UDP connect: Can't assign requested address or "how to determine the outgoing IP address?"

[Edwin Groothuis]

Hello,

I got a nice piece of code (see attachment) which could be used to
determine the outgoing IP address for an UDP socket. It works without
problem under Linux, but it fails on FreeBSD (-current and -stable)
during the call to connect() with the error: "Can't assign requested
address". The code looks like the udpcli09.c example in Richard
Stevens 'Unix Network Programming' book, which states it doesn't
work on SVR4 derived kernels.

So, the question is, how can I determine the outgoing IP address
for a socket?

Edwin

attached code can be compiled with "gcc -Wall -g -o a a.c && ./a"

-- 
Edwin Groothuis      |            Personal website: http://www.mavetju.org
[EMAIL PROTECTED]    |    Weblog: http://www.mavetju.org/weblog/weblog.php 

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#include <netdb.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int guess_local_address(char *address_to_reach,char **loc);

int main(void) {
    char *loc;

    guess_local_address("218.185.88.1",&loc);printf("%s\n",loc);
    guess_local_address("192.168.1.3",&loc);printf("%s\n",loc);
    guess_local_address("127.0.0.1",&loc);printf("%s\n",loc);
    return 0;
}

int guess_local_address(char *address_to_reach,char **loc){
    int s,err;
    struct addrinfo hints;
    struct addrinfo *res=NULL;
    struct sockaddr addr;
    int sock;
        
    *loc=NULL;

    printf("Trying %s: ",address_to_reach);
        
    memset(&hints,0,sizeof(hints));
    hints.ai_family=PF_UNSPEC;
    hints.ai_socktype=SOCK_DGRAM;
    hints.ai_flags=AI_NUMERICHOST|AI_CANONNAME;
        
    err=getaddrinfo(address_to_reach,NULL,&hints,&res);
    if (err<0){
        printf("getaddrinfo: %s\n",gai_strerror(err));
        return -1;
    }
    if (res==NULL){
        printf("getaddrinfo reported nothing !");
        return -1;
    }
    sock=socket(res->ai_family,SOCK_DGRAM,0);
    if (err<0){
        printf("Error in socket: %s\n",strerror(errno));
        return -1;
    }
    {
        struct sockaddr_in servaddr;
        memset(&servaddr,sizeof(servaddr),0);
        servaddr.sin_family=AF_INET;
        servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
        servaddr.sin_port=htons(0);
        err=bind(sock,(struct sockaddr *)&servaddr,sizeof(servaddr));
        if (err<0){
                printf("Error in bind: %s\n",strerror(errno));
                return -1;
        }
    }
    s=1;
    err=setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&s,sizeof(int));
    if (err<0){
        printf("Error in setsockopt: %s\n",strerror(errno));
        return -1;
    }
    err=connect(sock,res->ai_addr,res->ai_addrlen);
    if (err<0) {
        printf("Error in connect: %s\n",strerror(errno));
        return -1;
    }
    freeaddrinfo(res);
    res=NULL;
    s=sizeof(addr);
    err=getsockname(sock,(struct sockaddr*)&addr,&s);
    if (err<0) {
        printf("Error in getsockname: %s\n",strerror(errno));
        return -1;
    }
    *loc=malloc(30);
    err=getnameinfo(&addr,s,*loc,30,NULL,0,NI_NUMERICHOST);
    if (err<0){
        printf("getnameinfo error:%s",strerror(errno));
        return -1;
    }
    close(sock);
    return 0;
}

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