Chapter 17 routing sockets

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Chapter 17 routing sockets
2
abstract

• Introduction
• datalink socket address structure
• reading and writing
• sysctl operation
• get_ifi_info function
• interface name and index fuction

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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 gt cleaned up the interface to
  the kernels routing system
• only type of socket supported in the route domain
  gt raw socket

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

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datalink socket address structure

• Returned by routing socket ltnet/if_dl.hgt

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_data12/minimum work area,
name and
link-layer address/
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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 (figure 17-3)

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Figure 17-2
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Data exchanged with kernel across routing socket
for RTM_GET command
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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_infoRTAX_MAX
• struct sockaddr_in sin

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Figure 17-6(2)

• if (argc ! 2) err_quit("usage getrt
  ltIPaddressgt")
• sockfd Socket(AF_ROUTE, SOCK_RAW, 0) / need
  superuser privileges /
• buf Calloc(1, BUFLEN) / and initialized to 0
  /
• rtm (struct rt_msghdr ) buf
• rtm-gtrtm_msglen sizeof(struct rt_msghdr)
  sizeof(struct sockaddr_in)
• rtm-gtrtm_version RTM_VERSION
• rtm-gtrtm_type RTM_GET
• rtm-gtrtm_addrs RTA_DST
• rtm-gtrtm_pid pid getpid()
• rtm-gtrtm_seq SEQ
• sin (struct sockaddr_in ) (rtm 1)
• sin-gtsin_family AF_INET
• Inet_pton(AF_INET, argv1,
  sin-gtsin_addr)
• Write(sockfd, rtm, rtm-gtrtm_msglen)
• do
• n Read(sockfd, rtm, BUFLEN)
• while (rtm-gtrtm_type ! RTM_GET
  rtm-gtrtm_seq ! SEQ

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Figure 17-6(2)

• rtm (struct rt_msghdr ) buf
• sa (struct sockaddr ) (rtm 1)
• get_rtaddrs(rtm-gtrtm_addrs, sa, rti_info)
• if ( (sa rti_infoRTAX_DST) ! NULL)
• printf("dest s\n", Sock_ntop_host(sa,
  sa-gtsa_len))
• if ( (sa rti_infoRTAX_GATEWAY) ! NULL)
• printf("gateway s\n", Sock_ntop_host(sa,
  sa-gtsa_len))
• if ( (sa rti_infoRTAX_NETMASK) ! NULL)
• printf("netmask s\n", Sock_masktop(sa,
  sa-gtsa_len))
• if ( (sa rti_infoRTAX_GENMASK) ! NULL)
• printf("genmask s\n", Sock_masktop(sa,
  sa-gtsa_len))
• exit(0)

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rti_info structure filled in by our get_rtaddrs
function
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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-gtsa_len ? ROUNDUP(ap-gtsa_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 lt RTAX_MAX i)
• if (addrs (1 ltlt i))
• rti_infoi sa
• NEXT_SA(sa)
• else rti_infoi NULL

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sysctl operation

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

include ltsys/param.hgt include
ltsys/sysctl.hgt int sysctl(int name, u_int
namelen, void oldp, size_t oldlenp, void newp,
size_t newlen)
returns0 if OK, -1 on error
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Hierarchical arrangement of sysctl names
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Sysctl information returned for route domain
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Information returned for sysctl,CTL_NET,NET_RT_IFL
IST command
One per interface interface name, index, and
hardware address
One per address configured for the interface
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Figure 17-14

• include "unproute.h"
• include ltnetinet/udp.hgt
• include ltnetinet/ip_var.hgt
• include ltnetinet/udp_var.hgt / for UDPCTL_xxx
  constants /
• int main(int argc, char argv)
• int mib5, val
• size_t len
• mib0 CTL_NET
• mib1 AF_INET
• mib2 IPPROTO_UDP
• mib3 UDPCTL_CHECKSUM
• len sizeof(val)
• Sysctl(mib, 4, val, len, NULL, 0)
• printf("udp checksum flag d\n", val)
• exit(0)

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interface name and index fuction

• RFC 2133 defines four function that deal with
  interface names and indexes gtthese are used
  with IPv6 multicasting(chapter 19)
• each interface has a unique name and a unique
  positive index(0 is never used )

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include ltnet/if.hgt unsigned int
if_nametoindex(const char ifname)
returnspositive
interface index if OK, 0 on error char
if_indextoname(unsigned int ifindex, char
ifname)
returnspointer to interface name if OK, NULL on
error struct if_nameindex if_nameindex(void)

returnsnonnull pointer if OK, NULL on error void
if_freenameindex(struct if_nameindex ptr)
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• 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/