MaraDNS - a security-aware DNS server

MaraDNS A security-aware DNS server |  |
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# MaraDNS' security For people who just want to quickly get current with MaraDNS' security history should jump to the history section. MaraDNS should be a secure DNS server. The reason why I say should be instead of "is" is because I find claims of security very pretentious. There is no way that I can guarantee that a piece of code as complex as a recursive DNS server is completely free from security bugs. What I can guarantee, however, is that such bugs are very unlikely in MaraDNS, since the design has security checks and balances which minimize the chance of there being a security hazard in the code. ## Why code is insecure The main reason why code is insecure is because the code in question has undefined results when fed data which is in a different form than what the author of the code expected. The simplest case of this is the buffer overflow, where a program is fed a string far longer than the program was designed to handle. Another example is the "cache poison" bug which ancient versions of another DNS implementation had. With this bug, it was a trivial matter to tell the DNS server that, for example, www.yahoo.com had an ip address of say, 10.69.69.69, which really points to some sleazy site that installs spyware. Why did this bug exist? Because the original authors of this server did not expect remote servers to deliberately give out incorrect IP addresses. ## How MaraDNS avoids these problems First of all, MaraDNS uses a special string library which is resistant to buffer overflows. It does this by having a maximum allowed size for a string--any attempt to make a string larger than the maximum allowed size causes the string library to return an error instead of overflowing a buffer. In addition, the string library allocates an extra three bytes for each string object, as a cushion against possible boundary conditions (there was one case with another program which strived to be secure where a one-byte buffer overflow resulted in a remote root exploit). Second of all, MaraDNS protects herself from cache poisoning without ever discarding glue records is by handling out-of-bailiwick records differently, depending on where the offending records are. The actual rules are fairly complex, and detailed in a document called "cache.poison.protection ". Third of all, I have extensively read Dan Bernstein's notes on DNS implementation before implementing MaraDNS. Two ideas for implementing DNS security discussed on Dan's pages have been implemented in MaraDNS: When the recursive nameserver is performing queries, MaraDNS uses a securely generated pseudo-random-number-generator to generate the query ID, and the lower 12 bits of the query source port. This means that a given attempt to spoof a reply to MaraDNS has less than a 1 in 250 million chance of succeeding. MaraDNS uses a FIFO queue to delete unused records when the cache starts to fill up. The algorithm is simple: Every time a record is accessed, it is placed at the top of the list. When memory fills up, MaraDNS deletes records at the bottom of the list. This allows MaraDNS to act in a very graceful manner when the cache starts to fill up. Some known security problems are avoided: MaraDNS does not use printf() in a manner which would make format string vulnerabilities possible; MaraDNS does not use any of the potentially dangerous globing functions; and MaraDNS signal handlers are very short, avoiding the problems caused by poor signal handler construction. I go though the code, and when I find anything which has the possibility of allowing the data to be in an undefined state, I revise the code in question. For example, when I was looking through the code in the 0.8.35 release of MaraDNS; I found a case where the data could potentially become undefined. In more detail, if certain strings could not be allocated or copied over--cases which can never happen--the code which removes elements from the cache would attempt to free() unallocated memory later on. Even though this was a case where it was impossible to exploit the code in question, I felt it was prudent to update the code to not have this problem--hence the vaguely worded "security update" for the 0.8.99 release of MaraDNS. MaraDNS uses a minimum number of external libraries. This minimizes the impact of insecure code not under MaraDNS' direct control causing security problems with MaraDNS. This stops MaraDNS from having the kind of remote full-control exploit BIND9 had when it linked to an insecure version of the OpenSSL library. MaraDNS also mandates that the server is run as an unprivileged user, and strongly encourages MaraDNS to be run in a chroot() environment. In addition, the DNS cache uses a separate data structure than the local DNS records, making it difficult, if not impossible, for the cache at affect the local records. This design means that, even if there is a security hole in MaraDNS, the possibility of such a security hole allowing the attacker having elevated privileges is next to zero. ## The security history of MaraDNS No stable version of MaraDNS on a non-Linux system has ever had an exploitable security problem. To clarify: "stable version" is a version that has been marked stable on the MaraDNS download page (all Maradns 1.0 releases; Maradns 1.2.00, 1.2.01, 1.2.03.x, 1.2.07.x, and 1.2.12.x); "exploitable" means that the problem is one where a a practical exploit would be reasonably easy to perform. The issues I'm not considering "exploitable" are the cryptographic cache sniffing issues. There have been four security problems found with non-stable versions of MaraDNS: Two fairly major, and two minor security bugs. There was also a major security problem caused by broken behavior in Linux's kernel that affected stable versions of MaraDNS. There are also theoretical (less than minor) security problems caused by some research in to AES' implementation on modern cached processors which have been addressed. Here are all of the security problems ever discovered with MaraDNS: The first major security problem was discovered by Roy Arends, one of the BIND developers, and was caused by the fact that very early versions of MaraDNS did not check to see whether a given DNS packet was a question or an answer. This was fixed well before the 1.0 release of MaraDNS. Since a DNS answer looks like a DNS question, with the exception of a one-bit difference, this bug allowed an attacker to send a spoofed DNS packet which would result in MaraDNS sending an answer to herself (or to another MaraDNS server), which would result in another answer being sent, and so on. The reason this security glitch passed through was because of an overzealous effort to honor the RFC spirit to be "liberal in what one accepts and conservative in what one sends". Since then, I have gone over the DNS spec to see if their are any other cases where a malformed packet could cause this kind of security problem, and have made an effort that any such cases in MaraDNS no longer exist. Impact: Remote denial of service. Another major security problem was found by myself when performing an audit of the code during the beta test cycle. It involved the decompression code, and was caused because DNS compression is difficult to implement, and easy to create security holes with. I resolved this issue by completely rewriting the code in question with security in mind. This security problem was fixed in MaraDNS 0.9.01 (the rewrite came later). Impact: Remote denial of service. Another major problem, which allowed a remote denial of service, was caused by some broken behavior in the Linux kernel. People using BSD or other kernels were not affected by this security problem. This security problem was fixed in MaraDNS 1.0.28 and in MaraDNS 1.1.35. Impact: Remote denial of service. One old minor security problem involved the seed for the pseudo-random number generator. In cases where there was an ASCII null in the key for the pseudo-random number generator, MaraDNS would have less than a full 128 bits of entropy for the seed. There was also a related issue, even more minor, where, in certain rare circumstances, multiple instances of MaraDNS could potentially generate the same pseudo-random numbers, if two copies of MaraDNS used the same static random seed file, and were started at the same time. These issues were fixed before the release of MaraDNS 0.8.24. Impact: Theoretical spoofing of DNS records. Another minor problem, more recently discovered, was only present in the development (1.1) branch of MaraDNS. The `fetchzone` tool did not perform enough input validation and was vulnerable to specially formed packets that could send out-of-bailiwick data. I fixed this problem both ways; first by giving the fetchzone tool more input validation. Later on, I modified the csv2 parser to not accept the kind of data which would have triggered the `fetchzone` security bug. This issue was fixed in MaraDNS 1.1.38 (the csv2 improvments appeared in MaraDNS 1.1.47). Impact: Spoofing of DNS records. A theoretical security problem with the underlying random number generator that MaraDNS uses to generate secure random numbers was discovered by D. J. Bernstein. Since the underlying random number generator uses a fairly simple key schedule (well, a simple key schedule for a cryptographic routine), and since the random number generator uses table lookups, modern CPUs will very slighly vary (on the order of billionths of a second) in the amount of time used to generate a secure random number, depending on the underlying key. Bernstein needed to examine over 200 million packets, obtaining very precise timing information on each packet, to extract a key. I worked around this security problem by having the random number generator rekey every million packets. These changes were done in MaraDNS 1.0.27 and 1.1.35. Mailing list posting describing this issue Impact: Theoretical remote spoofing of DNS records. More recently, Dag Arne Osvik, Adi Shamir (the "S" in RSA), and Eran Tromer discovered some sophisticated cache data leakage attacks against AES, the algorithm from which MaraDNS' secure random number generator is derived. I have responded to this issue by tweaking MaraDNS' secure random number generator to essentially not leak sensitive key data via cache lookups. These changes were done for MaraDNS 1.0.35 and 1.1.56. Patch which describes the issue and how I work around it Impact: Theoretical local spoofing of DNS records. specially crafted DNS packets to a server running MaraDNS. MaraDNS would function normally once the DNS packets were no longer being sent. This affected all versions of MaraDNS, and was fixed in MaraDNS 1.2.12.05, 1.0.40, and the 20070216 snapshot of MaraDNS. Another problem was that MaraDNS would leak approximately 20 bytes whenever an invalid DNS packet was received. This leak would have allowed an attacker to cause MaraDNS to allocate an arbitrary large amount of memory by sending a very large number of invalid DNS packers to the server running MaraDNS. This affected the 1.2 and 1.3 branches of MaraDNS, and was fixed in MaraDNS 1.2.12.05 and the 20070216 snapshot of MaraDNS.