3 ms·

> Just yesterday I used it to troubleshoot a weird behavior in a recently upgraded Asterisk/FreePBX system which would have probably taken me days to guess my w

by zvmaz 3y ago

> Just yesterday I used it to troubleshoot a weird behavior in a recently upgraded Asterisk/FreePBX system which would have probably taken me days to guess my way through without packet captures Do you mind sharing with us what was the problem and how you solved it with packet captures, if you have time? A blog post would be very interesting too.

toast0 3y ago
For something like this firefox bug [1], getting down to pcaps helps determine where the problem is. Client is spinning on a request and server doesn't know about it could be a server problem or a client problem or a network in the middle problem. In this case, the problem was the client wasn't actually sending the request, and with a sizable request that's visible even without decoding the https; although to be totally clear on what was happening, decoding was needed. I've also debugged issued in remote networks where iirc, connections were being reset by some equipment local to the user. Seq/ack sequencing showed the resets were in response to a specific client sent packet and the timestamps showed it was impossible for that to have come from anywhere but equipment near the user. For this bug [2], it took a lot of luck and patience to get a good capture, but once I did, the immediate problem became obvious: the machine I controlled was getting an icmp needs frag but DF set at the same mtu it was already using, and responding by sending the whole sendqueue at once, packetized to the new MTU that was the same as the old one. There's actually three problems here: a) there's no reason for the other side to send this packet (I found this is an already fixed linux bug with forwarding and large receive offload, but no way to contact the administrator of that router), b) our side shouldn't resend the whole sendqueue when the mtu changes, c) if the mtu didn't change, then there's no need to take any action. We only fixed c, but that solved the major problem: these resends would trigger more resends and we'd have periods of unavailability as the network was really busy. This is pretty common when looking at wireshark; unless you work somewhere with full control of all clients and servers and a very network aware developer team, you're going to find lots of non-optimal or semi-broken stuff, and you've got to ignore it and focus on the majorly broken bit. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1740856 https://bugzilla.mozilla.org/show_bug.cgi?id=1740856 [2] https://reviews.freebsd.org/rS288412 https://reviews.freebsd.org/rS288412
wolrah 3y ago
I'm not much of a blogger, but here's the short version. If anyone happened to be on #freepbx yesterday morning they might already have seen this. I had just upgraded and migrated one of my clients from an on premise FreePBX system that was a few years out of date and running on a repurposed desktop computer with a failing fan to a brand new instance running on a VPS. Everything was working fine with basic phone functionality, but their main ring group was taking a few seconds to stop ringing when answered. Calls would ring in to all phones effectively simultaneously as expected, but when someone answered the call certain phones kept ringing for almost four full seconds after that point. In the past I had seen similar behaviors on AT&T DSL caused by their mandatory modem/router device having an anti-flood filter enabled by default which saw a bunch of nearly identical UDP packets hitting at once and dropped them after the first few. This site has cable internet through a dumb modem so I knew it wasn't that, but they had recently had their IT side taken over by a new company who put in a new firewall so that was a plausible answer. Their IT however had been taken over from us so I wasn't about to go accusing them of getting it wrong without strong evidence. I'm also just that kind of person, I hate when someone blames me or my gear for problems we're not causing so I do my best to never be that guy either. I'll waste an extra few hours of mine any day of the week to be sure I'm not accusing someone else of getting it wrong without a reason. I fired up sngrep on the server, waited for a call to come in, and saved all the SIP sessions that resulted. Download that file, load it up in Wireshark, and I see that while the INVITE messages to start ringing all went out more or less simultaneously (27 phones in ~5ms) the CANCEL messages that stop them from ringing once one answered were sent out sequentially, with the PBX waiting for the first one to respond and confirm it had stopped ringing before sending the next. Clearly this wasn't right, and it obviously wasn't a problem with the firewall either. At that point I started looking at the Asterisk logs and saw that an AGI script was being run for each line that was ringing which wasn't there previously. That script was associated with a new FreePBX module for missed call notifications which was installed but unconfigured on the new server. It didn't indicate it was doing anything in the UI, but it sure seemed to be doing something in the logs. I uninstalled that module and the next call all the CANCEL messages went out in ~5ms just like the INVITEs. I then filed a bug with FreePBX documenting what happened because I'm pretty sure it's not expected or desired for simply having that module installed to cause massive delays in ring groups. --- In this case the packet captures demonstrated conclusively that the problem was on the server itself and not in the network. If the capture at the server had looked reasonable my next step would have been to have the IT vendor capture traffic on their firewall at the same time as I was capturing at the server so we could compare and see if it's getting messed with along the way, but here it was not necessary. Like toast0 mentioned, captures help you narrow down where the problem is.
licebmi__at__ 3y ago
I had to troubleshoot an issue where several network routers restarted in group without a cause but only when connected to the big wan. The problem was a network discovery software which when poorly configured, would send ssh connection attempts to the management interface and a bug on the specific firmware would crash the router. A network capture was the only good clue.

by zvmaz 3y ago

toast0 3y ago

For something like this firefox bug [1], getting down to pcaps helps determine where the problem is. Client is spinning on a request and server doesn't know about it could be a server problem or a client problem or a network in the middle problem. In this case, the problem was the client wasn't actually sending the request, and with a sizable request that's visible even without decoding the https; although to be totally clear on what was happening, decoding was needed. I've also debugged issued in remote networks where iirc, connections were being reset by some equipment local to the user. Seq/ack sequencing showed the resets were in response to a specific client sent packet and the timestamps showed it was impossible for that to have come from anywhere but equipment near the user. For this bug [2], it took a lot of luck and patience to get a good capture, but once I did, the immediate problem became obvious: the machine I controlled was getting an icmp needs frag but DF set at the same mtu it was already using, and responding by sending the whole sendqueue at once, packetized to the new MTU that was the same as the old one. There's actually three problems here: a) there's no reason for the other side to send this packet (I found this is an already fixed linux bug with forwarding and large receive offload, but no way to contact the administrator of that router), b) our side shouldn't resend the whole sendqueue when the mtu changes, c) if the mtu didn't change, then there's no need to take any action. We only fixed c, but that solved the major problem: these resends would trigger more resends and we'd have periods of unavailability as the network was really busy. This is pretty common when looking at wireshark; unless you work somewhere with full control of all clients and servers and a very network aware developer team, you're going to find lots of non-optimal or semi-broken stuff, and you've got to ignore it and focus on the majorly broken bit. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1740856 https://bugzilla.mozilla.org/show_bug.cgi?id=1740856 [2] https://reviews.freebsd.org/rS288412 https://reviews.freebsd.org/rS288412

wolrah 3y ago

I'm not much of a blogger, but here's the short version. If anyone happened to be on #freepbx yesterday morning they might already have seen this. I had just upgraded and migrated one of my clients from an on premise FreePBX system that was a few years out of date and running on a repurposed desktop computer with a failing fan to a brand new instance running on a VPS. Everything was working fine with basic phone functionality, but their main ring group was taking a few seconds to stop ringing when answered. Calls would ring in to all phones effectively simultaneously as expected, but when someone answered the call certain phones kept ringing for almost four full seconds after that point. In the past I had seen similar behaviors on AT&T DSL caused by their mandatory modem/router device having an anti-flood filter enabled by default which saw a bunch of nearly identical UDP packets hitting at once and dropped them after the first few. This site has cable internet through a dumb modem so I knew it wasn't that, but they had recently had their IT side taken over by a new company who put in a new firewall so that was a plausible answer. Their IT however had been taken over from us so I wasn't about to go accusing them of getting it wrong without strong evidence. I'm also just that kind of person, I hate when someone blames me or my gear for problems we're not causing so I do my best to never be that guy either. I'll waste an extra few hours of mine any day of the week to be sure I'm not accusing someone else of getting it wrong without a reason. I fired up sngrep on the server, waited for a call to come in, and saved all the SIP sessions that resulted. Download that file, load it up in Wireshark, and I see that while the INVITE messages to start ringing all went out more or less simultaneously (27 phones in ~5ms) the CANCEL messages that stop them from ringing once one answered were sent out sequentially, with the PBX waiting for the first one to respond and confirm it had stopped ringing before sending the next. Clearly this wasn't right, and it obviously wasn't a problem with the firewall either. At that point I started looking at the Asterisk logs and saw that an AGI script was being run for each line that was ringing which wasn't there previously. That script was associated with a new FreePBX module for missed call notifications which was installed but unconfigured on the new server. It didn't indicate it was doing anything in the UI, but it sure seemed to be doing something in the logs. I uninstalled that module and the next call all the CANCEL messages went out in ~5ms just like the INVITEs. I then filed a bug with FreePBX documenting what happened because I'm pretty sure it's not expected or desired for simply having that module installed to cause massive delays in ring groups. --- In this case the packet captures demonstrated conclusively that the problem was on the server itself and not in the network. If the capture at the server had looked reasonable my next step would have been to have the IT vendor capture traffic on their firewall at the same time as I was capturing at the server so we could compare and see if it's getting messed with along the way, but here it was not necessary. Like toast0 mentioned, captures help you narrow down where the problem is.

licebmi__at__ 3y ago

I had to troubleshoot an issue where several network routers restarted in group without a cause but only when connected to the big wan. The problem was a network discovery software which when poorly configured, would send ssh connection attempts to the management interface and a bug on the specific firmware would crash the router. A network capture was the only good clue.