Using the latest libssh 0.10.4 ssh_connect operation is failing at the key exchange step. Following is the log.
[2023/01/09 16:37:44.874356, 2] ssh_connect: libssh 0.10.4 (c) 2003-2022 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_winlock
[2023/01/09 16:37:44.881334, 3] getai: host x.x.x.x matches an IP address
[2023/01/09 16:37:44.885324, 2] ssh_socket_connect: Nonblocking connection socket: 512
[2023/01/09 16:37:44.887321, 2] ssh_connect: Socket connecting, now waiting for the callbacks to work
[2023/01/09 16:37:44.887903, 3] ssh_connect: Actual timeout : 10000
[2023/01/09 16:37:45.139544, 3] ssh_socket_pollcallback: Received POLLOUT in connecting state
[2023/01/09 16:37:45.140542, 1] socket_callback_connected: Socket connection callback: 1 (0)
[2023/01/09 16:37:45.140542, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket
[2023/01/09 16:37:45.376008, 3] callback_receive_banner: Received banner: SSH-2.0-Cisco-1.25
[2023/01/09 16:37:45.377005, 2] ssh_client_connection_callback: SSH server banner: SSH-2.0-Cisco-1.25
[2023/01/09 16:37:45.377005, 2] ssh_analyze_banner: Analyzing banner: SSH-2.0-Cisco-1.25
[2023/01/09 16:37:45.390968, 3] ssh_client_select_hostkeys: Order of wanted host keys: "ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256"
[2023/01/09 16:37:45.391964, 1] ssh_known_hosts_read_entries: Failed to open the known_hosts file 'C:\Users\xxxxx/.ssh/known_hosts': No such file or directory
[2023/01/09 16:37:45.391964, 1] ssh_known_hosts_read_entries: Failed to open the known_hosts file '/etc/ssh/ssh_known_hosts': No such file or directory
[2023/01/09 16:37:45.391964, 3] ssh_client_select_hostkeys: No key found in known_hosts; changing host key method to "ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256"
[2023/01/09 16:37:45.391964, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket
[2023/01/09 16:37:45.391964, 3] packet_send2: packet: wrote [type=20, len=852, padding_size=7, comp=844, payload=844]
[2023/01/09 16:37:45.392965, 3] ssh_send_kex: SSH_MSG_KEXINIT sent
[2023/01/09 16:37:45.626340, 3] ssh_packet_socket_callback: packet: read type 20 [len=308,padding=4,comp=303,payload=303]
[2023/01/09 16:37:45.627339, 3] ssh_packet_process: Dispatching handler for packet type 20
[2023/01/09 16:37:45.627339, 1] ssh_kex_select_methods: kex error : no match for method kex algos: server [diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1], client [curve25519-sha256,curve25519-sha256#libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256]
[2023/01/09 16:37:45.627339, 3] ssh_connect: current state : 9
Any idea about why it is failing? It connects with old libssh 0.9.4 and also via putty without any issues.
I'm using the ssh.dll which I had compiled from source with openssl 1.1.1k.
Did I miss selecting anything from CMake?
The problem is that the default set of enabled key exchange algorithms and host key algorithms no longer work with some old ssh servers. They can be enabled in the ssh run-time config file by setting the KexAlgorithms and HostKeyAlgorithms properties.
For example:
KexAlgorithms curve25519-sha256,curve25519-sha256#libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
HostKeyAlgorithms ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa
You could also add ,ssh-dss to the end of the HostKeyAlgorithms list, but you might not need it and it would probably only work if cmake was run with the -DWITH_DSA=ON option.
Related
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.
I have timeout errors in a Oracle database and when I sniff it using Wireshark I get a packet with the following info: TCP segment of a reassembled PDU.
It has this TCP information:
Transmission Control Protocol, Src Port: ncube-lm (1521), Dst Port: 57861 (57861), Seq: 1, Ack: 1, Len: 1
I have 10 packets more with the same origin and destination as the mentioned above.
After the first packet with info 'TCP segment of a reassembled PDU' there come 9 identical packets:
2728 596.537143000 10.XX.XX.XX 10.YY.YY.YY TCP 55 [TCP Keep-Alive] ncube-lm > 57861 [ACK] Seq=1 Ack=1 Win=258 Len=1.
Then I have one last packet:
2746 605.585011000 10.XX.XX.XX 10.YY.YY.YY TCP 54 ncube-lm > 57861 [RST, ACK] Seq=2 Ack=1
Win=0 Len=0
This last packet appears at the exact time when the timeout occurs in the database.
How can a length 1 packet having been reassembled? Why does it have been reassembled when it comes from our own local machine?
This packet's source its our Oracle database (port 1521). Why does it send a packet with one byte of data (with value '00')?
Thank you!
By default the keep-alive(tcp_keepalive_probes) probe count is 9. The machine has sent 9 keep-alive probes but didn't get any response from the other end so it has reset the connection.
Usually, TCP packets with a len of 1 are control packets (ACK,SYN,FIN,RST). I'm guessing this is the case you encountered.
Moreover, the Wireshark packet extra info (in your case - 'TCP segment of a reassembled PDU') sometime labels packets incorrectly, so you should not rely on it entirely.
I don't think these packets are the reason for the timeout issue. could you supply more info about the rest of the session and the packets timing?
I am writing a proxy in C (Windows 8.1 Environment). I did the same in Linux without any trouble but in windows things look different.
First of all, I am aware of MS Windows restrictions for sending RAW TCP Data and read this post. I also found WinPcap as a potential solution. The difference is I need to add Ethernet frame (DstMac, SrcMac, and Protocol Type) and ran the following codes:
if ((fp = pcap_open_live(d->name, // name of the device
65536, // portion of the packet to capture. It doesn't matter in this case
1, // promiscuous mode (nonzero means promiscuous)
1000, // read timeout
errbuf // error buffer
)) == NULL)
{
fprintf(stderr, "\nUnable to open the adapter. %s is not supported by WinPcap\n", d->name);
}
then I created a packet:
u_char * data;
data = (u_char *)malloc(sizeof(u_char)*(ip_size+14));
memcpy((void*)data, (void*)dmac, 6);
memcpy((void*)(data + 6), (void*)smac, 6);
USHORT TmpType = 8;
memcpy((void*)(data + 12), (void*)&TmpType, 2);
memcpy(data+14, iphdr, ip_size); //iphdr is the full packet -(minus) ethernet header
//ip_size is the sizeof(iphdr)
pcap_sendpacket(fp, data, ip_size+14); //14: size of ethernet header
free(data);
In my test the first iphdr is a TCP Syn packet. When I send it to the network, I can capture it in Wireshark. However, the remote server does not respond to this packet and I got [TCP Out-of-Ordfer] messages and then a few RST packet and nothing happens.
my questions are:
1- Is winpcap the only feasible solution in windows with C?
2- If no, could you please provide any example/resource for developing RAW TCP sockets in C?
3- If yes, what causes the server does not response my manually crafted TCP Syn.
PS: I use the same code for creating TCP SYN in linux and it works perfectly. So, I have no doubt the tcphdr contain the IP header, TCP header and payload with correct checksums in both IP, and TCP headers.
I'm using OpenSSL in order to encrypt some emails, that a piece of hardware sends. But, whenever I try to call SSL_connect(), I get :
SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol
After sending "EHLO" and "STARTTLS" I call the following function:
SSL_CTX *ctx = NULL;
SSL *ssl = NULL;
void CreateTLSSession(int sockfd)
{
printf("///////////////creating TLS Session/////////////////////\n");
SSL_library_init();
SSL_load_error_strings();
OpenSSL_add_all_algorithms();
ctx = SSL_CTX_new(SSLv23_client_method());
if (ctx == NULL)
{
printf("failed to initialize context\n");
return;
}
SSL_CTX_set_options(ctx, SSL_OP_NO_SSLv2);
ssl = SSL_new(ctx);
if (!SSL_set_fd(ssl, sockfd))
{
printf("failed to bind to socket fd\n");
return;
}
if (SSL_connect(ssl) < 1)
{
ERR_print_errors_fp(stdout);
fflush(stdout);
printf("SSL_connect failed\n");
return;
}
}
I've tried connecting to :
smtp.live.com : 587 --> SSL routines:SSL23_GET_SERVER_HELLO:unknown
protocol s23_clnt.c:787:
smtp.live.com : 25 --> SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol s23_clnt.c:787:
smtp.gmail.com : 587 --> SSL routines:SSL23_GET_SERVER_HELLO:unknown
protocol s23_clnt.c:787:
smtp.gmail.com : 465 --> no response from server at all!
smtp.gmail.com : 25 --> SSL routines:SSL23_GET_SERVER_HELLO:unknown
protocol s23_clnt.c:787:
I've tried different ports, since some similar questions on this SO suggested, that such error is usually related to using the wrong port for SSL.
Am I missing something here?
UPDATE:
All other methods (i.e. TLSv1_1_method(), SSLv3_method() ...) lead to SSL3_GET_RECORD:wrong version number
UPDATE:
I was able to observe the following on wireshark:
"EHLO"
"at your service"
"STARTTLS"
"Ready to starttls"
-->now I call the above function
unreadable request (encrypted)
unreadable reply (encrypted)
--> ERROR
SMTP servers on ports 587 and 25 are usually plain text and will switch to TLS only after the initial SMTP dialog and a STARTTLS command from the client. And trying SSL_connect on the plain text socket will fail.
Another way to solve this problem may be to run your C program under Scott Gifford's sslclient (see http://www.superscript.com/ucspi-ssl/sslclient.html). sslclient will spawn your program and open an tcp connection to the server, and pipe your program's stdout to the server, and pipe output from the server to your program's stdin. He has a patched version for TLS that will start the connection off in plain text, then once the two sides have agreed on STARTTLS, your program can signal to sslcient to enable SSL encryption on the connection by writing a command to a file descriptor for this purpose. The nice thing about doing it this way is that you can let sslclient to all the heavy lifting as far as setting up the sockets and ssl, etc., and you can focus on the core function of your program.
The underlying socked was non-blocking. The problem was solved, by using select and waiting till the TLS handshake completes.
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.