Chapter 17 routing sockets

abstract

• Introduction

• datalink socket address structure

• reading and writing

• sysctl operation

• get_ifi_info function

• interface name and index fuction

Introduction• Access the Unix routing table within the kernel

– ioctl command (SIOCADDRT, SIODELRT)

– netstat: read the kernel memory to obtain the contents of the routing table

• Routing daemons (gated)– monitor ICMP

– AF_ROUTE domain => cleaned up the interface to the kernel’s routing system

– only type of socket supported in the route domain => raw socket

• Three type of operation on routing socket– process can send a message to the kernel by wri

ting to a routing socket(addition and deletion of routes, Only superuser)

– process can read a message from the kernel on a routing socket (Only superuser)

– process can use the sysctl function to either dump the routing table or to list all the configured interface

datalink socket address structure• Returned by routing socket <net/if_dl.h>

Structure sockaddr_dl{uint8_t sdl_len;sa_family_t sdl_family; /*AF_LINK*/uint16_t sdl_index;/*system assigned index*/uint8_t sdl_type; /*IFT_ETHER*/uint8_t sdl_nlen; /*name length*/uint8_t sdl_alen;/*link-layer address length*/uint8_t sdl_slen;/* link-layer selector length */char sdl_data[12];/*minimum work area, name and link-layer address*/};

reading and writing

• After a process create a routing socket, it can send commands to the kernel by writing to the socket and read information from the kernel by reading from the socket

• Three different structure are exchanged across a routing socket– rt_msghdr, if_msghdr, ifa_msghdr (fi

gure 17-3)

Figure 17-2

rt_msghdr{}

rtm _ typ e =R TM _ G E T

destina tionsocke t

add ressstruc tu re

buff er sentto kernel

RTA_DST

rt_msghdr{}

rtm _ typ e =R TM _ G E T

destinationsocket

addressstructure

genmasksocket

addressstructure

netmasksocket

addressstructure

gatewaysocket

addressstructure

buff er returnedby kernel

RTA_DST

RTA_GATEWAY

RTA_NETMASK

RTA_GENMASK

Data exchanged with kernel across routing socket

for RTM_GET command

Figure 17-6#include "unproute.h"

#define BUFLEN (sizeof(struct rt_msghdr) + 512)

/* 8 * sizeof(struct sockaddr_in6) = 192 */

#define SEQ 9999

int main(int argc, char **argv)

{

int sockfd;

char *buf;

pid_t pid;

ssize_t n;

struct rt_msghdr *rtm;

struct sockaddr *sa, *rti_info[RTAX_MAX];

struct sockaddr_in *sin;

Figure 17-6(2)if (argc != 2) err_quit("usage: getrt <IPaddress>");

sockfd = Socket(AF_ROUTE, SOCK_RAW, 0); /* need superuser privileges */

buf = Calloc(1, BUFLEN); /* and initialized to 0 */

rtm = (struct rt_msghdr *) buf;

rtm->rtm_msglen = sizeof(struct rt_msghdr) + sizeof(struct sockaddr_in);

rtm->rtm_version = RTM_VERSION;

rtm->rtm_type = RTM_GET;

rtm->rtm_addrs = RTA_DST;

rtm->rtm_pid = pid = getpid();

rtm->rtm_seq = SEQ;

sin = (struct sockaddr_in *) (rtm + 1);

sin->sin_family = AF_INET;

Inet_pton(AF_INET, argv[1], &sin->sin_addr);

Write(sockfd, rtm, rtm->rtm_msglen);

do {

n = Read(sockfd, rtm, BUFLEN);

} while (rtm->rtm_type != RTM_GET || rtm->rtm_seq != SEQ ||

rtm->rtm_pid != pid);

Figure 17-6(2)rtm = (struct rt_msghdr *) buf;

sa = (struct sockaddr *) (rtm + 1);

get_rtaddrs(rtm->rtm_addrs, sa, rti_info);

if ( (sa = rti_info[RTAX_DST]) != NULL)

printf("dest: %s\n", Sock_ntop_host(sa, sa->sa_len));

if ( (sa = rti_info[RTAX_GATEWAY]) != NULL)

printf("gateway: %s\n", Sock_ntop_host(sa, sa->sa_len));

if ( (sa = rti_info[RTAX_NETMASK]) != NULL)

printf("netmask: %s\n", Sock_masktop(sa, sa->sa_len));

if ( (sa = rti_info[RTAX_GENMASK]) != NULL)

printf("genmask: %s\n", Sock_masktop(sa, sa->sa_len));

exit(0);

}

rt_msghdr{}

rtm _ type =R TM _G E T

destinationsocket

addressstructure

genmasksocket

addressstructure

netmasksocket

addressstructure

gatewaysocket

addressstructure

buff er returnedby kernel

RTA_DST

RTA_GATEWAY

RTA_NETMASK

RTA_GENMASK

rti_ info [ RTAX_DST]

rti_ info [ RTAX_GATEWAY]

rti_ info [ RTAX_NETMASK]

rti_ info [ RTAX_GENMASK]

rti_ info [ RTAX_IFP ]

rti_ info [ RTAX_IFA]

rti_ info [ RTAX_AUTHOR]

rti_ info [ RTAX_BRD]

NULL

NULLNULL

NULL

rti_info structure filled in by our get_rtaddrs function

Figure 17-9#include "unproute.h"

#define ROUNDUP(a, size) (((a) & ((size)-1)) ? (1 + ((a) | ((size)-1))) : (a))

/* Step to next socket address structure;

if sa_len is 0, assume it is sizeof(u_long).

*/

#define NEXT_SA(ap) ap = (struct sockaddr *) \

((caddr_t) ap + (ap->sa_len ? ROUNDUP(ap->sa_len, sizeof (u_long)) : \

sizeof(u_long)))

void get_rtaddrs(int addrs, struct sockaddr *sa, struct sockaddr **rti_info)

{

int i;

for (i = 0; i < RTAX_MAX; i++) {

if (addrs & (1 << i)) {

rti_info[i] = sa;

NEXT_SA(sa);

} else rti_info[i] = NULL;

}

}

sysctl operation

• Any process can examine both the routing table and the interface list.

#include <sys/param.h>#include <sys/sysctl.h>

int sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp,void *newp, size_t newlen); returns:0 if OK, -1 on error

C TL_NETC TL_MAC HDEPC TL_KERNC TL_HWC TL_DEBUG C TL_VF SC TL_USER

AF _INET AF _LINE AF_ROUTE AF _UNSP EC

IP P RO TO _IC MP IP P RO TO _IG MP IP P RO TO _IP IP P RO TO _TC P IP P RO TO _UDP

Hierarchical arrangement of sysctl names

name[ ]

0

1

2

3

4

5

C TL_NET

AF_ROUTE

0

AF_INET

NET_RT_DUMP

0

C TL_NET

AF_ROUTE

0

AF_INET

NET_RT_FLAGS

RTF_LLINFO

C TL_NET

AF_ROUTE

0

AF_INET

NET_RT_IFLIST

0

return IP v4routing table

return IP v4arp cache

returninterface lis t

Sysctl information returned for route domain

if_msghdr{}

ifm_type =RTM_IFINFO

datalinksocket

addressstructure

unicast addrsocket

addressstructure

netmasksocket

addressstructure

ifa_msghdr{}ifam_type =

RTM_NEWADDR

buffer returnedby kernel

broadcast addrsocket

addressstructure

One per interface:

interface name, index, and hardware address

One per address:

configured for the interface

Information returned for sysctl,CTL_NET,NET_RT_IFLIST command

Figure 17-14#include "unproute.h"

#include <netinet/udp.h>

#include <netinet/ip_var.h>

#include <netinet/udp_var.h> /* for UDPCTL_xxx constants */

int main(int argc, char **argv)

{

int mib[5], val;

size_t len;

mib[0] = CTL_NET;

mib[1] = AF_INET;

mib[2] = IPPROTO_UDP;

mib[3] = UDPCTL_CHECKSUM;

len = sizeof(val);

Sysctl(mib, 4, &val, &len, NULL, 0);

printf("udp checksum flag: %d\n", val);

exit(0);

}

interface name and index fuction

• RFC 2133 defines four function that deal with interface names and indexes

==>these are used with IPv6 multicasting(chapter 19)

• each interface has a unique name and a unique positive index(0 is never used )

#include <net/if.h>unsigned int if_nametoindex(const char *ifname); returns:positive interface index if OK, 0 on error

char *if_indextoname(unsigned int ifindex, char *ifname); returns:pointer to interface name if OK, NULL on error

struct if_nameindex *if_nameindex(void); returns:nonnull pointer if OK, NULL on error

void if_freenameindex(struct if_nameindex *ptr);

• if_nameindex return a pointer to an array of if_nameindex structure

Struct if_nameindex{ unsigned int if_index;/*1, 2,…...*/ char *if_name;/*null terminated name*/}