profcane's networking revision
recommended listening for this post is do it (yves remix)
apparently wing keong is like an insane crackpot conspiracy theorist? either way the man’s a good professor and makes hard tests so if he says that vlan tags cause 9/11 then fuck it man encapsulation dot1q 9/11 my ass
question 1
consider the following setup. i have setup the trunk between S1 and S2, VLAN 10 on G1/0/6 S1 and G1/0/11 S2, and VLAN 20 on G1/0/18 as per the diagram i have also correctly set the router’s ip addresses on their interfaces. pc A can ping pc B, but no pcs can ping the router. this is because
explanation STP does not come into play here, and the default for a trunk is to allow all VLANs. the default for an interface on a switch is ONLY VLAN 1, so the switch cannot send packets from PC A to R1 (the VLAN has not been properly configured).
question 2
which of these is possible to do on a CISCO router?
explanation one VLAN+IP address per LAN subinterface. the key thing here is that an IP address is ‘more specific’ than a VLAN, and the router requires you to set the VLAN (with the encapsulation dot1q ..) command before configuring the IP address. (you’ll note that in each example in the labs, the encapsulation command is performed before setting the IP).
question 3
LAN subinterface G0/0/1.10 is currently configured to 192.168.1.10/28. i want to configure LAN subinterface G0/0/1.20 - which address + subnet mask would the router let me configure it to?
explanation: the only constraint that the router enforces is that no subinterface must overlap (when it receives a packet destined for a specific subnet, it must know which interface to send it to, there cannot be any ‘ambiguities’). for each IP address above starting with 192.168.1.x, the 192.168.1.10/28 will necessarily overlap (because that subnet covers every single 192.168.1.x addr). 192.168.2.0 255.255.254.0 covers from 192.168.2.0 - 192.168.3.255, hence no overlap.</div>
question 4
consider the network topology, noting that PC1 and PC2 are on separate VLANs. PC1 sends PC2 an ICMP ping request. assuming that all ARP tables are empty, which of the following is true?
explanation: PCs are ‘VLAN unaware’, the PC only sends an ethernet packet. VLAN tags are a layer 2 thing, so they are mainly dealt with by switches (as well as layer 3 routers, which need to decapsulate the packet to check its IP, then reapply the next VLAN tag during encapsulation). switches only learn MAC addresses from source MAC addresses and not destinations, and when the router decapsulates the packet, it modifies the source MAC address of the packet to be the router’s.
question 5
same question as above - which of the following is true? (remember the assumption that all ARP, MAC address, and routing tables are empty).
explanation:PC1 cannot send an ARP request to PC2 because they are on different subnets! PC1 would send it to the default gateway (the router), and the router would then send its own ARP request to PC2, and PC2 would respond. throughout this process only two ARP requests would be sent (PC1 would need to ARP for its default gateway, and the router would need to ARP for PC2). third option is wrong because a switch floods a broadcast packet out of all ports except ingress, so it would only send 2 packets max (if that).
question 6
after this finishes execution, what will the contents of rawr.txt be (assuming it starts off blank)?
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *fptr = fopen("rawr.txt", "r");
fprintf(fptr, "meow meow\n");
return 0;
}
explanation: it’ll just fail silently. ofc, the compiler doesn’t actually do a check to see if the file exists at compiletime, that’s crazy.
optional info so programs will actually segfault if they try to write to read only pages of memory, such as a binary’s Global Offset Table when the program is compiled w/ full Relocation Read-Only. this is relevant in the beautiful field of binary exploitation, which i dearly love, but is absolutely beyond the scope of a first year C class. please message me if you’d love to learn more
question 7
which of the following commands cannot be used to get input from stdin?
explanation:
fgets() reads from a FILE* ptr, and stdin is a FILE* ptr. read() reads from file descriptors, and stdin is also a file descriptor. getc() takes an argument for a FILE* ptr. the only thing here is strstr(), which is essentially python’s .find() method for strings. either way, strstr() can’t read from file pointers or file descriptors, just compare two strings.
question 8
what is the output of the following program? (assume long is 64-bit, and assume we’re on a 64-bit machine too)
#include <stdio.h>
#include <stdlib.h>
int main() {
int a = 10;
long b = (long)&a;
int c = *(int*)b;
printf("%ld %ld %ld\n", a, b, c);
return 0;
}
explanation: in C, this is perfectly valid - we’re casting the value of the pointer to a as if it were a long. we’re on a 64-bit machine so pointers are also 64-bit, the cast works out nice. the important thing here is that you don’t actually lose any information as long as you’re casting back and forth between the same sizes, hence the original value of int a remains at 10.
question 9
what is the output of the following program?
#include <stdio.h>
#include <stdlib.h>
int main() {
char buf[] = "what's so good about picking up the pieces?";
for (int i = 0; buf[i]; i++) {
printf("%c", buf[i]);
}
return 0;
}
explanation: for loops consist of three parts: the initialization, the condition, and the update. typically we see the standard format for a for loop as int i = 0; i<SOME_BOUND; i++ but that condition can really be anything! recall that all strings (char bufs) are nullterminated, and a nullterminator is just a 0 byte which is falsy.
question 10
what is the output of the following program?
#include <stdio.h>
#include <stdlib.h>
int main() {
char c = 'a';
while (c <= 'g') {
c = (c+1)^0x20;
printf("%c", c);
}
return 0;
}
explanation: ok yeah soz this is really tricky. we know that chars are just ints, but lowercase chars are also greater than uppercase chars, so when the xor 0x20 flips the case, the loop doesn’t terminate. we also have to keep in mind that the loop modifies the char BEFORE printing the string, which is why the initial ‘a’ value doesn’t get printed. finally, the conditional is c <= 'g', NOT c < g; so if c == 'g' then the while loop still continues.
question 11
which of the following choices for b[] would not result in the program outputting 1?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int are_the_same(char* a, char* b, int size) {
return (strncmp(a, b, size) == 0);
}
int main() {
char a[7] = "rawr\00xd";
char b[] = "...";
printf("%d", are_the_same(a, b, 7));
return;
}
explanation: again, strings are all null terminated. strncmp only checks at MAX the first n bytes (in this case, 7) but if both strings are null terminated before n then it just exits out. in this case, only raw would fail the check because strncmp would try to compare raw’s null terminator with rawr’s last ‘r’ character.</div>
question 12
which of the following correctly explains why this code won’t compile?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
printf("%d", atoi(getc(stdin)));
return 0;
}
explanation: honestly the answer explains it better than i can here. we just have to remember that a char and char* are distinct. also, atoi() returns an integer.
question 13
what is the output of the following program? assume that doesnotexist.txt does not exist.
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *fptr = fopen("doesnotexist.txt", "r");
int a = 0;
if (fptr == a) {
puts("yay!");
} else {
puts("awww.");
}
return 0;
}
explanationa null pointer is just the value 0, and you can compare the two perfectly fine. fopen will always return a null pointer when it fails.
question 14
which of the following is an invalid way to populate the name field of the Student struct?
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[16];
int age;
} Student;
int main(void) {
Student* student = malloc(sizeof(Student));
...
}
question 15 (bonus)
what’s wrong with the following program?
#include <stdio.h>
#include <stdlib.h>
int main(void) {
char buf[16];
fgets(buf, sizeof(buf), stdin);
printf(buf);
return 0;
}