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I have a problem with this line of code. I must take a packet to a port and resend to interface(ex:eth0). My program successfully take packet from the port but when I resend (with send() ) to interface I have an error:
send:Invalid argument

The line of code is:

    sock1=socket(AF_INET6,SOCK_DGRAM,0);
    sock2=socket(AF_INET6,SOCK_DGRAM,0);
    fd=fopen("port.txt","r+");
    if(fd) {
    while(!feof(fd)){
    fscanf(fd,"%d",&port);
    fscanf(fd, "%s",interface);
memset(&source_addr,0,sizeof(struct sockaddr_in6));
source_addr.sin6_family=AF_INET6;
source_addr.sin6_port=htons(port);
source_addr.sin6_addr=in6addr_any;
if(bind(sock1,(struct sockaddr*)&source_addr,sizeof(struct sockaddr_in6))==-1){perror("bind");}
//if(connect(sock1,(struct sockaddr*)&source_addr,sizeof(struct sockaddr_in6))==-1){perror("connect");}
//Device dove inviare
memset(&freq,0,sizeof(struct ifreq));
strncpy(freq.ifr_name,interface,IFNAMSIZ);
if(ioctl(sock2,SIOCGIFINDEX,&freq)==-1){perror("ioctl");}
 memset(&destination_addr,0,sizeof(struct sockaddr_in6));
 destination_addr.sin6_family=AF_INET6:
 destination_addr.sin6_scope_id=htonl(2)      
 inet_pton(AF_INET6,"2001::620:40b:555:110",(void*)&destination_addr.sin6_addr.s6_addr);
 if(bind(sock2,(struct sockaddr*)&destination_addr,sizeof(struct sockaddr_in6)==-1)
 {perror("bind");}
if((buff=malloc(BUFFER_LENGTH))==NULL){ perror("malloc");}
packet_length=recv(sock1,buff,BUFFER_LENGTH,0);
if(packet_length<0){perror("recv");}
printf("La lunghezza è %d\n",packet_length);
packet=(unsigned char*)buff;
Sniffer(packet,packet_length,' ');
packet_length2=send(sock2,buff,BUFFER_LENGTH,0);
buff is packet that I take from port! Where is the error?
You did not call connect() on your UDP socket, so it doesn't have a default destination (to be used by send()). either call connect() to set the default destination, or use sendto with the explicit destination.
You should also not be sending more than you actually received (ie. packet_length)
        
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