8.12 dg_cli Function
(Revisited)
We now return to the dg_cli function
from Figure 8.8 and recode
it to call connect. Figure 8.17 shows the new function.
Figure 8.17
dg_cli function that calls connect.
udpcliserv/dgcliconnect.c
1 #include "unp.h"
2 void
3 dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen)
4 {
5 int n;
6 char sendline[MAXLINE], recvline[MAXLINE + 1];
7 Connect(sockfd, (SA *) pservaddr, servlen);
8 while (Fgets(sendline, MAXLINE, fp) != NULL) {
9 Write(sockfd, sendline, strlen(sendline));
10 n = Read(sockfd, recvline, MAXLINE);
11 recvline[n] = 0; /* null terminate */
12 Fputs(recvline, stdout);
13 }
14 }
The changes are the new call to connect
and replacing the calls to sendto and recvfrom
with calls to write and read. This function is
still protocol-independent since it doesn't look inside the socket
address structure that is passed to connect. Our client
main function, Figure 8.7, remains
the same.
If we run this program on the host
macosx, specifying the IP address of the host
freebsd4 (which is not running our server on port 9877),
we have the following output:
macosx % udpcli04 172.24.37.94
hello, world
read error: Connection refused
The first point we notice is that we do
not receive the error when we
start the client process. The error occurs only after we send the
first datagram to the server. It is sending this datagram that
elicits the ICMP error from the server host. But when a TCP client
calls connect, specifying a server host that is not
running the server process, connect returns the error
because the call to connect causes the TCP three-way
handshake to happen, and the first packet of that handshake elicits
an RST from the server TCP (Section 4.3).
Figure
8.18 shows the tcpdump output.
Figure 8.18
tcpdump output when running Figure 8.17.
macosx % tcpdump
1 0.0 macosx.51139 > freebsd4.9877: udp 13
2 0.006180 ( 0.0062) freebsd4 > macosx: icmp: freebsd4 udp port 9877 unreachable
We also see in Figure A.15 that
this ICMP error is mapped by the kernel into the error
ECONNREFUSED, which corresponds to the message string
output by our err_sys function: "Connection refused."
Unfortunately, not all kernels return ICMP
messages to a connected UDP socket, as we have shown in this
section. Normally, Berkeley-derived kernels return the error, while
System V kernels do not. For example, if we run the same client on
a Solaris 2.4 host and connect to a host that is not
running our server, we can watch with tcpdump and verify
that the ICMP "port unreachable" error is returned by the server
host, but the client's call to read never returns. This
bug was fixed in Solaris 2.5. UnixWare does not return the error,
while AIX, Digital Unix, HP-UX, and Linux all return the error.
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