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One of many first low-level community instruments that any laptop consumer learns about is the venerable ping utility.
Named after the eponymous sound impact from any and each old-school struggle film scene involving submarines, the command is a metaphorical echo (see what we did there?) of the underwater model of RADAR referred to as SONAR.
You ship out a p-i-n-g (extra like a d-o-i-n-n-n-n-g noise, in actuality) into the briny depths, and by measuring how lengthy it takes for its eerie echo to return again to you, and by estimating the pace of sound within the surrounding ocean, you may compute the space to the thing that produced the echo.
Intriguingly, given that you just’ve most likely heard the abbreviation TCP/IP used as a generic description of the protocol glue that powers the web, ping doesn’t technically use TCP/IP in any respect.
The truth is, TCP/IP is brief for transmission management protocol over the web protocol, and refers to a reasonably high-level mechanism for sending knowledge over the web in such a means that the community itself places in numerous the “did that really work correctly?” effort for you.
For instance, in TCP connections, any chunks of information you ship are assured both to reach intact on the different finish or to trigger an error so you understand they didn’t make it.
Moreover, even when totally different knowledge chunks find yourself taking totally different routes throughout the web (for instance on account of load balancing, non permanent outages, or different recoverable errors), and even when some chunks take longer to reach than others, TCP knowledge will probably be accurately buffered up and offered in the suitable order on the different finish.
Ping is totally different
The ping command, nevertheless, is usually used to confirm if a pc you’re involved in is on-line in any respect, particularly if it’s not accepting the form of high-level TCP connections you’d anticipate, resembling receiving e-mail or permitting SSH logins.
This rapidly helps you establish whether or not an outage is prone to be because of the community or the server itself happening, or on account of particular person companies working on that server failing to begin accurately.
In consequence, ping makes use of a a lot lower-level protocol than TCP.
Certainly, ping doesn’t even use TCP’s extra informal cousin UDP, brief for consumer datagram protocol, which is a means of transmitting knowledge chunks that’s quick and simple, however is popularly known as send-and-hope (or, in case you are a cynical type, as spray-and-pray).
UDP itself doesn’t inform you whether or not your knowledge bought to the opposite finish or not, and even when it arrives intact, UDP doesn’t preserve monitor of the order wherein your packets had been initially despatched, so it may possibly’t rearrange them on the different finish in the event that they get there out of sequence.
Ping, for what it’s value, makes use of a very-low-level-indeed protocol, specifically designed for troubleshooting and community reconfiguration functions, referred to as ICMP, or web management message protocol.
Sometimes dealt with proper within the working system kernel, in order that ICMP/IP packets are virtually sure to get via even when no higher-level networking software program has come up accurately, ICMP notably consists of two particular message sorts:
Kind 0x08. Formally referred to as ICMP Echo, this form of packet is normally referred to as an Echo Request. It’s what the ping program sends out to be able to probe for energetic computer systems on the community.
Kind 0x00. Formally referred to as ICMP Echo Reply, this packet sort is strictly what it says. A pc that’s alive, on-line, and never configured to dam ICMP Echo site visitors is meant to ship this form of packet straight again to the pc that requested it.
Like this:
$ ping -c 3 -p 4E414B45445345435552495459 nakedsecurity.sophos.com
PATTERN: 0x4e414b45445345435552495459
PING news-sophos.go-vip.internet (192.0.66.227) 56(84) bytes of information.
64 bytes from 192.0.66.227 (192.0.66.227): icmp_seq=1 ttl=53 time=84.0 ms
64 bytes from 192.0.66.227 (192.0.66.227): icmp_seq=2 ttl=53 time=85.1 ms
64 bytes from 192.0.66.227 (192.0.66.227): icmp_seq=3 ttl=53 time=84.8 ms
— news-sophos.go-vip.internet ping statistics —
3 packets transmitted, 3 obtained, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 84.025/84.644/85.062/0.446 ms
To see a ping in motion at a barely decrease stage, we’ll use the Lua code that you’ll find on the finish of the article to assemble an ICMP Echo packet of our personal, and to learn the reply that comes again, if any:
$ sudo luax ping.lua nakedsecurity.sophos.com
Sending ICMP ECHO request to 192.0.66.227 –>
00000000 08 00 03 02 bb 5a 6f 1d 50 69 6e 67 52 65 71 75 |…..Zo.PingRequ|
00000010 65 73 74 4d 65 73 73 61 67 65 20 42 42 35 41 36 |estMessage BB5A6|
00000020 46 31 44 |F1D |
Acquired back–>
00000000 45 00 00 37 f6 af 00 00 35 01 94 7f c0 00 42 e3 |E..7….5…..B.|
00000010 XX XX XX XX 00 00 0b 02 bb 5a 6f 1d 50 69 6e 67 |………Zo.Ping|
00000020 52 65 71 75 65 73 74 4d 65 73 73 61 67 65 20 42 |RequestMessage B|
00000030 42 35 41 36 46 31 44 |B5A6F1D |
By the way in which, we would have liked to make use of sudo above to run our script with superuser privileges, as a result of we created what’s referred to as a uncooked IP socket – one that may be crafted into any underlying format we like, together with TCP, UDP and, as wanted right here, ICMP.
On many Linux/Unix techniques, the ping command equipped by your distro works with out explicitly being given root privileges, normally as a result of it’s put in with particular safety capabilities, or with its setuid flag set, which means it begins off with working underneath a distinct consumer account than the consumer who ran it.
Effectively-designed ping applications, in fact, will mechanically discard their further privileges as soon as they’ve opened up the uncooked socket they want.
We omitted this privilege-dropping code from our pattern script for the sake of brevity. You should use the posix.unistd.setpid() operate to change to an unprivileged account after creating the socket, however earlier than sending or receiving any knowledge.
Analyzing the reply
As you would possibly recognise within the knowledge dump from our script above, the community socket operate we’re utilizing to learn again knowledge from the responding server consists of not solely the ICMP Echo Reply knowledge, but in addition the low-level IP (web protocol headers) within the underlying packet.
We haven’t tried to parse or in any other case course of this knowledge, however the FreeBSD ping program wants to take action to be able to make sense of the reply, together with making sense of any error messages that come again.
If the ping will get rejected not directly, the Echo Reply will sometimes embody not solely its personal IP headers (as seen above) but in addition a reference copy of the IP headers and the ICMP knowledge that appeared within the authentic outbound request.
IPv4 packet headers normally look very similar to you see above, the place the IP headers begin with 45 00 00 37… and proceed for 20 bytes in whole, as much as and together with the bytes proven as …XX XX XX XX, which is the IP deal with of my laptop computer.
Like this:
00000000 45 00 00 37 f6 af 00 00 35 01 94 7f c0 00 42 e3 |E..7….5…..B.|
00000010 XX XX XX XX |…. |
IP model and header size: 0x45 (4 = IPv4, 5 = 5 32-bit phrases, i.e. 20 bytes
Service sort and congestion knowledge: 0x00
Complete size of packet: 0x0037 (decimal 55)
Sequence info: F6 AF 00 00
Time-to-live (hops left): 0x35 (decimal 53)
Protocol sort: 0x01 (ICMP)
Checksum: 0x947F (decimal 38015)
Sending laptop’s IP quantity: C0 00 42 E3 (192.0.66.227 = nakedsecurity.sophos.com)
Recipient’s IP (my laptop computer): XX XX XX XX (REDACTED = my very own IP quantity)
FreeBSD’s ping programmers, it appears, assumed that headers of this type would, certainly, at all times be precisely 20 bytes lengthy, primarily based on that header-length worth within the first byte of 0x45, denoting IPv4 (0x4?) with a 5-DWORD (0x?5), or 20-byte, header.
With simply 20 bytes to fret about, the programmers allotted fixed-size buffers on the stack the place they might make a copy of the IP headers within the reply, plus any embedded IP headers from the unique request, if there was an error situation to deal with.
You’ll be able to guess the place that is going.
That first byte within the IPv4 header can legally have any worth from 0x45 (the minimal header dimension of 5 DWORDs, or 20 bytes, as proven) as much as 0x4F (denoting 15 DWORDs, as a result of 0xF is decimal 15, or 60 bytes of header knowledge in whole), thus neatly permitting for an non-compulsory further 40 bytes of header knowledge.
These uncommon, however authorized, further header bytes can be utilized for varied funky and weird “options” with curious names resembling Selective Directed Broadcast, Experimental Movement Management and Upstream Multicast Packet – issues that we’ve heard of however by no means knowingly used, and even seen.
Beware cybercriminals who put you to the check
As you may think about, on condition that these further fields are virtually by no means used, you would possibly by no means see an IPv4 packet with something apart from 0x45 initially, and with 20 bytes of header knowledge in whole, except you’ve run right into a bunch of cybercriminals who’re able to put you to the check.
Sadly, there’s not a lot to cease an attacker from rigging up a server that guesses whether or not you’re utilizing FreeBSD, and intentionally generates outsized ICMP/IP Echo Reply packets to be able to provoke a stack buffer overflow inside your ping program.
If ever you examine to see if their server is energetic (which you would possibly do even, or maybe particularly, when you assume it’s suspicious!), you can be focused with a booby-trapped reply.
At greatest, your ping program will crash; at worst, nevertheless, because the FreeBSD safety advisory generously admits, “it might be attainable for a malicious host to set off distant code execution in ping.”
Happily, because the FreeBSD authors additionally level out, “[t]he ping course of runs in a functionality mode sandbox on all affected variations of FreeBSD and is thus very constrained in the way it can work together with the remainder of the system on the level the place the bug can happen.”
In different phrases, you positively have to patch, however the dangers will be thought-about modest.
Notably, the ping program just isn’t solely locked in a sandbox, however isn’t working as root when the buggy code will get reached, as confirmed within the safety advisory: “When ping runs, it creates the uncooked socket wanted to do its work, after which revokes its elevated privileges.”
As described above, superuser powers are required solely to amass a uncooked IP socket from the working system, to not use the sendto() and recvfrom() features on that socket afterwards.
This bug has been given the official identifier CVE-2022-23093; it’s documented within the safety advisory FreeBSD-SA-22:15.ping.
What to do?
Should you’re a FreeBSD consumer, merely replace the affected releases (FreeBSD 12 and FreeBSD 13) to their newest variations, the place this bug is mounted.
Should you’re a community programmer, at all times guarantee that you’ve accounted for packet headers that might point out uncommon dimension variations. The truth that you’ve by no means seen any variation your self doesn’t cease you dealing with a packet tomorrow that’s out-of-the-ordinary but completely authorized.
Should you’re a community supervisor, contemplate blocking IPv4 packets with IP headers that aren’t 20 bytes in dimension. Should you genuinely appear to want to permit some software program merchandise to make use of uncommon IPv4 header choices, contemplate logging these uncommon packets to be taught why.
Take care on the market!
EXAMPLE CODE TO DEMONSTRATE PING TRAFFIC
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