Re: Connman v1.33 with systemd v230 : experiencing delay in IP assignment

Hi Shrikant, On 12/09/2016 02:37 PM, Shrikant Bobade wrote: > Getting connman waiting/stuck at /getrandom > (_rnd_get_system_entropy_getrandom) (by default gnutls enabled) > Hmm, getrandom is part of gnutls? Maybe post the stack trace, that would help to understand the situation better.

So tried to increase entropy with help of /dev/urandom & and with > sufficient ~3k+ entropy count observed the getrandom call completed > successfully. > Is this know behaviour Or experienced by anyone.. ? > Haven't seen this problem before but that doesn't mean it doesn't exists. :~# rngd -r /dev/urandom -o > /dev/random > > :~# cat /proc/sys/kernel/random/entropy_avail > 3087 > > > Please advise, we suppose to use only /dev/random for better security > reasons and not /dev/urandom ? thoughts if any for other ways to deal > with less/no entropy available with /dev/random? > http://security.stackexchange.com/questions/122155/how-bad-i > t-is-to-feed-dev-random-with-dev-urandom > http://stackoverflow.com/questions/3690273/did-i-understand- > dev-urandom#comment30985465_3709644 > I had to read up on the random and urandom. I also checked if any of our calls to __connman_util_get_random() is a problem (see below [1]). It looks like that our code base is using urandom correctly. So it is how we use gnutls. I wonder how this could be a problem in assigning IP addresses. Could you past a complete log?

Thanks, Daniel

[1] ** src/config.c: __connman_util_get_random(&rand); generate_random_string(rstr, 11); vfile = g_strdup_printf("service_mutable_%s.config", rstr); ** src/dhcpv6.c: __connman_util_get_random(&rand); /* Initial timeout, RFC 3315, 18.1.5 */ __connman_util_get_random(&rand); delay = rand % 1000; RFC 3314 chapter 18.1.5 """ The first Confirm message from the client on the interface MUST be delayed by a random amount of time between 0 and CNF_MAX_DELAY. """ ** src/dhcpv6.c: __connman_util_get_random(&rand); /* Initial timeout, RFC 3315, 17.1.2 */ __connman_util_get_random(&rand); delay = rand % 1000; RFC 3314 chapter 17.1.2 """ The first Solicit message from the client on the interface MUST be delayed by a random amount of time between 0 and SOL_MAX_DELAY. In the case of a Solicit message transmitted when DHCP is initiated by IPv6 Neighbor Discovery, the delay gives the amount of time to wait after IPv6 Neighbor Discovery causes the client to invoke the stateful address autoconfiguration protocol (see section 5.5.3 of RFC 2462). This random delay desynchronizes clients which start at the same time (for example, after a power outage). """ ** src/dhcpv6.c: __connman_util_get_random(&rand); static guint compute_random(guint val) { uint64_t rand; __connman_util_get_random(&rand); return val - val / 10 + ((guint) rand % (2 * 1000)) * val / 10 / 1000; } /* Calculate a random delay, RFC 3315 chapter 14 */ /* RT and MRT are milliseconds */ static guint calc_delay(guint RT, guint MRT) { if (MRT && (RT > MRT / 2)) RT = compute_random(MRT); else RT += compute_random(RT); return RT; } RFC 3315 chapter 14 """ Each of the computations of a new RT include a randomization factor (RAND), which is a random number chosen with a uniform distribution between -0.1 and +0.1. The randomization factor is included to minimize synchronization of messages transmitted by DHCP clients. The algorithm for choosing a random number does not need to be cryptographically sound. The algorithm SHOULD produce a different sequence of random numbers from each invocation of the DHCP client. """ ** src/dnsproxy.c: __connman_util_get_random(&rand); req->dstid = get_id(); req->altid = get_id(); RFC 5625 chapter 6.1 """ It has been standard guidance for many years that each DNS query should use a randomly generated Query ID. However, many proxies have been observed picking sequential Query IDs for successive requests. It is strongly RECOMMENDED that DNS proxies follow the relevant recommendations in [RFC5452], particularly those in Section 9.2 relating to randomisation of Query IDs and source ports. This also applies to source port selection within any NAT function """ RFC 5452 chapter 9.2. Extending the Q-ID Space by Using Ports and Addresses """ Resolver implementations MUST: o Use an unpredictable source port for outgoing queries from the range of available ports (53, or 1024 and above) that is as large as possible and practicable; o Use multiple different source ports simultaneously in case of multiple outstanding queries; o Use an unpredictable query ID for outgoing queries, utilizing the full range available (0-65535). Resolvers that have multiple IP addresses SHOULD use them in an unpredictable manner for outgoing queries. Resolver implementations SHOULD provide means to avoid usage of certain ports. Resolvers SHOULD favor authoritative nameservers with which a trust relation has been established; stub-resolvers SHOULD be able to use Transaction Signature (TSIG) ([RFC2845]) or IPsec ([RFC4301]) when communicating with their recursive resolver. In case a cryptographic verification of response validity is available (TSIG, SIG(0)), resolver implementations MAY waive above rules, and rely on this guarantee instead. Proper unpredictability can be achieved by employing a high quality (pseudo-)random generator, as described in [RFC4086]. """ RFC 4086 chapter 7. Randomness Generation Examples and Standards """ Several public standards and widely deployed examples are now in place for the generation of keys or other cryptographically random quantities. Some, in section 7.1, include an entropy source. Others, described in section 7.2, provide the pseudo-random number strong-sequence generator but assume the input of a random seed or input from a source of entropy. """ RFC 4086 chapter 7.1.2. The /dev/random Device """ Several versions of the UNIX operating system provide a kernel- resident random number generator. Some of these generators use events captured by the Kernel during normal system operation. For example, on some versions of Linux, the generator consists of a random pool of 512 bytes represented as 128 words of 4 bytes each. When an event occurs, such as a disk drive interrupt, the time of the event is XOR'ed into the pool, and the pool is stirred via a primitive polynomial of degree 128. The pool itself is treated as a ring buffer, with new data being XOR'ed (after stirring with the polynomial) across the entire pool. Each call that adds entropy to the pool estimates the amount of likely true entropy the input contains. The pool itself contains a accumulator that estimates the total over all entropy of the pool. Input events come from several sources, as listed below. Unfortunately, for server machines without human operators, the first and third are not available, and entropy may be added slowly in that case. 1. Keyboard interrupts. The time of the interrupt and the scan code are added to the pool. This in effect adds entropy from the human operator by measuring inter-keystroke arrival times. 2. Disk completion and other interrupts. A system being used by a person will likely have a hard-to-predict pattern of disk accesses. (But not all disk drivers support capturing this timing information with sufficient accuracy to be useful.) 3. Mouse motion. The timing and mouse position are added in. When random bytes are required, the pool is hashed with SHA-1 [SHA*] to yield the returned bytes of randomness. If more bytes are required than the output of SHA-1 (20 bytes), then the hashed output is stirred back into the pool and a new hash is performed to obtain the next 20 bytes. As bytes are removed from the pool, the estimate of entropy is correspondingly decremented. To ensure a reasonably random pool upon system startup, the standard startup and shutdown scripts save the pool to a disk file at shutdown and read this file at system startup. There are two user-exported interfaces. /dev/random returns bytes from the pool but blocks when the estimated entropy drops to zero. As entropy is added to the pool from events, more data becomes available via /dev/random. Random data obtained from such a /dev/random device is suitable for key generation for long term keys, if enough random bits are in the pool or are added in a reasonable amount of time. /dev/urandom works like /dev/random; however, it provides data even when the entropy estimate for the random pool drops to zero. This may be adequate for session keys or for other key generation tasks for which blocking to await more random bits is not acceptable. The risk of continuing to take data even when the pool's entropy estimate is small in that past output may be computable from current output, provided that an attacker can reverse SHA-1. Given that SHA-1 is designed to be non-invertible, this is a reasonable risk. To obtain random numbers under Linux, Solaris, or other UNIX systems equipped with code as described above, all an application has to do is open either /dev/random or /dev/urandom and read the desired number of bytes. (The Linux Random device was written by Theodore Ts'o. It was based loosely on the random number generator in PGP 2.X and PGP 3.0 (aka PGP 5.0).) """ man page on urandom: A read from the /dev/urandom device will not block waiting for more entropy. If there is not sufficient entropy, a pseudorandom number generator is used to create the requested bytes. As a result, in this case the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current unclassified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead. O_NONBLOCK has no effect when opening /dev/urandom. When calling read(2) for the device /dev/urandom, signals will not be handled until after the requested random bytes have been generated.