I often find myself in need to test if an internet connection is online or not. In particular, sometimes it happens (on very, very peculiar connections) that UDP is disabled. Now, I figured that one of the most simple ways to test if UDP is available to my connection is through a DNS query.
Now, using some function like getaddrinfo abstracts away UDP from the query, so that (as far as my understanding goes) if UDP is not available, TCP will be used instead. Now, since I need to test for UDP connections, I will need to forge the UDP packet with the query myself. Moreover, I would like to make the query to a public DNS server (like 8.8.8.8), so that I am certain that the UDP connection is available to talk to external hosts.
So I am wondering: what is the most simple DNS query that I can send via an UDP packet? Could you show me an example that just asks for the ip, say, of google.com to a DNS server?
I'm not sure I understand your question fully, but the simplest query would be something like SOA for . (i.e. the start of authority for the root zone), like this:
$ dig SOA . #8.8.8.8
; <<>> DiG 9.8.3-P1 <<>> SOA . #8.8.8.8
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46366
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;. IN SOA
;; ANSWER SECTION:
. 4350 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2015102500 1800 900 604800 86400
;; Query time: 36 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sun Oct 25 21:07:19 2015
;; MSG SIZE rcvd: 92
If you are asking what that looks like as a UDP packet, simply capture it with tcpdump. However, such trivial DNS packets are relatively easy to construct in C - see RFC1035. You may want to use a pre-existing library like c-ares or adns.
Note that as far as I remember, whether your resolver library falls back to TCP is implementation dependent. IIRC glibc on linux does not fall back to TCP, and libresolv on OpenBSD only does if /etc/resolv.conf contains options tcp. When using libresolv programatically you merely need to ensure RES_USEVC is clear.
If you are looking for example code, 'Stevens' TCP/IP illustrated' is the canonical answer.
Related
Today, I was handed a 300 million entry csv file of netflow records, and my objective is to convert the netflow data to synthesized packets by any means necessary. After a bit of researching, I've decided Scapy would be an incredible tool for this process. I've been fiddling with some of the commands and attempting to create accurate packets that depict that netflow data, but I'm struggling and would really appreciate help from someone whose dabbled with Scapy before.
Here is an example entry from my dataset:
1526284499.233,1526284795.166,157.239.11.35,41.75.41.198,443,55915,6,1,24,62,6537,1419,1441,32934,65535,
Below is what each comma separated value represents:
Start Timestamp (Epoch Format): 1526284499.233
End Timestamp (Epoch Format): 1526284795.166
Source IP: 157.239.11.35
Destination IP: 41.75.41.198
IP Header Protocol Number: 443 (HTTPS)
Source Port Number: 55915
Destination Port Number: 6 (TCP)
TOS Value in IP Header: 1 (FIN)
TCP Flags: 24 (ACK & PSH)
Number of Packets: 62
Number of Bytes: 6537
Router Ingress Port: 1419
Router Egress Port: 1441
Source Autonomous System: 32934 (Facebook)
Destination Autonomous System: 65535
My Current Scapy Representation of this Entry:
>>> size = bytes(6537)
>>> packet = IP(src="157.240.11.35", dst="41.75.41.200", chksum=24, tos=1, proto=443) / TCP(sport=55915, dport=6, flags=24) / Raw(size)
packet.show():
###[ IP ]###
version= 4
ihl= None
tos= 0x1
len= None
id= 1
flags=
frag= 0
ttl= 64
proto= 443
chksum= 0x18
src= 157.240.11.35
dst= 41.75.41.200
\options\
###[ TCP ]###
sport= 55915
dport= 6
seq= 0
ack= 0
dataofs= None
reserved= 0
flags= PA
window= 8192
chksum= None
urgptr= 0
options= []
###[ Raw ]###
load= '6537'
My Confusion:
Frankly, I'm not sure if this is right. Where I get confused is that the IP Protocol Header is 443, indicating HTTPS, however the destination port is 6, indicating TCP. Therefore, I'm not sure if I should include TCP or not, or if including the proto IP attribute is gratuitous. Furthermore, I'm not sure if Raw() is the correct way to include the size of each packet, let alone if I defined size in a proper manner.
Please be so kind as to let me know where I've gone wrong, or if I actually miraculously created a perfect synthesized packet for this particular entry. Thank you so much!
I think the columns might be wrong. HTTPS is TCP port 443 (usually), so the protocol number should be 6 (TCP) and one of the ports should be 443. My GUESS is that 443 is the source port, since the source IP belongs to Facebook, making 55915 the destination port. So, I think the columns there go: source IP, dest IP, source port, dest port, protocol.
I'm trying to broadcast a message from a server to all clients, but only one client receives the message.
I want to run this server and two or more instances of this client (taken from Donahoo, Calvert, "TCP/IP Sockets in C", 1e; I can paste the code into this question on request).
The programs work fine with a single client, but when running two clients only one (the first) ever receives a message, while the second instance just gets stuck (on bind).
I don't know what I'm doing wrong, I'm sure the program is correct so perhaps I'm running it wrong. I start the server as:
$ ./BroadcastSender localhost 1337 hey &
As for the clients, I have tried two variations, the first:
$ ./BroadcastReceiver 1337 & ./BroadcastReceiver 1337 &
In the second variation I have added while (1) {} after close(sock) and then run as:
$ ./BroadcastReceiver 1337 &
$ ./BroadcastReceiver 1337 &
Both variations give the same result, namely that the first client receives the message, the other one doesn't, but instead gets stuck trying to bind.
Am I running the server/clients the wrong way, or is there something missing in the code? I'm new to sockets so I don't really see if there's anything in, say, the server code saying "I'm gonna broadcast to one client only".
Could you give me some pointers in the right direction? There are other questions and answers about broadcasting but I haven't found one that addresses this particular issue. Thank you.
you can't have 2 processes bind on the same port. Not familiar with the broadcaster, but generally you have 2 options - either run the 2 processes on 2 machines on the same network or run the clients on separate ports and have the broadcaster broadcast on several ports
The command line when running 2 processes on 2 machines should be something like:
$ ./BroadcastSender 127.0.255.255 1337 hey &
when 127.0.255.255 is your subnet mask
--- edit(thanks #Jeremy) ---
you can also bind two sockets to the same UDP port using setsockopt
with SO_REUSEADDR/SO_REUSEPORT flags
My current project is a bare-metal webserver. For this I'm using no libraries, and programming directly onto the chip. What I'm trying to do right now is send a single piece of HTTP data:
HTTP/1.1 200 OK\r\n
Content-Length: 45\r\n
Content-Type: text/html\r\n
Server: microserver\r\n
Connection: close\r\n
\r\n
<!DOCTYPE html><html>Hello<br>world!<hr></html>
My server tries to go through the following steps:
Receive SYN
Send [SYN+ACK]
Receive ACK
Receive ACK containing HTTP GET
Send [ACK,PUSH,FIN] with HTTP data (this one changed a lot, I've tried sending ACK PUSH and FIN seperately (with the content in PUSH), tried [ACK+PUSH],FIN and ACK,[PUSH+FIN] as well.
Receive [ACK+FIN] <<--- Here's where it goes wrong, this one is never even sent, according to wireshark.
Send ACK.
As said, it goes wrong at step 6. every single time. No matter what combination of ACK, PUSH and FIN I use in step 5. When looking at it with wireshark all the SEQ and ACK numbers are correct.
My server is able to close connections once the [FIN+ACK] finally does get sent, which sometimes happens on sockets that are kept open by the browser synchronously.
Pcap file of what wireshark records: https://www.dropbox.com/s/062k9hkgkenaqij/httpdump.pcap with as filter: (tcp.dstport == 80 || tcp.srcport == 80) && (ip.dst == 169.254.100.100 || ip.src == 169.254.100.100)
I know there is a 4 year old very similar question, Building a webserver, client doesn't acknowledge HTTP 200 OK frame, but I've tried pretty much everything that was suggested in there, and it didn't seem to get me any further, so I'm out of ideas.
EDIT:
I have found the problem, after studying sections of wireshark captures for hours an end. Apparently the mistake was that I did not use data to calculate the TCP checksum... But well, found the solution.
FreeBSD: network interface address: dhcp or static
Followup question now:
I've decided to go with looking at leases files: /var/db/dhclient.leases.. What does it tell me exactly? Existence of /var/db/dhclient.leases.em0 signifies em0 has address by DHCP? This file does not seem to go away with reboot.
You should read the manual page for dhclient. This will answer most of your questions. And if that fails, you can browse the source in /usr/src/sbin/dhclient.
Another possibility might be to to use devd(8). This is a daemon that can execulte a script or program if a certain event occurs. It can e.g. note when a network interface goes "up" or "down". From the default /etc/devd.conf (see also devd.conf(5)):
# Try to start dhclient on Ethernet-like interfaces when the link comes
# up. Only devices that are configured to support DHCP will actually
# run it. No link down rule exists because dhclient automatically exits
# when the link goes down.
#
notify 0 {
match "system" "IFNET";
match "type" "LINK_UP";
media-type "ethernet";
action "/etc/rc.d/dhclient quietstart $subsystem";
};
A client is supposed to remember a DHCP lease across reboots and is supposed to remember past leases on a particular network when requesting an address. Therefore, the file should not go away across boots.
We are trying to implement a software based on Moxa UC-7112-LX embedded computer (uClinux OS). We use Cinteron MC52i GSM modem (regular GPRS service) and standart pppd to connect to the Internet.
Everything seems to be fine, right after the connection. Ping utility is working, Socket functions in my program work normally too. However after some time ppp connection brokes in a very peculiar way. These are the symptoms of that situation:
When I call ping utility with some host name as parameter the system is able to resolve it's IP and starts sending ICMP packets but gets no response. I am trying different web resources names, so that the system cannot have their addresses cached or something. Whatever I choose, the system correctly resolves IP but can't get any ping responce.
connect() and write() functions in my application give no error return but when it comes to read() the function returns with errno set to ECONNRESET (Connection reset by peer). The program uses standard socket functions (TCP protocol)
the ppp link is shown as running (ifconfig ppp0)
So, the situation that I have is: the link is good enough to maintain DNS resolving service (UDP is working?) but NOT good enough to run TCP connection and receive ping echoes...
The situation does not appear all the time. Sometimes the system can work normally for days without any problem. Whenever the problem appears, simple reset solves everything.
I know that the system we use is quite exotic, and the situation described here may be connected with some buggy tcp stack or pppd implementation. Considering that the system is preconfigured by the manufacturer I don't have any options to rebuild/change the OS firmware.
Still I hope that someone have seen the similar situation on any linux-like system. Is there any way to test why DNS name resolving is working while the other network stuff does not? Is it possible to remove such connection state with some pppd settings?
Edit:
First of all, I'd like to address the possibility of local caching of the IP addresses. I don't have dig utility and I have no idea how to check which host gives the result to getaddrinfo(). Still I'm sure that the addresses are not cached cause I'm trying to ping totally random URLs. Also given the slow GPRS response time it is not necessary to have the time measuring utility to see that ping takes 1-2 seconds or more to resolve IP before starting sending out packets. Furthermore ncsd, BIND or any dns servers do not run locally on the machine. I understand that you may not see that as proof, but that's what I have given the utility set available on my system.
I'd like to give some additional information concerning the internet connection operation.
Normal connection state
The rc script at system load runs another script as background process:
sh /etc/connect &
The connect script is as follows:
#!/bin/sh
echo First connect attempt > /etc/ppp/conn.info
while true
do
date >> /etc/ppp/conn.info
pppd call mts
echo Reconnecting... >> /etc/ppp/conn.info
done
The reason that I've made a loop here is simple: the connection persists for several hours and after that it always breaks. Unfortunately my implementation of pppd does not support the logfile option (so I can't see why is it broken). persist does not seem to work either so I've come to the connect script above. The pppd options are:
/dev/ttyM0 115200 crtscts
connect 'chat -f /etc/ppp/peers/mts.chat'
noauth
user mts
password mts
noipdefault
usepeerdns
defaultroute
ifconfig ppp0 gives:
ppp0 Link encap:Point-Point Protocol
inet addr:172.22.22.109 P-t-P:192.168.254.254 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:34 errors:0 dropped:0 overruns:0 frame:0
TX packets:36 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:3130 (3.0 KiB) TX bytes:2250 (2.1 KiB)
And thats where it starts getting strange. Whenever I connect I'm getting different inet addr but P-t-p is always the same: 192.168.254.254. This is the same address that appears in default gateway entry, as given by netstat -rn:
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.254.254 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
192.168.4.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
192.168.15.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.0.0 192.168.15.1 255.255.0.0 UG 0 0 0 eth0
0.0.0.0 192.168.254.254 0.0.0.0 UG 0 0 0 ppp0
route -Cevn is unavailable on my system, route gives the same info as above.
But I'm never able to ping the 192.168.254.254, not even when everything is working as intended: tcp connection, ping, DNS etc. Here is the result of traceroute:
traceroute to kernel.org (149.20.4.69), 30 hops max, 40 byte packets
1 172.16.4.210 (172.16.4.210) 528.765 ms 545.269 ms 616.67 ms
2 172.16.4.226 (172.16.4.226) 563.034 ms 526.176 ms 537.07 ms
3 10.250.85.161 (10.250.85.161) 572.805 ms 564.073 ms 556.766 ms
4 172.31.250.9 (172.31.250.9) 556.513 ms 563.383 ms 580.724 ms
5 172.31.250.10 (172.31.250.10) 518.15 ms 526.403 ms 537.574 ms
6 pub2.kernel.org (149.20.4.69) 538.058 ms 514.222 ms 538.575 ms
7 pub2.kernel.org (149.20.4.69) 537.531 ms 538.52 ms 537.556 ms
8 pub2.kernel.org (149.20.4.69) 568.695 ms 523.099 ms 570.983 ms
9 pub2.kernel.org (149.20.4.69) 526.511 ms 534.583 ms 537.994 ms
##### traceroute loops here - why?? #######
So, I can assume that 172.16.4.210 is peer's address. Such address is pingable in any case (see below). I have no idea why the structure of traceroute output is like this (packets come from internal network of ISP right to the destination, 'loop' at the destination address - it just should not be like this).
Also I would like to note that I can ping DNS server but traceroute does not go all the way up to it.
You may notice that there are eth0 and eth1 devices. They are irrelevant to the case. eth1 is not connected and eth0 is connected to lan without internet access.
Bad connection state
So, some time passes and the situation under question appears. I can't ping anything but DNS server (and peer, the address for which I get from traceroute result for the DNS) and cant communicate with remote host via tcp. DNS resolving is working
The network utilites give the same output as in normal state. I have the same unpingable peer (192.168.254.254 from ifconfig result), the routing table is the same:
# ifconfig ppp0
ppp0 Link encap:Point-Point Protocol
inet addr:172.22.22.109 P-t-P:192.168.254.254 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:297 errors:0 dropped:0 overruns:0 frame:0
TX packets:424 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:33706 (32.9 KiB) TX bytes:27451 (26.8 KiB)
# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.254.254 * 255.255.255.255 UH 0 0 0 ppp0
192.168.4.0 * 255.255.255.0 U 0 0 0 eth1
192.168.15.0 * 255.255.255.0 U 0 0 0 eth0
192.168.0.0 192.168.15.1 255.255.0.0 UG 0 0 0 eth0
default 192.168.254.254 0.0.0.0 UG 0 0 0 ppp0
Note that the original ppp connection (one which I used to provide the output from normal state) persisted. My /etc/connect script did not loop (there was no new record in a makeshift log the script makes).
Here goes the ping to DNS server:
# cat /etc/resolv.conf
#search moxa.com
nameserver 213.87.0.1
nameserver 213.87.1.1
# ping 213.87.0.1
PING 213.87.0.1 (213.87.0.1): 56 data bytes
64 bytes from 213.87.0.1: icmp_seq=0 ttl=59 time=559.8 ms
64 bytes from 213.87.0.1: icmp_seq=1 ttl=59 time=509.9 ms
64 bytes from 213.87.0.1: icmp_seq=2 ttl=59 time=559.8 ms
And traceroute:
# traceroute 213.87.0.1
traceroute to 213.87.0.1 (213.87.0.1), 30 hops max, 40 byte packets
1 172.16.4.210 (172.16.4.210) 542.449 ms 572.858 ms 595.681 ms
2 172.16.4.214 (172.16.4.214) 590.392 ms 565.887 ms 676.919 ms
3 * * *
4 217.8.237.62 (217.8.237.62) 603.1 ms 569.078 ms 553.723 ms
5 * * *
6 * * *
## and so on ###
*** lines may look like trouble but im getting the same traceroute for that DNS in normal situation
ping to 172.16.4.210 works fine as well.
Now to TCP. I've started a simple echo server on my PC and tried to connect via telnet to it (the actual ip address is not shown):
# telnet XXX.XXX.XXX.XXX 9060
Trying XXX.XXX.XXX.XXX(25635)...
Connected to XXX.XXX.XXX.XXX.
Escape character is '^]'.
aaabbbccc
Connection closed by foreign host.
So thats what happened here. Successfull connect() just like in my custom application is followed by Connection closed... when telnet called read(). The actual server did not receive any incoming connection. Why did 'connect()' return normally (it could not get the handshake response from the host!) is beyond my scope of knowledge.
Sure enough same telnet test works fine in normal state.
Note:
I did not publish this on serverfault cause of the embedded nature of my system. serverfault as far as I understand deals with more conventional systems (like x86s running 'normal' linux). I just hope that stackoverflow has more embedded experts who know such systems as my Moxa.
Q: How can I have DNS name resolving running while other protocols seem to be down?
A: Your local DNS resolver (bind is another possibility besides ncsd) might be caching the first response. dig will tell you where you are getting the response from:
[mpenning#Bucksnort ~]$ dig cisco.com
; <<>> DiG 9.6-ESV-R4 <<>> +all cisco.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 22106
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0
;; QUESTION SECTION:
;cisco.com. IN A
;; ANSWER SECTION:
cisco.com. 86367 IN A 198.133.219.25
;; AUTHORITY SECTION:
cisco.com. 86367 IN NS ns2.cisco.com.
cisco.com. 86367 IN NS ns1.cisco.com.
;; Query time: 1 msec <----------------------- 1msec is usually cached
;; SERVER: 127.0.0.1#53(127.0.0.1) <--------------- Answered by localhost
;; WHEN: Wed Dec 7 04:41:21 2011
;; MSG SIZE rcvd: 79
[mpenning#Bucksnort ~]$
If you are getting a very quick (low milliseconds) answer from 127.0.0.1, then it's very likely that you're getting a locally cached answer from a prior query of the same DNS name (and it's quite common for people to use caching DNS resolvers on a ppp connection to reduce connection time, as well as achieving a small load reduction on the ppp link).
If you suspect a cached answer, do a dig on some other DNS name to see whether it can resolve too.
If random DNS names continue resolution and you still cannot make a TCP connection to a certain host, this is worthy of noting when you edit the question after this investigation.
If random DNS names don't resolve, then this is indicative of something like the loss of your default route, or the ppp connection going down.
Other diagnostic information
If you find yourself in either of the last situations I described, you need to do some IP and ppp-level debugs before this can be isolated further. As someone mentioned, tcpdump is quite valuable at this point, but it sounds like you don't have it available.
I assume you are not making a TCP connection to the same IP address of your DNS server. There are many possibilities at this point... If you can still resolve random DNS names, but TCP connections are failing, it is possible that the problem you are seeing is on the other side of the ppp connection, that the kernel routing cache (which holds a little TCP state information like MSS) is getting messed up, you have too much packet loss for tcp, or any number of things.
Let's assume your topology is like this:
10.1.1.2/30 10.1.1.1/30
[ppp0] [pppX]
uCLinux----------------------AccessServer---->[To the reset of the network]
When you initiate your ppp connection, take note of your IP address and the address of your default gateway:
ip link show ppp0 # display the link status of your ppp0 intf (is it up?)
ip addr show ppp0 # display the IP address of your ppp0 interface
ip route show # display your routing table
route -Cevn # display the kernel's routing cache
Similar results can be found if you don't have the iproute2 package as part of your distro (iproute2 provides the ip utility):
ifconfig ppp0 # display link status and addresses on ppp0
netstat -rn # display routing table
route -Cevn # display kernel routing table
For those with the iproute2 utilities (which is almost everybody these days), ifconfig has been deprecated and replaced by the ip commands; however, if you have an older 2.2 or 2.4-based system you may still need to use ifconfig.
Troubleshooting steps:
When you start having the problem, first check whether you can ping the address of pppX on your access server.
If you can not ping the ip address of pppX on the other side, then it is highly unlikely your DNS is getting resolved by anything other than a cached response on your uCLinux machine.
If you can ping pppX, then try to ping the ip address of your TCP peer and the IP address of the DNS (if it is not on localhost). Unless there is a firewall involved, you must be able to ping it successfully for any of this to work.
If you can ping the ip address of pppX but you cannot ping your TCP peer's ip address, check your routing table to see whether your default route is still pointing out ppp0
If your default route points through ppp0, check whether you can still ping the ip address of the default route.
If you can ping your default route and you can ping the remote host that you're trying to connect to, check the kernel's routing cache for the IP address of the remote TCP host.... look for anything odd or suspicious
If you can ping the remote TCP host (and you need to do about 200 pings to be sure... tcp is sensitive to significant packet loss & GPRS is notoriously lossy), try making a successful telnet <remote_host> <remote_port>. If both are successful, then it's time to start looking inside your software for clues.
If you still can't untangle what is happening, please include the output of the aforementioned commands when you come back... as well as how you're starting the ppp connection.
Pings should never be part of an end-user application(see note), and no program should rely on ping to function. At best ping might tell us that a part of the TCP/IP stack was running on the remote. See my argument here.
What the OP describes as a problem doesn't seem to be a problem. All network connections fail, the resolver may or may not use the network, and ping isn't really helpful. I would guess that the OP can check that the modem is connected or not, and if it isn't connect again.
edit: Pseudo code
do until success
try
connect "foobar.com"
try
write data
read response
catch
not success
endtry
catch error
'modem down - reconnect
not success
end try
loop
Note: the exception would be if you are writing a network monitoring application for a networking person.