CODEWITHSUNDEEP
objective-ctelnet
owen gerig
owen gerig

Reputation: 6172

Telnet over a socket with GCDAsyncSocket

I'm trying to connect to a Cisco C40 codec via telnet from objective c. When using the terminal on my computer I get:

Password:

However when doing a socket connection there are telnet negotiations that need to be made. Which I am but for some reason I cannot get to the "Password:" prompt above.

- (void)socket:(GCDAsyncSocket *)sock didReadData:(NSData *)data withTag:(long)tag
{
    NSLog(@"RECEIVE BUFFER %@",data);

    //store read bytes to rawData
    self.rawData = [[NSMutableData alloc] initWithData:data];

    //cast bytes
    const uint8_t *bytes = [self.rawData bytes];

    //go through rawdata format and save it to networkbuffer
    for (int i =0; i < [self.rawData length]; i++)
    {
        if (![[NSString stringWithFormat:@"%02X", bytes[i]]isEqual:@"0D"])
        {
            [self.networkBuffer addObject:[NSString stringWithFormat:@"%02X", bytes[i]]];
        }
    }

    //negotiate any telnet protocal stuff (just accept options )
    //example:
    //FF:FD:18 returns FF:FB:18
    while([[self.networkBuffer objectAtIndex:0]isEqualToString:@"FF"] && [[self.networkBuffer objectAtIndex:1]isEqualToString:@"FD"] ) {
//        NSLog(@"HERE");

        NSData * tempData =[data subdataWithRange:NSMakeRange(0, 3)];

        NSMutableData * tempMutData = [NSMutableData dataWithData:tempData];

        const unsigned char replacement[] = {
                0xFC
            };

        [tempMutData replaceBytesInRange:NSMakeRange(1, 1) withBytes:replacement];

        [self sendCommand:tempMutData];

        data = [data subdataWithRange:NSMakeRange(3, [data length]-3)];

        self.networkBuffer = [NSMutableArray arrayWithArray:[self.networkBuffer subarrayWithRange:NSMakeRange(3, [self.networkBuffer count]-3)]];

//        NSLog(@"network buffer after removal: %@", data);

        if ([self.networkBuffer count]<3) {
            [self.networkBuffer insertObject:@" " atIndex:0];
        }
    }

    //decode from bytes to text
    for ( NSString * component in self.networkBuffer)
    {
        int value = 0;
        sscanf([component cStringUsingEncoding:NSASCIIStringEncoding], "%x", &value);
        [self.dataString appendFormat:@"%c", (char)value];
        NSLog(@"data byte: %c",(char)value);
    }
    [self telnetResponse:self.dataString];

    [self.networkBuffer removeAllObjects];
    [self.socket readDataToData:[GCDAsyncSocket CRLFData] withTimeout:-1 tag:0];//CRLFData
}

Here is a breakdown of the telnet negotiation options Im receiving and sending:

server sending me:

FF,FD,18 (0x18 = 24dec) (Do terminal type)

FF,FD,20 (0x20 = 32dec) (Do terminal speed)

FF,FD,23 (0x23 = 35dec) (Do X Display Location)

FF,FD,27 (0x27 = 39dec) (Do New Environment Option)

My attempt at a response that doesnt work (no prompt for further input):

FF,FC,18 (0x18 = 24dec) (Wont terminal type)

FF,FC,20 (0x20 = 32dec) (Wont terminal speed)

FF,FC,23 (0x23 = 35dec) (Wont X Display Location)

FF,FC,27 (0x27 = 39dec) (Wont New Environment Option)

If you look at the code you will see that I am checking for FF and if so responding with similar bytes (replacing FD with FC), in hopes that wont accept the options but that does not seem to be working.

Links that helped me:

https://stackoverflow.com/a/2913991/530933

Telnet IAC commands (NSStream socket)

http://www.iprodeveloper.com/forums/aft/52910

UPDATE

I did a wireshark with a command shell and the cisco codec. After which I duplicated those negotiation setting/packets. Now the only problem is that Im only getting the echo. So i will get nothing, send a command, then get back a prompt plus my text. (Example. get nothing - send username "admin" - get back "login: admin") Hence what I mean by only getting the echo. I should get a prompt "login:" then send "admin" then it should prompt me for the password.

here are the negotiation options Im sending on connect:

//will terminal type
    //will negotiate about window size
    const unsigned char nineteen[] = {
        0xFF, 0xFB, 0x18, 0xFF, 0xFB, 0x1F
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:nineteen length:sizeof(nineteen)];
    [self sendCommand:self.dataToBeSent];

    //wont terminal speed
    //wont X display location
    //will new environment option
    const unsigned char twenty[] = {
        0xFF, 0xFC, 0x20, 0xFF, 0xFC, 0x23, 0xFF, 0xFB, 0x27
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twenty length:sizeof(twenty)];
    [self sendCommand:self.dataToBeSent];

    //Suboption being: negotiate about window size
    //end
    const unsigned char twentyOne[] = {
        //0xFF,0xFC, 0x18
        0xFF, 0xFA, 0x1F, 0x00, 0x50, 0x00, 0x19, 0xFF, 0xF0
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentyOne length:sizeof(twentyOne)];
    [self sendCommand:self.dataToBeSent];

    //new enviroment option
    //end
    const unsigned char twentyThree[] = {
        0xFF,0xFA, 0x27, 0x00, 0xFF, 0xF0
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentyThree length:sizeof(twentyThree)];
    [self sendCommand:self.dataToBeSent];

    //Terminal Type (ANSI)
    //end
    const unsigned char twentySeven[] = {
        0xFF,0xFA, 0x18, 0x00, 0x41, 0x4E, 0x53, 0x49, 0xFF, 0xF0
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentySeven length:sizeof(twentySeven)];
    [self sendCommand:self.dataToBeSent];

    //do suppress go ahead
    const unsigned char twentyEight[] = {
        0xFF, 0xFD, 0x03
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentyEight length:sizeof(twentyEight)];
    [self sendCommand:self.dataToBeSent];

    //will echo
    //dont status
    //wont remote flow control
    const unsigned char twentyFour[] = {
        0xFF, 0xFB, 0x01, 0xFF, 0xFE, 0x05, 0xFF,0xFC, 0x21
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentyFour length:sizeof(twentyFour)];
    [self sendCommand:self.dataToBeSent];

    //wont echo
    const unsigned char twentyFive[] = {
        0xFF, 0xFC, 0x01
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentyFive length:sizeof(twentyFive)];
    [self sendCommand:self.dataToBeSent];

    //Do echo
    const unsigned char twentySix[] = {
        0xFF,0xFD, 0x01
    };
    self.dataToBeSent = [[NSData alloc]initWithBytes:twentySix length:sizeof(twentySix)];
    [self sendCommand:self.dataToBeSent];

Upvotes: 7

Views: 3092

Answers (1)

owen gerig
owen gerig

Reputation: 6172

So a big problem came from the fact that the prompts (login: or password:) do no end the line with CR NL (0D:0A). And I was doing

[self.socket readDataToData:[GCDAsyncSocket CRLFData] withTimeout:-1 tag:0];

so I was never reading the data that held the prompt (a big problem also was that wireshark wasnt working (fixed that myself too)). Once I figured this out I changed the line above to:

[self.socket readDataWithTimeout:-1 tag:0];

Which successfully gave me my prompt. Below are the negotiations Im sending to get to this point and what the original questions entailed (same as above in the update):

will terminal type - 0xFF, 0xFB, 0x18

will negotiate about window size - 0xFF, 0xFB, 0x1F

wont terminal speed - 0xFF, 0xFC, 0x20

wont X display location - 0xFF, 0xFC, 0x23

will new environment option - 0xFF, 0xFB, 0x27

Suboptions

 negotiate about window size - 0xFF, 0xFA, 0x1F, 0x00, 0x50, 0x00, 0x19
 end - 0xFF, 0xF0

 new enviroment option - 0xFF,0xFA, 0x27, 0x00, 
 end - 0xFF, 0xF0

 Terminal Type (ANSI) -  0xFF,0xFA, 0x18, 0x00, 0x41, 0x4E, 0x53, 0x49, 
 end - 0xFF, 0xF0

do suppress go ahead - 0xFF, 0xFD, 0x03

will echo - 0xFF, 0xFB, 0x01

dont status - 0xFF, 0xFE, 0x05

wont remote flow control - 0xFF,0xFC, 0x21

wont echo - 0xFF, 0xFC, 0x01

Do echo - 0xFF,0xFD, 0x01

This might also help. It removes the negotiation bytes from the stream so when your encoding to make the string it doesnt include negotiation bytes. 

while([[self.networkBuffer objectAtIndex:0]isEqualToString:@"FF"])
    {
        if ([[self.networkBuffer objectAtIndex:1]isEqualToString:@"FD"] || [[self.networkBuffer objectAtIndex:1]isEqualToString:@"FB"] || [[self.networkBuffer objectAtIndex:1]isEqualToString:@"FE"] || [[self.networkBuffer objectAtIndex:1]isEqualToString:@"FA"]) {

            //most negotiation options are 3 bytes long
            int indexToRemoveFromBuffer = 3;

            //if FA then they are longer then 3 bytes
            if ([[self.networkBuffer objectAtIndex:1]isEqualToString:@"FA"]) {
                //look for indicator of END (F0)
                indexToRemoveFromBuffer = [self.networkBuffer indexOfObject:@"F0"]+1;

            }

            //remove these bytes from networkbuffer
            self.networkBuffer = [NSMutableArray arrayWithArray:[self.networkBuffer subarrayWithRange:NSMakeRange(indexToRemoveFromBuffer, [self.networkBuffer count]-indexToRemoveFromBuffer)]];

            if ([self.networkBuffer count] == 0) {
                if (self.isLoggedIn) {
                    [self.socket readDataToData:[GCDAsyncSocket CRLFData] withTimeout:-1 tag:0];//CRLFData
                }else{
                    [self.socket readDataWithTimeout:-1 tag:0];
                }
                return;
            }
        }else{
            break;
        }
}

Upvotes: 1

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