked to the ifnet structure by ifa_ifp and if_addrlist. Finally, the sockaddr_dl structure is connected to the ifnet structure with ifa_addr. Ethernet interfaces replace the default function, link_rtrequest with arp_rtrequest. The loopback interface installs loop_rtrequest. We describe ifa_rtrequest and arp_rtrequest in Chapters 19 and 21. link_rtrequest and loop_rtrequest are left for readers to investigate on their own. This completes the initialization of the first sockaddr_dl structure.

The mask


The second sockaddr_dl structure is a bit mask that selects the text name that appears in the first structure. ifa_netmask from the ifaddr structure points to the mask structure (which in this case selects the interface text name and not a network mask). The while loop turns on the bits in the bytes corresponding to the name.

Figure 3.38 shows the two initialized sockaddr_dl structures for our example Ethernet interface, where if_name is "le", if_unit is 0, and if_index is 1.

03fig16.gifFigure 3.16. The initialized Ethernet sockaddr_dl structures (sdl_ prefix omitted).

In Figure 3.38, the address is shown after ether_ifattach has done additional initialization of the structure (Figure 3.41).

Figure 3.39 shows the structures after the first interface has been attached by if_attach.

03fig39.gifFigure 3.39. The ifaddr and sockaddr_dl structures after if_attach is called for the first time.

At the end of if_attach, the ether_ifattach function is called for Ethernet devices, as shown in Figure 3.40.

03fig40.gifFigure 3.40. if_attach function: Ethernet initialization.

03fig41.gifFigure 3.41. ether_ifattach function.


ether_ifattach isn't called earlier (from leattach, for example) because it copies the Ethernet hardware address into the sockaddr_dl allocated by if_attach.

The XXX comment indicates that the author found it easier to insert the code here once than to modify all the Ethernet drivers.

ether_ifattach function

The ether_ifattach function performs the ifnet structure initialization common to all Ethernet devices.


For an Ethernet device, if_type is IFT_ETHER, the hardware address is 6 bytes long, the entire Ethernet header is 14 bytes in length, and the Ethernet MTU is 1500 (ETHERMTU).

The MTU was already assigned by leattach, but other Ethernet device drivers may not have performed this initialization.

Section 4.3 discusses the Ethernet frame organization in more detail. The for loop locates the link-level address for the interface and then initializes the Ethernet hardware address information in the sockaddr_dl structure. The Ethernet address that was copied into the arpcom structure during system initialization is now copied into the link-level address.

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